Patent Publication Number: US-9906391-B2

Title: Methods and apparatus for packet acquisition in mixed-rate wireless communication networks

Description:
PRIORITY CLAIM 
     This application claims the benefit of U.S. Provisional Application No. 62/051,171, filed Sep. 16, 2014, which is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD 
     Certain aspects of the present disclosure generally relate to wireless communications, and more particularly, to methods and apparatus for mixed-rate communication in a wireless network. 
     BACKGROUND 
     In many telecommunication systems, communications networks are used to exchange messages among several interacting spatially-separated devices. Networks can be classified according to geographic scope, which could be, for example, a metropolitan area, a local area, or a personal area. Such networks can 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. 
     As the volume and complexity of information communicated wirelessly between multiple devices continues to increase, overhead bandwidth required for physical layer control signals continues to increase at least linearly. The number of bits utilized to convey physical layer control information has become a significant portion of required overhead. Thus, with limited communication resources, it is desirable to reduce the number of bits required to convey this physical layer control information, especially as multiple types of traffic are concurrently sent from an access point to multiple terminals. For example, when a wireless device sends low-rate uplink communications to an access point, it is desirable to minimize the number of bits used for signaling and packet acquisition while maintaining backwards compatibility. Thus, there is a need for an improved protocol for mixed-rate transmissions. 
     SUMMARY 
     Various implementations of systems, methods and devices within the scope of the appended claims each have several aspects, no single one of which is solely responsible for the desirable attributes described herein. Without limiting the scope of the appended claims, some prominent features are described herein. 
     Details of one or more implementations of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages can become apparent from the description, the drawings, and the claims. Note that the relative dimensions of the following figures may not be drawn to scale. 
     One aspect of the present disclosure provides a method of wireless communication. The method includes generating, at a wireless device, a packet including a first portion having a first symbol duration. The packet further includes a second portion having a second symbol duration greater than the first. The first portion includes a first training field. The method further includes prepending or appending a second training field to the first portion. The second training field has the second symbol duration. The method further includes transmitting the packet. 
     In various embodiments, said prepending or appending can include prepending the second training field to the first portion. The second training field can have a periodicity different than a periodicity of the first training field. In various embodiments, the method can further include inverting a sign of one or more samples of every other multiple of periodicity for the second training field. In various embodiments, the second training field can have a periodicity at least two times the periodicity of the first training field. 
     In various embodiments, the packet can further include a signal field after the first portion, the signal field having the second symbol duration. In various embodiments, the packet can further include a one or more symbols after the first portion. The symbols can have the first symbol duration. 
     In various embodiments, the first portion can include a signal field. The second portion can include a plurality of repeated portions of the signal field. The repeated portions can have the second symbol duration. 
     In various embodiments, n repeated portions of the signal field can each include a subset of carrier tones of the signal field. In various embodiments, said prepending or appending can include appending one or more symbols having the first symbol duration to the first portion, boosting a gain of the second training field, and appending the second training field to the one or more symbols. 
     In various embodiments, the packet can include a scheduled multi-user (MU) orthogonal frequency division multiple access (OFDMA) uplink (UL) packet. In various embodiments, the packet initiates an unscheduled single-user (SU) orthogonal frequency division multiple access (OFDMA) uplink (UL) packet. 
     Another aspect provides an apparatus of wireless communication. The apparatus includes a processor configured to generate a packet including a first portion having a first symbol duration. The packet further includes a second portion having a second symbol duration greater than the first. The first portion includes a first training field. The processor is further configured to prepend or append a second training field to the first portion. The second training field has the second symbol duration. The apparatus further includes a transmitter configured to transmit the packet. 
     In various embodiments, the processor can be configured to prepend the second training field to the first portion. The second training field can have a periodicity different than a periodicity of the first training field. In various embodiments, the processor can be further configured to invert a sign of one or more samples of every other multiple of periodicity for the second training field. In various embodiments, the second training field can have a periodicity at least two times the periodicity of the first training field. 
     In various embodiments, the packet can further include a signal field after the first portion. The signal field can have the second symbol duration. In various embodiments, the packet can further include a one or more symbols after the first portion. The symbols can have the first symbol duration. 
     In various embodiments, the first portion can include a signal field. The second portion can include a plurality of repeated portions of the signal field. The repeated portions can have the second symbol duration. 
     In various embodiments, n repeated portions of the signal field can each include a subset of carrier tones of the signal field. In various embodiments, the processor can be configured to append one or more symbols having the first symbol duration to the first portion, boost a gain of the second training field, and append the second training field to the one or more symbols. 
     In various embodiments, the packet can include a scheduled multi-user (MU) orthogonal frequency division multiple access (OFDMA) uplink (UL) packet. In various embodiments, the packet initiates an unscheduled single-user (SU) orthogonal frequency division multiple access (OFDMA) uplink (UL) packet. 
     Another aspect provides an apparatus of wireless communication. The apparatus includes means for generating a packet including a first portion having a first symbol duration. The packet further includes a second portion having a second symbol duration greater than the first. The first portion includes a first training field. The apparatus further includes means for prepending or appending a second training field to the first portion. The second training field has the second symbol duration. The apparatus further includes means for transmitting the packet. 
     In various embodiments, said means for prepending or appending can include means for prepending the second training field to the first portion. The second training field has a periodicity different than a periodicity of the first training field. In various embodiments, the apparatus can further include means for inverting a sign of one or more samples of every other multiple of periodicity for the second training field. In various embodiments, the second training field can have a periodicity at least two times the periodicity of the first training field. 
     In various embodiments, the packet can further include a signal field after the first portion. The signal field can have the second symbol duration. In various embodiments, the packet can further include a one or more symbols after the first portion. The symbols can have the first symbol duration. 
     In various embodiments, the first portion can include a signal field. The second portion can include a plurality of repeated portions of the signal field. The repeated portions can have the second symbol duration. 
     In various embodiments, n repeated portions of the signal field can each include a subset of carrier tones of the signal field. In various embodiments, said means for prepending or appending can include means for appending one or more symbols having the first symbol duration to the first portion, means for boosting a gain of the second training field, and means for appending the second training field to the one or more symbols. 
     In various embodiments, the packet can include a scheduled multi-user (MU) orthogonal frequency division multiple access (OFDMA) uplink (UL) packet. In various embodiments, the packet initiates an unscheduled single-user (SU) orthogonal frequency division multiple access (OFDMA) uplink (UL) packet. 
     Another aspect provides a non-transitory computer-readable medium. The medium includes code that, when executed, causes an apparatus to generate a packet including a first portion having a first symbol duration. The packet further includes a second portion having a second symbol duration greater than the first. The first portion includes a first training field. The medium further includes code that, when executed, causes the apparatus to prepend or append a second training field to the first portion. The second training field has the second symbol duration. The medium further includes code that, when executed, causes the apparatus to transmit the packet. 
     In various embodiments, said prepending or appending can include prepending the second training field to the first portion. The second training field can have a periodicity different than a periodicity of the first training field. In various embodiments, the medium can further include code that, when executed, causes the apparatus to invert a sign of one or more samples of every other multiple of periodicity for the second training field. In various embodiments, the second training field can have a periodicity at least two times the periodicity of the first training field. 
     In various embodiments, the packet can further include a signal field after the first portion. The signal field can have the second symbol duration. In various embodiments, the packet can further include a one or more symbols after the first portion. The symbols can have the first symbol duration. 
     In various embodiments, the first portion can include a signal field. The second portion can include a plurality of repeated portions of the signal field. The repeated portions can have the second symbol duration. 
     In various embodiments, n repeated portions of the signal field each comprise a subset of carrier tones of the signal field. In various embodiments, said prepending or appending can include appending one or more symbols having the first symbol duration to the first portion, boosting a gain of the second training field, and appending the second training field to the one or more symbols. 
     In various embodiments, the packet can include a scheduled multi-user (MU) orthogonal frequency division multiple access (OFDMA) uplink (UL) packet. In various embodiments, the packet initiates an unscheduled single-user (SU) orthogonal frequency division multiple access (OFDMA) uplink (UL) packet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of a wireless communication system in which aspects of the present disclosure can be employed. 
         FIG. 2  illustrates various components that can be utilized in a wireless device that can be employed within the wireless communication system of  FIG. 1 . 
         FIG. 3  illustrates a channel allocation for channels available for 802.11 systems. 
         FIGS. 4 and 5  illustrate data packet formats for several currently existing IEEE 802.11 standards. 
         FIG. 6  illustrates a frame format for the currently existing IEEE 802.11ac standard. 
         FIG. 7  illustrates an exemplary structure of a physical-layer packet which can be used to enable backward-compatible multiple access wireless communications. 
         FIG. 8  illustrates an exemplary structure of an uplink physical-layer packet which can be used to enable backward-compatible low rate multiple-access wireless communications. 
         FIG. 9  illustrates another exemplary structure of an uplink physical-layer packet which can be used to enable backward-compatible low rate multiple-access wireless communications. 
         FIG. 10  illustrates another exemplary structure of an uplink physical-layer packet which can be used to enable backward-compatible low rate multiple-access wireless communications. 
         FIG. 11  illustrates another exemplary structure of an uplink physical-layer packet which can be used to enable backward-compatible low rate multiple-access wireless communications. 
         FIG. 12  shows an example detector that can be used to detect a hidden low rate short training field. 
         FIG. 13  shows example normalized correlator output over time for both legacy and high-efficiency wireless receivers. 
         FIG. 14  shows another flowchart for an exemplary method of wireless communication that can 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 disclosed can, 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 can be implemented or a method can 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 can 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 can include various types of wireless local area networks (WLANs). A WLAN can be used to interconnect nearby devices together, employing widely used networking protocols. The various aspects described herein can 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 can be used as part of an IEEE 802.11 protocol, such as an 802.11 protocol which supports orthogonal frequency-division multiple access (OFDMA) communications. 
     It can be beneficial to allow multiple devices, such as STAs, to communicate with an AP at the same time. For example, this can allow multiple STAs to receive a response from the AP in less time, and to be able to transmit and receive data from the AP with less delay. This can also allow an AP to communicate with a larger number of devices overall, and can also make bandwidth usage more efficient. By using multiple access communications, the AP can be able to multiplex OFDM symbols to, for example, four devices at once over an 80 MHz bandwidth, where each device utilizes 20 MHz bandwidth. Thus, multiple access can be beneficial in some aspects, as it can allow the AP to make more efficient use of the spectrum available to it. 
     It has been proposed to implement such multiple access protocols in an OFDM system such as the 802.11 family by assigning different subcarriers (or tones) of symbols transmitted between the AP and the STAs to different STAs. In this way, an AP could communicate with multiple STAs with a single transmitted OFDM symbol, where different tones of the symbol were decoded and processed by different STAs, thus allowing simultaneous data transfer to multiple STAs. These systems are sometimes referred to as OFDMA systems. 
     Such a tone allocation scheme is referred to herein as a “high-efficiency” (HE) system, and data packets transmitted in such a multiple tone allocation system can be referred to as high-efficiency (HE) packets. Various structures of such packets, including backward compatible preamble fields are described in detail below. 
     Various aspects of the novel systems, apparatuses, and methods are described more fully hereinafter with reference to the accompanying drawings. This disclosure can, 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 can be implemented or a method can 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 can 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. 
     Popular wireless network technologies can include various types of wireless local area networks (WLANs). A WLAN can be used to interconnect nearby devices together, employing widely used networking protocols. The various aspects described herein can apply to any communication standard, such as a wireless protocol. 
     In some aspects, wireless signals can be transmitted according to an 802.11 protocol. In some implementations, a WLAN includes various devices which are the components that access the wireless network. For example, there can be two types of devices: access points (APs) and clients (also referred to as stations, or STAs). In general, an AP can serve as a hub or base station for the WLAN and an STA serves as a user of the WLAN. For example, an STA can 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 compliant wireless link to obtain general connectivity to the Internet or to other wide area networks. In some implementations an STA can also be used as an AP. 
     An access point (AP) can also include, be implemented as, or known as a base station, wireless access point, access node or similar terminology. 
     A station “STA” can also include, 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. Accordingly, one or more aspects taught herein can 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 for network communication via a wireless medium. 
     As discussed above, certain of the devices described herein can implement an 802.11 standard, for example. Such devices, whether used as an STA or AP or other device, can be used for smart metering or in a smart grid network. Such devices can provide sensor applications or be used in home automation. The devices can instead or in addition be used in a healthcare context, for example for personal healthcare. They can 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 can be employed. The wireless communication system  100  can operate pursuant to a wireless standard, for example at least one of the 802.11ah, 802.11ac, 802.11n, 802.11g and 802.11b standards. The wireless communication system  100  can operate pursuant to a high-efficiency wireless standard, for example the 802.11ax standard. The wireless communication system  100  can include an AP  104 , which communicates with STAs  106 A- 106 D (which can be generically referred to herein as STA(s)  106 ). 
     A variety of processes and methods can be used for transmissions in the wireless communication system  100  between the AP  104  and the STAs  106 A- 106 D. For example, signals can be sent and received between the AP  104  and the STAs  106 A- 106 D in accordance with OFDM/OFDMA techniques. If this is the case, the wireless communication system  100  can be referred to as an OFDM/OFDMA system. Alternatively, signals can be sent and received between the AP  104  and the STAs  106 A- 106 D in accordance with code division multiple access (CDMA) techniques. If this is the case, the wireless communication system  100  can 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 A- 106 D can be referred to as a downlink (DL)  108 , and a communication link that facilitates transmission from one or more of the STAs  106 A- 106 D to the AP  104  can be referred to as an uplink (UL)  110 . Alternatively, a downlink  108  can be referred to as a forward link or a forward channel, and an uplink  110  can be referred to as a reverse link or a reverse channel. 
     The AP  104  can 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 A- 106 D associated with the AP  104  and that use the AP  104  for communication can be referred to as a basic service set (BSS). It can be noted that the wireless communication system  100  may not have a central AP  104 , but rather can function as a peer-to-peer network between the STAs  106 A- 106 D. Accordingly, the functions of the AP  104  described herein can alternatively be performed by one or more of the STAs  106 A- 106 D. 
     In some aspects, a STA  106  can be required to associate with the AP  104  in order to send communications to and/or receive communications from the AP  104 . In one aspect, information for associating is included in a broadcast by the AP  104 . To receive such a broadcast, the STA  106  can, for example, perform a broad coverage search over a coverage region. A search can also be performed by the STA  106  by sweeping a coverage region in a lighthouse fashion, for example. After receiving the information for associating, the STA  106  can transmit a reference signal, such as an association probe or request, to the AP  104 . In some aspects, the AP  104  can use backhaul services, for example, to communicate with a larger network, such as the Internet or a public switched telephone network (PSTN). 
     In an embodiment, the AP  104  includes an AP high efficiency wireless controller (HEW)  154 . The AP HEW  154  can perform some or all of the operations described herein to enable communications between the AP  104  and the STAs  106 A- 106 D using the 802.11 protocol. The functionality of the AP HEW  154  is described in greater detail below with respect to  FIGS. 4-20 . 
     Alternatively or in addition, the STAs  106 A- 106 D can include a STA HEW  156 . The STA HEW  156  can perform some or all of the operations described herein to enable communications between the STAs  106 A- 106 D and the AP  104  using the 802.11 protocol. The functionality of the STA HEW  156  is described in greater detail below with respect to  FIGS. 2-11 . 
       FIG. 2  illustrates various components that can be utilized in a wireless device  202  that can be employed within the wireless communication system  100  of  FIG. 1 . The wireless device  202  is an example of a device that can be configured to implement the various methods described herein. For example, the wireless device  202  can include the AP  104  or one of the STAs  106 A- 106 D. 
     The wireless device  202  can include a processor  204  which controls operation of the wireless device  202 . The processor  204  can also be referred to as a central processing unit (CPU) or hardware processor. Memory  206 , which can 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  can 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  can be executable to implement the methods described herein. 
     The processor  204  can include or be a component of a processing system implemented with one or more processors. The one or more processors can 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 processor  204  or the processor  204  and the memory  206  can correspond to the packet generator  124  of  FIG. 1 , which can be utilized to generate a packet including a value in a packet type field and to allocate a plurality of bits of the packet to each of a plurality of subsequent fields based at least in part on the value in the packet type field, as can be described in more detail below. 
     The processing system can also include non-transitory 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 can 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  can also include a housing  208  that can include a transmitter  210  and 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  can be combined into a transceiver  214 . An antenna  216  can be attached to the housing  208  and electrically coupled to the transceiver  214 . The wireless device  202  can also include (not shown) multiple transmitters, multiple receivers, multiple transceivers, and/or multiple antennas, which can be utilized during multiple-input multiple-output (MIMO) communications, for example. 
     The wireless device  202  can also include a signal detector  218  that can be used in an effort to detect and quantify the level of signals received by the transceiver  214 . The signal detector  218  can detect such signals as total energy, energy per subcarrier per symbol, power spectral density and other signals. The wireless device  202  can also include a digital signal processor (DSP)  220  for use in processing signals. The DSP  220  can be configured to generate a data unit for transmission. In some aspects, the data unit can include a physical layer data unit (PPDU). In some aspects, the PPDU is referred to as a packet. 
     The wireless device  202  can further include a user interface  222  in some aspects. The user interface  222  can include a keypad, a microphone, a speaker, and/or a display. The user interface  222  can 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  can be coupled together by a bus system  226 . The bus system  226  can 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. Those of skill in the art can appreciate the components of the wireless device  202  can 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 , those of skill in the art can recognize that one or more of the components can be combined or commonly implemented. For example, the processor  204  can 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  can be implemented using a plurality of separate elements. 
     As discussed above, the wireless device  202  can include the AP  104  or one of the STAs  106 A- 106 D, and can be used to transmit and/or receive communications. The communications exchanged between devices in a wireless network can include data units which can include packets or frames. In some aspects, the data units can include data frames, control frames, and/or management frames. Data frames can be used for transmitting data from an AP and/or a STA to other APs and/or STAs. Control frames can be used together with data frames for performing various operations and for reliably delivering data (e.g., acknowledging receipt of data, polling of APs, area-clearing operations, channel acquisition, carrier-sensing maintenance functions, etc.). Management frames can be used for various supervisory functions (e.g., for joining and departing from wireless networks, etc.). 
       FIG. 3  illustrates a channel allocation for channels available for 802.11 systems. Various IEEE 802.11 systems support a number of different sizes of channels, such as 5, 10, 20, 40, 80, and 160 MHz channels. For example, and 802.11ac device can support 20, 40, and 80 MHz channel bandwidth reception and transmission. A larger channel can include two adjacent smaller channels. For example, an 80 MHz channel can include two adjacent 40 MHz channels. In the currently implemented IEEE 802.11 systems, a 20 MHz channel contains 64 subcarriers, separated from each other by 312.5 kHz. Of these subcarriers, a smaller number can be used for carrying data. For example, a 20 MHz channel can contain transmitting subcarriers numbered −1 to −28 and 1 to 28, or 56 subcarriers. Some of these carriers can also be used to transmit pilot signals. 
       FIGS. 4 and 5  illustrate data packet formats for several currently existing IEEE 802.11 standards. Turning first to  FIG. 4 , a packet format for IEEE 802.11a, 11b, and 11g is illustrated. This frame includes a short training field  422 , a long training field  424 , and a signal field  426 . The training fields do not transmit data, but they allow synchronization between the AP and the receiving STAs for decoding the data in the data field  428 . 
     The signal field  426  delivers information from the AP to the STAs about the nature of the packet being delivered. In IEEE 802.11a/b/g devices, this signal field has a length of 24 bits, and is transmitted as a single OFDM symbol at a 6 Mb/s rate using BPSK modulation and a code rate of ½. The information in the SIG field  426  includes 4 bits describing the modulation scheme of the data in the packet (e.g., BPSK, 16 QAM, 64 QAM, etc.), and 12 bits for the packet length. This information is used by a STA to decode the data in the packet when the packet is intended for the STA. When a packet is not intended for a particular STA, the STA can defer any communication attempts during the time period defined in the length field of the SIG symbol  426 , and can, to save power, enter a sleep mode during the packet period of up to about 5.5 msec. 
     As features have been added to IEEE 802.11, changes to the format of the SIG fields in data packets were developed to provide additional information to STAs.  FIG. 5  shows the packet structure for the IEEE 802.11n packet. The 11n addition to the IEEE.802.11 standard added MIMO functionality to IEEE.802.11 compatible devices. To provide backward compatibility for systems containing both IEEE 802.11a/b/g devices and IEEE 802.11n devices, the data packet for IEEE 802.11n systems also includes the STF, LTF, and SIG fields of these earlier systems, noted as L-STF  422 , L-LTF  424 , and L-SIG  426  with a prefix L to denote that they are “legacy” fields. To provide the needed information to STAs in an IEEE 802.11n environment, two additional signal symbols  440  and  442  were added to the IEEE 802.11n data packet. In contrast with the SIG field and L-SIG field  426 , however, these signal fields used rotated BPSK modulation (also referred to as QBPSK modulation). When a legacy device configured to operate with IEEE 802.11a/b/g receives such a packet, it can receive and decode the L-SIG field  426  as a normal 11/b/g packet. However, as the device continued decoding additional bits, they may not be decoded successfully because the format of the data packet after the L-SIG field  426  is different from the format of an 11/b/g packet, and the CRC check performed by the device during this process can fail. This causes these legacy devices to stop processing the packet, but still defer any further operations until a time period has passed defined by the length field in the initially decoded L-SIG. In contrast, new devices compatible with IEEE 802.11n would sense the rotated modulation in the HT-SIG fields, and process the packet as an 802.11n packet. Furthermore, an 11n device can tell that a packet is intended for an 11/b/g device because if it senses any modulation other than QBPSK in the symbol following the L-SIG  426 , it can ignore it as an 11/b/g packet. After the HT-SIG 1  and SIG 2  symbols, additional training fields suitable for MIMO communication are provided, followed by the data  428 . 
       FIG. 6  illustrates a frame format for the currently existing IEEE 802.11ac standard, which added multi-user MIMO functionality to the IEEE 802.11 family. Similar to IEEE 802.11n, an 802.11ac frame contains the same legacy short training field (L-STF)  422  and long training field (L-LTF)  424 . An 802.11ac frame also contains a legacy signal field L-SIG  426  as described above. 
     Next, an 802.11ac frame includes a Very High Throughput Signal (VHT-SIG-A 1   450  and A 2   452 ) field two symbols in length. This signal field provides additional configuration information related to 11ac features that are not present in 11/b/g and 11n devices. The first OFDM symbol  450  of the VHT-SIG-A can be modulated using BPSK, so that any 802.11n device listening to the packet can believe the packet to be an 802.11a packet, and can defer to the packet for the duration of the packet length as defined in the length field of the L-SIG  426 . Devices configured according to 11/g can be expecting a service field and MAC header following the L-SIG  426  field. When they attempt to decode this, a CRC failure can occur in a manner similar to the procedure when an 11n packet is received by an 11a/b/g device, and the 11/b/g devices can also defer for the period defined in the L-SIG field  426 . The second symbol  452  of the VHT-SIG-A is modulated with a 90-degree rotated BPSK. This rotated second symbol allows an 802.11ac device to identify the packet as an 802.11ac packet. The VHT-SIGA 1   450  and A 2   452  fields contain information on a bandwidth mode, modulation and coding scheme (MCS) for the single user case, number of space time streams (NSTS), and other information. The VHT-SIGA 1   450  and A 2   452  can also contain a number of reserved bits that are set to “1.” The legacy fields and the VHT-SIGA 1  and A 2  fields can be duplicated over each 20 MHz of the available bandwidth. Although duplication may be constructed to mean making or being an exact copy, certain differences may exist when fields, etc. are duplicated as described herein. 
     After the VHT-SIG-A, an 802.11ac packet can contain a VHT-STF, which is configured to improve automatic gain control estimation in a multiple-input and multiple-output (MIMO) transmission. The next 1 to 8 fields of an 802.11ac packet can be VHT-LTFs. These can be used for estimating the MIMO channel and then equalizing the received signal. The number of VHT-LTFs sent can be greater than or equal to the number of spatial streams per user. Finally, the last field in the preamble before the data field is the VHT-SIG-B  454 . This field is BPSK modulated, and provides information on the length of the useful data in the packet and, in the case of a multiple user (MU) MIMO packet, provides the MCS. In a single user (SU) case, this MCS information is instead contained in the VHT-SIGA 2 . Following the VHT-SIG-B, the data symbols are transmitted 
     Although 802.11ac introduced a variety of new features to the 802.11 family, and included a data packet with preamble design that was backward compatible with 11/g/n devices and also provided information necessary for implementing the new features of 11ac, configuration information for OFDMA tone allocation for multiple access is not provided by the 11ac data packet design. New preamble configurations are desired to implement such features in any future version of IEEE 802.11 or any other wireless network protocol using OFDM subcarriers. 
       FIG. 7  illustrates an exemplary structure of a physical-layer packet which can be used to enable backward-compatible multiple access wireless communications. In this example physical-layer packet, a legacy preamble including the L-STF  422 , L-LTF  426 , and L-SIG  426  are included. In various embodiments, each of the L-STF  422 , L-LTF  426 , and L-SIG  426  can be transmitted using 20 MHz, and multiple copies can be transmitted for each 20 MHz of spectrum that the AP  104  ( FIG. 1 ) uses. A person having ordinary skill in the art can appreciate that the illustrated physical-layer packet can include additional fields, fields can be rearranged, removed, and/or resized, and the contents of the fields varied. 
     This packet also contains an HE-SIG 0  symbol  455 , and one or more HE-SIG 1 A symbols  457  (which can be variable in length), and an optional HE-SIG 1 B symbol  459  (which can be analogous to the VHT-SIG 1 B field  454  of  FIG. 4 ). In various embodiments, the structure of these fields can be backward compatible with IEEE 802.11a/b/g/n/ac devices, and can also signal OFDMA HE devices that the packet is an HE packet. To be backward compatible with IEEE 802.11a/b/g/n/ac devices, appropriate modulation can be used on each of these symbols. In some implementations, the HE-SIG 0  field  455  can be modulated with BPSK modulation. This can have the same effect on 802.11a/b/g/n devices as is currently the case with 802.11ac packets that also have their first SIG symbol BPSK modulated. For these devices, it does not matter what the modulation is on the subsequent HE-SIG symbols  457 . In various embodiments, the HE-SIG 0  field  455  can be modulated and repeated across multiple channels. 
     In various embodiments, the HE-SIG 1 A field  457  can be BPSK or QBPSK modulated. If BPSK modulated, an 11ac device can assume the packet is an 802.11a/b/g packet, and can stop processing the packet, and can defer for the time defined by the length field of L-SIG  426 . If QBPSK modulated, an 802.11ac device can produce a CRC error during preamble processing, and can also stop processing the packet, and can defer for the time defined by the length field of L-SIG. To signal HE devices that this is an HE packet, at least the first symbol of HE-SIG 1 A  457  can be QBPSK modulated. 
     The information necessary to establish an OFDMA multiple access communication can be placed in the HE-SIG fields  455 ,  457 , and  459  in a variety of positions. In various embodiments, the HE-SIG 0   455  can include one or more of: a duration indication, a bandwidth indication (which can be, for example, 2 bits), a BSS color ID (which can be, for example, 3 bits), an UL/DL indication (which can be, for example, a 1-bit flag), a cyclic redundancy check (CRC) (which can be, for example, 4 bits), and a clear channel assessment (CCA) indication (which can be, for example, 2 bits). 
     In various embodiments, the HE-SIG 1  field  457  can include a tone allocation information for OFDMA operation. The example of  FIG. 7  can allow four different users to be each assigned a specific sub-band of tones and a specific number of MIMO space time streams. In various embodiments, 12 bits of space time stream information allows three bits for each of four users such that 1-8 streams can be assigned to each one. 16 bits of modulation type data allows four bits for each of four users, allowing assignment of any one of 16 different modulation schemes (16 QAM, 64 QAM, etc.) to each of four users. 12 bits of tone allocation data allows specific sub-bands to be assigned to each of four users. 
     One example SIG field scheme for sub-band (also referred to herein as sub-channel) allocation includes a 6-bit Group ID field as well as 10 bits of information to allocate sub-band tones to each of four users. The bandwidth used to deliver a packet can be allocated to STAs in multiples of some number of MHz. For example, the bandwidth can be allocated to STAs in multiples of B MHz. The value of B can be a value such as 1, 2, 5, 10, 15, or 20 MHz. The values of B can be provided by a two bit allocation granularity field. For example, the HE-SIG 1 A  457  can contain one two-bit field, which allows for four possible values of B. For example, the values of B can be 5, 10, 15, or 20 MHz, corresponding to values of 0-3 in the allocation granularity field. In some aspects, a field of k bits can be used to signal the value of B, defining a number from 0 to N, where 0 represents the least flexible option (largest granularity), and a high value of N represents the most flexible option (smallest granularity). Each B MHz portion can be referred to as a sub-band. 
     The HE-SIG 1 A  457  can further use 2 bits per user to indicate the number of sub-bands allocated to each STA. This can allow 0-3 sub-bands to be allocated to each user. The group-id (G_ID) can be used in order to identify the STAs, which can receive data in an OFDMA packet. This 6-bit G_ID can identify up to four STAs, in a particular order, in this example. 
     The training fields and data which are sent after the HE-SIG symbols can be delivered by the AP according to the allocated tones to each STA. This information can potentially be beamformed. Beamforming this information can have certain advantages, such as allowing for more accurate decoding and/or providing more range than non-beamformed transmissions. 
     Depending on the space time streams assigned to each user, different users can use a different number of HE-LTFs  465 . Each STA can use a number of HE-LTFs  465  that allows channel estimation for each spatial stream associated with that STA, which can be generally equal to or more than the number of spatial streams. LTFs can also be used for frequency offset estimation and time synchronization. Because different STAs can receive a different number of HE-LTFs, symbols can be transmitted from the AP  104  ( FIG. 1 ) that contain HE-LTF information on some tones and data on other tones. 
     In some aspects, sending both HE-LTF information and data on the same OFDM symbol can be problematic. For example, this can increase the peak-to-average power ratio (PAPR) to too high a level. Thus, it can be beneficial to instead to transmit HE-LTFs  465  on all tones of the transmitted symbols until each STA has received at least the required number of HE-LTFs  465 . For example, each STA can need to receive one HE-LTF  465  per spatial stream associated with the STA. Thus, the AP can be configured to transmit a number of HE-LTFs  465  to each STA equal to the largest number of spatial streams assigned to any STA. For example, if three STAs are assigned a single spatial stream, but the fourth STA is assigned three spatial streams, in this aspect, the AP can be configured to transmit four symbols of HE-LTF information to each of the four STAs before transmitting symbols containing payload data. 
     It is not necessary that the tones assigned to any given STA be adjacent. For example, in some implementations, the sub-bands of the different receiving STAs can be interleaved. For example, if each of user- 1  and user- 2  receive three sub-bands, while user- 4  receives two sub-bands, these sub-bands can be interleaved across the entire AP bandwidth. For example, these sub-bands can be interleaved in an order such as 1,2,4,1,2,4,1,2. In some aspects, other methods of interleaving the sub-bands can also be used. In some aspects, interleaving the sub-bands can reduce the negative effects of interferences or the effect of poor reception from a particular device on a particular sub-band. In some aspects, the AP can transmit to STAs on the sub-bands that the STA prefers. For example, certain STAs can have better reception in some sub-bands than in others. The AP can thus transmit to the STAs based at least in part on which sub-bands the STA can have better reception. In some aspects, the sub-bands can also not be interleaved. For example, the sub-bands can instead be transmitted as 1,1,1,2,2,2,4,4. In some aspects, it can be pre-defined whether or not the sub-bands are interleaved. 
     In the example of  FIG. 7 , HE-SIG 0   455  symbol modulation can be used to signal HE devices that the packet is an HE packet. Other methods of signaling HE devices that the packet is an HE packet can also be used. In the example of  FIG. 7 , the L-SIG  426  can contain information that instructs HE devices that an HE preamble can follow the legacy preamble. For example, the L-SIG  426  can contain a low-energy, 1-bit code on the Q-rail which indicates the presence of a subsequent HE preamble to HE devices sensitive to the Q signal during the L-SIG  426 . A very low amplitude Q signal can be used because the single bit signal can be spread across all the tones used by the AP to transmit the packet. This code can be used by high efficiency devices to detect the presence of an HE-preamble/packet. The L-SIG  426  detection sensitivity of legacy devices need not be significantly impacted by this low-energy code on the Q-rail. Thus, these devices can be able to read the L-SIG  426 , and not notice the presence of the code, while HE devices can be able to detect the presence of the code. In this implementation, all of the HE-SIG fields can be BPSK modulated if desired, and any of the techniques described herein related to legacy compatibility can be used in conjunction with this L-SIG signaling. 
     In various embodiments, any HE-SIG field  455 - 459  can contain bits defining user-specific modulation type for each multiplexed user. For example, the optional HE-SIG 1 B  459  field can contain bits defining user-specific modulation type for each multiplexed user. 
     In some aspects, wireless signals can be transmitted in a low-rate (LR) mode, for example according the 802.11ax protocol. Particularly, in some embodiments, the AP  104  can have a greater transmit power capability compared to the STAs  106 . In some embodiments, for example, the STAs  106  can transmit at several dB lower than the AP  104 . Thus, DL communications from the AP  104  to the STAs  106  can have a higher range than UL communications from the STAs  106  to the AP  104 . In order to close the link budget, the LR mode can be used. In some embodiments, the LR mode can be used in both DL and UL communications. In other embodiments, the LR mode is only used for UL communications. 
     In some embodiments, backwards compatibility with a legacy communications system can be provided by prefixing a hidden LR preamble in front of a legacy preamble. In some embodiments, backwards compatibility can be provided by post-fixing an LR preamble after the legacy preamble. In various embodiments, the legacy preamble can be repeated and the LR-SIG simplified or omitted. In various embodiments, the legacy preamble is not used and the LR-SIG is provided. 
     In some embodiments, the HEW STAs  106  can communicate using a symbol duration four times that of a legacy STA. Accordingly, each symbol which is transmitted may be four times as long in duration. When using a longer symbol duration, each of the individual tones may only require one-quarter as much bandwidth to be transmitted. For example, in various embodiments, a 1× symbol duration can be 4 ms and a 4× symbol duration can be 16 ms. Thus, in various embodiments, 1× symbols can be referred to herein as legacy symbols and 4× symbols can be referred to as HEW symbols. In other embodiments, different durations are possible. 
       FIG. 8  illustrates an exemplary structure of an uplink physical-layer packet  800  which can be used to enable backward-compatible low rate multiple-access wireless communications. In this example physical-layer packet  800 , a legacy preamble  805  including the L-STF  422 , L-LTF  426 , and L-SIG  426  are included. In various embodiments, each of the L-STF  422 , L-LTF  426 , and L-SIG  426  can be transmitted using 20 MHz, and multiple copies can be transmitted for each 20 MHz of spectrum that the AP  104  ( FIG. 1 ) uses. A person having ordinary skill in the art can appreciate that the illustrated physical-layer packet  800  can include additional fields, fields can be rearranged, removed, and/or resized, and the contents of the fields varied. 
     This packet  800  also contains one or more symbols  810  preceding a LR portion  815 . In various embodiments, the symbols  810  can have a legacy duration such as, for example, 1× duration. The symbols  810  can be variously referred to as dummy symbols, padding symbols, etc. The symbols  810  can serve to provide symbols of a duration expected by legacy devices such as, for example, 802.11ac or non-LR 802.11ax devices. In various embodiments, the symbols  810  can be 8 μs in length. In other embodiments, the symbols  810  can have other lengths. 
     The LR portion  815  of the packet  800  can include LR-STFs  820 , LR-LTFs  830 , and SIG/Data fields  840  for each of one or more OFDMA users. In the illustrated embodiment, three users  1 - 3  share the bandwidth. In the illustrated embodiment, the LR-STF  820 A of user- 1  can be boosted by 3 dB, the LR-STF  820 B of user- 2  can be boosted by 3 dB, and the LR-STF  820 C of user- 3  can be optionally boosted by 3 dB. In various other embodiments, different boosts, including no boost, can be applied. In some embodiments, the LR-STFs  820  and the LR-LTFs  830  can each be 16 μs in length. In other embodiments, LR-STFs  820  and the LR-LTFs  830  can have other lengths. 
       FIG. 9  illustrates another exemplary structure of an uplink physical-layer packet  900  which can be used to enable backward-compatible low rate multiple-access wireless communications. In this example physical-layer packet  900 , a legacy preamble  905  including the L-STF  422 , L-LTF  426 , and L-SIG  426  are included. In various embodiments, each of the L-STF  422 , L-LTF  426 , and L-SIG  426  can be transmitted using 20 MHz, and multiple copies can be transmitted for each 20 MHz of spectrum that the AP  104  ( FIG. 1 ) uses. A person having ordinary skill in the art can appreciate that the illustrated physical-layer packet  900  can include additional fields, fields can be rearranged, removed, and/or resized, and the contents of the fields varied. 
     This packet  900  also contains a hidden LR-STF  910  preceding the legacy preamble  905 . The hidden LR-STF  910  can be encoded such that legacy devices do not interpret the LR-STF  910  as the beginning of the packet  1000 . For example, in some embodiments, the hidden LR-STF  910  can have a different periodicity than the L-STF  422 . In various embodiments, the hidden LR-STF  910  can have other parameters or characteristics that cause non-LR devices to ignore or refrain from decoding the LR-STF  910 . 
     After the legacy preamble  905 , the packet  900  includes an LR-SIG field  920 . In various embodiments, the LR-SIG field  920  can have a legacy duration such as, for example, 1× duration. In some embodiments, the LR-SIG field  920  can be dummy symbols, padding symbols, etc. In various embodiments, the LR-SIG field  920  can each be at least 4 μs in length, for example 8 μs. In other embodiments, the LR-SIG field  920  can have other lengths. In some embodiments, the LR-SIG field  920  can have an LR duration such as, for example, 4× duration. The LR-SIG field  920  can include define modulation characteristics for the LR portion  915 . 
     The LR portion  1015  of the packet  1000  can include the LR-LTFs  830  and Data fields  840  for each of one or more OFDMA users. In the illustrated embodiment, three users  1 - 3  share the bandwidth. In the illustrated embodiment, the hidden LR-STF  910  can be transmitted across the entire channel bandwidth. In some embodiments, the LR-STFs  910  and the LR-LTFs  830  can each be 16 μs in length. In other embodiments, LR-STFs  910  and the LR-LTFs  830  can have other lengths. 
       FIG. 10  illustrates another exemplary structure of an uplink physical-layer packet  1000  which can be used to enable backward-compatible low rate multiple-access wireless communications. In this example physical-layer packet  1000 , a legacy preamble  1005  including the L-STF  422 , L-LTF  426 , and L-SIG  426  are included. In various embodiments, each of the L-STF  422 , L-LTF  426 , and L-SIG  426  can be transmitted using 20 MHz, and multiple copies can be transmitted for each 20 MHz of spectrum that the AP  104  ( FIG. 1 ) uses. A person having ordinary skill in the art can appreciate that the illustrated physical-layer packet  1000  can include additional fields, fields can be rearranged, removed, and/or resized, and the contents of the fields varied. 
     This packet  1000  also contains a hidden LR-STF  910  preceding the legacy preamble  905 . The hidden LR-STF  910  can be encoded such that legacy devices do not interpret the LR-STF  910  as the beginning of the packet  1000 . For example, in some embodiments, the hidden LR-STF  910  can have a different periodicity than the L-STF  422 . In various embodiments, the hidden LR-STF  910  can have other parameters or characteristics that cause non-LR devices to ignore or refrain from decoding the LR-STF  910 . 
     After the legacy preamble  1005 , the packet  1000  repeats the L-SIG  426  in two symbols  1010 A- 1010 B. In particular, the STA  106  can split the L-SIG  426  according to even and odd subcarrier tones. Odd tones of the L-SIG  426  are repeated in a first symbol  1010 A. Even tones of the L-SIG  426  are repeated in a second symbol  1010 B. In various embodiments, the symbols  1010 A- 1010 B can have a legacy duration such as, for example, 1× duration. In some embodiments, the LR-SIG field  920  can be dummy symbols, padding symbols, etc. In various embodiments, the symbols  1010 A- 1010 B can each be 4 μs in length. In other embodiments, the symbols  1010 A- 1010 B can have other lengths. 
     The LR portion  1015  of the packet  1000  can include the LR-LTFs  830  and Data fields  840  for each of one or more OFDMA users. In the illustrated embodiment, three users  1 - 3  share the bandwidth. In the illustrated embodiment, the hidden LR-STF  910  can be transmitted across the entire channel bandwidth. In some embodiments, the LR-STFs  910  and the LR-LTFs  830  can each be 16 μs in length. In other embodiments, LR-STFs  910  and the LR-LTFs  830  can have other lengths. 
       FIG. 11  illustrates another exemplary structure of an uplink physical-layer packet  1100  which can be used to enable backward-compatible low rate multiple-access wireless communications. In this example physical-layer packet  1100 , a legacy preamble  1105  including the L-STF  422 , L-LTF  426 , and L-SIG  426  are included. In various embodiments, each of the L-STF  422 , L-LTF  426 , and L-SIG  426  can be transmitted using 20 MHz, and multiple copies can be transmitted for each 20 MHz of spectrum that the AP  104  ( FIG. 1 ) uses. A person having ordinary skill in the art can appreciate that the illustrated physical-layer packet  1100  can include additional fields, fields can be rearranged, removed, and/or resized, and the contents of the fields varied. 
     This packet  1100  also contains a hidden LR-STF  910  preceding the legacy preamble  905 . The hidden LR-STF  910  can be encoded such that legacy devices do not interpret the LR-STF  910  as the beginning of the packet  1100 . For example, in some embodiments, the hidden LR-STF  910  can have a different periodicity than the L-STF  422 . In various embodiments, the hidden LR-STF  910  can have other parameters or characteristics that cause non-LR devices to ignore or refrain from decoding the LR-STF  910 . 
     After the legacy preamble  1105 , the packet  1100  repeats the L-SIG  426  in four symbols  1110 A- 1010 D. Although  FIGS. 10 and 11  show two and four repetitions of the L-SIG  426 , respectively, other numbers of repetition are possible. In an embodiment, the STA  106  can split the L-SIG  426  according to the number of subcarrier tones modulus the number of repetitions. Thus, tones having a modulus of 0 are repeated in a first symbol  1110 A, tones having a modulus of 1 are repeated in a second symbol  1110 B, tones having a modulus of 2 are repeated in a third symbol  1110 C, tones having a modulus of 3 are repeated in a fourth symbol  1110 D, and so on. In various embodiments, the symbols  1110 A- 1010 D can have a legacy duration such as, for example, lx duration. In some embodiments, the LR-SIG field  920  can be dummy symbols, padding symbols, etc. In various embodiments, the symbols  1110 A- 1010 B can each be 4 μs in length. In other embodiments, the symbols  1110 A- 1010 B can have other lengths. 
     The LR portion  1115  of the packet  1100  can include the LR-LTFs  830  and Data fields  840  for each of one or more OFDMA users. In the illustrated embodiment, three users  1 - 3  share the bandwidth. In the illustrated embodiment, the hidden LR-STF  910  can be transmitted across the entire channel bandwidth. In some embodiments, the LR-STFs  910  and the LR-LTFs  830  can each be 16 μs in length. In other embodiments, LR-STFs  910  and the LR-LTFs  830  can have other lengths. 
     In various embodiments, the STA  106  can encode LR-SIG information in a polarity of repeated symbols. For example, to encode a 1, the STA  106  can multiply the repeated bits in the symbol  1010 A by −1, to encode a 0, the STA  106  can multiply the repeated bits in the symbol  1010 B by 1, and so on. In various embodiments, positive and negative repetition polarities can represent 0 and 1, respectively. In other embodiments, different encodings are possible. Note that information bit [0, 1] become modulation bit [1, −1] in one embodiment. Changing the polarity of a symbol means multiply it with +−1 instead of [0, 1]. 
     In various embodiments, the hidden LR-STF  910  can be longer than the L-STF  424 . For example, the hidden LR-STF  910  can be sufficiently long for detection by HEW devices at a reduced SNR. In some embodiments, the STAs  106  can flip the sign of one or more samples of every other hidden LR-STF  910  short, which can also be referred to herein as a period in the STF sequence. For example, in some embodiments with a period P=4, the STAs  106  can flip the sign of hidden LR-STF  910  samples as follows: ++++, +−−+, ++++, +−−+, ++++, +−−+, etc. In various embodiments, the STA  106  can choose a flipping pattern sufficient to reduce or minimize an output of a period-P auto-correlator. For example, an exemplary flipping pattern can include [1,−1,1,−1,−1,1,−1,1,−1,1,−1,1,1,−1,1,−1] for 20 MHz, and alternating [1,−1] for higher BW. The foregoing examples are for purposes of illustration. A person having ordinary skill in the art will appreciate that the foregoing examples can be applied to other periods, for example periods P&gt;4. 
     One advantageous effect of such alternating samples based on the periodicity of 2 shorts, according to one embodiment, is that the PAPR does not change. Another advantageous effect, according to one embodiment, is that that the hardware processing design can be reused (with a doubling of the periodicity). Thus, design and debugging effort can be reduced with a relatively small increase in hardware block size. 
       FIG. 12  shows an example detector  1300  that can be used to detect a hidden low rate short training field. For example, the detector  1300  can be implemented at the STAs  106  and/or the AP  104  to detect the hidden LR-STF  910 . The illustrated detector  1300  includes a delay  910 , accumulators  920 A- 920 B, a comparator  930 , a combiner  940 , and mixers  950 A- 950 D. As shown, the delay  910  has length 2P, and the accumulators  920 A- 920 B each have length Q-2P. 
       FIG. 13  shows example normalized correlator output over time for both legacy and high-efficiency wireless receivers. As shown, a legacy device and HEW device both receive a HEW packet, such as the packet  1100  of  FIG. 11 . The HEW device will continue to accumulate during the hidden LR-STF  910 , thus starting detection of the LR packet  1100  earlier. On the other hand, the legacy device will only begin to accumulate during the L-STF  1105 , thus starting detection of the packet later. 
       FIG. 14  shows another flowchart  1400  for an exemplary method of wireless communication that can be employed within the wireless communication system  140  of  FIG. 1 . The method can be implemented in whole or in part by the devices described herein, such as the wireless device  202  shown in  FIG. 2 . Although the illustrated method is described herein with reference to the wireless communication system  100  discussed above with respect to  FIG. 1  and the packets  800 ,  900 ,  1000 , and  1100  discussed above with respect to  FIGS. 8-11 , a person having ordinary skill in the art will appreciate that the illustrated method can be implemented by another device described herein, or any other suitable device. Although the illustrated method is described herein with reference to a particular order, in various embodiments, blocks herein can be performed in a different order, or omitted, and additional blocks can be added. 
     First, at block  1402 , a wireless device generates a packet including a first portion having a first symbol duration. The packet further includes a second portion having a second symbol duration greater than the first. The first portion includes a first short training field (STF). 
     For example, the STA  106  can generate the packet  1100  of  FIG. 11  for transmission to the AP  104 . The packet  1100  includes the legacy portion  1105 , which can be a first portion. The legacy portion  1105  can have a 1× symbol duration, which can be the first symbol duration. 
     The packet  1100  further includes the LR portion  1115 , which can be the second portion. The LR portion  1115  can have a 4× symbol duration, which can be the second symbol duration. The legacy portion  1105  can include the L-STF  422 , which can be the first STF. 
     Next, at block  1404 , the device prepends or appends a second STF to the first portion. The second STF has the second symbol duration. For example, the STA  106  can prepend the LR-STF  910 , which can be the second STF, to the legacy portion  1105 . As another example, the STA  106  can append the LR-STF  820 A, which can be the second STF, to the legacy portion  805 . 
     Although various fields are described herein as prepending (adding to the front) an/or appending (adding to the back), in some embodiments, appended fields can be created first, with subsequent fields prepended thereto, prepended fields can be created first, with subsequent fields appended thereto, fields can be created simultaneously or substantially concurrently, or fields can be created in any other order within the scope of this disclosure. In some embodiments, appending/prepending can occur sequentially. In some embodiments, appended/prepended fields are directly appended/prepended and in other embodiments, intervening fields can be contemporaneously, subsequently, or otherwise inserted between the appended/prepended fields. 
     In various embodiments, said prepending or appending can include prepending the second STF to the first portion. The second STF can have a periodicity different than a periodicity of the first STF. In various embodiments, the method can further include inverting a sign of one or more samples of every other multiple of periodicity for the second STF. 
     In various embodiments, the second training field can have a periodicity at least two times the periodicity of the first training field. In some embodiments, the device can flip signs of some samples in every other cycle. In some embodiments, the device can flip signs of every cycle in certain manner. In some embodiments, the device can generate a sequence that has tripled periodicity. In some embodiments, the device can generate a totally different sequence, not based on flipping signs of existing legacy STF sequence. 
     In various embodiments, the packet can further include a signal field after the first portion, the signal field having the second symbol duration. In various embodiments, the packet can further include a one or more symbols after the first portion. The symbols can have the first symbol duration. 
     In various embodiments, the first portion can include a signal field. The second portion can include a plurality of repeated portions of the signal field. The repeated portions can have the second symbol duration. In various embodiments, n repeated portions of the signal field can each include a subset of carrier tones of the signal field. For example, n repeated portions of the signal field can include every nth subcarrier tone of the signal field. For example, the STA  106  can repeat the L-SIG  426  one or more times including the repetitions  1010 A- 1010 D. 
     In various embodiments, said prepending or appending can include appending one or more symbols having the first symbol duration to the first portion, boosting a gain of the second STF, and appending the second STF to the one or more symbols. For example, the STA  106  can optionally boost the gain of the LR-STF  820 . 
     Then, at block  1406 , the wireless device transmits the packet. For example, the STA  106  can transmit the packet  1100  to the AP  104 . In various embodiments, the packet can include a scheduled multi-user (MU) orthogonal frequency division multiple access (OFDMA) uplink (UL) packet. In various embodiments, the packet initiates an unscheduled single-user (SU) orthogonal frequency division multiple access (OFDMA) uplink (UL) packet. 
     In an embodiment, the method shown in  FIG. 14  can be implemented in a wireless device that can include a generating circuit, a prepending/appending circuit, and a transmitting circuit. Those skilled in the art will appreciate that a wireless device can have more components than the simplified wireless device described herein. The wireless device described herein includes only those components useful for describing some prominent features of implementations within the scope of the claims. 
     The generating circuit can be configured to generate the packet. In some embodiments, the generating circuit can be configured to perform at least block  1402  of  FIG. 14 . The generating circuit can include one or more of the processor  204  ( FIG. 2 ), the memory  206  ( FIG. 2 ), and the DSP  220  ( FIG. 2 ). In some implementations, means for generating can include the generating circuit. 
     The prepending/appending circuit can be configured to prepend and/or append the second STF. In some embodiments, the prepending/appending circuit can be configured to perform at least block  1404  of  FIG. 14 . The prepending/appending circuit can include one or more of the processor  204  ( FIG. 2 ), the memory  206  ( FIG. 2 ), and the DSP  220  ( FIG. 2 ). In some implementations, means for prepending/appending can include the prepending/appending circuit. 
     The transmitting circuit can be configured to transmit the first and second messages together. In some embodiments, the transmitting circuit can be configured to perform at least block  1406  of  FIG. 14 . The transmitting circuit can include one or more of the transmitter  210  ( FIG. 2 ), the antenna  216  ( FIG. 2 ), and the transceiver  214  ( FIG. 2 ). In some implementations, means for transmitting can include the transmitting circuit. 
     A person/one having ordinary skill in the art would understand that information and signals can be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that can be referenced throughout the above description can be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
     Various modifications to the implementations described in this disclosure can be readily apparent to those skilled in the art, and the generic principles defined herein can be applied to other implementations without departing from the spirit or scope of this disclosure. Thus, the disclosure is not intended to be limited to the implementations shown herein, but is to be accorded the widest scope consistent with the claims, the principles and the novel features disclosed herein. The word “exemplary” is used exclusively herein to mean “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other implementations. 
     Certain features that are described in this specification in the context of separate implementations also can be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation also can be implemented in multiple implementations separately or in any suitable sub-combination. Moreover, although features can be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination can be directed to a sub-combination or variation of a sub-combination. 
     The various operations of methods described above can 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 can 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 can 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 can be a microprocessor, but in the alternative, the processor can be any commercially available processor, controller, microcontroller or state machine. A processor can 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 can be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions can 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 can be any available media that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can include 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 can include non-transitory computer readable medium (e.g., tangible media). In addition, in some aspects computer readable medium can include transitory computer readable medium (e.g., a signal). Combinations of the above can also be included within the scope of computer-readable media. 
     The methods disclosed herein include one or more steps or actions for achieving the described method. The method steps and/or actions can 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 can be modified without departing from the scope of the claims. 
     Further, it can 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. 
     While the foregoing is directed to aspects of the present disclosure, other and further aspects of the disclosure can be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.