Patent Publication Number: US-9843654-B2

Title: Systems and methods for wireless communication of packets having a plurality of formats

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/466,929, titled “SYSTEMS AND METHODS FOR WIRELESS COMMUNICATION OF PACKETS HAVING A PLURALITY OF FORMATS,” filed May 8, 2012, which claims the benefit of U.S. Provisional Patent Application No. 61/585,557 entitled “SYSTEMS AND METHODS FOR WIRELESS COMMUNICATION OF PACKETS HAVING A PLURALITY OF FORMATS” filed on Jan. 11, 2012, which claims the benefit of U.S. Provisional Patent Application No. 61/580,613 entitled “SYSTEMS AND METHODS FOR WIRELESS COMMUNICATION OF PACKETS HAVING A PLURALITY OF FORMATS” filed on Dec. 27, 2011, which claims the benefit of U.S. Provisional Patent Application No. 61/577,442 entitled “SYSTEMS AND METHODS FOR WIRELESS COMMUNICATION OF PACKETS HAVING A PLURALITY OF FORMATS” filed on Dec. 19, 2011, which claims the benefit of U.S. Provisional Patent Application No. 61/488,714 entitled “SYSTEMS AND METHODS FOR WIRELESS COMMUNICATION OF PACKETS HAVING A PLURALITY OF FORMATS” filed on May 21, 2011, which claims the benefit of U.S. Provisional Patent Application No. 61/486,107 entitled “SYSTEMS AND METHODS FOR WIRELESS COMMUNICATION OF PACKETS HAVING A PLURALITY OF FORMATS” filed on May 13, 2011. The above applications are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Field 
     The present application relates generally to wireless communications, and more specifically to systems and methods for communicating packets having a plurality of different formats. 
     Background 
     In many telecommunication systems, communications networks are used to exchange messages among several interacting spatially-separated devices. Networks may be classified according to geographic scope, which could be, for example, a metropolitan area, a local area, or a personal area. Such networks would be designated respectively as a wide area network (WAN), metropolitan area network (MAN), local area network (LAN), or personal area network (PAN). Networks also differ according to the switching/routing technique used to interconnect the various network nodes and devices (e.g. circuit switching vs. packet switching), the type of physical media employed for transmission (e.g. wired vs. wireless), and the set of communication protocols used (e.g. Internet protocol suite, SONET (Synchronous Optical Networking), Ethernet, etc.). 
     Wireless networks are often preferred when the network elements are mobile and thus have dynamic connectivity needs, or if the network architecture is formed in an ad hoc, rather than fixed, topology. Wireless networks employ intangible physical media in an unguided propagation mode using electromagnetic waves in the radio, microwave, infra-red, optical, etc. frequency bands. Wireless networks advantageously facilitate user mobility and rapid field deployment when compared to fixed wired networks. 
     The devices in a wireless network may transmit/receive information between each other. The information may comprise packets, which in some aspects may be referred to as data units. The packets may include overhead information (e.g., header information, packet properties, etc.) that helps in routing the packet through the network, identifying the data in the packet, processing the packet, etc., as well as data, for example user data, multimedia content, etc. as might be carried in a payload of the packet. 
     After a packet is received, portions of the overhead or control information in a packet may be used to determine parameters for processing data carried in the packet. The packet, however, may be formatted in a plurality of ways. Accordingly, it is advantageous for a transmitting node to be able to determine which format to use for a given communication and to generate the communication. Similarly, it is advantageous for a receiving node to be able to determine the format of the packet and process the data in the packet accordingly. Thus, improved systems, methods, and devices for communicating packets having a plurality of formats are desired. 
     SUMMARY 
     The systems, methods, and devices of the invention each have several aspects, no single one of which is solely responsible for its desirable attributes. Without limiting the scope of this invention as expressed by the claims which follow, some features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled “Detailed Description” one will understand how the features of this invention provide advantages that include improved approaches for communicating packets having a plurality of formats. 
     One aspect of the disclosure provides an apparatus for wireless communication. The apparatus comprises a receiver configured to receive a wireless communication comprising a physical layer preamble and a payload. The preamble may include a first field indicating whether the preamble includes an extension field. The apparatus further comprises a processor configured to process the payload based on modulation coding parameters included in the first field when the indicator signifies that the preamble does not include the extension field, and configured to process the payload based on coding parameters included in the extension field when the indicator signifies that the preamble includes the extension field. 
     Another aspect of the disclosure provides a method for wireless communication. The method comprises receiving a wireless communication comprising a physical layer preamble and a payload. The preamble includes a first field indicating whether the preamble includes an extension field. The method further comprises processing the payload based on modulation coding parameters included in the first field when the indicator signifies that the preamble does not include the extension field, and processing the payload based on coding parameters included in the extension field when the indicator signifies that the preamble includes the extension field. 
     Another aspect of the disclosure provides an apparatus for wireless communication. The apparatus comprises means for receiving a wireless communication comprising a physical layer preamble and a payload. The preamble may include a first field indicating whether the preamble includes an extension field. The apparatus further comprises means for processing the payload based on modulation coding parameters included in the first field when the indicator signifies that the preamble does not include the extension field, and means for processing the payload based on coding parameters included in the extension field when the indicator signifies that the preamble includes the extension field. 
     Another aspect of the disclosure provides a computer readable medium comprising instructions that when executed cause an apparatus to receive a wireless communication comprising a physical layer preamble and a payload. The preamble may include a first field indicating whether the preamble includes an extension field. The medium further comprises instructions that when executed cause an apparatus to process the payload based on modulation coding parameters included in the first field when the indicator signifies that the preamble does not include the extension field, and process the payload based on coding parameters included in the extension field when the indicator signifies that the preamble includes the extension field. 
     Another aspect of the disclosure provides an apparatus for wireless communication. The apparatus comprises a processor configured to generate a communication comprising a physical layer preamble and a payload and to determine whether to include an extension field in the preamble. The preamble includes a first field indicating whether the extension field is included. The processor is configured to include modulation coding parameters for the payload in the first field when it is determined not to include the extension field, and to include coding parameters for the payload in the extension field when it is determined to include the extension field. The apparatus further comprises a transmitter configured to wirelessly transmit the generated communication. 
     Another aspect of the disclosure provides a method of wireless communication. The method comprises determining whether to include an extension field in a physical layer preamble of a communication, generating the communication, and wirelessly transmitting the generated communication. The communication comprises the preamble and a payload, and the preamble includes a first field indicating whether the extension field is included. The generation may comprise including modulation coding parameters for the payload in the first field when it is determined not to include the extension field, and including coding parameters for the payload in the extension field when it is determined to include the extension field. 
     Another aspect of the disclosure provides an apparatus for wireless communication. The apparatus comprises means for determining whether to include an extension field in a physical layer preamble of a communication, means for generating the communication, and means for wirelessly transmitting the generated communication. The communication comprises the preamble and a payload, and the preamble includes a first field indicating whether the extension field is included. The means for generating may comprise means for including modulation coding parameters for the payload in the first field when it is determined not to include the extension field, and means for including coding parameters for the payload in the extension field when it is determined to include the extension field. 
     Another aspect of the disclosure provides a computer readable medium comprising instructions that when executed cause an apparatus to determine whether to include an extension field in a physical layer preamble of a communication, generate the communication, and wirelessly transmit the generated communication. The communication may comprise the preamble and a payload, and the preamble may include a first field indicating whether the extension field is included. The generation may comprise including modulation coding parameters for the payload in the first field when it is determined not to include the extension field, and including coding parameters for the payload in the extension field when it is determined to include the extension field. 
     One aspect of the disclosure provides an apparatus for wireless communication. The apparatus comprises a receiver configured to wirelessly receive data packets having at least two formats. The receiver comprises a first detector configured to detect data packets of at least one of the two formats and a second detector configured to detect data packets of another of the two formats. The apparatus further comprises a processor configured to process a received data packet based at least in part on whether the received data packet was detected by the first detector or the second detector. 
     Another aspect of the disclosure provides a method for wireless communication. The method comprises wirelessly receiving a data packet having one of at least two formats, detecting a format of the received data packet using one of at least two detectors configured to detect respective data packet formats, and processing the received data packet based on the detected format. 
     Another aspect of the disclosure provides an apparatus for wireless communication. The apparatus comprises means for wirelessly receiving a data packet having one of at least two formats, first means for detecting whether the received data packet has a first format, second means for detecting whether the received data packet has a second format, and means for processing the received data packet based on the first detecting means an the second detecting means. 
     Another aspect of the disclosure provides a computer readable medium comprising instructions that when executed cause an apparatus to wirelessly receive a data packet having one of at least two formats, detect a format of the received data packet using one of at least two detectors configured to detect respective data packet formats, and process the received data packet based on the detected format. 
     Another aspect of the disclosure provides an apparatus for wireless communication. The apparatus comprises a processor configured to select a data packet format from at least two data packet formats comprising a training field, and a transmitter configured to transmit a wireless communication using the selected data packet format. The training field of one of the data packet formats includes a sequence repeated a greater number of times than in the training field of another of the data packet formats. 
     Another aspect of the disclosure provides a method of wireless communication. The method comprises selecting a data packet format from at least two data packet formats comprising a training field, and transmitting a wireless communication using the selected data packet format. The training field of one of the data packet formats includes a sequence repeated a greater number of times than in the training field of another of the data packet formats. 
     Another aspect of the disclosure provides an apparatus for wireless communication. The apparatus comprises means for selecting a data packet format from at least two data packet formats comprising a training field, and means for transmitting a wireless communication using the selected data packet format. The training field of one of the data packet formats includes a sequence repeated a greater number of times than in the training field of another of the data packet formats. 
     Another aspect of the disclosure provides a computer readable medium comprising instructions that when executed cause an apparatus to select a data packet format from at least two data packet formats comprising a training field, and transmit a wireless communication using the selected data packet format. The training field of one of the data packet formats includes a sequence repeated a greater number of times than in the training field of another of the data packet formats. 
     Another aspect of the disclosure provides an apparatus for wireless communication. The apparatus comprises a receiver configured to receive at least a physical layer preamble of a wireless communication. The preamble includes a first field indicating whether the preamble also includes an extension field. The apparatus further comprises a processor configured to abort reception of a remainder of the communication when the first field indicates that the preamble includes the extension field. The processor may be configured to determine whether the extension field is included based on the first field. 
     Another aspect of the disclosure provides a method of wireless communication. The method comprises receiving at least a physical layer preamble of a wireless communication. The preamble includes a first field indicating whether the preamble also includes an extension field. The method further comprises aborting reception of a remainder of the communication when the first field indicates that the preamble includes the extension field. The method may further comprise determining whether the extension field is included based on the first field. 
     Another aspect of the disclosure provides an apparatus for wireless communication. The apparatus comprises means for receiving at least a physical layer preamble of a wireless communication. The preamble includes a first field indicating whether the preamble also includes an extension field. The apparatus further comprises means for aborting reception of a remainder of the communication when the first field indicates that the preamble includes the extension field. The apparatus may further comprise means for determining whether the extension field is included based on the first field. 
     Another aspect of the disclosure provides a computer readable medium comprising instructions that when executed cause an apparatus to receive at least a physical layer preamble of a wireless communication. The preamble includes a first field indicating whether the preamble also includes an extension field. The instructions further cause the apparatus to abort reception of a remainder of the communication when the first field indicates that the preamble includes the extension field. The instructions may further cause the apparatus to determine whether the extension field is included based on the first field. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of a wireless communication system in which aspects of the present disclosure may be employed. 
         FIG. 2  illustrates various components that may be utilized in a wireless device that may be employed within the wireless communication system of  FIG. 1 . 
         FIG. 3  illustrates various components that may be utilized in the wireless device of  FIG. 2  to transmit wireless communications. 
         FIG. 4  illustrates various components that may be utilized in the wireless device of  FIG. 2  to receive wireless communications. 
         FIG. 5  is a functional block diagram of an example MIMO system that may be implemented in wireless devices such as the wireless device of  FIG. 2  to transmit wireless communications. 
         FIG. 6  is a functional block diagram of an example MIMO system that may be implemented in wireless devices such as the wireless device of  FIG. 2  to receive wireless communications. 
         FIG. 7  is a block diagram showing an example structure of a preamble and payload of a physical layer packet. 
         FIG. 8A  is a block diagram showing an example structure of a preamble and payload of a physical layer packet for transmission over a bandwidth of substantially 1 MHz. 
         FIG. 8B  is a block diagram showing an example structure of a preamble and payload of a physical layer packet for transmission over a bandwidth of substantially 2 MHz according to a single user mode. 
         FIG. 8C  is a block diagram showing an example structure of a preamble and payload of a physical layer packet for transmission over a bandwidth of substantially 2 MHz according to a multi user mode. 
         FIG. 9  illustrates an example of a format of a packet having a signal field. 
         FIG. 10  illustrates an example of a format of a packet having a signal field and an extension field. 
         FIG. 11  illustrates an example of a format of a packet having a signal field. 
         FIG. 12  illustrates an example of the signal field of  FIG. 9 or 10 . 
         FIG. 13A  illustrates an example of the signal field of  FIG. 9 or 10 . 
         FIG. 13B  illustrates an example of the signal field of  FIG. 9 or 10 . 
         FIG. 14  illustrates an example of the signal field of  FIG. 9 or 10 . 
         FIG. 15  illustrates an example of the extension field of  FIG. 10 . 
         FIG. 16  illustrates an example of the extension field of  FIG. 10 . 
         FIG. 17  illustrates an example format of a packet having a signal field and an extension field. 
         FIG. 18  illustrates an example packet format. 
         FIGS. 19A and 19B  illustrate example formats of packets having one or more signal fields. 
         FIG. 20  illustrates an example of a signal field in  FIG. 19A or 19B . 
         FIG. 21  illustrates an example of a signal field in  FIG. 19B . 
         FIGS. 22A, 22B, and 22C  illustrate example formats of packets having one or more signal fields. 
         FIG. 23  illustrates an example of a signal field of  FIGS. 22A-22C . 
         FIG. 24  illustrates an example of a signal field of  FIG. 22A or 22B . 
         FIGS. 25A and 25B  illustrate example formats of packets having one or more signal fields. 
         FIG. 26  illustrates an example of a signal field of  FIGS. 25A and 25B . 
         FIG. 27  illustrates an example of an extension field of  FIGS. 25A and 25B . 
         FIG. 28  illustrates an aspect of a method for transmitting a packet. 
         FIG. 29  is a functional block diagram of an example wireless device that may be employed within the wireless communication system of  FIG. 1 . 
         FIG. 30  illustrates an aspect of a method for receiving a packet. 
         FIG. 31  is a functional block diagram of an example wireless device that may be employed within the wireless communication system of  FIG. 1 . 
         FIG. 32  illustrates various example components that may be utilized in the receiver of  FIG. 2 . 
         FIG. 33  illustrates an example of the signal field of  FIG. 11 . 
         FIG. 34  illustrates an aspect of a method for transmitting a packet. 
         FIG. 35  is a functional block diagram of an example wireless device that may be employed within the wireless communication system of  FIG. 1 . 
         FIG. 36  illustrates an aspect of a method for receiving a packet. 
         FIG. 37  is a functional block diagram of an example wireless device that may be employed within the wireless communication system of  FIG. 1 . 
         FIG. 38  illustrates an aspect of a method for receiving a portion of a packet. 
         FIG. 39  is a functional block diagram of an example wireless device that may be employed within the wireless communication system of  FIG. 1 . 
         FIG. 40  illustrates an aspect of a method for transmitting a packet. 
         FIG. 41  is a functional block diagram of an example wireless device that may be employed within the wireless communication system of  FIG. 1 . 
         FIG. 42  illustrates an aspect of a method for receiving a portion of a packet. 
         FIG. 43  is a functional block diagram of an example wireless device that may be employed within the wireless communication system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Various aspects of the novel systems, apparatuses, and methods are described more fully hereinafter with reference to the accompanying drawings. The teachings disclosure may, however, be embodied in many different forms and should not be construed as limited to any specific structure or function presented throughout this disclosure. Rather, these aspects are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Based on the teachings herein one skilled in the art should appreciate that the scope of the disclosure is intended to cover any aspect of the novel systems, apparatuses, and methods disclosed herein, whether implemented independently of or combined with any other aspect of the invention. For example, an apparatus may be implemented or a method may be practiced using any number of the aspects set forth herein. In addition, the scope of the invention is intended to cover such an apparatus or method which is practiced using other structure, functionality, or structure and functionality in addition to or other than the various aspects of the invention set forth herein. It should be understood that any aspect disclosed herein may be embodied by one or more elements of a claim. 
     Although particular aspects are described herein, many variations and permutations of these aspects fall within the scope of the disclosure. Although some benefits and advantages of the preferred aspects are mentioned, the scope of the disclosure is not intended to be limited to particular benefits, uses, or objectives. Rather, aspects of the disclosure are intended to be broadly applicable to different wireless technologies, system configurations, networks, and transmission protocols, some of which are illustrated by way of example in the figures and in the following description of the preferred aspects. The detailed description and drawings are merely illustrative of the disclosure rather than limiting, the scope of the disclosure being defined by the appended claims and equivalents thereof. 
     Wireless network technologies may include various types of wireless local area networks (WLANs). A WLAN may be used to interconnect nearby devices together, employing widely used networking protocols. The various aspects described herein may apply to any communication standard, such as WiFi or, more generally, any member of the IEEE 802.11 family of wireless protocols. For example, the various aspects described herein may be used as part of the IEEE 802.11 ah protocol, which uses sub-1 GHz bands. 
     In some aspects, wireless signals in a sub-gigahertz band may be transmitted according to the 802.11ah protocol using orthogonal frequency-division multiplexing (OFDM), direct-sequence spread spectrum (DSSS) communications, a combination of OFDM and DSSS communications, or other schemes. Implementations of the 802.11ah protocol may be used for sensors, metering, and smart grid networks. Advantageously, aspects of certain devices implementing the 802.11ah protocol may consume less power than devices implementing other wireless protocols, and/or may be used to transmit wireless signals across a relatively long range, for example about one kilometer or longer. 
     Certain of the devices described herein may further implement Multiple Input Multiple Output (MIMO) technology and be implemented as part of the 802.11ah standard. A MIMO system employs multiple (N T ) transmit antennas and multiple (N R ) receive antennas for data transmission. A MIMO channel formed by the N T  transmit and N R  receive antennas may be decomposed into N S  independent channels, which are also referred to as spatial channels or streams, where N S ≦min{N T , N R }. Each of the N S  independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized. 
     In some implementations, a WLAN includes various devices which are the components that access the wireless network. For example, there may be two types of devices: access points (“APs”) and clients (also referred to as stations, or “STAs”). In general, an AP serves as a hub or base station for the WLAN and an STA serves as a user of the WLAN. For example, an STA may be a laptop computer, a personal digital assistant (PDA), a mobile phone, etc. In an example, an STA connects to an AP via a WiFi (e.g., IEEE 802.11 protocol such as 802.11ah) compliant wireless link to obtain general connectivity to the Internet or to other wide area networks. In some implementations an STA may also be used as an AP. 
     An access point (“AP”) may also comprise, be implemented as, or known as a NodeB, Radio Network Controller (“RNC”), eNodeB, Base Station Controller (“BSC”), Base Transceiver Station (“BTS”), Base Station (“BS”), Transceiver Function (“TF”), Radio Router, Radio Transceiver, or some other terminology. 
     A station “STA” may also comprise, be implemented as, or known as an access terminal (“AT”), a subscriber station, a subscriber unit, a mobile station, a remote station, a remote terminal, a user terminal, a user agent, a user device, user equipment, or some other terminology. In some implementations an access terminal may comprise a cellular telephone, a cordless telephone, a Session Initiation Protocol (“SIP”) phone, a wireless local loop (“WLL”) station, a personal digital assistant (“PDA”), a handheld device having wireless connection capability, or some other suitable processing device connected to a wireless modem. Accordingly, one or more aspects taught herein may be incorporated into a phone (e.g., a cellular phone or smartphone), a computer (e.g., a laptop), a portable communication device, a headset, a portable computing device (e.g., a personal data assistant), an entertainment device (e.g., a music or video device, or a satellite radio), a gaming device or system, a global positioning system device, or any other suitable device that is configured to communicate via a wireless medium. 
     As discussed above, certain of the devices described herein may implement the 802.11ah standard, for example. Such devices, whether used as an STA or AP or other device, may be used for smart metering or in a smart grid network. Such devices may provide sensor applications or be used in home automation. The devices may instead or in addition be used in a healthcare context, for example for personal healthcare. They may also be used for surveillance, to enable extended-range Internet connectivity (e.g. for use with hotspots), or to implement machine-to-machine communications. 
       FIG. 1  illustrates an example of a wireless communication system  100  in which aspects of the present disclosure may be employed. The wireless communication system  100  may operate pursuant to a wireless standard, for example the 802.11ah standard. The wireless communication system  100  may include an AP  104 , which communicates with STAs  106   a ,  106   b ,  106   c ,  106   d  (collectively STAs  106 ). 
     A variety of processes and methods may be used for transmissions in the wireless communication system  100  between the AP  104  and the STAs  106 . For example, signals may be sent and received between the AP  104  and the STAs  106  in accordance with OFDM/OFDMA techniques. If this is the case, the wireless communication system  100  may be referred to as an OFDM/OFDMA system. Alternatively, signals may be sent and received between the AP  104  and the STAs  106  in accordance with CDMA techniques. If this is the case, the wireless communication system  100  may be referred to as a CDMA system. 
     A communication link that facilitates transmission from the AP  104  to one or more of the STAs  106  may be referred to as a downlink (DL)  108 , and a communication link that facilitates transmission from one or more of the STAs  106  to the AP  104  may be referred to as an uplink (UL)  110 . Alternatively, a downlink  108  may be referred to as a forward link or a forward channel, and an uplink  110  may be referred to as a reverse link or a reverse channel. 
     The AP  104  may act as a base station and provide wireless communication coverage in a basic service area (BSA)  102 . The AP  104  along with the STAs  106  associated with the AP  104  and that use the AP  104  for communication may be referred to as a basic service set (BSS). It should be noted that the wireless communication system  100  may not have a central AP  104 , but rather may function as a peer-to-peer network between the STAs  106 . Accordingly, the functions of the AP  104  described herein may alternatively be performed by one or more of the STAs  106 . 
       FIG. 2  illustrates various components that may be utilized in a wireless device  202  that may be employed within the wireless communication system  100 . The wireless device  202  is an example of a device that may be configured to implement the various methods described herein. For example, the wireless device  202  may comprise the AP  104  or one of the STAs  106  of  FIG. 1 . 
     The wireless device  202  may include a processor  204  which controls operation of the wireless device  202 . The processor  204  may also be referred to as a central processing unit (CPU). Memory  206 , which may include both read-only memory (ROM) and random access memory (RAM), provides instructions and data to the processor  204 . A portion of the memory  206  may also include non-volatile random access memory (NVRAM). The processor  204  typically performs logical and arithmetic operations based on program instructions stored within the memory  206 . The instructions in the memory  206  may be executable to implement the methods described herein. 
     When the wireless device  202  is implemented or used as a transmitting node, the processor  204  may be configured to select one of a plurality of packet formats, and to generate a packet having that format. For example, the processor  204  may be configured to generate a packet comprising a preamble, such as a physical layer preamble and a payload and to determine whether to include an extension field in the preamble, as discussed in further detail below. The processor  204  may further be configured to generate a packet having a training field with a repeated sequence. 
     When the wireless device  202  is implemented or used as a receiving node, the processor  204  may be configured to process packets having a plurality of formats. For example, the processor  204  may be configured to process a payload of a packet based on a preamble of the packet. In some aspects, the preamble includes an extension field, as discussed in further detail below. 
     The processor  204  may comprise or be a component of a processing system implemented with one or more processors. The one or more processors may be implemented with any combination of general-purpose microprocessors, microcontrollers, digital signal processors (DSPs), field programmable gate array (FPGAs), programmable logic devices (PLDs), controllers, state machines, gated logic, discrete hardware components, dedicated hardware finite state machines, or any other suitable entities that can perform calculations or other manipulations of information. 
     The processing system may also include machine-readable media for storing software. Software shall be construed broadly to mean any type of instructions, whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. Instructions may include code (e.g., in source code format, binary code format, executable code format, or any other suitable format of code). The instructions, when executed by the one or more processors, cause the processing system to perform the various functions described herein. 
     The wireless device  202  may also include a housing  208  that may include a transmitter  210  and/or a receiver  212  to allow transmission and reception of data between the wireless device  202  and a remote location. The transmitter  210  and receiver  212  may be combined into a transceiver  214 . An antenna  216  may be attached to the housing  208  and electrically coupled to the transceiver  214 . The wireless device  202  may also include (not shown) multiple transmitters, multiple receivers, multiple transceivers, and/or multiple antennas. 
     The transmitter  210  may be configured to wirelessly transmit packets having a plurality of different formats. For example, the transmitter  210  may be configured to transmit different types of packets generated by the processor  204 , discussed above. 
     The receiver  212  may be configured to wirelessly receive packets having a plurality of different formats. In some aspects, the receiver  212  is configured to detect a type of a received packet, as discussed in further detail below. For example, the receiver may implement an auto-detect procedure to determine a format of a received packet prior to the processing system processing the packet or a payload thereof. 
     The wireless device  202  may also include a signal detector  218  that may be used to detect and quantify the level of signals received by the transceiver  214 . The signal detector  218  may detect such signals as total energy, energy per subcarrier per symbol, power spectral density, and other signals. The wireless device  202  may also include a digital signal processor (DSP)  220  for use in processing signals. The DSP  220  may be configured to generate a packet for transmission. In some aspects, the packet may comprise a physical layer data unit (PPDU). 
     The wireless device  202  may further comprise a user interface  222  in some aspects. The user interface  222  may comprise a keypad, a microphone, a speaker, and/or a display. The user interface  222  may include any element or component that conveys information to a user of the wireless device  202  and/or receives input from the user. 
     The various components of the wireless device  202  may be coupled together by a bus system  226 . The bus system  226  may include a data bus, for example, as well as a power bus, a control signal bus, and a status signal bus in addition to the data bus. The components of the wireless device  202  may further be coupled together or accept or provide inputs to each other using some other mechanism. 
     Although a number of separate components are illustrated in  FIG. 2 , one or more of the components may be combined or commonly implemented. For example, the processor  204  may be used to implement not only the functionality described above with respect to the processor  204 , but also to implement the functionality described above with respect to the signal detector  218  and/or the DSP  220 . Further, each of the components illustrated in  FIG. 2  may be implemented using a plurality of separate elements. Furthermore, the processor  204  may be used to implement any of the components, modules, circuits, or the like described, or each may be implemented using a plurality of separate elements. 
     For ease of reference in this disclosure, when the wireless device  202  is configured as a transmitting node, it may hereinafter be referred to as a wireless device  202   t . Similarly, when the wireless device  202  is configured as a receiving node, it may hereinafter be referred to as a wireless device  202   r . A device in the wireless communication system  100  of  FIG. 1  may implement only functionality of a transmitting node, only functionality of a receiving node, or functionality of both a transmitting node and a receive node. 
     The wireless device  202   t  of  FIG. 3  may comprise a modulator  302  configured to modulate bits for transmission. For example, the modulator  302  may determine a plurality of symbols from bits received from the processor  204  ( FIG. 2 ) or the user interface  222  ( FIG. 2 ), for example by mapping bits to a plurality of symbols according to a constellation. The bits may correspond to user data or to control information. In some aspects, the bits are received in codewords. In one aspect, the modulator  302  comprises a QAM (quadrature amplitude modulation) modulator, for example a 16-QAM modulator or a 64-QAM modulator. In other aspects, the modulator  302  comprises a binary phase-shift keying (BPSK) modulator or a quadrature phase-shift keying (QPSK) modulator. 
     The wireless device  202   t  may further comprise a transform module  304  configured to convert symbols or otherwise modulated bits from the modulator  302  into a time domain. In  FIG. 3 , the transform module  304  is illustrated as being implemented by an inverse fast Fourier transform (IFFT) module. In some implementations, there may be multiple transform modules (not shown) that transform units of data of different sizes. In some implementations, the transform module  304  may be itself configured to transform units of data of different sizes. For example, the transform module  304  may be configured with a plurality of modes, and may use a different number of points to convert the symbols in each mode. For example, the IFFT may have a mode where 32 points are used to convert symbols being transmitted over 32 tones (i.e., subcarriers) into a time domain, and a mode where 64 points are used to convert symbols being transmitted over 64 tones into a time domain. The number of points used by the transform module  304  may be referred to as the size of the transform module  304 . 
     In  FIG. 3 , the modulator  302  and the transform module  304  are illustrated as being implemented in the DSP  320 . In some aspects, however, one or both of the modulator  302  and the transform module  304  are implemented in the processor  204  or in another element of the wireless device  202  (e.g., see description above with reference to  FIG. 2 ). 
     As discussed above, the DSP  320  may be configured to generate a data unit for transmission. In some aspects, the modulator  302  and the transform module  304  may be configured to generate a data unit comprising a plurality of fields including control information and a plurality of data symbols. The fields including the control information may comprise one or more training fields, for example, and one or more signal (SIG) fields. Each of the training fields may include a known sequence of bits or symbols. Each of the SIG fields may include information about the data unit, for example a description of a length or data rate of the data unit. 
     In some aspects, the DSP  320  is configured to insert one or more training fields between a plurality of data symbols. The DSP  320  may determine a position or location of the one or more training fields in the data unit based on information received from the processor  204  ( FIG. 2 ), and/or stored in the memory  206  ( FIG. 2 ) or in a portion of the DSP  320 . Inserting the training fields in the data unit will be discussed in additional detail. 
     Returning to the description of  FIG. 3 , the wireless device  202   t  may further comprise a digital to analog converter  306  configured to convert the output of the transform module into an analog signal. For example, the time-domain output of the transform module  306  may be converted to a baseband OFDM signal by the digital to analog converter  306 . The digital to analog converter  306  may be implemented in the processor  204  or in another element of the wireless device  202  of  FIG. 2 . In some aspects, the digital to analog converter  306  is implemented in the transceiver  214  ( FIG. 2 ) or in a data transmit processor. 
     The analog signal may be wirelessly transmitted by the transmitter  310 . The analog signal may be further processed before being transmitted by the transmitter  310 , for example by being filtered or by being unconverted to an intermediate or carrier frequency. In the aspect illustrated in  FIG. 3 , the transmitter  310  includes a transmit amplifier  308 . Prior to being transmit, the analog signal may be amplified by the transmit amplifier  308 . In some aspects, the amplifier  308  comprises a low noise amplifier (LNA). 
     The transmitter  310  is configured to transmit one or more packets or data units in a wireless signal based on the analog signal. The data units may be generated using the processor  204  ( FIG. 2 ) and/or the DSP  320 , for example using the modulator  302  and the transform module  304  as discussed above. Data units that may be generated and transmitted as discussed above are described in additional detail in this disclosure. 
     In some aspects, the transmitter  310  is configured to transmit the data units over a bandwidth of approximately 2.5 MHz or 1.25 MHz, or lower. When using such bandwidths, transmission of the data unit may be performed over a relatively lengthy period of time. For example, a data unit composed of 500 bytes may be transmitted over a period of approximately 11 milliseconds. Such transmission is approximately sixteen times slower than comparable transmissions implemented pursuant to the 802.11ac standard over bandwidths of approximately 20 MHz. 
       FIG. 4  illustrates various components that may be utilized in the wireless device  202  of  FIG. 2  to receive wireless communications. The components illustrated in  FIG. 4  may be used, for example, to receive OFDM communications. For example, the components illustrated in  FIG. 4  may be used to receive data units transmitted by the components discussed above with respect to  FIG. 3 . 
     The receiver  412  of wireless device  202   r  is configured to receive one or more packets or data units in a wireless signal. Data units that may be received and decoded or otherwise processed as discussed below are described in additional detail in this disclosure. 
     In some aspects, the receiver  412  is configured to receive the data units over a bandwidth of approximately 2.5 MHz or 1.25 MHz, or lower. When using such bandwidths, reception of the data unit may be performed over a relatively lengthy period of time, for example approximately 11 milliseconds when the data unit is composed of 500 bytes. During this time, the channel over which the data unit is received may be changing. For example, conditions of the channel may change due to movement of the wireless device  202   r  or of a device transmitting the data unit, or due to weather or other environmental conditions such as the introduction of various obstacles. In such circumstances, information near the end of the data unit may not be correctly decoded if the wireless device  202   r  uses settings determined when reception of the data unit began. As described in additional detail below, however, the wireless device  202   r  may use the training fields interposed between the plurality of data symbols to form an updated estimate of the channel in order to properly decode one or more of the data symbols. 
     In the aspect illustrated in  FIG. 4 , the receiver  412  includes a receive amplifier  401 . The receive amplifier  401  may be configured to amplify the wireless signal received by the receiver  412 . In some aspects, the receiver  412  is configured to adjust the gain of the receive amplifier  401  using an automatic gain control (AGC) procedure. In some aspects, the automatic gain control uses information in one or more received training fields, such as a received short training field (STF), for example, to adjust the gain. Those having ordinary skill in the art will understand methods for performing AGC. In some aspects, the amplifier  401  comprises an LNA. 
     The wireless device  202   r  may comprise an analog to digital converter  410  configured to convert the amplified wireless signal from the receiver  410  into a digital representation thereof. Further to being amplified, the wireless signal may be processed before being converted by the digital to analog converter  410 , for example by being filtered or downconverted to an intermediate or baseband frequency. The analog to digital converter  410  may be implemented in the processor  204  or in another element of the wireless device  202  ( FIG. 2 ). In some aspects, the analog to digital converter  410  is implemented in a transceiver or in a data receive processor. 
     The wireless device  202   r  may further comprise a transform module  404  configured to convert the representation of the wireless signal into a frequency spectrum. In  FIG. 4 , the transform module  404  is illustrated as being implemented by a fast Fourier transform (FFT) module. In some aspects, the transform module may identify a symbol for each point that it uses. As described above with reference to transform module  304  of  FIG. 3 , the transform module  404  may be configured with a plurality of modes, and may use a different number of points to convert the signal in each mode. For example, the transform module  404  may have a mode where 32 points are used to convert a signal received over 32 tones into a frequency spectrum, and a mode where 64 points are used to convert a signal received over 64 tones into a frequency spectrum. The number of points used by the transform module  404  may be referred to as the size of the transform module  404 . In some aspects, the transform module  404  may identify a symbol for each point that it uses. 
     The wireless device  202   r  may further comprise a channel estimator and equalizer  405  configured to form an estimate of the channel over which the data unit is received, and to remove certain effects of the channel based on the channel estimate. For example, the channel estimator may be configured to approximate a function of the channel, and the channel equalizer may be configured to apply an inverse of that function to the data in the frequency spectrum. 
     In some aspects, the channel estimator and equalizer  405  uses information in one or more received training fields, such as a long training field (LTF) for example, to estimate the channel. The channel estimate may be formed based on one or more LTFs received at the beginning of the data unit. This channel estimate may thereafter be used to equalize data symbols that follow the one or more LTFs. After a certain period of time or after a certain number of data symbols, one or more additional LTFs may be received in the data unit. The channel estimate may be updated or a new estimate formed using the additional LTFs. This new or updated channel estimate may be used to equalize data symbols that follow the additional LTFs. In some aspects, the new or updated channel estimate is used to re-equalize data symbols preceding the additional LTFs. Those having ordinary skill in the art will understand methods for forming a channel estimate. 
     The wireless device  202   r  may further comprise a demodulator  406  configured to demodulate the equalized data. For example, the demodulator  406  may determine a plurality of bits from symbols output by the transform module  404  and the channel estimator and equalizer  405 , for example by reversing a mapping of bits to a symbol in a constellation. The bits may be processed or evaluated by the processor  204  ( FIG. 2 ), or used to display or otherwise output information to the user interface  222  ( FIG. 2 ). In this way, data and/or information may be decoded. In some aspects, the bits correspond to codewords. In one aspect, the demodulator  406  comprises a QAM (quadrature amplitude modulation) demodulator, for example a 16-QAM demodulator or a 64-QAM demodulator. In other aspects, the demodulator  406  comprises a binary phase-shift keying (BPSK) demodulator or a quadrature phase-shift keying (QPSK) demodulator. 
     In  FIG. 4 , the transform module  404 , the channel estimator and equalizer  405 , and the demodulator  406  are illustrated as being implemented in the DSP  420 . In some aspects, however, one or more of the transform module  404 , the channel estimator and equalizer  405 , and the demodulator  406  are implemented in the processor  204  or in another element of the wireless device  202  (e.g., see description above with reference to  FIG. 2 ). 
     As discussed above, the wireless signal received at the receiver  412  comprises one or more data units. Using the functions or components described above, the data units or data symbols therein may be decoded evaluated or otherwise evaluated or processed. For example, the processor  204  ( FIG. 2 ) and/or the DSP  420  may be used to decode data symbols in the data units using the transform module  404 , the channel estimator and equalizer  405 , and the demodulator  406 . 
     Data units exchanged by the AP  104  and the STA  106  may include control information or data, as discussed above. At the physical (PHY) layer, these data units may be referred to as physical layer protocol data units (PPDUs). In some aspects, a PPDU may be referred to as a packet or physical layer packet. Each PPDU may comprise a preamble and a payload. The preamble may include training fields and a SIG field. The payload may comprise a Media Access Control (MAC) header or data for other layers, and/or user data, for example. The payload may be transmitted using one or more data symbols. The systems, methods, and devices herein may utilize data units with training fields that are also interposed between data symbols in the payload. 
     The wireless device  202   t  shown in  FIG. 3  shows an example of a single transmit chain to be transmitted over an antenna. The wireless device  202   r  shown in  FIG. 4  shows an example of a single receive chain to be received over an antenna. In some implementations, the wireless devices  202   t  and  202   r  may implement a portion of a MIMO system using multiple antennas to simultaneously transmit data. 
       FIG. 5  is a functional block diagram of a MIMO system that may be implemented in wireless devices such as the wireless device  202  of  FIG. 2  to transmit and receive wireless communications. The MIMO system may make use of some or all of the components described with reference to  FIG. 3 . Bits for transmission that are to be received at an output of the receiver are provided to an encoder  504 . The encoder  504  may apply a forward error correcting (FEC) code on the bit stream. The FEC code may be a block code, a convolutional code, or the like. The encoded bits are provided to an interleaving system  505  that distributes the encoded bits into N transmit streams. 
     The interleaving system  505  includes a stream parser  506  that parses an input bit stream from the encoder  504  to N spatial stream interleavers  508   a ,  508   b , and  508   n  (collectively interleaver  508 ). The stream parser  506  may be provided with the number of spatial streams and parse bits on a round-robin basis. Other parsing functions may also be used. One parsing function that may be used is k n =N TX *k+n (i.e., round-robin with one bit per spatial stream, then on to the next spatial stream where k n  is the input bit index and N TX  is the number of transmitters/spatial streams). Another more general function f(k,n) may also be used, for example, sending two bits to a spatial stream, then moving on to the next spatial stream. Each interleaver  508   a ,  508   b , and  508   n  may each thereafter distribute bits so that errors may be recovered due to fading or other channel conditions. 
     Each transmit stream may then be modulated by a modulator  502   a ,  502   b , or  502   n . As described above with reference to  FIG. 3 , the bits may be modulated using modulation techniques such as QPSK (Quaternary Phase Shift Keying) modulation, BPSK (mapping one bit at a time), 16-QAM (mapping group of six bits), 64-QAM, and the like. The modulated bits for each stream may be provided to transform modules  510   a ,  510   b , and  510   n . In some implementations, the transform modules  510   a ,  510   b , and  510   n  may perform an inverse discrete time fourier transform (IDFT) to convert the modulated bits from a frequency domain into a time domain. The transform modules  510   a ,  510   b , and  510   n  may operate according to different modes as described above with reference to  FIG. 3 . For example, the transform modules  510   a ,  510   b , and  510   n  may be configured to operate according to a 32 point mode or a 64 point mode. In some implementations, the modulated bits may be encoded using space time block coding (STBC) and spatial mapping may be performed before being provided to transform modules  510   a ,  510   b , and  510   n . After the modulated bits have been converted into time domain signals for each spatial stream, the time domain signal may be converted into an analog signal via converters  512   a ,  512   b , and  512   n  as described above with reference to  FIG. 3 . The signals may then be transmitted using transmitters  514   a ,  514   b , and  514   c  and using antennas  516   a ,  516   b , or  516   n , into a wireless radio space over a desired frequency bandwidth (e.g., 1 MHz, 2 MHz, 4 MHz, 8 MHz, and 16 MHz, or higher). 
     In some embodiments, antennas  516   a ,  516   b , and  516   n  are distinct and spatially separated antennas. In other embodiments, distinct signals may be combined into different polarizations off of fewer than N antennas. An example of this is where spatial rotation or spatial spreading is done and multiple spatial streams are mapped on a single antenna. Further, distinct spatial streams can be organized in different manners. For example, a transmit antenna may carry data from more than one spatial stream or several transmit antennas may carry data from a spatial stream. For example, consider the case of a transmitter with four transmit antennas and two spatial streams. Each spatial stream can be mapped onto two transmit antennas, so two antennas are carrying data from just one spatial stream. 
       FIG. 6  is a functional block diagram of an exemplary MIMO system that may be implemented in wireless devices such as the wireless device  202  of  FIG. 2  to receive wireless communications. The MIMO system may make use of some or all of the components described with reference to  FIG. 4 . The wireless device  202   r  may be configured to receive transmissions from the antennas  516   a ,  516   b , and  516   n  of  FIG. 5 . A wireless device  202   r  receives signals from the channel at N antennas  518   a ,  518   b , and  518   n  or  618   a ,  618   b , and  618   n  (counting separate polarizations, as appropriate) coupled to N receive circuits. The signals are then provided to receivers  620   a ,  620   b , and  620   n  that each may include an amplifier configured to amplify the received signals. The signals may then be converted into a digital form via converters  622   a ,  622   b , and  622   n.    
     Converted signals may then be converted into a frequency spectrum via transform modules  624   a ,  624   b , and  624   n . As described above, the transform modules  624   a ,  624   b , and  624   n  may operate according to various modes and according to the size and bandwidth used (e.g., 32 point 64 point, etc.). The transformed signals may be provided to respective channel estimator and equalizer blocks  626   a ,  626   b , and  626   n  that may function similarly as described above with reference to  FIG. 4 . After channel estimation, the outputs may be provided to a MIMO detector  628  (e.g., corresponding to MIMO detector  528  of  FIG. 5 ) which may thereafter provide its output to demodulators  630   a ,  630   b , and  630   n  which may demodulate the bits according to one of the modulation techniques as described above. Demodulated bits may then be provided to deinterleavers  632   a ,  632   b , and  632   n  which may pass bits into a stream de-parser  634  which may provide the bits into a single bit stream into a decoder  636  (e.g., corresponding to decoder  536  of  FIG. 5 ) that may decode the bits into an appropriate data stream. 
     As described above, data units exchanged by the AP  104  and the STA  106  may include control information or data in the form of physical (PHY) layer packets or physical layer protocol data units (PPDUs). 
       FIG. 7  is a block diagram showing an example structure of a preamble  702  and payload  710  of a physical layer packet  700 . The preamble  702  may include a short training field (STF)  704  that includes an STF sequence of known values. In some aspects, the STF may be used for packet detection (e.g., to detect the start of a packet) and for coarse time/frequency estimation. The STF sequence may be optimized to have a low PAPR and include a subset of non-zero tones with a particular periodicity. The STF  704  may span one or multiple OFDM symbols. In some aspects, the preamble  702  may include a long training field (LTF)  706  that may span one or multiple OFDM symbols and may include one or more LTF sequences of known non-zero values. The LTF may be used for channel estimation, fine time/frequency estimation, and mode detection. Further, in some aspects, the preamble  702  may include a signal field (SIG)  708  as described above that may include a number of bits or values used in one aspect for mode detection purposes and determination of transmission parameters. 
     Certain implementations described herein may be directed to wireless communication systems that may be used for smart metering or in a smart grid network. These wireless communication systems may be used to provide sensor applications or in home automation. Wireless devices used in such systems may instead or in addition be used in a healthcare context, for example, for personal healthcare. They may also be used for surveillance, to enable extended-range Internet connectivity (e.g., for use with hotspots), or to implement machine-to-machine communications. Accordingly, some implementations may use low data rates such as approximately 150 Kbps. Implementations may further have increased link budget gains (e.g., around 20 dB) over other wireless communications such as 802.11b. In accordance with low data rates, if wireless nodes are configured for use in a home environment, certain aspects may be directed to implementations with good in-home coverage without power amplification. Furthermore, certain aspects may be directed to single-hop networking without using a MESH protocol. In addition, certain implementations may result in significant outdoor coverage improvement with power amplification over other wireless protocols. Furthermore, certain aspects may be directed to implementations that may accommodate large outdoor delay-spread and reduced sensitivity to Doppler. Certain implementations may achieve similar LO accuracy as traditional WiFi. 
     Accordingly, certain implementations are directed to transmitting and receiving wireless signals in sub-gigahertz bands. In one aspect, this may result in a propagation gain of, for example, 8.5 dB (e.g., available due to 900 MHz vs. 2.4 GHz). In another aspect, obstruction loss may be reduced by using sub-gigahertz signal which may result in, for example, a 3 dB gain. 
     Certain implementations are further directed to sending wireless signals with low bandwidths in sub-gigahertz bands. This may further allow achieving greater link budget gains over other wireless communication systems. For example, in one implementation, a symbol may be configured to be transmitted or received using a bandwidth of 1 MHz. The wireless device  202  of  FIG. 2  may be configured to operate in one of several modes. In one mode, symbols such as OFDM symbols may be transmitted or received using a bandwidth of 1 MHz. In another mode, symbols may be transmitted or received using a bandwidth of 2 MHz. Additional modes may also be provided for transmitting or receiving symbols using a bandwidth of 4 MHz, 8 MHz, 16 MHz, and the like. The bandwidth may also be referred to as the channel width. 
     Each mode may use a different number of tones/subcarriers for transmitting the information. For example, in one implementation, a 1 MHz mode (corresponding to transmitting or receiving symbols using a bandwidth of 1 MHz) may use 32 tones. In one aspect, using a 1 MHz mode may provide for a 13 dB noise reduction as compared to a bandwidth such as 20 MHz. In addition, low rate techniques may be used to overcome effects such as frequency diversity losses due to a lower bandwidth which could result in 4-5 dB losses depending on channel conditions. To generate/evaluate symbols sent or received using 32 tones, a transform module  304  or  404  as described in  FIGS. 3 and 4  may be configured to use a 32 point mode (e.g., a 32 point IFFT or FFT). The 32 tones may be allocated as data tones, pilot tones, guard tones, and a DC tone. In one implementation, 24 tones may be allocated as data tones, 2 tones may be allocated as pilot tones, five tones may be allocated as guard tones, and 1 tone may be reserved for the DC tone. In this implementation, the symbol duration may be configured to be 40 μs including cyclic prefix. 
     For example, a wireless device  202   t  of  FIG. 3  may be configured to generate a packet for transmission via a wireless signal using a bandwidth of 1 MHz. In one aspect, the bandwidth may be approximately 1 MHz where approximately 1 MHz may be within a range of 0.8 MHz to 1.2 MHz. The packet may be formed of one or more OFDM symbols having 32 tones allocated as described using a DSP  320  ( FIG. 3 ). A transform module  304  ( FIG. 3 ) in a transmit chain may be configured as an IFFT module operating according to a thirty-two point mode to convert the packet into a time domain signal. A transmitter  310  ( FIG. 3 ) may then be configured to transmit the packet. 
     Likewise, a wireless device  202   r  of  FIG. 4  may be configured to receive the packet over a bandwidth of 1 MHz. In one aspect, the bandwidth may be approximately 1 MHz where approximately 1 MHz may be within a range of 0.8 MHz to 1.2 MHz. The wireless device  202   r  may include a DSP  420  ( FIG. 4 ) including a transform module  404  ( FIG. 4 ) in a receive chain that may be configured as an FFT module operating according to a thirty-two point mode to transform the time domain signal into a frequency spectrum. A DSP  420  may be configured to evaluate the packet. The 1 MHz mode may support a modulation and coding scheme (MCS) for both a low data rate and a “normal” rate. According to some implementations, the preamble  702  may be designed for a low rate mode that offers reliable detection and improved channel estimation as will be further described below. Each mode may be configured to use a corresponding preamble configured to optimize transmissions for the mode and desired characteristics. 
     In addition to a 1 MHz mode, a 2 MHz mode may additionally be available that may be used to transmit and receive symbols using 64 tones. In one implementation, the 64 tones may be allocated as 52 data tones, 4 pilot tones, 1 DC tone, and 7 guard tones. As such, a transform module  304  or  404  of  FIGS. 3 and 4  may be configured to operate according to a 64 point mode when transmitting or receiving 2 MHz symbols. The symbol duration may also be 40 μs including cyclic prefix. Additional modes with different bandwidths (e.g., 4 MHz, 8 MHz, and 16 MHz) may be provided that may use transform modules  304  or  404  operating in modes of corresponding different sizes (e.g., 128 point FFT, 256 point FFT, 512 point FFT, etc.). In addition, each of the modes described above may be configured additionally according to both a single user mode and a multi user mode. Wireless signals using bandwidths less than or equal to 2 MHz may provide various advantages for providing wireless nodes that are configured to meet global regulatory constraints over a broad range of bandwidth, power, and channel limitations. 
     In some aspects, the wireless device  202  of  FIG. 2  is configured to operate according to several wireless standards, for example, according to one of the 802.11 standards. In this configuration, the wireless device  202  may have a mode for operating in a 20 MHz channel width in the 2.4 GHz or 5 GHz band, as well as a mode for operating in a 40 MHz channel width in the 2.4 GHz band. In another aspect, the wireless device  202  is configured to operate pursuant to the 802.11ac standard. In this configuration, the wireless device  202  has a mode for operating in each of a 20 MHz, 40 MHz, and 80 MHz channel width. Generally, the transform module  304  or  404  may use 64 tones when the wireless device  202  is operating in the 20 MHz band, may use 128 tones when the wireless device  202  is operating in the 40 MHz band, and may use 256 tones when the wireless device  202  is operating in the 80 MHz band. 
     In some aspects, a controller (e.g., such as processor  204  or DSP  220  of  FIG. 2 ) is configured to adjust operation of the wireless device  202  of  FIG. 2  so as to operate in a sub-gigahertz band as described above. In one implementation, to operate according to a mode such as 1 MHz, 2 MHz, 4 MHz, etc. as described above, a controller may be configured to downclock one or more of the components in the wireless device  202  such that the wireless device  202  will operate in a 1 MHz, 2 MHz, 4 MHz, 8 MHz, or 16 MHz. In addition, the processor  204  may be configured to downclock operation of one or more of the components in the wireless device  202  such that the wireless device  202  will operate in modes corresponding to using bandwidths of 5 MHz, 2.5 MHz, 1.25 MHz, and/or 0.625 MHz channel width. During such downclocked operation, the number of tones used by the transform module  304  or  404  may remain the same in some aspects. 
     Downclocking operation of the wireless device  202  may comprise operating one or more of the components illustrated in  FIG. 2  at a reduced clock rate. For example, the downclocking may comprise operating the processor  204 , the signal detector  218 , the DSP  220 , and/or any other digital signal circuitry at a lower rate, for example by adjusting, modifying, or assigning the timing settings of one or more of these components. In some aspects, the downclocked operation is performed in response to a command from the processor  204 . In some aspects, the processor  204  provides a clock signal which is reduced in comparison to a clock signal used when operating in the 20 MHz, 40 MHz, or 80 MHz channel width. 
     In some aspects, the processor  204  is configured to cause the operation of the wireless device  202  of  FIG. 2  to be downclocked by a factor of 10 (e.g., by 10×). In such configuration, operation in the 20 MHz channel width will be downclocked to operation in a 2 MHz channel width, and operation in the 40 MHz channel width will be downclocked to operation in a 4 MHz channel width. Furthermore, operation in the 80 MHz channel width will be downclocked to operation in an 8 MHz channel width, and operation in the 160 MHz channel width will be downclocked to operation in a 16 MHz channel width. 
     Similarly as described above, in one aspect, when a 1 MHz bandwidth for transmission or reception of OFDM symbols is used, a 32 point transform module  304  or  404  may be used. In this case, tones may be allocated as 24 data tones, 2 pilot tones, 5 guard tones, and a DC tone. In another aspect, when a 2 MHz bandwidth for transmission or reception of OFDM symbols is used, a 64 point transform module  304  or  404  may be used. In this case, tones may be allocated as 52 data tones, 4 pilot tones, 7 guard tones, and a DC tone. In yet another aspect, when a 4 MHz bandwidth for transmission or reception of OFDM symbols is used, a 64 point transform module  304  or  404  of  FIGS. 3  and  4  may be used. In this case tones may be allocated as 108 data tones, 6 pilot tones, 11 guard tones, and three DC tones. In yet a further aspect, when a 8 MHz bandwidth for transmission or reception of OFDM symbols is used, a 256 point transform module  304  or  404  may be used. In this case tones may be allocated as 234 data tones, 8 pilot tones, 11 guard tones, and three DC tones. Accordingly, the spacing between tones for these bandwidths may be 31.25 KHz. In addition, the symbol duration may be 40 μs including a cyclic prefix of either 4 μs (for short cyclic prefixes) or 8 μs (for long cyclic prefixes). A longer cyclic prefix may be used to accommodate outdoor delay spreads. Furthermore, large symbol durations may be needed to keep cyclic prefix overhead manageable. 
     In some aspects, the amount by which operation of the wireless device  202  of  FIG. 2  is downclocked is predetermined. For example, the downclocking factor may be stored in the memory  206 , and loaded at startup of the wireless device  202 . In such configuration, the processor  204  may cause the wireless device  202  to operate in a downclocked mode according to the predetermined or loaded downclocking factor. 
     In some aspects, the amount by which operation of the wireless device  202  of  FIG. 2  is downclocked at any given time may be determined in situ. For example, the signal detector  218  may determine a downclocking factor from a beacon or pilot received by the receiver  212 . In some aspects, this factor is determined at startup of the device, or when connecting to the network for the first time. In some aspects, a new factor is determined during handoff of the wireless device  202  or each time the wireless device  202  connects to a new network. In some aspects, a predetermined factor may be modified or updated based on a received signal, such as based on a received beacon or pilot. In this way, the wireless device  202  may operate in different bandwidths pursuant to a location of the device or a network to which the device is connecting, for example. The processor  204  may cause the wireless device  202  to operate in a downclocked mode according to the determined downclocking factor. 
     In some aspects, the wireless device  202  of  FIG. 2  is permanently configured to operate in the downclocked mode. For example, the components of the wireless device  202  may be hardwired or have firmware installed therein that causes the device to always perform downclocked operation. In such aspects, the wireless device  202  may be incapable of communicating in the 20 MHz, 40 MHz, and 80 MHz channel widths. Further, the factor of downclocking may be fixed in such aspects. For example, the components may be manufactured and/or installed so as to implement only the fixed downclocking factor. In other aspects, the wireless device may be operated in any of the 20 MHz, 40 MHz, and 80 MHz channel widths, or may be selectively downclocked by the processor  204  to operate in the 1 MHz, 2 MHz,  4 , MHz, 8 MHz, and 16 MHz channel width. 
     In some implementations, when transmitting in a sub-gigahertz range (e.g., 900 MHz), a repetition mode may be used where repetition coding is implemented. A repetition mode may allow for accurate transmission over long distances without sacrificing too much preamble overhead. In some implementations 2× repetition encoding may be used. For example, repetition encoding may allow for as little as 105 dB of pathloss to provide good in-home coverage. When using a wireless sensor network, without repetition coding, customers may have to install higher-power sensors in difficult to reach places. It may not be practical to sell two types of sensors (sensors for “easy to reach places” versus “difficult to reach places”). Furthermore, high-power sensors may not be able to work with low power batteries (e.g., coin-cell batteries) due to peak current drain. Alternatively, without repetition, multiple APs could be installed. However, choosing location and configuration of the APs could be non-trivial for an average consumer. As such, repetition coding may provide various advantages for certain implementations for low data rate applications such as sensor networks. 
     As an example, in one aspect BPSK rate ½ coding may be used with 4× repetition yielding 94 Kbps. In another aspect, BPSK rate ½ coding may be used with 2× repetition yielding 188 Kbps. In yet another aspect, BPSK rate ½ coding may be used yielding 375 Kbps. In a further aspect, 64 QAM rate ¾ coding may be used resulting in 3.75 Mbps. 
     In some implementations, the 1 MHz mode and the 2 MHz mode may be required and configured to be interoperable. Using two required modes may avoid issues where devices could be configured for some regulatory regions but may not work for other regulatory regions and may allow for devices to have more options if regulatory constraints change allowing for less restrictive communications. Higher bandwidths (e.g., 8 MHz) may be used for cellular offload. 
     With reference to  FIG. 7 , when transmitting packets in sub-gigahertz bands with bandwidths as described above, the preamble  702  may be designed to have robust mode detection in an early state of the preamble to detect between different modes. The preamble  702  may further be optimized to minimize overhead and provide adequate coexistence of devices transmitting using the 1 MHz mode and devices transmitting using greater than or equal to 2 MHz modes. The preamble  702  may be designed to have robust mode detection in an early state of the preamble to detect between 1 MHz transmissions (32 pt FFT) and 2 MHz transmissions (64 pt FFT). The physical layer packet  700  may be generated for transmission for different data rates to allow in one aspect for transmission of data over greater distances. For example, the physical layer packet  700  may be generated for a low data rate along with another “normal” data rate as described above. 
       FIG. 8A  is a block diagram showing an example structure of a preamble  802   a  and payload  810   a  of a physical layer packet  800   a  for transmission over a bandwidth of substantially 1 MHz according to certain implementations. The physical layer packet  800   a  may be generated using a transform module  304  ( FIG. 3 ) that is configured according to a 32 point FFT mode for transmitting an OFDM symbol with 32 tones as described above. 
     The preamble  802   a  may include a short training field (STF)  804   a . The STF  804   a  may include a sequence of known values with a subset of non-zero values corresponding to a subset of non-zero tones with a particularly chosen periodicity. The periodicity of the non-zero tones may be the same as used for STF sequences used in higher bandwidths such as 2 MHz. In some implementations, the STF field  804   a  may be boosted, such as by 3 dB for repetition coding. The STF  804   a  may be sent over four OFDM symbols where each symbol repeats a known STF sequence. 
     The preamble  802   a  may include a long training field (LTF)  806   a . The LTF  806   a  may be formed of four OFDM symbols and may include an LTF sequence transmitted in each symbol. The LTF sequences may be formed of known non-zero values corresponding to non-zero tones for all pilot and data tones. In some implementations, the LTF sequences may therefore include 26 non-zero values. 
     The preamble  802   a  may include a signaling field (SIG)  808   a . In some implementations, the SIG field  808   a  may be repetition coded or 2× repetition coded. The physical layer packet  800   a  may further include the payload  810   a  that may be generated using 24 tones in each OFDM symbol allocated for data. The preamble  802   a  may be used for generating either a low rate or a normal rate 1 MHz transmission. The preamble  802   a  may be used according to a single user mode. 
     As described above, the SIG field  808   a  for a 1 MHz mode may be two symbols. In one implementation, the entries into the SIG field  808   a  may correspond to the entries shown in Table 1 below. As such, the SIG field  808   a  may include 36 bits. The SIG field  808   a  may be coded at BPSK-rate ½ repetition 2×. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Field 
                 Bits 
                 Description 
               
               
                   
               
             
            
               
                 Space Time 
                 1 
                 May indicate whether Space Time Block 
               
               
                 Coding Block 
                   
                 Coding is used 
               
               
                 Number of 
                 2 
               
               
                 Spatial Streams 
               
               
                 Short Guard 
                 1 
               
               
                 Interval 
               
               
                 Coding 
                 2 
                 1 st  bit may be coding type (LDPC/BCC) 
               
               
                   
                   
                 while 2 nd  bit may be for LDPC N sym   
               
               
                   
                   
                 ambiguity 
               
               
                 Modulation Coding 
                 4 
               
               
                 Scheme (MCS) 
               
               
                 Aggregation Bit 
                 1 
                 Signals use of AMPDU 
               
               
                 Length 
                 9 
                 My be in symbols when aggregation is on 
               
               
                   
                   
                 or in bytes when aggregation is off. An 
               
               
                   
                   
                 AMPDU may be required for packet sizes 
               
               
                   
                   
                 greater than 511 bytes 
               
               
                 Reserved 
                 6 
                 May be used for MAC bits 
               
               
                 CRC 
                 4 
               
               
                 Tail 
                 6 
                 May be needed for BCC but could 
               
               
                   
                   
                 be less bits 
               
               
                   
               
            
           
         
       
     
       FIG. 8B  is a block diagram showing an example structure of a preamble  802   b  and payload  810   b  of a physical layer packet  800   b  for transmission over a bandwidth of substantially 2 MHz according to a single user mode. The physical layer packet  800   b  may be generated using a transform module  304  ( FIG. 3 ) that is configured according to a 64 point FFT mode for transmitting an OFDM symbol with 64 tones as described above. 
     The preamble  802   b  may include a short training field (STF)  804   b . The STF  804   b  may include a sequence of known values with a subset of non-zero values corresponding to a subset of non-zero tones over 64 tones with a determined periodicity. The periodicity of the non-zero tones may be the same as used for STF sequences used for 1 MHz transmissions. The preamble  802   b  may further include a long training field (LTF)  806   b . The LTF  806   b  may be formed of two OFDM symbols and may include LTF sequences transmitted in each symbol. The LTF sequences may comprise non-zero values corresponding to non-zero tones for all pilot and data tones. The LTF sequences may therefore include 56 non-zero values in some implementations. The preamble  802   b  may further include a signaling field (SIG)  808   b . The SIG field  808   b  may be formed from two OFDM symbols. The two OFDM symbols of the SIG field  808   b  may each be QBPSK rotated. If more than one spatial streams are being used, the preamble  802   b  may include additional long training fields (LTFs)  816   b  for each of the additional spatial streams being used (e.g., as the LTF  804   b  may correspond to the first spatial stream if there are more than one). The physical layer packet  800   b  may further include the payload  810   b  that may be generated using 52 tones in each OFDM symbol allocated for data. The preamble  802   b  may be used according to a single user mode. 
       FIG. 8C  is a block diagram showing an example structure of a preamble  802   c  and payload  810   c  of a physical layer packet  800   c  for transmission over a bandwidth of 2 MHz according to a multi-user mode. As described above with reference to  FIG. 8B , the physical layer packet  800   c  may be generated using a transform module  304  ( FIG. 3 ) that is configured according to a 64 point FFT mode for transmitting an OFDM symbol with 64 tones. 
     The preamble  802   c  may include a short training field (STF)  804   c . The STF  804   c  may include a sequence of known values with a subset of non-zero values corresponding to a subset of non-zero tones over 64 tones with a determined periodicity. The periodicity of the non-zero tones may be the same as used for STF sequences used for 1 MHz transmissions. The preamble  802   c  may further include a long training field (LTF)  806   c . The LTF  806   c  may be formed of two OFDM symbols and may include LTF sequences transmitted in each symbol. The LTF sequences may comprise non-zero values corresponding to non-zero tones for all pilot and data tones. The LTF sequences may therefore include 56 non-zero values according to some implementations. The preamble  802   c  may further include a signaling field (SIG)  808   c . The SIG field  808   c  may be formed from two OFDM symbols. The first of the two OFDM symbols of the SIG field  808   c  may be QBPSK rotated. In one aspect, this allows for the receiver to detect whether the packet  800   c  is multi-user mode packet or a single user mode packet based on whether only one of the SIG field symbols is QBPSK rotated. The preamble  802   c  may further include a very high throughput short training field (VHT-STF)  814   c . The VHT-STF  814   c  may correspond to a VHT-STF used for IEEE 802.11ac transmissions. The preamble  802   c  may further include one or more very high throughput long training fields (VHT-LTFs)  816   c  corresponding to each spatial stream being used. The VHT-LTFs  816   c  may correspond to VHT-LTFs used for IEEE 802.11ac transmissions. The preamble  802   c  may further include a very high throughput signal field (VHT-SIG-B)  818   c . The VHT-SIG-B  818   c  may correspond to the VHT-SIG-B used for IEE 802.11ac transmissions. The physical layer packet  800   c  may further include the payload  810   c  that may be generated using 52 tones in each OFDM symbol allocated for data. The preamble  802   c  may be used according to a multi-user mode. 
     Differentiating between a 32 point mode (i.e., 1 MHz) and a 64 point mode (2 MHz) may be done by using an LTF sequence that is orthogonal in frequency across 32 and 64 tone mode, or by detecting the QBPSK rotation on the 1 st  SIG symbol. 
     As described above, a wireless device  202  of  FIG. 2  may be configured to generate OFDM symbols for transmission over bandwidths greater than 2 MHz, such as for 4 MHz, 8 MHz, 16 MHz, and 32 MHz. In some implementations, when sending OFDM symbols over bandwidths greater than 2 MHz, the SIG field  808   b  ( FIG. 8B ) may be duplicated in every 2 MHz segment of the OFDM symbol and may be used to be able to determine the bandwidth of the symbol. As the OFDM symbol for the SIG field may use 52 tones allocated for data, duplication of the SIG field may leave 7 guard tones (3 and 4 tones on the ends of the symbol) for higher bandwidths (4 MHz, 8 MHz, 16 MHz). 
     In some cases, it may be desirable to use additional guard tones for the LTF  806   b  and/or SIG  808   b  fields ( FIG. 8B ). For example, it may be desirable for the 4 MHz, 8 MHz, and 16 MHz preamble symbols to correspond to corresponding symbols used for 40 MHz, 80 MHz, and 160 MHz of 802.11ac transmissions. As one example, the LTF  806   b  may use the VHT-LTFs for 40 MHz, 80 MHz, and 160 MHz 802.11ac transmissions depending on whether the OFDM symbol is for 4 MHz, 8 MHz, and 16 MHz respectively. As the VHT-LTFs for 40 MHz, 80 MHz, and 160 MHz have 11 guard tones (5/6), using these VHT-LTFs may not provide non-zero values for channel estimation for 2 tones at each edge, for example if the SIG  808   b  field allocated 52 tones for data. Furthermore, there may be stricter filtering requirements for symbols being transmitted using greater bandwidths (4 MHz, 8 MHz, and 16 MHz) if the LTF  806   b  and SIG  808   b  are transmitted using 52 data tones (i.e., having less guard tones). Duplicating the LTF  806   b  used for 2 MHz transmissions may inadequately address these issues as the LTF uses 52 non-zero tones, and thus the same guard tone issue remains. As such, an optimized LTF  806   b  and SIG  808   b  may be provided for 2, 4, and 8 MHz transmissions. In one aspect, the fields are chosen so as to be able to re-use 20, 40, and 80 MHz LTF sequences used for IEEE 802.11 ac packets. 
     As such, in one implementation, for the 2 MHz packets shown in  FIGS. 8B and 8C , the SIG fields  808   b  and  808   c  may be transmitted using a different tone allocation than the rest of the fields of the packets  800   b  and  800   c . For example, The SIG fields  808   b  and  808   c  may be transmitted using 48 data tones rather than 52 data tones. This may correspond to the tone allocation used for an L-SIG of 802.11a tone allocation. This SIG field  808   b  and  808   c  may then be duplicated for each 2 MHz segment for transmissions over 2 MHz. In another implementation, the STFs  804   b  and  804   c , the LTFs  806   b  and  806   c , and the SIG fields  808   b  and  808   c  may be generated for transmission using a different tone allocation than the rest of the fields of the packet. For example the STFs  804   b  and  804   c , the LTFs  806   b  and  806   c , and the SIG fields  808   b  and  808   c  may be generated for transmission using 48 tones allocated for data. 
     As described above, the SIG fields  808   b  and  808   c  for a 2 MHz mode may use two symbols transmitting up to 52 bits of data. The entries into the SIG fields  808   b  and  808   c  may correspond to the entries shown in Table 2 below. The first 26 bits including the bits for the fields in the first eight rows of Table 2 may correspond to the first symbol, while the last 26 bits including the bits for the fields in the last seven rows of Table 2 may correspond to the second symbol. It should be appreciated that while 52 bits of data are shown in the table below, however as described above, in some implementations, the SIG fields  808   b  and  808   c  may be sent using 48 data tones and as such the SIG field may correspond to 48 bits. In one corresponding implementation, the number of reserved bits shown in Table 2 below may be reduced so that 48 bits are sent or received. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Field 
                 Bits 
                 Description 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Bandwidth 
                 2 
                 This may indicate a bandwidth mode  
               
               
                   
                   
                 (e.g., 2 MHz, 4 MHz, 8 MHz, or 16 MHz) 
               
               
                 Reserved 
                 1 
                   
               
               
                 Space Time  
                 1 
                 Indicates whether Space Time Block  
               
               
                 Block Coding 
                   
                 Coding is used 
               
               
                 Nsts/GID/AID 
                 14 
                 For Single User (SU) Mode - 2 bits may 
               
               
                   
                   
                 indicate Nsts, 0-12 bits may indicate  
               
               
                   
                   
                 partial AID 
               
               
                   
                   
                 For Multi User (MU) Mode - 8 bits may 
               
               
                   
                   
                 indicate Nsts, 6 bit GID 
               
               
                 Reserved 
                 1 
                   
               
               
                 Short Guard  
                 1 
                   
               
               
                 Interval (SGI) 
                   
                   
               
               
                 Coding 
                 2 
                 1 st  bit may indicate a coding type for SU (or  
               
               
                   
                   
                 for user zero for MU) while 2 nd  bit may be  
               
               
                   
                   
                 used for LDPC Nsym ambiguity 
               
               
                 Modulation Coding 
                 4 
                 For MU mode, the first 3 bits may indicate 
               
               
                 Scheme (MCS) 
                   
                 coding type for users 1-3 while the last is 
               
               
                   
                   
                 reserved) 
               
               
                 Beamformed 
                 1 
                 May indicate to the receiver if a  
               
               
                   
                   
                 beamforming steering matrix is applied to  
               
               
                   
                   
                 the waveform in a SU mode 
               
               
                 Aggregation Bit 
                 1 
                 Reserved for MU 
               
               
                 Length 
                 9 
                 Length field (in symbols when aggregation is  
               
               
                   
                   
                 on and in bytes when aggregation is off) May 
               
               
                   
                   
                 mandate AMPDU for packet sizes &gt;511 bytes 
               
               
                   
                   
                 and for MU 
               
               
                 Reserved 
                 4 
                 Doppler bit may be indicated here 
               
               
                 Midamble/Doppler 
                 1 
                   
               
               
                 CRC 
                 4 
                   
               
               
                 Tail 
                 6 
                 May be needed for BCC 
               
               
                   
               
            
           
         
       
     
       FIG. 9  illustrates an example format of a packet  900 . The packet  900  may comprise a PPDU for use in the wireless communication system  100  of  FIG. 1 . In some aspects, the packet  900  is used when the wireless device  202  ( FIG. 2 ) is operating in a basic mode. In some aspects, the packet  900  is referred to as a basic packet. The packet  900  may be used for sensors, and may support operation in one or two bandwidths, for example, the two lowest bandwidths used according to the 802.11ah standard. 
     The packet  900  includes a preamble  910  and a payload  920 . The preamble  910  includes a short training field (STF)  912 , a long training field (LTF)  914 , and a signal (SIG) field  916 . In the aspect illustrated in  FIG. 9 , the SIG field  916  is referred to as an Omni-SIG. The payload  920  may include user information or data and directly follow the SIG field  916 , as in the aspect illustrated in  FIG. 9 . 
     The STF  912  may comprise one or more sequences. In some aspects, the sequence in the STF  912  is repeated a plurality of times. The STF  912  may be used by the receiver  212  of the wireless device  202  ( FIG. 2 ) to set or adjust a gain of a receive amplifier. For example, automatic gain control may be performed to set a gain of a LNA. Further, the receiver  212  or the wireless device  202  may use the STF  912  to detect a beginning of the packet  900 . As shown, the STF  912  may comprise 2 symbols. 
     The LTF  914  may also comprise one or more sequences. The LTF  914  may be used by the processor  204 , the signal detector  218 , or the DSP  220  of the wireless device  202  ( FIG. 2 ) to estimate a channel over which the packet  900  is received and/or to equalize symbols received in the payload  920 . As shown, the LTF  914  may comprise one or two symbols. 
     The SIG field  916  may comprise information regarding parameters of the packet  900  and the payload  920 . For example, the SIG field  916  may indicate a length of the packet  900  or a modulation coding scheme (MCS) of the payload  920 . As shown, the SIG field  916  may comprise one or two symbols. The contents and format of the SIG field  916  are described in additional detail in this disclosure. 
       FIG. 10  illustrates an example format of a packet  1000 . The packet  1000  may comprise a PPDU for use in the wireless communication system  100  of  FIG. 1 . In some aspects, the packet  1000  is used when the wireless device  202  ( FIG. 2 ) is operating in an advanced mode. In some aspects, the packet  1000  is referred to as an advanced packet. The packet  1000  may be implemented for non-sensor uses and/or for uses that require more than two bandwidths. As will be discussed further below, the packet  1000  may support multi-user multiple input multiple output (MU-MIMO) communications. 
     The packet  1000  includes a preamble  1010  and a payload  1020 . The preamble  1010  includes the STF  912 , the LTF  914 , and the SIG field  916  illustrated in  FIG. 9 . In contrast to the preamble  910 , however, the preamble  1010  further includes an extension field  1012 . In  FIG. 10 , the extension field  1012  is illustrated as an extension SIG field. In some aspects, the SIG field  916  indicates whether the extension field  1012  is included in a packet. Thus, the SIG field  916  may be used to distinguish between a basic packet and an advanced packet in some aspects. The payload  1020  may include user information or data, and may be configured similar to the payload  920 . In some aspects, the payload  1020  may be longer than the payload  920 . 
     The extension SIG field  1012  may comprise parameters of the packet  1000  or the payload  1020  in addition to the parameters included in the SIG field  916 . In some aspects, the extension SIG field  1012  includes information that is not included in the SIG field  916 . In some aspects, the extension SIG field  1012  includes information relating to the parameters in the SIG field  916 , which information may be used to supplement the SIG field  916 . The extension SIG field  1012  may comprise one or two symbols, and may be disposed between the SIG field  916  and the payload  1020 . The contents and format of the extension SIG field  1012  are described in additional detail in this disclosure. 
       FIG. 11  illustrates an example format of a packet  1100 . The packet  1100  may comprise a PPDU for use in the wireless communication system  100  of  FIG. 1 . In some aspects, the packet  1100  is used when the wireless device  202  ( FIG. 2 ) is operating in an extended range (XR) mode. In some aspects, the packet  1100  is referred to as an extended range or XR packet. The packet  1100  may provide a robust preamble and data encoding so as to increase the range over which the packet  1100  may be correctly received and decoded. 
     The packet  1100  includes a preamble  1110  and a payload  1120 . The preamble  1110  includes a short training field (STF)  1112 , a long training field (LTF)  1114 , and a signal (SIG) field  1116 . In the aspect illustrated in  FIG. 11 , the SIG field  1116  is referred to as an Omni-SIG. The payload  1120  may include user information or data, and may be configured similar to the payload  920  or  1020 . In some aspects, the payload  1120  may be shorter than the payload  920  or  1020 . 
     Similar to the STF  912 , the STF  1112  may comprise one or more sequences. The sequence included in the STF  1112 , however, may be repeated a greater number of times than the sequence in the STF  912 . The STF  912  may be used to set or adjust a gain of a receive amplifier or to detect a beginning of the packet  900 . As shown, the STF  1112  may be longer than the STF  912 . For example, the STF  1112  may comprise 3 symbols. 
     The format of the STF  1112  may be formatted in any number of ways. In one aspect, the format of the STF  1112  may be based on a Chui sequence. In some aspects, the format may be based on a quantum-dot cellular automata (QCA) design, for example by populating every tone with a 32 point fast Fourier transform (FFT). In other aspects, every other tone may be populated by a 64 point FFT. 
     The LTF  1114  may also comprise one or more sequences. The LTF  1114  may be used to estimate a channel over which the packet  1100  is received, and/or to equalize symbols received in the payload  1120 . As shown, the LTF  1114  may be longer than the LTF  914 . For example, the LTF  1114  may comprise two or more symbols. In some aspects, one of the symbols of the LTF  1114  is flipped when compared to a respective symbol in the LTF  914 . The LTF  1114  may be repeated a plurality of times in some aspects. 
     The SIG field  1116  may comprise information regarding parameters of the packet  1100  and the payload  1120 . For example, the SIG field  1116  may indicate a length of the packet  1100  or a modulation coding scheme (MCS) of the payload  1120 . The SIG field  1116  may comprise two or more symbols. In some aspects, a plurality of bits representing sub-field of the SIG field  1116  are repeated four or more times in the SIG field  1116 . For example, the SIG field  1116  may be represented by 19 bits, as will be discussed in further detail below, which bits may be repeated four times to occupy three symbols. In some aspects, the SIG field  1116  is modulated using a form of binary phase-shift keying (BPSK) such as BPSK 1/2. In some aspects, a different coding may be used instead of repetition of the bits or instead of using a binary convolutional code (BCC), which may reduce the length of the SIG field  1116 , for example to two symbols. The different coding may include a block code. The contents and format of the SIG field  1116  are described in additional detail in this disclosure. 
     The wireless device  202   t  may be configured to determine which of the packets discussed in this disclosure to transmit. This determination may be based on any number of factors. For example, network congestion may be considered, as may the type or amount of data being transmitted. 
     In some aspects, the processor  204  of the wireless device  202  ( FIG. 2 ) determines to transmit the packet  1000  instead of the packet  900  when MU-MIMO is used, when the length of the packet will be greater than a threshold amount, when a default mode is not being used for data, when the wireless device  202  is not operating at one of two lowest bandwidths, or when the forward error correction (FEC) being used is not BCC. In some aspects, the threshold amount is approximately 4096 bytes. In some aspects, the default mode relates to whether a short guard interval (SGI) or long guard interval (LGI) is being used. 
     The processor  204  of the wireless device  202  ( FIG. 2 ) may further generate a packet, indicating whether the packet is formatted as the packet  900  or the packet  1000  with the SIG field  916 . In some aspects, the wireless device  202  may rotate a modulation, such as the BPSK, of the SIG field  916  to indicate the type of the packet. In some aspects, a bit or other indicator may be transmitted over a quadrature phase (e.g. on the Q rail) during one of the symbols of the SIG field  916  to indicate whether the packet  900  or  1000  is being transmitted. 
     The processor  204  of the wireless device  202  ( FIG. 2 ) may determine a formatting of a received packet based on the SIG field  916  and process the payload accordingly. For example, when the extension field  1012  is included in the preamble  1010 , the wireless device  202  may decode or otherwise process the payload  1020  using parameters in the extension field  1012  such as an MCS or number of spatial streams. In some aspects, the wireless device  202  may be configured to decode packets having one of the formats  900  and  1000 , and to ignore packets having the other format. For example, some devices may not implement multi-user (MU) functionality which utilizes information in the extension field  1012 . If these devices determine that the extension field  1012  is included based on the SIG field  916 , the processor  204  may cease further processing of the packet  1000  or abort receiving any further portion of the packet  1000 . In this way, the device may identify packets that are not intended for the device, and may save power by aborting reception of those packets. 
       FIG. 12  illustrates an example  916   a  of the SIG field  916 . The SIG field  916   a  may be used with the aspects described in this disclosure where the type of packet is indicated using a rotated BPSK or Q-rail bit, for example. The SIG field  916   a  comprises a length sub-field  1202  including 12 bits, an MCS sub-field  1204  including 4 bits, a bandwidth (BW) sub-field  1206  including 1 bit, a parity sub-field  1208  including 1 bit, a reserved sub-field  1212  including 2 bits, and a tail sub-field  1214  including 6 bits. The length sub-field  1202  may indicate a length of the packet  900  or  1000  in bytes. The MCS sub-field  1204  may indicate an MCS used for the payload  920 ,  1020 . The bandwidth sub-field  1206  may indicate which bandwidth is being used. In the illustrated aspect, the SIG field  916   a  comprises one symbol. 
     In some aspects, a format of the packet generated by the processor  204  ( FIG. 2 ) may be indicated by one or more sub-fields or bits in the SIG field  916 . For example, when generating the packet, the processor  204  of the wireless device  202  ( FIG. 2 ) may include an explicit indicator in the SIG field  916  to distinguish the packet  900  from the packet  1000 . When another wireless device  202  receives the packet, the processor  204  of the wireless device  202  may determine a formatting of the packet based on a subfield of the SIG field  916  and process the payload accordingly. 
     In one aspect, the inclusion of the extension field  1012  is determined using a mode sub-field of the SIG field  916 . The mode sub-field may comprise two bits and may be used to indicate a number of spatial streams or a number of bandwidth portions used for the packet. In some aspects, the packet  900  is utilized when the payload  920  is transmitted over one spatial stream. In some aspects, the packet  1000  is utilized when the payload  1020  is transmitted over more than one spatial stream. For example, the extension sub-field  1012  may be included when single user MIMO (SU-MIMO) or MU-MIMO is used. The mode sub-field is described in additional detail below. 
       FIG. 13A  illustrates an example  916   b  of the SIG field  916 . The SIG field  916   b  may be used in the packet  900  with the aspects described in this disclosure where the type of the packet is indicated using a subfield of the SIG field  916   b , for example. The SIG field  916   b  comprises the length sub-field  1202 , the MCS sub-field  1204 , a mode sub-field  1302  as discussed above, a SGI sub-field  1304  including 1 bit, the parity sub-field  1208 , and the tail sub-field  1214 . In the aspect illustrated in  FIG. 13A , the length sub-field  1202  may indicate a length of the packet  900  in bytes or symbols. The parity sub-field  1208  may only apply to the mode sub-field  1302  and the SGI sub-field  1304  in some aspects. In the illustrated aspect, the SIG field  916   b  comprises one symbol. 
     The following table illustrates example values of the mode sub-field  1302 . The table further enumerates a number of spatial streams and bandwidths that may be used for each of the values of the mode sub-field  1302 , and the table further describes whether the length of the packet  900  is described in the bytes or symbols in the length sub-field  1202 . 
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Mode 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 “00” 
                 Basic BW, 1 ss 
                 Length in bytes 
               
               
                   
                 “01” 
                 BW × 2, 1 ss 
                 Length in symbols 
               
               
                   
                 “10” 
                 BW × 4, 1 ss 
                 Length in symbols 
               
               
                   
                 “11” 
                 Ext SIG 
                 Ext SIG present 
               
               
                   
                   
               
            
           
         
       
     
     As can be seen from the table above, the length may be indicated in symbols when more than one bandwidth is used. In some aspects, an aggregate MAC protocol data unit (A-MPDU) is used when more than one bandwidth is used, the length of which may be sufficiently indicated in symbols. As can also be seen above, the extension field  1012  may be included when the mode sub-field  1302  is set to “1 1.” Thus, the SIG field  916   b  may be used when the mode sub-field  1302  is set to “0 0,” “0 1,” or “1 0.” 
       FIG. 13B  illustrates an example  916   c  of the SIG field  916 . The SIG field  916   c  may be used in the packet  1000  with the mode sub-field  1302  described above. Thus, the SIG field  916   c  may be used when the mode sub-field  1302  is set to “1 1” and the extension field  1012  is included. The SIG field  916   c  comprises a length sub-field  1312 , a bandwidth sub-field  1314 , a reserved sub-field  1316  including 4 bits, the mode sub-field  1302  as discussed above, the parity sub-field  1208 , the SGI sub-field  1304 , and the tail sub-field  1214 . In the aspect illustrated in  FIG. 13B , the length sub-field  1312  may indicate a length of the packet  1000  in symbols. In contrast to the length sub-field  1202 , however, the length sub-field  1312  includes 10 bits. The bandwidth sub-field  1314  may indicate a number of bandwidths being used and may include 2 bits. In the illustrated aspect, the SIG field  916   c  comprises one symbol. 
       FIG. 14  illustrates an example  916   d  of the SIG field  916 . The SIG field  916   d  may be used with the aspects described in this disclosure where the type of packet is indicated using a subfield of the SIG field  916   d . For example, the inclusion of the extension field  1012  may be indicated by an MU-extension sub-field  1414 . In the aspect illustrated in  FIG. 14 , the MU-extension sub-field  1414  includes one bit, may be set to “0” to indicate that the extension field  1012  is not included, and may be set to “1” to indicate that the extension field  1012  is included. In some aspects, the extension field  1012  comprises a SIG field and is included for MU transmissions. In such aspects, the extension field  1012  may be referred to as an MU-SIG. In the illustrated aspect, the SIG field  916   d  comprises two symbols. 
     The SIG field  916   d  comprises a rate sub-field  1402  including 4 bits, a spatial streams sub-field  1404 , the short guard interval (SGI) sub-field  1304 , a length sub-field  1406  including 18 bits, a cyclic redundancy check (CRC) sub-field  1408  including 4 bits, the tail sub-field  1214 , a bandwidth sub-field  1412 , the MU-extension sub-field  1414 , an aggregation sub-field  1416  including 1 bit, and a reserved sub-field  1418 . When the SIG field  916   d  is used for SU, the length sub-field  1406  may indicate a length of the packet  900  in bytes or octets. This allows the PHY layer to determine the boundary of the packet  900  when an A-MPDU is not used. When the SIG field  916   d  is used for MU, however, the length sub-field  1406  may indicate a maximum length of the packet  1000  among users in symbols. In this situation, an A-MPDU may be used with transmission of the packet  1000 . Similar to the bandwidth sub-field  1314 , the bandwidth sub-field  1412  may be used to indicate a number of bandwidths or modes being used, except that the bandwidth sub-field  1412  may include 2 or 3 bits. 
     In some aspects, the rate sub-field  1402  may indicate the MCS of the payload  920 . The spatial streams sub-field  1404  may indicate a number of spatial streams for SU operation and/or number of spatial streams reserved for MU operation. The length sub-field may indicate the length of the packet  900  in octets if the MU extension sub-field  1414  is 0 and indicate length in symbols if the MU extension sub-field  1414  is 1. The aggregation sub-field  1416  may be reserved if the MU extension sub-field  1414  is 1 and may indicate the packet  900  is an A-MPDU if the MU extension sub-field  1414  is 0. 
       FIG. 15  illustrates an example  1012   a  of the extension field  1012 . In the illustrated aspect, the extension field  1012   a  comprises a two symbol extension SIG field. The extension SIG field  1012   a  comprises an MCS sub-field  1502  including 16 bits, a length sub-field  1504  including 4 bits, a bandwidth sub-field  1506  including 1 bit, an SGI/LGI sub-field  1508  including 1 bit, a coding sub-field  1512  including 4 bits, a spatial streams sub-field  1514  including 8 bits, a group ID (GID) sub-field  1516  including 6 bits, a CRC sub-field  1518  including 4 bits, a reserved sub-field  1522  including 2 bits, and a tail sub-field  1524  including 6 bits. 
     The MCS sub-field  1502  may indicate an MCS for each of a plurality of users. In the illustrated embodiment, there may be up to four users. The length sub-field  1504  may indicate a length of the packet  1000  in symbols. The bandwidth sub-field  1506  may indicate a bandwidth used for the packet  1000 . The SGI/LGI sub-field  1508  may indicate whether an SGI or LGI is used. The coding sub-field  1512  may indicate a coding for each of a plurality of users. In the illustrated embodiment, there may be up to four users. The spatial streams sub-field  1514  may indicate a number of spatial streams for each of a plurality of users. In the illustrated embodiment, there may be up to four users. 
     In some aspects, any of the MCS sub-field  1502 , the length sub-field  1504 , the bandwidth sub-field  1506 , and the SGI/LGI sub-field  1508  may indicate a parameter of the packet  1000  rather than a corresponding sub-field in the SIG field  916  indicating that parameter. For example, when the extension field  1012   a  is included, the wireless device  202   r  may use the MCS sub-field  1502  to determine an MCS for one or more users instead of using the MCS sub-field  1204 . In other aspects, one or more sub-fields in the SIG field  916  may indicate parameters for a first user, while any of the MCS sub-field  1502 , the length sub-field  1504 , the bandwidth sub-field  1506 , and the SGI/LGI sub-field  1508  may indicate parameters for one or more other users. 
     In some aspects, the length of the packet  1000  is indicated by a combination of the bits in the length sub-field  1504  with the bits in a length sub-field of the SIG field  916 . For example, the length sub-field  1312  may be set to the value “0000000010” and the length sub-field  1504  may be set to the value “1111” to indicate that a length of the packet  1000  is 47 symbols. Similarly, the number of bandwidths used for the packet  1000  may be indicated by a combination of the bit in the bandwidth sub-field  1506  with the bits of a bandwidth sub-field in the SIG field  916 . 
       FIG. 16  illustrates an example  1012   b  of the extension field  1012 . In the illustrated aspect, the extension field  1012   b  comprises a two symbol extension SIG field. The extension SIG field  1012   b  comprises the MCS sub-field  1502 , the spatial streams sub-field  1514 , the GID sub-field  1516 , the CRC sub-field  1518 , a reserved sub-field  1602  including 10 bits, and the tail sub-field  1524 . As can be seen in  FIG. 16 , the extension SIG field  1012   b  is formatted similar to the extension SIG field  1012   a , except that the sub-fields  1504 - 1512  are omitted in the extension SIG field  1012   b , and the reserved sub-field  1602  includes a greater number of bits than the reserved sub-field  1522 . 
       FIG. 17  illustrates an example format of a packet  1700 . The packet  1700  may comprise a PPDU for use in the wireless communication system  100  of  FIG. 1 . In some aspects, the packet  1700  is used when the wireless device  202  ( FIG. 2 ) is operating in the advanced mode, and the packet  1700  may be referred to as an advanced packet. 
     The packet  1700  includes a plurality of extension fields  1732 - 1738  in a preamble  1710  of the packet  1700 . The extension fields may include an MU-SIG field  1732 , a precoded STF  1734 , one or more LTFs  1736 , and a SIG-B field  1738 . In some aspects, the packet  1700  may be used instead of the packet  1000 . 
     In addition to the extension fields  1732 - 1738 , the preamble  1710  includes a high throughput (HT) STF  1712 , an HT-LTF 1   1714 , and the signal (SIG) field  916 . In the aspect illustrated in  FIG. 9 , the SIG field  916  is referred to as an Omni-SIG. In some aspects, the SIG field  916  indicates whether the extension fields  1732 - 1738  are included in a packet. For example, one or more bits in the SIG field  916 , a BPSK rotation of the SIG field  916 , and/or a bit on the Q-rail during a symbol of the SIG field  916  may indicate that the extension fields  1732 - 1738  are included. 
     The HT-STF  1712  may comprise one or more sequences. In some aspects, the sequence in the STF  1712  is repeated a plurality of times. The HT-STF  1712  may be used by the receiver  212  of the wireless device  202  ( FIG. 2 ) to set or adjust a gain of a receive amplifier or used to detect a beginning of the packet  1700 . As shown, the HT-STF  1712  may comprise 2 symbols. 
     The HT-LTF  1714  may also comprise one or more sequences. The HT-LTF  1714  may be used by the processor  204 , the signal detector  218 , or the DSP  220  of the wireless device  202  ( FIG. 2 ) to estimate a channel over which the packet  1700  is received and/or to equalize symbols received in a payload  1720 . As shown, the HT-LTF  1714  may comprise two symbols. 
     In some aspects, the MU-SIG field  1732  includes one or more of the sub-fields illustrated in  FIGS. 15 and 16 . In some aspects, the MU-SIG field  1732  and the SIG-B field  1738  are collapsed together to create a two symbol field. When the MU-SIG field  1732  and the SIG-B field  1738  are collapsed, the combined contents may include a GID sub-field, an N sts  (number of space time streams) sub-field, and/or an MCS sub-field. In some aspects, the MCS sub-field includes an MCS for each user. In some aspects, one or more of the SIG fields illustrated in  FIG. 17  can be used as an additional LTF. 
     As alluded to above, the packet  1700  may further include the payload  1720 . The payload  1720  may include user information or data, and may be configured similar to the payload  920 . 
       FIG. 18  illustrates an example generalized format of a packet  1841  which may be used within the wireless communication system  100  of  FIG. 1 . The packet  1841  may comprise a PPDU, and may be selectively formatted according to either the basic mode or advanced mode described above. In some aspects, the packet  1841  may be formatted according to a plurality of other modes. 
     The packet  1841  includes a preamble  1851  and a payload  1861 . The preamble  1851  includes the HT-STF  1712 , the HT-LTF  1714 , and the SIG field  916 . In some modes or formats, the packet  1841  may additionally include an extension  1853 . 
     The HT-STF  1712  and the HT-LTF  1714  allow for data transmission on 52 tones. The extension  1853  may include one or more optional or extension fields. The SIG field  916  may be used to indicate whether the extension  1853  is included in the preamble  1851  and, when the extension  1853  is included, to indicate whether certain fields are included in the extension  1853 . For example, for sensor transmissions using one spatial stream, the SIG field  916  may indicate that the extension  1853  is omitted and the SIG field  916  may be directly followed by the payload  1861 . The payload  1861  may include SU data or MU data, and/or aggregated or non-aggregated MPDU information, for example, and may be configured similar to the payloads discussed in this disclosure. 
     In some aspects, the STF  912  discussed above with respect to  FIG. 9  may be configured similar to the HT-STF  1712 . Further, the LTF  914  discussed above with respect to  FIG. 9  may be configured similar to the HT-LTF  1714 . 
     The SIG field  916  is labeled as a SIG-A field in  FIG. 18 . In some aspects, the SIG-A field  916  may be configured similar to the Omni-SIG fields illustrated or discussed in this disclosure. In other aspects, the SIG-A field  916  may differ in configuration from the Omni-SIG fields illustrated or discussed in this disclosure. For example, the SIG-A field  916  may be configured as discussed with respect to  FIGS. 20 and 23 . 
     The packets discussed above may be formatted pursuant to the generalized format of the packet  1841 . For example, when the extension  1853  is omitted, the packet  900  may be formatted similar to the packet  1841 . As another example, when the extension  1853  is included, the packet  1000  may be formatted similar to the packet  1841 . In this example, the extension field  1012  may be included in the extension  1853 . Similarly, when the extension  1853  is included, the packet  1700  may be formatted similar to the packet  1841 . In this example, one or more of the plurality of extension fields  1732 - 1738  may be included in the extension  1853 . 
     The packet  1841  may be formatted to reduce overhead for devices that do not support or are not using MU-MIMO, for example, by omitting one or more fields from the extension  1853  or by omitting the extension  1853  altogether. Similarly, the extension  1853  or one or more fields of the extension may be omitted for devices that do not support or are not using SU transmit beamforming (Tx-BF). Thus, sensors and other such devices may utilize non-AMPDU transmissions. Therefore, the packet  1841 , and the implementations of the packet  1841  described below, support both MU-MIMO and Tx-BF as optional features with little or no additional overhead for devices that do not support such features. 
       FIGS. 19A and 19B  illustrate a first implementation showing a plurality of formats that may be used for the packet  1841  discussed above. Each of the formats illustrated in  FIGS. 19A and 19B  include the HT-STF  1712 , the HT-LTF  1714 , and an example  916   e  of the SIG field  916 . The SIG-A field  916   e  may include two symbols. 
       FIG. 19A  illustrates an example packet format  1941  of the packet  1841  according to the first implementation, and  FIG. 19B  illustrates an example packet format  1961  of the packet  1841  according to the first implementation. The wireless device  202  ( FIG. 2 ) may distinguish between the packet  1941  and the packet  1961  based on the SIG-A field  916   e , for example. 
     With reference to  FIG. 19A , the packet  1941  includes a preamble  1951  and the payload  1861 . The preamble  1951  includes the HT-STF  1712 , the HT-LTF  1714 , and the SIG-A field  916   e  discussed above. The preamble  1951  optionally includes one or more additional LTFs  1953 . 
     In some aspects, the packet  1941  is used for SU open loop transmission. In such aspects, the additional LTFs  1953  are omitted when one spatial stream is used for the packet  1941 . When additional spatial streams are used, an additional LTF  1953  for each additional spatial stream may be included in the preamble  1951 . In some aspects, 1, 2, or 4 spatial streams may be used. In these aspects, 0, 1, or 2 additional LTFs  1953  will be included in the preamble  1951 . 
     In some aspects, an indicator in the SIG-A  916   e  signifies whether the additional LTFs  1953  are included. An example of such an indicator is described with respect to  FIG. 20 . 
     With reference to  FIG. 19B , the packet  1961  includes a preamble  1971  and the payload  1861 . The preamble  1971  includes the HT-STF  1712 , the HT-LTF  1714 , and the SIG-A field  916   e  discussed above. The preamble  1971  further includes a precoded STF  1973  comprising one symbol, and a SIG field comprising one symbol. The precoded STF  1973  may be used in automatic gain control (AGC) process. In  FIG. 19B , the SIG field is illustrated as a SIG-B field  1977 . The preamble  1971  optionally includes one or more precoded LTFs  1975 . The precoded LTFs  1975  may be used to for training purposes, for example, to estimate the channel over which the packet  1961  is received. Precoding may allow additional amounts of data to be transmitted per symbol. In some aspects, the SIG-B field  1977  is precoded. 
     In some aspects, the packet  1961  is used for MU-MIMO or Tx-BF transmission. Indicators in the SIG-A field  916   e  may be used to differentiate between such transmissions as will be discussed in additional detail below. In some aspects, the precoded LTFs  1975  are included when more than one spatial stream is used, similar to how the additional LTFs  1953  are included in the packet  1941  when more than one spatial stream is used. The inclusion or omission of the precoded LTFs  1975  may be indicated in the same way as the inclusion or omission of the additional LTFs  1953 . 
     In some aspects, a modulation of at least one of the symbols of the SIG-A field  916   e  is used to identify whether the packet  1941  or the packet  1961  is being transmitted. For example, the wireless device  202   t  may transmit the first symbol of the SIG-A field  916   e  using a rotated BPSK. When the wireless device  202   r  receives the SIG-A field  916   e , the wireless device  202   r  may determine that the packet  1961  is being received. In some aspects, a QBPSK rotation is used. Thus, a rotation of a symbol in the SIG-A field  916   e  may indicate that the pre-coded STF  1973  follows the SIG-A field  916   e , as well as indicating that the SIG-B field  1977  is included in the preamble  1971 . 
     In some uses of the wireless communication system  100  of  FIG. 1 , SU open loop transmissions will be used with a greater frequency than either MU-MIMO or Tx-BF transmissions. For example, certain sensors configured for 802.11 ah transmission may use SU open loop transmission. Thus, in the first implementation, the packet  1941  may be used more often than the packet  1961 , and the SIG-B field  1977  therefore omitted from many communicated packets. 
       FIG. 20  illustrates an example of the SIG-A field  916   e . The SIG-A field  916   e  includes an MCS sub-field  2051  comprising 4 bits, a spatial streams sub-field  2053  comprising 2 bits, an SGI sub-field  2055  comprising 1 bit, a length sub-field  2057  comprising 12 bits, a bandwidth sub-field  2059  comprising 2 bits, an aggregation sub-field  2061  comprising 1 bit, a coding sub-field  2063  comprising 1 bit, an MU sub-field  2065  comprising 1 bit, a space-time block code (STBC) sub-field  2067  comprising 1 bit, an AID/GID sub-field  2069  comprising 16 bits, a reserved sub-field  2071  comprising 1 bit, a CRC sub-field  2073  comprising 4 bits, and a tail sub-field  2075  comprising 6 bits. 
     The MCS sub-field  2051  indicates an MCS used when the SIG-A field  916   e  is used in a SU transmission. The MCS sub-field  2051  is reserved for MU transmission because the MCS for an MU transmission may be indicated in the SIG-B field  1977 . In some aspects, the SU transmission may be indicated by the symbols of the SIG-A field  916   e  being transmitted without a rotated modulation, or by the MU sub-field  2065  being set to zero when a symbol of the SIG-A field  916   e  is transmitted with a rotated modulation. 
     The spatial streams sub-field  2053  may indicate the number of spatial streams used in a SU transmission. When the spatial streams sub-field  2053  indicates that more than one spatial stream is used, the additional LTFs  1953  or precoded LTFs  1975  may be included. Thus, a value of the spatial streams sub-field  2053  may indicate whether one or more LTFs are included after the SIG-A field  916   e , as well as how many of the additional LTFs are included. The spatial streams sub-field  2053  may be reserved for MU transmissions. 
     The length sub-field  2057  may indicate a length of the packet, or of the payload of the packet, in which the SIG-A field  916   e  is included. The length sub-field  2057  may indicate the length of the packet in bytes when a non-aggregated MPDU is used with SU transmission. This ensures that the PHY layer of the wireless device  202   r  may properly determine the length of the packet. If MU is used or if A-MPDU is used, the length sub-field  2057  indicates the length of the packet in symbols. In some aspects, A-MPDU is always used for MU transmission. In some aspects, A-MPDU is always used for packets having a length that is greater than 4095 bytes. When the length sub-field  2057  indicates the length in symbols, the length of the packet may be accurately determined because delimiters within the A-MPDU may carry an exact byte length. Further, the bandwidth sub-field  2059  may indicate a bandwidth used for the packet  1941  or the  1961 , for example. 
     The aggregation sub-field  2061  indicates whether MPDUs are being aggregated when SU transmission is used. Thus, the aggregation sub-field indicates whether an A-MPDU is used, as well as indicates whether the length sub-field  2057  should be interpreted as bytes or symbols. The aggregation sub-field  2061  may be reserved for MU transmissions in some aspects. 
     The coding sub-field  2063  may indicate a coding for a plurality of users. The coding sub-field  2063  may indicate a coding type for SU, and may be reserved in the case of MU. 
     As alluded to above, the MU sub-field  2065  indicates whether the SIG-A field  916   e  is included an MU transmission or an SU transmission. In the illustrated aspect, a value of “1” in the MU sub-field  2065  indicates that MU is being used, while a value of zero indicates that SU is being used. 
     The STBC sub-field  2067  indicates STBC for some or all spatial streams. Further, the STBC sub-field  2067  may be used as in the 802.11 ac standard. 
     The AID/GID sub-field  2069  will carry different information depending on whether MU or SU is being used. When MU transmissions are not being used, the AID/GID sub-field  2069  may indicate an association identifier (AID) of the device to which the packet carrying the SIG-A field  916   e  is directed. When MU transmission are being used, the AID/GID sub-field  2069  may indicate a group identifier (GID) of the devices to which the packet carrying the SIG-A field  916   e  is directed, as well as a number of spatial streams being used. When the AID/GID sub-field  2069  indicates that more than one spatial stream is used, the precoded LTFs  1975  may be included. Thus, a value of the AID/GID sub-field  2069  may indicate whether one or more precoded LTFs  1975  are included after the SIG-A field  916   e , as well as how many of the precoded LTFs  1975  are included. 
       FIG. 21  illustrates an example  1977   a  of the SIG-B field  1977 . The SIG-B field  1977   a  includes an MCS sub-field  2151  comprising 4 bits, a coding sub-field  2153  comprising 1 bit, a reserved sub-field  2155  comprising 11 bits, a CRC sub-field  2157  comprising 4 bits, and a tail sub-field  2159  comprising 6 bits. In some aspects, a SIG-B field  1977   a  is included for each user transmission. Thus, each of the sub-fields  2151 - 2159  may include information for one user. 
     In some aspects, the SIG-B field  1977  may be omitted for SU Tx-BF transmissions. This aspect, however, may involve an additional mode to properly receive a packet omitting the SIG-B field. Thus, rather than a wireless device implementing two modes, for example, the wireless device may implement three modes. 
     The first implementation discussed above with respect to  FIG. 19  provides support for SU-MIMO, STBC, short GI, AID-based power save, and bandwidths using only the SIG-A field. The preamble for such communications may comprise only six symbols. Additional information may be included for MU-MIMO or Tx-BF in an extension field, for example, a SIG-B field or one or more additional LTFs. 
       FIGS. 22A, 22B, and 22C  illustrate a second implementation showing a plurality of formats that may be used for the packet  1841  discussed above. Each of the formats illustrated in  FIGS. 22A, 22B, and 22C  includes the HT-STF  1712 , the HT-LTF  1714 , and an example  916   f  of the SIG field  916 . The SIG-A field  916   f  includes one symbol. 
       FIG. 22A  illustrates an example  2241  of a format of the packet  1841  according to the second implementation,  FIG. 22B  illustrates an example  2261  of another format of the packet  1841  according to the second implementation, and  FIG. 22C  illustrates an example  2281  of yet another format of the packet  1841  according to the second implementation. The wireless device  202   r  may distinguish between the packets  2241 ,  2261  and  2281  based on at least the SIG-A field  916   f.    
     With reference to  FIG. 22A , the packet  2241  includes a preamble  2251  and the payload  1861 . The preamble  2251  includes the HT-STF  1712 , the HT-LTF  1714 , and the SIG-A field  916   f  discussed above. In some aspects, the packet  2241  is used for open loop transmissions over one spatial stream. For example, certain sensors configured for 802.11ah transmission may utilize the packet  2241 . 
     With reference to  FIG. 22B , the packet  2261  includes a preamble  2271  and the payload  1861 . The preamble  2271  includes the HT-STF  1712 , the HT-LTF  1714 , the SIG-A field  916   f  discussed above, and the extension field  1012 . In  FIG. 22B , the extension field  1012  is illustrated as an extension SIG field comprising two symbols. The preamble  2271  optionally includes the one or more additional LTFs  1953 . 
     In some aspects, the packet  2261  is used for open loop MIMO transmission. In such aspects, the additional LTFs  1953  are omitted when one spatial stream is used for the packet  2261 . When additional spatial streams are used, an additional LTF  1953  for each additional spatial stream may be included in the preamble  2271 . In some aspects, 1, 2, or 4 spatial streams may be used. In these aspects, 0, 1, or 2 additional LTFs  1953  will be included in the preamble  2271 . 
     In some aspects, an indicator in the extension field  1012  signifies whether the additional LTFs  1953  are included. An example of such an indicator is described below with respect to  FIG. 24 . 
     With reference to  FIG. 22C , the packet  2281  includes a preamble  2291  and the payload  1861 . The preamble  2291  includes the HT-STF  1712 , the HT-LTF  1714 , the SIG-A field  916   f  discussed above, the extension field  1012 , and the precoded STF  1973 . In  FIG. 22C , the extension field  1012  is illustrated as an extension SIG field comprising two symbols. The preamble  2291  optionally includes the one or more precoded LTFs  1975 . 
     In some aspects, the packet  2281  is used for MU-MIMO or Tx-BF transmission. Indicators in the extension field  1012  may be used to differentiate between such transmissions, as will be discussed in additional detail below. In some aspects, the precoded LTFs  1975  are included when more than one spatial stream is used, similar to how the additional LTFs  1953  are included in the packet  2261  when more than one spatial stream is used. The inclusion or omission of the precoded LTFs  1975  may be indicated in the same way as the inclusion or omission of the additional LTF  1953 . 
     In some aspects, a modulation of the SIG-A field  916   f  is used to identify whether the packet  2241  or either of the packets  2261 ,  2281  is being transmitted. For example, the wireless device  202   t  may transmit the SIG-A field  916   f  using a rotated BPSK. When the wireless device  202   r  receives the SIG-A field  916   f , the wireless device  202   r  may determine that either the packet  2261  or the packet  2281  is being received. In order to differentiate between the packet  2261  and  2281 , the wireless device  202   r  may evaluate the extension field  1012 . In some aspects, a QBPSK rotation is used to delineate between the packet  2241  and either of the packets  2261 ,  2281 . Thus, a rotation of the SIG-A field  916   f  may indicate that the extension field  1012  follows the SIG-A field  916   e . The extension field  1012  may indicate whether an additional LTF  1953 , a precoded STF  1973 , or the payload  1861  is next. 
     In some aspects, the processor  204  of the wireless device  202  ( FIG. 2 ) determines to include the extension field  1012  when MIMO, MU-MIMO, STBC, or SU-BF is used. In some aspects, the processor  204  of the wireless device  202  determines to include the extension field  1012  when the packet being transmitted is greater than 4096 bytes, a short GI is used, or a low-density parity-check (LDPC) code is used. Thus, the extension field  1012  may be included for certain open loop SU modes (e.g. short GI, STBC, MIMO, aggregation). In some aspects, A-MPDU is used when the extension field  1012  is included, and aggregation is not used when the extension field  1012  is omitted. 
     As discussed above, an open loop transmission may be used in the wireless communication system  100  of  FIG. 1 . For example, certain sensors configured for 802.11ah transmission may use an open loop transmission. Thus, in the second implementation, a packet including only five symbols in the preamble (e.g. the packet  2241 ) may be used for typical sensor transmissions. 
       FIG. 23  illustrates an example of the SIG-A field  916   f  The SIG-A field  916   f  includes a length sub-field  2351  comprising 12 bits, an MCS sub-field  2353  comprising 4 bits, the bandwidth sub-field  2059 , a reserved sub-field  2355  comprising 1 bit, a parity sub-field  2357  comprising 1 bit, and the tail sub-field  2075 . 
     The length sub-field  2351  may indicate a length of the packet, or of the payload of the packet, in which the SIG-A field  916   f  is included. The length field  2351  may indicate the length of the packet in when the extension field  1012  is omitted. When the extension field  1012  is included, the length may be indicated in symbols. As discussed above, the inclusion of the extension field  1012  may be indicated by BPSK rotation of the SIG-A field  916   f . Thus, a modulation rotation of the SIG-A field  916   f  may delineate whether the length field  2351  should be interpreted as bytes or symbols. 
     The MCS sub-field  2353  indicates an MCS used for a user. If SU is being used, the MCS is for the single user. If MU is being used, the MCS is for one of the multiple users, for example, the first user. 
       FIG. 24  illustrates an example  1012   c  of the extension field  1012 . In the illustrated aspect, the extension field  1012   c  comprises a two symbol extension SIG field. The extension SIG field  1012   c  includes an MCS sub-field  2451  comprising 12 bits, an N sts  sub-field  2453  comprising 8 bits, a BF sub-field  2455  comprising 1 bit, an SGI/LGI sub-field  2457  comprising 1 bit, a coding sub-field  2459  comprising 4 bits, an STBC sub-field  2461  comprising 1 bit, a GID sub-field  2463  comprising 6 bits, a CRC sub-field  2465  comprising 4 bits, a reserved sub-field  2467  comprising 9 bits, and a tail sub-field  2469  comprising 6 bits. 
     For MU transmissions, the MCS sub-field  2451  may indicate an MCS for each of a plurality of users. In the illustrated aspect, there may be up to three users. As discussed above, an MCS for one user may be included in the SIG-A field  916   f  The MCSs in the MCS sub-field  2451  may be for users in addition to the user for which the MCS is included in the SIG-A field  916   f  Thus, between the SIG-A field  916   f  and the extension SIG field  1012   c , MCS for four different users may be included. 
     For MU transmission, the N sts  sub-field  2453  may indicate a number of spatial streams being used. For SU transmissions, however, the MCS sub-field  2451  in combination with the N sts , sub-field  2453  may be used to indicate an AID of the single user. For example, the bits of the MCS sub-field  2451  and six bits of the N sts , sub-field  2453  may carry the AID. 
     The BF sub-field  2455 , the SGI/LGI sub-field  2457 , and the STBC sub-field  2461  may indicate whether beamforming is being used, whether an SGI or LGI is being used, and whether STBC is being used, respectively. Thus, the BF sub-field  2455  may be used to distinguish between SU open loop transmissions and SU-BF transmissions. 
     The GID sub-field  2463  may indicate a GID for devices to which a packet including the extension SIG field  1012   c  is addressed. In some aspects, a value of the GID sub-field  2463  is reserved for SU open loop transmissions and/or a value of the GID sub-field  2463  is reserved for SU-BF transmissions. In such aspects, SU open loop an SU-BF transmissions may be distinguished without evaluating the BF sub-field  2455 . In some such aspects, the BF sub-field  2455  is omitted. 
     The coding sub-field  2459  may indicate a coding for each of a plurality of users. In the illustrated aspect, there may be up to four users. In one aspect, each bit of the coding sub-field  2459  indicates a coding used for a respective user. 
     In some aspects, the SGI/LGI sub-field  2457  and/or the STBC sub-field  2461  may be included in the SIG-A field  916   f  rather than the extension SIG field  1012   c . The STBC sub-field  2461  indicates an STBC for some or all spatial streams. 
       FIGS. 25A and 25B  illustrate another implementation showing a plurality of formats that may be used for the packet  1841  discussed above. Each of the formats illustrated in  FIGS. 25A and 25B  include an STF  912 , an LTF  914 , and a SIG-A field  916 . In this example, the LTF  914  and the SIG-A field  916  each include four symbols.  FIG. 25A  illustrates an example of a format of a packet, and  FIG. 25B  illustrates an example of another format of a packet. The wireless device  202   r  may distinguish between the packets of  FIGS. 25A and 25B  based on at least the LTF field  914 . 
     With reference to  FIG. 25A , the packet  2500  includes a preamble  2510  and the payload  2520 . The preamble  2510  includes STF  912 , LTF  916 , and SIG-A field  916 . The SIG-A field  916  may be repetition coded. These fields may be similar to the corresponding fields discussed in this disclosure. In some aspects, the packet  2500  is used for open loop transmissions over one spatial stream. For example, certain sensors configured for 802.11ah transmission may utilize the packet  2500 . 
     With reference to  FIG. 25B , the packet  2550  includes a preamble  2560  and the payload  2520 . The preamble  2560  includes STF  912 , LTF  914 , SIG-A field  916 , and extension field  1012 . The SIG-A field  916  may be repetition coded. These fields may be similar to the corresponding fields discussed in this disclosure. In  FIG. 25B , the extension field  1012  is illustrated as an extension SIG field comprising three symbols. 
     In some aspects, the packet  2550  is used when advanced features, such as open loop MIMO transmission, LDPC, single-user MIMO, Midamble, STBC, and PAID are used or when the payload is greater than 511 bytes. The extension field  1012  of the packet  2550  may communicate information for the advanced features. In some aspects, an indicator in the LTF field  914  signifies whether the extension SIG field  1012  are included. An example of such an indicator is described below with respect to  FIG. 26 . 
       FIG. 26  illustrates an example of the SIG-A field  916   g . In the illustrated aspect, the SIG field  916   g  comprises four symbols. The SIG-A field  916   g  includes a Length sub-field  2651  comprising 9 bits, an MCS sub-field  2653  comprising 4 bits, an SGI sub-field  2655  comprising 1 bit, a 4-bit CRC sub-field  2657 , and a 6-bit tail field  2659 . In some embodiments, instead of the 4-bit CRC sub-field  2657 , 1 parity bit and 3 reserved bits may be included. Alternatively, in some aspects, rather than the 4-bit CRC sub-field  2657 , 1 parity bit, 2 reserved bits, and 1-bit Doppler/Midamble sub-field may be included. 
     The Length sub-field  2651  may indicate a length of the packet, or of the payload of the packet, in which the SIG-A field  916   g  is included. The Length sub-field  2651  may indicate the length of the packet in bytes when the extension field  1012  is omitted. When the extension field  1012  is included, the length may be indicated in symbols. Whether the extension SIG field  1012  is included in the packet may be indicated by symbol rotation of the LTF field  914  or a portion of the LTF field  914 . For example, rotations of the last two symbols of the LTF field  914  may indicate whether or not the extension SIG field  1012  is included. Thus, a modulation rotation of the LTF field  914  may delineate whether the Length sub-field  2651  should be interpreted as bytes or symbols. 
     The MCS sub-field  2653  may indicate a MCS for a user. If SU mode is being used, the MCS may be for the single user. If MU mode is being used, the MCS may be for one of the multiple users, for example, the first user. The SGI sub-field  2655  may indicate where the short guard interval is used. For instance, a short guard interval may be 2 μs and a normal guard interval may be 8 μs. In some aspects, a short guard interval may be 2 μs and a normal guard interval may be 4 μs. 
     The SIG-A field  916   g  may include the information needed for 1ss sensor traffic for payloads up to 511 bytes and include the information needed for deferral. Therefore, devices not implementing advanced features may shut-off after decoding the SIG-A field to save power. 
       FIG. 27  illustrates an example of the extension field  1012   d . In the illustrated example, the extension field  1012   d  comprises a three symbol extension SIG field. The extension SIG field  1012   d  includes a 2-bit SS sub-field  2751 , a 1-bit Doppler/Midamble sub-field  2753 , a 2 bit Coding sub-field  2755 , a 5-bit PAID (partial association identifier) sub-field  2757 , a 1-bit STBC sub-field  2759 , 1 parity sub-field  2761 , and 6 tail sub-field  2763 . 
     The num SS sub-field  2751  may indicate the number of spatial streams used. The Doppler/Midamble sub-field  2753  may be included to indicate that the receiver should mitigate the impact of high temporal channel variation, or to indicate the presence of a midamble. The coding sub-field  2755  may indicate a coding for each of a plurality of users. In the illustrated aspect, there may be up to four users. In one aspect, each bit of the coding sub-field  2755  may indicate a coding used for a respective user. 
     The PAID sub-field  2757  includes a partial identifier for one or more receivers. The PAID sub-field  2757  may be used by each receiver  202   r  as an early indicator of whether the receiver should receive and decode the remainder of the packet. For example, if the PAID sub-field  2757  indicates that the packet is not intended for a particular receiver, the particular receiver may discontinue processing the packet in order to save power. The STBC sub-field  2759  may indicate an STBC for one or more spatial streams. In some aspects, the parity sub-field  2761  covers only the extension SIG field  1012   d.    
     In some aspects, a SGI bit can be included in the extension SIG field  1012   d  instead of in the SIG-A field  916   g , and the Doppler/Midamble bit can be included in the SIG-A field  916   g  instead of in the extension SIG field  1012 . In some embodiments, the extension SIG field  1012   d  includes four symbols. In such embodiments the additional symbol may be included, for example, for reserved bits and/or additional PAID bits. The additional symbol may alternatively include other sub-fields. 
     The packets  2500  and  2550  of  FIGS. 25A and 25B  are particularly advantageous for 1 MHz transmission modes. The packet of  FIG. 25A  may be sufficient for most traffic. In some embodiments, the longer packets of  FIG. 25B  are used only when advanced features are used. 
     In some embodiments, the packets  2500  and  2550  of  FIGS. 25A and 25B  have alternate configurations. For example, in some embodiments, the SIG-A field  916   g  may be three symbols in length and the extension field  1012   d  may be only one symbol. In such embodiments, the three symbol SIG-A field  916   g  of may include a Length sub-field  2651  comprising 9 bits, an MCS sub-field  2653  comprising 3 or 4 bits, a 1 or 0 bit Coding sub-field  2755 , a SGI sub-field  2655  comprising 1 bit, and a 4-bit CRC sub-field  2657 . In some aspects, tail bits may be omitted when tail-biting convolutional codes are used. Further, a one symbol extension field  1012   d  may comprise a 2-bit SS sub-field  2751 , a 1-bit Doppler/Midamble sub-field  2753 , a 1 bit Coding sub-field  2755 , a 1-bit STBC sub-field  2759 , and 1 parity sub-field  2761 . 
     In some aspects, a payload, such as the payload  2520  of  FIGS. 25A and 25B , may be repetition coded. Whether the payload is repetition coded may be indicated by symbol rotation of the LTF field  914  or a portion of the LTF field  914 . For example, BPSK rotations of the last two symbols of the LTF field  914  may indicate whether the payload is BPSK rate ½ coded or is BPSK rate ½ repetition coded. In some embodiments, if the rotation of the LTF field  914  indicates that the payload is BPSK rate ½ repetition coded, bits in a field of a preamble that may otherwise be used to indicate MCS of the payload may be used for another purpose, such as, but not limited to reserved bits, parity bits, or a CRC field. In some embodiments, a preamble includes a SIG field, which may be BPSK rate ½ 2× repetition encoded whenever the payload is BPSK rate 1/2 2× repetition encoded, and may be BPSK rate ½ encoded whenever the payload is not BPSK rate 1/2 2× repetition encoded. 
       FIG. 28  illustrates an aspect of a method  2800  for transmitting a packet. The method  2800  may be used to selectively generate the packets discussed in this disclosure, such as packets  700 ,  800   a ,  800   b ,  800   c ,  900 ,  1941 ,  2241 ,  1000 ,  1700 ,  1941 ,  1961 ,  2261 ,  2281 , for example. The packet may be generated at the AP  104  or the STA  106  and transmitted to another node in the wireless network  100 . Although the method  2800  is described below with respect to elements of the wireless device  202   t , other components may be used to implement one or more of the steps. 
     At block  2802 , it is determined whether to include an extension field in a physical layer preamble of a communication. The extension field may comprise an extension SIG field and/or a SIG-B field, for instance. In some aspects, a plurality of extension fields may be included. The determination may be performed by the processor  204  and/or the DSP  220 , for example. In some aspects, the processor  204  determines to include the extension field when MU-MIMO is used, when the length of the packet will be greater than a threshold amount, when a default mode is not being used for data, when the wireless device  202   t  is not operating at one of two lowest bandwidths, or when the forward error correction (FEC) being used is not BCC. 
     At block  2804 , the communication is generated. The communication may comprise the physical layer preamble and a payload, and the preamble may include a first field indicating whether the extension field is included. The first field may comprise a SIG field, for example, a SIG-A field. The inclusion of the extension field may be indicated, for example, by one or more bits in the SIG field, a BPSK rotation of the SIG field, and/or a bit on the Q-rail during a symbol of the SIG field. The generation may be performed by the processor  204  and/or the DSP  220 , for instance. In some aspects, the processor  204  includes coding parameters for the payload in the first field when it is determined not to include the extension field, and includes coding parameters for the payload in the extension field when it is determined to include the extension field. In some aspects, the coding parameters in an MCS may be for one or more users. 
     At block  2806 , the packet is wirelessly transmitted. The transmission may be performed by the transmitter  210 , for example. 
       FIG. 29  is a functional block diagram of an example wireless device  2900  that may be employed within the wireless communication system  100  of  FIG. 1 . The device  2900  comprises a determining module  2902  for determining whether to include an extension field in a physical layer preamble of a communication. The determining module  2902  may be configured to perform one or more of the functions discussed above with respect to the block  2802  illustrated in  FIG. 28 . The determining module  2902  may correspond to one or more of the processor  204  and the DSP  220  of  FIG. 2 , for instance. The device  2900  further comprises a generating module  2904  for generating the communication. The generating module  2904  may be configured to perform one or more of the functions discussed above with respect to the block  2804  illustrated in  FIG. 28 . The generating module  2904  may correspond to one or more of the processor  204  and the DSP  220 , for instance. The device  2900  further comprises a transmitting module  2906  for wirelessly transmitting the generated communication. The transmitting module  2906  may be configured to perform one or more of the functions discussed above with respect to the block  2806  illustrated in  FIG. 28 . The transmitting module  2906  may correspond to the transmitter  210  of  FIG. 2 , for instance. 
       FIG. 30  illustrates an aspect of a method  3000  for receiving and processing a packet. The method  3000  may be used to receive and process the packets discussed in this disclosure, such as packets  700 ,  800   a ,  800   b ,  800   c ,  900 ,  1941 ,  2241 ,  1000 ,  1700 ,  1941 ,  1961 ,  2261 ,  2281 , for example. The packet may be received at either the AP  104  or the STA  106  from another node in the wireless network  100  of  FIG. 1 . Although the method  3000  is described below with respect to elements of the wireless device  202   r , other components may be used to implement one or more of the steps. 
     At block  3002 , a wireless communication comprising a physical layer preamble and a payload is received. The reception may be performed by the receiver  212 , for example. In some aspects, the preamble includes a first field indicating whether the preamble also includes an extension field. The first field may comprise a SIG field, for example, a SIG-A field. The inclusion of the extension field may be indicated, for example, by one or more bits in the SIG field, a BPSK rotation of the SIG field, and/or a bit on the Q-rail during a symbol of the SIG field. The extension field may comprise an extension SIG field and/or a SIG-B field. In some aspects, a plurality of extension fields may be included. 
     At block  3004 , the payload is processed based on modulation coding parameters included in the first field when the indicator signifies that the preamble does not include the extension field, and based on coding parameters included in the extension field when the indicator signifies that the preamble includes the extension field. The processing may be performed by the processor  204 , the signal detector  218 , and/or the DSP  220 , for example. In some examples, the payload is processed using an MCS included in the first field and/or the extension field. In some aspects, when the extension field is included, the payload is processed by combining one or more sub-fields of the first field with one or more sub-fields of the extension field. In some aspects, the payload is processed for a plurality of users based on information in the extension field. 
       FIG. 31  is a functional block diagram of an example wireless device  3100  that may be employed within the wireless communication system  100  of  FIG. 1 . The device  3100  comprises a receiving module  3102  for wirelessly receiving a wireless communication comprising a physical layer preamble and a data unit. In some aspects, the preamble includes a first field indicating whether the preamble also includes an extension field. The receiving module  3102  may be configured to perform one or more of the functions discussed above with respect to the block  3002  illustrated in  FIG. 30 . The receiving module  3002  may correspond to the receiver  212  of  FIG. 2 , for example. The device  3100  further comprises a processing module  3104  for processing the payload based on modulation coding parameters included in the first field. The processing module  3104  may be configured to perform one or more of the functions discussed above with respect to the block  3004  illustrated in  FIG. 30 . The processing module  3104  may correspond to one or more of the processor  204 , the signal detector  218 , and the DSP  220  of  FIG. 2 , for example. The device  3100  further comprises a processing module  3106  for processing the payload based on coding parameters included in the extension field. The processing module  3104  may be configured to perform one or more of the functions discussed above with respect to the block  3004  illustrated in  FIG. 30 . The processing module  3106  may correspond to one or more of the processor  204 , the signal detector  218 , and the DSP  220  of  FIG. 2 , for example. 
       FIG. 32  illustrates various components that may be utilized in the receiver  212  of the wireless device  202  of  FIG. 2 . The components illustrated in  FIG. 32  may be used to receive and distinguish between packets, for example, such as the packets  900 ,  1000  and the packet  1100 . 
     In the aspect illustrated in  FIG. 32 , the receiver  212  comprises a first detector  3202  and a second detector  3204 . The first detector  3202  is configured to detect the STF  912  of  FIG. 9 , for instance. The second detector  3204  is configured to detect the STF  1112  of  FIG. 11 , for instance. The first detector  3202  and the second detector  3204  may run in parallel to detect a packet and the format of the packet. 
     By using both the first detector  3202  and the second detector  3204 , the receiver  212  may auto-detect whether the packet  1100  is received, or whether the packets  900 ,  1000  are received. If the first detector  3202  detects that the packet  900  or  1000  is being received, the wireless device  202   r  may use one or more of the mechanisms described in this disclosure to determine whether the packet  900  or the packet  1000  is being received. The payload of a received packet may be processed based on which of the packets  900 - 1100  are received and based on a SIG and/or extension field in the received packet. In this way, the wireless device  202   r  may be configured to receive and process packets that are formatted in multiple configurations as illustrated in  FIGS. 9-11 , for example. 
     The processor of the wireless device  202   t  may be configured to select, for instance, between the packets  900 ,  1000 , and the packet  1100  based on which packet includes a sequence repeated a greater number of times than in the packets  900  and  1000 . Thus, a longer, more robust STF and/or preamble may be transmitted when advantageous while maintaining the STF and/or preamble at an efficient length in other transmissions. 
     In addition to or in place of the STF detection described in this disclosure, the wireless device  202   r  may distinguish, for instance, between the packets  900 ,  1000  and the packet  1100  using an auto-detect procedure of the LTF. For example, when one of the symbols of the LTF  1114  is flipped when compared to a respective symbol in the LTF  914 , as described in this disclosure, the wireless device  202   r  may detect whether a received packet is formatted as the packet  900  or  1000 , or as the packet  1100 . In some such aspects, the STF may be formatted similarly in different packet formats. For example, the STF  912  in the packets  900  and  1000  may be replaced by the STF  1112  in the packet  1100 . In these aspects, a single detector may be implemented in the receiver  212  to detect the start of a packet using the STF and the type of packet using the LTF. In these aspects, however, packets may use the extended STF  1112 , which may increase the length of the preamble. 
       FIG. 33  illustrates an example  1116   a  of the SIG field  1116 . The SIG field  1116   a  comprises a length sub-field  3302  including 10 bits, a repetition factor sub-field  3304  including 1 bit, a parity sub-field  3306  including 1 bit, a reserved sub-field  3308  including 1 bit, and a tail sub-field  3312  including 6 bits. The length sub-field  3302  may indicate a length of the packet  1100  in bytes. The repetition factor sub-field  3304  may indicate a number of times that the plurality of bits in the SIG field  1116   a  is repeated. In the illustrated aspect, the repetition factor sub-field includes a bit which may be used to indicate whether the plurality of bits in the SIG field  1116  is repeated two times or four times. If an 8× downclock factor is used for the preamble  1110  and the plurality of bits may be repeated twice, the PHY rate may be approximately 400 Kbps. In such an aspect, transmitting 1024 bytes may take more than approximately 20 milliseconds. 
     The packets and fields illustrated in  FIGS. 7-27 and 33  are examples and are not limiting on any of the packets or fields discussed in this disclosure. The packets and fields illustrated in  FIGS. 7-27 and 33  may include one or more additional fields or sub-fields or may omit one or more fields or sub-fields. 
       FIG. 34  illustrates an aspect of a method  3400  for transmitting a packet. The method  3400  may be used to selectively generate the packets illustrated in  FIGS. 7, 8, 9, 10, 11, 17-19, 22, 25 , for example. The packet may be generated at the AP  104  or the STA  106  and transmitted to another node in the wireless network  100 . Although the method  3400  is described below with respect to elements of the wireless device  202   t , other components may be used to implement one or more of the steps. 
     At block  3402 , a packet format is selected from at least two packet formats comprising a training field. In some aspects, the training field of one of the data packet formats includes a sequence repeated a greater number of times than in the training field of another of the data packet formats. In some aspects, the training field comprises an STF or LTF. The selection may be performed by the processor  204  and/or the DSP  220 , for example. 
     At block  3404 , a wireless communication is transmitted using the selected data packet format. The transmission may be performed by the transmitter  210 , for example. 
       FIG. 35  is a functional block diagram of another example wireless device  3500  that may be employed within the wireless communication system  100  of  FIG. 1 . The device  3500  comprises a selecting module  3502  for selecting a data packet format from at least two data packet formats comprising a training field. The selecting module  3502  may be configured to perform one or more of the functions discussed above with respect to the block  3402  illustrated in  FIG. 34 . The selection module  3502  may correspond to one or more of the processor  204  and the DSP  220  of  FIG. 2 , for example. The device  3500  further comprises a transmitting module  3504  for transmitting a wireless communication using the selected packet format. The transmitting module  3504  may be configured to perform one or more of the functions discussed above with respect to the block  3404  illustrated in  FIG. 34 . The transmitting module  3504  may correspond to the transmitter  210  of  FIG. 2 , for example. 
       FIG. 36  illustrates an aspect of a method  3600  for receiving and processing a packet. The method  3600  may be used to receive and process the packets illustrated in  FIGS. 7, 8, 9, 10, 11, 17-19, 22, 25 , for example. The packet may be received at the AP  104  or the STA  106  from another node in the wireless network  100  of  FIG. 1 . Although the method  3600  is described below with respect to elements of the wireless device  202   r , other components may be used to implement one or more of the steps. 
     At block  3602 , a packet having one of at least two formats is wirelessly received. The reception may be performed by the receiver  212 , for example. At block  3604 , a format of the packet is detected using one of at least two detectors configured to detect respective data packet formats. For example, the first detector  3202  and the second detector  3204  of the receiver  212  may be used to detect either the packet format  900  or the packet format  1100 . 
     At block  3606 , the received data packet is processed based on the detected format. The processing may be performed by the processor  204 , the signal detector  218 , and/or the DSP  220 , for example. 
       FIG. 37  is a functional block diagram of another example wireless device  3700  that may be employed within the wireless communication system  100 . The device  3700  comprises a receiving module  3702  for wirelessly receiving a packet having one of at least two formats. The receiving module  3702  may be configured to perform one or more of the functions discussed above with respect to the block  3602  illustrated in  FIG. 36 . The receiving module  3702  may correspond to the receiver  212  of  FIG. 2 , for example. The device  3700  further comprises a first detecting module  3704  for detecting whether the received data packet has a first format. The first detecting module  3704  may be configured to perform one or more of the functions discussed above with respect to the block  3604  illustrated in  FIG. 36 . The first detecting module  3704  may correspond to the first detector  3202  in the receiver  212  of  FIG. 32 , for example. The device  3700  further comprises a second detecting module  3706  for detecting whether the received data packet has a second format. The second detecting module  3706  may be configured to perform one or more of the functions discussed above with respect to the block  3604  illustrated in  FIG. 36 . The second detecting module  3706  may correspond to the second detector  3204  in the receiver  212  of  FIG. 32 , for example. The device  3700  further comprises a processing module  3708  for processing the packet based on the first detecting module  3704  and the second detecting module  3706 . The processing module  3708  may be configured to perform one or more of the functions discussed above with respect to the block  3606  illustrated in  FIG. 36 . The processing module  3708  may correspond to one or more of the processor  204 , the signal detector  218 , and the DSP  220  of  FIG. 2 , for example. 
       FIG. 38  illustrates an aspect of a method  3800  for receiving a portion of a packet. The method  3800  may be used to receive a physical layer preamble of the packet and cease further processing of the packet after determining that the packet is not intended for the device that received the packet. The packet may be received at the AP  104  or the STA  106  from another node in the wireless network  100  of  FIG. 1 . Although the method  3800  is described below with respect to elements of the wireless device  202   r , other components may be used to implement one or more of the steps. 
     At block  3802 , at least the preamble of a packet is wirelessly received. The reception may be performed by the receiver  212 , for example. In some aspects, the preamble includes a first field indicating whether the preamble also includes an extension field. The first field may comprise a SIG field, for example a SIG-A field. The inclusion of the extension field may be indicated, for example, by one or more bits in the SIG field, a BPSK rotation of the SIG field, and/or a bit on the Q-rail during a symbol of the SIG field. The extension field may comprise an extension SIG field and/or a SIG-B field. In some aspects, a plurality of extension fields may be included. 
     At block  3804 , reception of a remainder of the packet is aborted when the first field indicates that the preamble includes the extension field. The aborting may be performed by the processor  204 , the receiving  212 , the signal detector  218 , and/or the DSP  220 , for example. In this way, power that may otherwise be used to fully receive and/or process the packet may be conserved. 
       FIG. 39  is a functional block diagram of another example wireless device  3900  that may be employed within the wireless communication system  100 . The device  3900  comprises a receiving module  3902  for wirelessly receiving at least a physical layer preamble of a wireless communication. In some aspects, the preamble includes a first field indicating whether the preamble also includes an extension field. The receiving module  3902  may be configured to perform one or more of the functions discussed above with respect to the block  3802  illustrated in  FIG. 38 . The receiving module  3902  may correspond to the receiver  212  of  FIG. 2 , for example. The device  3900  further comprises an aborting module  3904  for aborting reception of a remainder of the packet when the first field indicates that the preamble includes the extension field. The aborting module  3904  may be configured to perform one or more of the functions discussed above with respect to the block  3804  illustrated in  FIG. 38 . The aborting module  3904  may correspond to one or more of the processor  204 , the receiver  212 , the signal detector  218 , and the DSP  220  of  FIG. 2 , for example. 
       FIG. 40  illustrates an aspect of a method  4000  for transmitting a packet. The method  4000  may be used to generate the packets discussed in this disclosure, such as packets  2500  and  2550 , for example. The packet may be generated at the AP  104  or the STA  106  and transmitted to another node in the wireless network  100 . Although the method  4000  is described below with respect to elements of the wireless device  202   t , other components may be used to implement one or more of the steps. 
     At block  4002 , a wireless communication comprising a physical layer preamble and payload is generated. The preamble includes a LTF indicating whether the payload includes data which is repetition coded. For instance, the indication may be provided by symbol rotation of the LTF or a portion of the LTF. The generation may be performed by the processor  204  and/or the DSP  220 , for example. 
     At block  4004 , the generated communication is transmitted wirelessly. The transmission may be performed by the transmitter  210 , for example. 
       FIG. 41  is a functional block diagram of an example wireless device  4100  that may be employed within the wireless communication system of  FIG. 1 . The device  4100  comprises a generating module  4102  for generating a wireless communication including a physical layer preamble and a payload. The preamble may include a LTF indicating whether the payload includes data which is repetition coded. The generating module  4102  may be configured to perform one or more of the functions discussed above with respect to the block  4002  illustrated in  FIG. 40 . The generating module  4102  may correspond to one or more of the processor  204  and the DSP  220  of  FIG. 2 , for instance. The device  4100  further comprises a transmitting module  4104  for wirelessly transmitting the generated communication. The transmitting module  4104  may be configured to perform one or more of the functions discussed above with respect to the block  4004  illustrated in  FIG. 40 . The transmitting module  4104  may correspond to the transmitter  210  of  FIG. 2 , for instance. 
       FIG. 42  illustrates an aspect of a method  4200  for receiving a portion of a packet. The method  4200  may be used to receive and process the packets discussed in this disclosure, such as packets  2500  and  2550 , for example. The packet may be received at either the AP  104  or the STA  106  from another node in the wireless network  100 . Although the method  4200  is described below with respect to elements of the wireless device  202   r , other components may be used to implement one or more of the steps. 
     At block  4202 , a wireless communication comprising a physical layer preamble and a payload is received. The reception may be performed by the receiver  212 , for example. 
     At block  4204 , the payload is processed based on a LTF included in the preamble that indicates whether the payload includes data which is repetition coded. For instance, the indication may comprise a symbol rotation of the LTF or a portion of the LTF. The processing may be performed by the processor  204 , the signal detector  218 , and/or the DSP  220 , for example. 
       FIG. 43  is a functional block diagram of an example wireless device  4300  that may be employed within the wireless communication system of  FIG. 1 . The device  4300  comprises a receiving module  4302  for receiving a wireless communication comprising a physical layer preamble and a payload. The receiving module  4302  may be configured to perform one or more of the functions discussed above with respect to the block  4202  illustrated in  FIG. 42 . The receiving module  4302  may correspond to the receiver  212  of  FIG. 2 , for example. The device  4200  further comprises a processing module  4304  for processing the payload based on a LTF included in the preamble that indicates whether the payload includes data which is repetition coded. The processing module  4304  may be configured to perform one or more of the functions discussed above with respect to block  4204  illustrated in  FIG. 42 . The processing module  4304  may correspond to one or more of the processor  204 , the signal detector  218 , and the DSP  220  of  FIG. 2 , for example. 
     As used herein, the term “determining” encompasses a wide variety of actions. For example, “determining” may include calculating, computing, processing, deriving, investigating, looking up (e.g., looking up in a table, a database or another data structure), ascertaining and the like. Also, “determining” may include receiving (e.g., receiving information), accessing (e.g., accessing data in a memory) and the like. Also, “determining” may include resolving, selecting, choosing, establishing and the like. Further, a “channel width” as used herein may encompass or may also be referred to as a bandwidth in certain aspects. 
     As used herein, a phrase referring to “at least one of” a list of items refers to any combination of those items, including single members. As an example, “at least one of: a, b, or c” is intended to cover: a, b, c, a-b, a-c, b-c, and a-b-c. 
     The various operations of methods described above may be performed by any suitable means capable of performing the operations, such as various hardware and/or software component(s), circuits, and/or module(s). Generally, any operations illustrated in the Figures may be performed by corresponding functional means capable of performing the operations. 
     The various illustrative logical blocks, modules and circuits described in connection with the present disclosure may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array signal (FPGA) or other programmable logic device (PLD), discrete gate or transistor logic, discrete hardware components or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any commercially available processor, controller, microcontroller or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     In one or more aspects, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Computer-readable media includes both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. A storage media may be any available media that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Thus, in some aspects computer readable medium may comprise non-transitory computer readable medium (e.g., tangible media). In addition, in some aspects computer readable medium may comprise transitory computer readable medium (e.g., a signal). Combinations of the above should also be included within the scope of computer-readable media. 
     The methods disclosed herein comprise one or more steps or actions for achieving the described method. The method steps and/or actions may be interchanged with one another without departing from the scope of the claims. In other words, unless a specific order of steps or actions is specified, the order and/or use of specific steps and/or actions may be modified without departing from the scope of the claims. 
     The functions described may be implemented in hardware, software, firmware or any combination thereof. If implemented in software, the functions may be stored as one or more instructions on a computer-readable medium. A storage media may be any available media that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. Disk and disc, as used herein, include compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and Blu-ray® disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. 
     Thus, certain aspects may comprise a computer program product for performing the operations presented herein. For example, such a computer program product may comprise a computer readable medium having instructions stored (and/or encoded) thereon, the instructions being executable by one or more processors to perform the operations described herein. For certain aspects, the computer program product may include packaging material. 
     Software or instructions may also be transmitted over a transmission medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of transmission medium. 
     Further, it should be appreciated that modules and/or other appropriate means for performing the methods and techniques described herein can be downloaded and/or otherwise obtained by a user terminal and/or base station as applicable. For example, such a device can be coupled to a server to facilitate the transfer of means for performing the methods described herein. Alternatively, various methods described herein can be provided via storage means (e.g., RAM, ROM, a physical storage medium such as a compact disc (CD) or floppy disk, etc.), such that a user terminal and/or base station can obtain the various methods upon coupling or providing the storage means to the device. Moreover, any other suitable technique for providing the methods and techniques described herein to a device can be utilized. 
     It is to be understood that the claims are not limited to the precise configuration and components illustrated above. Various modifications, changes and variations may be made in the arrangement, operation and details of the methods and apparatus described above without departing from the scope of the claims. 
     While the foregoing is directed to aspects of the present disclosure, other and further aspects of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.