Non-legacy preamble for wireless local area networks

Method and apparatus for transmission and reception of a Greenfield preamble are provided. In the method and apparatus, the Greenfield preamble may be a single user (SU) preamble or a multi user (MU) preamble. As an MU preamble, the Greenfield preamble includes a short training field (STF), a first long training field (LTF), a first signal (SIG) field, at least one additional LTF, and a second SIG field. Additionally, the Greenfield preamble may be utilized for efficient transmission and reception of control information to wireless devices, whereby the control information may be indicated using the STF, the first LTF, or the first or second SIG fields.

BACKGROUND

The usage of preambles is very common in wireless communication systems. Preambles provide efficient and effective ways for communications devices to obtain channel conditions. In addition, preambles are also useful for relaying control information to communications devices, including modes of operation and the manner in which data is to be transmitted or received in a communications system.

In some communications systems, preambles are transmitted before every data transmission and, therefore, the preambles often occupy a large portion of the volume of traffic in a communications system. Additionally, as communications systems become more advanced and incorporate various technologies, preambles are expected to carry more control information often using shrinking bandwidth resources.

It is, therefore, desirable to have a method and apparatus for preamble transmission and reception in which preambles efficiently relay information between communications devices. It is also desirable for the preambles to be compliant with advanced communications protocols.

SUMMARY

Method and wireless transmit/receive unit (WTRU) for receiving a preamble in multi-user (MU) multiple input multiple output (MIMO) communications are provided. In the method and WTRU, the preamble comprising a short training field (STF), a first long training field (LTF), a first signal (SIG) field, one or more additional LTFs, and a second SIG field is received. The preamble may be a multi-user (MU) preamble. Further in the method and WTRU, time or frequency acquisition may be performed based on the STF and channel estimation may be performed based on the first LTF. Additionally, in the method and WTRU, a first control information is obtained from the first SIG field and a second control information is obtained from the second SIG field, whereby the first control information is associated with multiple receivers and the second control information is associated with a subset of the multiple receivers.

In one embodiment, the first LTF may comprise two long training symbols preceded by a double length cyclic prefix and in another embodiment, the first SIG field may indicate whether the preamble is a single user (SU) preamble or a multi user (MU) preamble. In a further embodiment, cyclic redundancy check (CRC) masking of the first field may indicate whether the preamble is an SU preamble or an MU preamble. In an additional embodiment, the first SIG field or the second SIG field may indicate an operating bandwidth or whether data transmission is aggregated.

DETAILED DESCRIPTION

FIG.1Cis a system diagram of the RAN104and the core network106according to an embodiment. The RAN104may be an access service network (ASN) that employs IEEE 802.16 radio technology to communicate with the WTRUs102a,102b,102cover the air interface116. As will be further discussed below, the communication links between the different functional entities of the WTRUs102a,102b,102c, the RAN104, and the core network106may be defined as reference points.

As shown inFIG.1C, the RAN104may include base stations140a,140b,140c, and an ASN gateway142, though it will be appreciated that the RAN104may include any number of base stations and ASN gateways while remaining consistent with an embodiment. The base stations140a,140b,140cmay each be associated with a particular cell (not shown) in the RAN104and may each include one or more transceivers for communicating with the WTRUs102a,102b,102cover the air interface116. In one embodiment, the base stations140a,140b,140cmay implement MIMO technology. Thus, the base station140a, for example, may use multiple antennas to transmit wireless signals to, and receive wireless signals from, the WTRU102a. The base stations140a,140b,140cmay also provide mobility management functions, such as handoff triggering, tunnel establishment, radio resource management, traffic classification, quality of service (QoS) policy enforcement, and the like. The ASN gateway142may serve as a traffic aggregation point and may be responsible for paging, caching of subscriber profiles, routing to the core network106, and the like.

The air interface116between the WTRUs102a,102b,102cand the RAN104may be defined as an R1 reference point that implements the IEEE 802.16 specification. In addition, each of the WTRUs102a,102b,102cmay establish a logical interface (not shown) with the core network106. The logical interface between the WTRUs102a,102b,102cand the core network106may be defined as an R2 reference point, which may be used for authentication, authorization, IP host configuration management, and/or mobility management.

The communication link between each of the base stations140a,140b,140cmay be defined as an R8 reference point that includes protocols for facilitating WTRU handovers and the transfer of data between base stations. The communication link between the base stations140a,140b,140cand the ASN gateway215may be defined as an R6 reference point. The R6 reference point may include protocols for facilitating mobility management based on mobility events associated with each of the WTRUs102a,102b,102c.

As shown inFIG.1C, the RAN104may be connected to the core network106. The communication link between the RAN104and the core network106may defined as an R3 reference point that includes protocols for facilitating data transfer and mobility management capabilities, for example. The core network106may include a mobile IP home agent (MIP-HA)144, an authentication, authorization, accounting (AAA) server146, and a gateway148. While each of the foregoing elements are depicted as part of the core network106, it will be appreciated that any one of these elements may be owned and/or operated by an entity other than the core network operator.

The MIP-HA may be responsible for IP address management, and may enable the WTRUs102a,102b,102cto roam between different ASNs and/or different core networks. The MIP-HA144may provide the WTRUs102a,102b,102cwith access to packet-switched networks, such as the Internet110, to facilitate communications between the WTRUs102a,102b,102cand IP-enabled devices. The AAA server146may be responsible for user authentication and for supporting user services. The gateway148may facilitate interworking with other networks. For example, the gateway148may provide the WTRUs102a,102b,102cwith access to circuit-switched networks, such as the PSTN108, to facilitate communications between the WTRUs102a,102b,102cand traditional land-line communications devices. In addition, the gateway148may provide the WTRUs102a,102b,102cwith access to the networks112, which may include other wired or wireless networks that are owned and/or operated by other service providers.

Although not shown inFIG.1C, it will be appreciated that the RAN104may be connected to other ASNs and the core network106may be connected to other core networks. The communication link between the RAN104the other ASNs may be defined as an R4 reference point, which may include protocols for coordinating the mobility of the WTRUs102a,102b,102cbetween the RAN104and the other ASNs. The communication link between the core network106and the other core networks may be defined as an R5 reference, which may include protocols for facilitating interworking between home core networks and visited core networks.

When referred to hereinafter, the term transmitter may mean a WTRU, a station (STA), a base station, a node B, or an access point (AP), among others. Further, when referred to hereinafter, the term receiver may mean a WTRU, an STA, a base station, a node B, or an AP, among others. Further, the transmitter or the receiver may communicate using any communications protocol including, but not limited to, an Institute for Electrical and Electronics Engineers (IEEE) 802 communications protocol, such as 802.11n, 802.11ac, 802.11af, or 802.11ah, among others. The transmitter or the receiver may also operate in any spectrum including, but not limited to, a television (TV) whitespace spectrum or a sub-1 gigahertz (GHz) spectrum.

Preambles are widely used in communications systems. A preamble may be transmitted stand-alone, (i.e., without a subsequent data transmission), or may be a header for a data transmission. The preamble allows for training a receiver to obtain information about channel conditions between a transmitter and the receiver, and thereby improved reception of subsequent data transmissions (for example, user data) is facilitated. In addition to training the receiver, a preamble may be used for sending control information to the receiver that may be necessary for reception of subsequent data transmissions.

FIG.2shows a preamble for a data transmission. InFIG.2, the preamble201is transmitted prior to a data transmission202. The preamble201may include training symbols that may be used for automatic gain control (AGC), timing and frequency acquisition, or channel estimation by the receiver. Furthermore, the training symbols may be used for frequency or time synchronization between the transmitter and the receiver.

In addition to the training symbols, the preamble may include dedicated bits that carry control information to the receiver. The dedicated bits may indicate to the receiver a transmission bandwidth, modulation or coding information, beamforming information, or space-time coding information.

As may be recognized, for proper communication between a transmitter and a receiver, a communications protocol may define a preamble for use in the communications protocol. A communications protocol may define the training symbols of the preamble and the meaning of the dedicated bits of the preamble. For example, the transmission protocol may define the length of the preamble (in bits, bytes, or symbols), the time or frequency resources used for transmission of the preamble, the encoding or modulation of the preamble, or the interpretation of the dedicated bits of the preamble. A transmitter or a receiver that is compliant with the communications protocol is aware of the manner in which the preamble is defined and may, therefore, successfully interpret and utilize the preamble.

As communications protocols are updated in order to allow for increased data rates and very high throughput (VHT) or enable usage of additional frequency bandwidths, preambles may also be updated according to the requirements of the communications protocols. Furthermore, transmitters and receivers using the communications protocols are required to be updated in order to be compliant with the latest communications protocols and in order to receive and properly process preambles of the communications protocols. However, it may be desirable for a communications protocol to allow legacy transmitters and receivers, (i.e., transmitters and receivers that are not compliant with the updated communications protocol), to communicate using the updated communications protocols.

A mixed format preamble may be used to allow legacy transmitters and receivers to communicate using the updated communications protocols. The mixed format preamble has both a legacy portion for use by legacy transmitters and receivers and a non-legacy portion for use by non-legacy transmitters and receivers. The non-legacy portion of the preamble is referred to herein as a very high throughput (VHT) portion. Furthermore, a transmitter or a receiver that is compliant with the updated communications protocols is referred to herein as a VHT transmitter or a VHT receiver or a non-legacy transmitter or a non-legacy receiver.

FIG.3Ashows a mixed format preamble300. The mixed format preamble300comprises a legacy portion311and a VHT portion312. The legacy portion311of the mixed format preamble300may comprise training fields and control information fields for use by legacy receivers. Legacy receivers may process the legacy portion311of the mixed format preamble300and may subsequently be able to interpret the VHT portion312using information provided in the legacy portion311. The VHT portion312of the mixed format preamble300may comprise training fields and control information fields for use by non-legacy receivers or VHT receivers. Non-legacy receivers or VHT receivers may ignore or skip the legacy portion311and only use the VHT portion312of the mixed format preamble300.

FIG.3Bshows an example of a mixed format preamble. The mixed format preamble300comprises a legacy portion311and a VHT portion312. The legacy portion311comprises a legacy short training field (STF), referred to herein as L-STF321, a legacy long training field (LTF), referred to herein as L-LTF322, and a legacy signal (SIG) field, referred to herein as L-SIG323. The VHT portion312comprises a first VHT SIG field, referred to herein as VHT-SIG-A324, and a second VHT SIG field, referred to herein as VHT-SIG-B327, an STF, referred to herein as VHT-STF325, and one or more LTFs, referred to herein as VHT-LTFs3261-Nand collectively referred to hereinafter as VHT-LTFs326.

In the legacy portion311of the mixed format preamble300, L-STF321comprises one or more short training symbols and may be used for AGC and timing and frequency acquisition by a legacy receiver. Further, L-LTF322comprises one or more long training symbols and may be used for channel estimation by a receiver. L-SIG323may include dedicated bits that signal to a receiver control information, such as bandwidth information, modulation or coding information, and the like.

Legacy receivers may train for reception of subsequent data transmission based on L-STF321and L-LTF322. Further, legacy receivers may receive control information included in L-SIG323.

In the VHT portion312of the mixed format preamble300, VHT-STF325comprises one or more short training symbols and may be used for AGC and timing and frequency acquisition by a non-legacy or a VHT receiver. Further, VHT-LTFs326comprise one or more long training symbols and may be used for antenna calibration by a non-legacy receiver. VHT-SIG-A324and VHT-SIG-B327include control information intended for a non-legacy receiver.

Non-legacy receivers may train using the training symbols of VHT-STF325and VHT-LTFs326. The non-legacy receivers may also receive control information included in VHT-SIG-A324and VHT-SIG-B327. Further, non-legacy receivers may perform AGC and time and frequency acquisition based on VHT-STF325, and antenna calibration and the like based on VHT-LTFs326.

VHT-SIG-A324of the VHT portion312of the mixed format preamble300may include information intended for multiple non-legacy receivers, whereas VHT-SIG-B327of the VHT portion312of the mixed format preamble300may include information intended for one non-legacy receiver. For example, the multiple non-legacy receivers may acquire control information intended for the multiple non-legacy receivers from VHT-SIG-A324, such as a group identity (ID). However, one non-legacy receiver may acquire information intended to the non-legacy receiver such as modulation and coding scheme (MCS) from VHT-SIG-B327. Thus, the VHT portion312of the mixed format preamble300may include an Omni portion intended for multiple non-legacy receivers and a multi-user (MU) portion intended for one non-legacy receiver.

A mixed format preamble300is associated with an increased signaling overhead due to the inclusion of the legacy portion311. An alternative to the signaling overhead of the mixed format preamble300is a Greenfield preamble. A Greenfield preamble does not include a legacy portion and instead includes only a VHT portion for use by non-legacy receivers or VHT receivers. Because the Greenfield preamble does not include a legacy portion, additional resources may be allocated to the Greenfield preamble. The additional resources allocated to the Greenfield preamble result in improved channel estimation and time and frequency acquisition, among other benefits. For example, the Greenfield preamble may include STFs and LTFs having longer training symbols than a mixed format preamble without adding additional overhead. Further, the STFs and LTFs of the Greenfield preamble may have longer guard intervals than a counterpart mixed format preamble.

FIG.4Ashows a Greenfield preamble. The Greenfield preamble400comprises a Greenfield (GF) STF (GF-STF)401, a first GF LTF, referred to herein as GF-LTF1402, and additional GF LTFs, referred to herein as GF-LTFs4041-Nand collectively referred to hereinafter as GF-LTFs404, and singularly referred to hereinafter as GF-LTF404i. GF-LTFs404may be data or expansion LTFs. Further, the Greenfield preamble400also comprises a first SIG field, referred to herein GF-SIG-A403, and a second SIG field, referred to herein as GF-SIG-B405.

GF-STF401, GF-LTF1402, and GF-SIG-A403may be intended for multiple non-legacy receivers, and thus may form an Omni portion of the Greenfield preamble400. On the other hand, GF-LTFs404and GF-SIG-B405may be intended for one or more specific non-legacy receivers, and thus may form an MU portion of the Greenfield preamble400.

GF-STF401may comprise one or more short training symbols and may be used for AGC and timing and frequency acquisition by a non-legacy receiver, and a non-legacy receiver may perform AGC and timing and frequency acquisition based on GF-STF401.

Further, GF-LTF1402may comprise one or more long training symbols and may be used for channel estimation by a non-legacy receiver, and a non-legacy receiver may perform channel estimation based on GF-LTF1402.

GF-SIG-A403may include dedicated bits that signal control information to multiple non-legacy receivers. The multiple non-legacy receivers may receive the control information, such as group ID, from GF-SIG-A403. Further, GF-SIG-A403may provide an indication as to whether the Greenfield preamble400is an MU preamble or an SU preamble. A non-legacy receiver may know whether to receive or process GF-SIG-B405of the MU portion of the Greenfield preamble400based on the indication in GF-SIG-A403. Further, in contrast to GF-SIG-A403, GF-SIG-B405includes information intended for a subset of one or more specific non-legacy receivers of the multiple receivers, such as modulation and coding scheme (MCS) information of subsequent data transmissions.

GF-LTFs404, on the other hand, comprise long training symbols and may be used for additional training of a non-legacy receiver, such as antenna calibration.

Referring to GF-STF401of the Greenfield preamble400inFIG.4. GF-STF401may be constructed from an orthogonal frequency division multiplexing (OFDM) sequence denoted as S−x,x, where 2x+1 represents the number of OFDM subcarriers. Because the Greenfield preamble400does not include a legacy portion, additional resources for a longer OFDM sequence may be allocated to the GF-STF401of the Greenfield preamble400than used in L-STF321or VHT-STF325of the mixed format preamble300.

For example, for usage in a 5, 10, or 20 megahertz (MHz) bandwidth, the GF-STF sequence may be described as:

A time domain waveform having a 0.8 microseconds (μs) period may be obtained from the sequence S−x,xby applying an inverse fast Fourier transform (IFFT) and adding a cyclic prefix. The time domain waveform may be repeated ten times to form a GF-STF401with a duration of 8 μs. It is noted that a GF-STF401with a duration of 16 μs may be obtained by utilizing half-clocking and a GF-STF401with a 32 μs duration may be obtained by utilizing quarter-clocking.

Table 1 shows a tone scaling factor and duration of GF-STF401for various bandwidths. It is noted that the tone scaling factor and GF-STF401duration may be similarly obtained for any other bandwidth, such as 1, 2, 4, or 8 MHz.

TABLE 1Tone scaling factor and duration of GF-STF for various bandwidthsBandwidth (MHz)510204080160Tone scaling factor121212244896GF-STF duration (32 μs)32168888

Referring to GF-LTF1402of the Greenfield preamble400ofFIG.4, GF-LTF1402may be 8 μs in duration, as compared with a GF-LTF404iof 4 μs in duration. Further, GF-LTF1402may comprise two periods of long training symbols preceded by a double length 1.6 μs cyclic prefix. The usage of two periods of long training symbols and a double length cyclic prefix is facilitated by the fact that the Greenfield preamble400does not include a legacy portion, as compared to a mixed format preamble300and, therefore, more resources may be allocated to the Greenfield preamble400without incurring overhead.

Table 2 shows a tone scaling factor, GF-LTF1 duration, and guard interval (GI) time for various bandwidths. It is noted that the tone scaling factor, GF-LTF1 duration, and GI time may be similarly obtained for any other bandwidth, such as 1, 2, 4, 8, or 16 MHz.

It is worth noting that the training symbols for GF-LTF1402may be half-clocked for 5 MHz operation and quarter-clocked for 10 MHz operation.

As described herein, in multiple input multiple output (MIMO) communications and communications protocols employing MIMO, a preamble may include information intended for multiple receivers, whereby the preamble is said to include a multi-user (MU) portion or the preamble is said to be an MU preamble. Further, a preamble may be intended for a single receiver, whereby the preamble is said to be an SU preamble. Accordingly, the Greenfield preamble400may be used as the basis for an SU preamble, as described with reference toFIG.4B.

FIG.4Bshows an SU preamble in accordance with the Greenfield preamble. The SU preamble410comprises STF411, a first LTF, denoted LTF1412, a signal (SIG)413field, and one or more additional LTFs, denoted as LTF24141, . . . , LTFN414N-1and referred to collectively hereinafter as LTFs414.

STF411of the SU preamble410may be the same as GF-STF401of the Greenfield preamble400. Further, LTF1412may be the same as GF-LTF1402, SIG413may be the same as GF-SIG-A403, and LTFs414may be the same as GF-LTFs404of the Greenfield preamble400. It is noted that because an SU preamble only needs to signal control information to a single receiver, the signaling of control information may consolidated in SIG413and, thus, there is no need for an additional MU SIG field, such as GF-SIG-B405of the Greenfield preamble400.

Further, the Greenfield preamble400may also be used as the basis for an MU preamble, as described with reference toFIG.4C. Unlike the SU preamble410that is intended for a specific receiver, an MU preamble may include a first portion intended for multiple receivers, referred to herein as an Omni portion, and a second portion intended for a subset of the multiple receivers, referred to herein as an MU portion.

FIG.4Cshows an MU preamble in accordance with the Greenfield preamble. The MU preamble420comprises an Omni portion421and an MU portion425. The Omni portion421comprises an STF422, a first LTF, referred to herein as LTF1423, and a first SIG field, referred to herein as SIG-A424. Further, STF422may be the same as GF-STF401of the Greenfield preamble400, LTF1423may be the same as GF-LTF1402of the Greenfield preamble400, and SIG-A424may be the same as GF-SIG-A403of the Greenfield preamble400.

As described herein, the Omni portion421of the MU preamble420is intended for multiple receivers and the multiple receivers may receive and utilize the Omni portion421of the MU preamble420as described herein. For example, the multiple receivers may perform AGC and timing and frequency acquisition based on STF422of the Omni portion421, and channel estimation based on LTF1423of the Omni portion421. Further, the multiple receivers may acquire control information intended for the multiple receivers, such as a group identity (ID), from SIG-A424of the Omni portion421of the MU preamble420.

The MU portion425of the MU preamble420comprises an additional STF, referred to herein as MU-STF426, one or more additional LTFs, denoted as LTF24271, . . . , LTFN427N-1and referred to collectively herein as LTFs427, and a second SIG field, referred to herein as SIG-B428.

LTFs427of the MU portion may be the same as GF-LTFs404of the Greenfield preamble400, and SIG-B428may also be the same as GF-SIG-B405of the Greenfield preamble400.

However, although not included in the Greenfield preamble400, MU-STF426may alternatively be included in the MU preamble420. MU-STF426may comprise short training symbols and may be used for performing finer AGC, and time and frequency acquisition than performed based on STF422of the Omni portion421.

As described herein, a Greenfield preamble400may be used by non-legacy receivers or VHT receivers. Further, a mixed format preamble300may be used by both VHT receivers and legacy receivers.

FIG.5shows a method for preamble transmission. In the method500, a transmitter determines whether to transmit a Greenfield preamble400or a mixed format preamble300501. If the transmitter determines that a Greenfield preamble400is to be transmitted, then the transmitter transmits the Greenfield preamble400comprising GF-STF401, GF-LTF1402, GF-SIG-A403, GF-LTFs404, and GF-SIG-B405502. If the transmitter determines that a mixed format preamble300is to be transmitted, then the transmitter transmits the legacy portion311of the mixed format preamble300comprising L-STF321, L-LTF322, and L-SIG323503. The transmitter also transmits the VHT portion312of the mixed format preamble300comprising VHT-SIG-A324, VHT-STF325, VHT-LTFs326, and VHT-SIG-B327504.

A receiver may be either a VHT receiver that is capable of processing both a VHT preamble and a mixed format preamble or a legacy receiver that is capable of processing only a mixed format preamble.

FIG.6shows a method for processing a preamble. In the method600, a receiver receives a preamble601. The receiver determines the type of SIG field of the preamble602. If the receiver determines that the SIG field is a GF-SIG-A403, then the receiver processes the GF-SIG-A403603. The receiver also processes GF-SIG-B405604. If, on the other hand, the receiver determines that the SIG field is an L-SIG323, the receiver processes the L-SIG323605.

When referred to hereinafter, a preamble may mean any preamble, such as mixed format preamble300, Greenfield preamble400, SU preamble410, or MU preamble420. Further, when referred to hereinafter, an STF may mean any STF of any preamble, such as L-STF321or VHT-STF325of mixed format preamble300, GF-STF401of Greenfield preamble400, STF411of SU preamble410, or STF422or MU-STF426of MU preamble420.

Additionally, when referred to hereinafter, an LTF may mean any LTF of any preamble, such as L-LTF322of mixed format preamble300, or GF-LTF1402of Greenfield preamble400, LTF1412of SU preamble410, or LTF1423of MU preamble420. Furthermore, when referred to hereinafter a SIG field may mean any SIG field of a preamble, such as L-SIG323, VHT-SIG-A324, or VHT-SIG-B327mixed format preamble300, GF-SIG-A403or GF-SIG-B405of Greenfield preamble400, SIG413field of SU preamble410, or SIG-A424or SIG-B428of MU preamble420. Additionally, when referred to hereinafter, a receiver may mean a legacy receiver, or a non-legacy or VHT receiver.

A preamble may include an indication as to whether the preamble is an SU preamble intended for a specific receiver or an MU preamble intended for multiple receivers.

The SIG field may include one or more bits or a field to indicate whether the preamble is an SU preamble or an MU preamble, and a receiver may determine from the SIG field whether the preamble is an SU preamble or an MU preamble. If the receiver determines that the preamble is an MU preamble, the receiver may obtain control information (for example, group ID or NSTS) related to multiple receivers from the SIG field. Further, the receiver may obtain information specific to the receiver (for example, MCS) from a SIG field of an MU portion of the preamble.

If, on the other hand, the receiver determines that the preamble is an SU preamble, the receiver may obtain control information (for example, MCS or NSTS) specific to the receiver from the SIG field.

In another embodiment, an STF or an LTF of a preamble may be used to indicate whether the preamble is an SU preamble or an MU preamble. A sequence or subcarrier mapping of the STF or LTF may be used to indicate whether the preamble is an SU preamble or an MU preamble. When an STF is used to indicate whether the preamble is an SU preamble or an MU preamble, a receiver may be aware of a first STF sequence or subcarrier mapping used to indicate an SU preamble and a second STF sequence or subcarrier mapping used to indicate an MU preamble. A receiver may receive an STF and may apply frequency domain correlation to the received STF and the first STF sequence, and may also apply frequency domain correlation to the received STF and the second STF sequence, in order to determine whether the preamble is an SU preamble or an MU preamble.

Similar to determining whether the preamble is an SU preamble or an MU preamble based on the STF, a receiver may determine whether the preamble is an SU preamble or an MU preamble based on an LTF.

In another embodiment, cyclic redundancy check (CRC) masking of a SIG field may be performed to indicate whether the preamble is an SU preamble or an MU preamble. As may be recognized, error protection for a SIG field may be provided through a CRC having a length of L bits and denoted as CL-1, CL-2, . . . , C0. The CRC may be masked with a sequence xL-1, xL-2, . . . . , x0to indicate whether the preamble is an SU preamble or an MU preamble. For example, if the CRC is masked with sequence {0, 0, . . . , 0}, then an SU preamble is indicated, whereas if the CRC is masked with sequence {1, 1, . . . , 1}, then an MU preamble is indicated, and vice-versa. It is noted that CRC masking may be performed by applying a modulo 2 operation to the respective bit positions of the CRC and the sequence.

A receiver may receive the preamble and calculate L CRC bits based on the SIG field. The receiver may further determine whether the preamble is an SU preamble or an MU preamble by comparing the received CRC bits with the calculated CRC bits. It is noted that the masked CRC may be included in the SIG field or elsewhere in the preamble (for example, in a SERVICE field in IEEE 802.11 communications).

Further, a CRC mask may only be associated with a particular transmission mode and the presence of the CRC mask may indicate the transmission mode. For example, a CRC mask may only be associated with a 1 MHZ transmission mode.

In a further embodiment, tail bits that terminate a convolutional code for a SIG field may be used to indicate whether a preamble is an MU preamble or an SU preamble. For example, the SIG field may be terminated with the tail bits [0 0 0 0 0 0] to indicate that the preamble is an SU preamble, or the SIG field may be terminated with the tail bits [1 1 1 1 1 1] to indicate that the preamble is an MU preamble. Further, in addition to indicating whether the preamble is an MU preamble or an SU preamble, tail bits may carry additional information bits. For example, two bits of information may be carried by choosing among four different termination sequences (e.g., termination sequences [0 0 0 0 0 0], [1 1 1 0 0 0], [0 0 0 1 1 1] or [1 1 1 1 1 1]). In addition, the termination sequences may be chosen to maximize a Hamming distance between resultant codewords. It is noted that three bits may indicate eight possible termination states, whereas four bits may indicate 16 possible termination states up to a maximum of 6 bits, which may be equivalent to a code that is not terminated.

A receiver may perform convolutional decoding to decode the SIG field. Further, the convolutional decoding may be performed assuming that the tail bits are either [0 0 0 0 0 0] or [1 1 1 1 1 1]. After convolutional decoding is performed, a receiver may utilize a maximum likelihood function to determine whether the tail bits of the SIG field are [0 0 0 0 0 0] or [1 1 1 1 1 1], and thus determine whether the preamble is an MU preamble or an SU preamble. Similarly, when more than one bit of information is indicated using the tail bits, a decoding process may choose the best state to trace back from given possible alternatives. In the example above, a convolution decoder may choose the best metric from the four states of [0 0 0 0 0 0], [1 1 1 0 0 0], [0 0 0 1 1 1], and [1 1 1 1 1 1] in order to perform tracing back. It is noted that if tail biting is utilized in an encoder, an appropriate tail-biting decoder may be required to be used in a decoder.

In one embodiment, the SIG field of a preamble may include an indication of an operating bandwidth or a mode of operation for preamble transmission or data transmission. For example, the SIG field may indicate whether a 1 MHz bandwidth or a 2 MHz bandwidth or mode of operation is used. A bit in the SIG field may be used to indicate the operating bandwidth or the mode of operation.

A receiver that is capable of operating in either a first bandwidth or a second bandwidth may determine the bandwidth based on the SIG field and the receiver may appropriately process a remainder of the preamble and subsequent data transmissions based on the determined bandwidth (i.e., the receiver may perform detection, or frequency and time synchronization, among others). For example, in IEEE 802.11 communications, if the SIG field indicates 1 MHz bandwidth on either or both an upper or lower 1 MHz band, a receiver may process a preamble or a data transmission according to the 1 MHz bandwidth on either or both of the upper or the lower 1 MHz bands. Further, subcarrier demapping may be performed according to a 1 MHz location of received preamble or data and the receiver may set a Network Allocation Vector (NAV) for either or both of the upper or the lower 1 MHz bands and may ignore a packet based on configurations.

Further, a receiver that is only capable of operating in the first bandwidth may determine that the second bandwidth is used and may operate accordingly. For example, the receiver may cease receiving on the second bandwidth in order to conserve battery life.

In another embodiment, the SIG field of a preamble may include an indication of whether packet aggregation is performed. Packet aggregation may be utilized for reducing signaling overhead when available bandwidths are relatively small. Further, packet aggregation results in gain when a large number of receivers or transmitters are available.

The SIG field may include an indication that a preamble or a data transmission is aggregated, an indication that an aggregated preamble or an aggregated data transmission is intended for one or more specific receivers or multiple receivers, or information for processing or de-aggregation of data transmissions, such as an order of receivers or timing information.

Aggregation may be performed over contiguous or non-contiguous bandwidths. Further, the contiguous or non-contiguous bandwidths may have the same or different bandwidths. For example, any 5, 10, 40, or 80 MHz bandwidth may be aggregated with any other 5, 10, 40, or 80 MHz bandwidth.

In one embodiment, the SIG field of a preamble may indicate MCS information for a subchannel (i.e., on a subchannel basis). The MCS information may include the modulation or coding utilized for each subchannel within a transmission bandwidth. For example, the MCS information may indicate a modulation, a coding rate, or a binary convolutional code (BCC) or a low density parity check (LDPC) coding indicator. A receiver may receive MCS information for each subchannel and may demodulate or decode a transmission over each subchannel based on the MCS information.

It is noted that subchannels in a transmission bandwidth or channel width may be non-contiguous or contiguous. Further, multiple contiguous or non-contiguous subchannels may be used simultaneously for wide-bandwidth transmission or multichannel transmission. Further, each of the subchannels may have its own MCS that is indicated using the SIG field.

For example, in a transmission bandwidth or channel width of 8 MHz, a subchannel may have a width of less than 8 MHz. Channel conditions, such as interference, of a first subchannel may be different than the channel conditions of a second subchannel. Further, when the channels are non-contiguous, the difference between the channel conditions of the first subchannel and the second subchannel is expected to be larger than when the first subchannel and the second subchannel are contiguous. A subchannel-specific MCS may take into account signal-to-noise ratio (SNR) or bit error rate (BER) conditions for the subchannel or other subchannels.

In one embodiment, the SIG of an Omni portion of a preamble (for example, GF-SIG-A403of Greenfield preamble400, or SIG-A424of MU preamble420) may include MCS information on a subchannel basis that is intended for multiple users (for example, a BCC or LDPC coding indicator for multiple users). However, the SIG field of an MU portion of the preamble (for example, GF-SIG-B405of Greenfield preamble400, or SIG-A428of MU preamble420) may include MCS information on a subchannel basis intended for a subset of users (for example, a modulation and coding rate for a specific user).

In an embodiment, a SIG field of a preamble may include an indication of transmit power control information. Further, a receiver may adjust its transmission power according to the power control information included in the SIG field. The SIG field may indicate a power up command indicating that a transmission power should be increased, a power down command indicating that a transmission power should be decreased, or an absolute power level indicating that a transmission power of the receiver should be adjusted to match the absolute power level.

Additionally, a SIG field of an Omni portion of a preamble (for example, SIG-A424of MU preamble420or GF-SIG-A403of Greenfield preamble400) may include a reference transmission power level intended for multiple receivers, whereas a SIG field of an MU portion of a preamble (for example, SIG-B428of MU preamble420or GF-SIG-B405of Greenfield preamble400) may include an offset transmission power level relative to the reference transmission power level. The offset transmission power level may be intended for one or more specific receivers of the MU portion of the preamble and the specific receiver may adjust its transmission power level to match a power level that is the aggregate of both the reference transmission power level and the offset transmission power level.

Transmit power control information may include a quantized representation of an absolute power level, a quantized representation of power up or power down indication, a power difference between two measurement intervals for a specific receiver, a power difference between two receivers, or a power difference between a transmitter and a receiver.

To improve time and frequency acquisition and channel estimation by a receiver, a midamble or a postamble may be utilized in a transmission as described with reference toFIG.7.

FIG.7shows a transmission including a preamble, midamble, and a postamble. The transmission700comprises a preamble701, where the preamble701includes a SIG field701A, as described herein. The transmission also includes data fields702,704,706which may include any data that is intended for a receiver, a first midamble703, a second midamble705(collectively referred to herein as midambles703,705), and a postamble707. The midambles703,705are placed amidst data fields702,704,706and may comprise STFs or LTFs that may be used by the receiver for time and frequency acquisition, channel estimation, and the like. The midambles703,705may be necessary when the aggregate of the length of data fields702,704,706is large and the time and frequency acquisition and channel estimation acquired by the receiver based on the preamble701have become stale or non-applicable to current communications conditions.

To achieve diversity, the preamble701and the midambles703,705may each use different or orthogonal subcarriers. Further, if available subcarriers are limited, the preamble701or the midambles703,705may re-use the subcarriers. Additionally, the preamble701and the midambles703,705may be transmitted on different antennas or on orthogonal antennas, or using a spatially orthogonal covering code. Further, each data field702,704,706may have an MCS associated with the data field702,704,706and the MCSs may be the same or different for the data fields702,704,706.

The SIG field701Aof the preamble701may include an indication of the presence of or a location of a midamble (for example, first midamble703or second midamble705) or the postamble707in a transmission700. The location may be indicated by a symbol offset to the midamble, a number of symbols between the midambles703,705, time between midambles703,705, or an index to a pre-determined midamble location (for example, every nth OFDM symbol). Further, the indication of a location of the midamble may be determined based on an antenna index or an antenna number.

The SIG field701Aof the preamble701may also include an indication of a format of the midamble, an index to a format of each midamble, or an index to the format of all preambles. The SIG field701Aof the preamble701may also include an indication of a subcarrier pattern of a midamble. Alternatively, a subcarrier pattern may be implicitly indicated using a location of the midamble.

In an alternative embodiment, midambles703,705may each have a subsequently transmitted midamble SIG (MSIG) field. Each MSIG field of the midambles703,705may indicate the length of subsequent data fields704,706, respectively. In addition, the length of data field702may be indicated by the SIG field701Aof the preamble701. Furthermore, SIG field701Aor an MSIG field associated with midambles703,705may indicate the MCS associated with data fields702,704,706.

A receiver may interpolate channel estimates obtained based on preamble701, midambles703,705, or postamble707, and may utilize the interpolated channel estimates to process data fields702,704,706. For example, a receiver may interpolate channel estimates obtained based on midambles703,705and utilize the interpolated channel estimates to process data field704that is received between the midambles703,705. The interpolation may allow for more robust channel estimation.

Further, a receiver may obtain a Doppler estimate of a channel based on preamble701, midambles703,705, or postamble707. The receiver may also request increasing or decreasing the number of midambles based on the Doppler estimate. The Doppler estimate may be sent to a transmitter and may be used to determine whether to increase or decrease the number of midambles sent to the receiver. Doppler estimates may also be used for formation of a group of receivers, whereby the group of receivers may have requested the same number of midambles.

In another embodiment, the SIG field of a preamble may indicate usage of a short guard interval (GI) for a subsequent data transmission. The SIG field may indicate usage of a short GI using a bit indicator. Additionally, short GI usage may be indicated using a polarity of the pilot tone values of the SIG field. For example, where a SIG field has four pilot tones, pilot tone values [1 1 1 −1] may indicate the absence of a short GI, whereas pilot tone values [−1 −1 −1 1] may indicate the presence of a short GI.

A receiver may process the SIG field and may determine the pilot tone values of the SIG field using, for example, a mean squared error (MSE) metric or another metric. The receiver may further determine the presence or absence of a short GI based on the pilot tone values and may process a data transmission accordingly. It is noted that when multiple SIG fields are used to indicate usage of a short GI, the MSE or any other metric may be averaged to increase robustness.

FIG.8shows a preamble indicating a short GI and a data transmission. The preamble810comprises an STF811, an LTF812, and a SIG field813which indicates usage of a short GI. Because of the usage of a short GI, the SIG field813is followed by a data transmission820.

In an embodiment, the SIG field of a preamble may indicate whether beamforming is utilized on a subsequent data transmission and whether the preamble is a beamforming preamble. Further, when beamforming is utilized, the beamforming preamble may include an additional STF for beamforming, an additional LTF for beamforming, or both an additional STF and an additional LTF for beamforming.

FIG.9shows a beamforming preamble. The beamforming preamble900comprises an STF901, an LTF902, and a SIG field903. In addition, the beamforming preamble900includes a beamforming STF (BF-STF)904, and one or more beamforming LTFs (BF-LTFs), denoted BF-LTF19051, . . . , BF-LTFN905Nand referred to collectively herein as BF-LTFs905. BF-STF904may be used for AGC correction, whereas BF-LTFs905may be used for improved frequency offset and channel estimation by a receiver. The SIG field903of the beamforming preamble900may include a one-bit indicator for beamforming. It is noted that BT-STF904may be replaced by a beamforming LTF, for example, BF-LTF, and a receiver may perform AGC and time and frequency acquisition based on the BT-LTF.

In an embodiment, the SIG field of a preamble may include a length field indicating the length of a data transmission in bits, bytes, or OFDM symbols, or in multiples of bits, bytes, or OFDM symbols (for example, in pairs of OFDM symbols). For example, the length field may comprise n bits and may, thus, indicate any length of the data transmission field between 0 and 2n−1 bits, bytes, or OFDM symbols, or multiples of bits, bytes, or OFDM symbols.

Whether the length field represents the length of the data transmission in bits, bytes, or OFDM symbols, or in multiples of bits, bytes, or OFDM symbols may depend upon the modulation scheme used. Thus, when a first modulation scheme is used, the length field may represent the length of the data transmission in bits, whereas when a second modulation scheme is used, the length field may represent the length of the data transmission in OFDM symbols. Further and by way of example, the SIG field may represent the length of the data transmission in bytes only for modulation scheme MCS0-Rep2 of IEEE 802.11ah, whereas for all other IEEE 802.11ah modulation schemes, the length field may represent the length of the data transmission in OFDM symbols.

By way of yet another example, the length field of a SIG field may denote the length of a data transmission in OFDM symbols, whereby a length field of a SERVICE field of an IEEE 802.11 data transmission may indicate the length in bytes of the last OFDM symbol of the data transmission.

Whether the length field represents a length of transmission in bits or bytes may depend on an aggregation indication. For example, when the SIG field indicates that aggregation is not performed, then the length field indicates the length of the data transmission field in bytes, whereas when the SIG field indicates that aggregation is performed, then the length field indicates the length of the data transmission field in OFDM symbols. Further, in IEEE 802.11 it may be required that an aggregated medium access control (MAC) protocol data unit (AMPDU) be used when a data transmission exceeds 2047 bytes in length.

In order to allocate additional bits in the SIG field for a length field, space time block coding (STBC), which is typically indicated by one or two bits in the SIG field, may be implicitly indicated and the one or two bits used to indicate STBC may be used instead as additional bits for the length field. Furthermore, to allocate additional bits to the length field, short GI indication, aggregation indication, and NSTSindication, which are typically indicated using bits of the SIG field, may instead be implicitly indicated and the bits that were formerly used to indicate a short GI, aggregation, and NSTSmay be used as additional bits for a length field. The modulation scheme of symbols of the SIG field may indicate a short GI, aggregation, and NSTS. For example, the modulation scheme of the first symbol of the SIG field may indicate whether STBC is performed, the modulation scheme of the second symbol of the SIG field may indicate whether a short GI is used, the modulation scheme of the third symbol of the SIG field may indicate whether aggregation is performed, and the modulation scheme of the fourth and fifth symbols of the SIG field may indicate NSTS.

In one embodiment, a preamble may be transmitted stand-alone without a subsequent data transmission in order to perform channel sounding. A SIG field of the preamble may indicate that the preamble is used for the purpose of channel sounding. The SIG field may indicate that the preamble is used for the purpose of channel sounding if the length field of the SIG field is set to zero.

In one embodiment, an STF may use every other available frequency bin in any mode of operation, such as a 1 MHz mode of operation. For example, when using every other available frequency bin, the STF may use twelve tones out of a total of twenty four tones. By using every other tone, the twelve tones of the STF may be [−12 −10 −8 −6 −4 −2 2 4 6 8 10 12]. Further, the values of the twelve tones may be [−1 −1 −1 1 1 1 −1 1 −1 −1 1 −1]*(1+i).

Using a fast Fourier transform (FFT) of size 32, the 12-tone STF has a peak-to-average power ratio (PAPR) of 2.06 decibels (dB). Further, the number of repetitions per OFDM symbol for the 12-tone STF is two and the 12-tone STF results in improved autocorrelation properties and improved packet timing detection.

In another embodiment, an STF may use every fourth available frequency bin in addition to the direct current (DC) bin in any mode of operation, such as a 1 MHz mode of operation. For example, when using every fourth available frequency bin in addition to DC bin, the STF may use seven tones out of a total of twenty four tones. The tones may be [−12 −8 −4 0 4 8 12]. Further, the values of the tones may be [−1 −1 −1 1 1 −1 1]*(1+i).

Using a fast Fourier transform (FFT) of size 32, the STF has a PAPR of 1.32 dB. Further, the number of repetitions per OFDM symbol for the STF is four. The STF has improved autocorrelation properties and improved packet timing detection.

A preamble may be constructed using 12-tone STFs enabling robust detection, frequency and time synchronization, and channel estimation as described with reference toFIG.10A. In the preamble ofFIG.10A, a maximum range of frequency offset estimated may be ±Δf, where Δf is frequency spacing.

FIG.10Ashows a preamble having four 12-tone STFs and four LTFs. The preamble1000comprises STFs10011-4and LTFs10021-4. The STFs10011-4are each 12-tone STFs and have a duration of 40 μs. The LTFs10021-4are each 26-tone LTFs and have a duration of 40 μs.

In addition, a preamble may be constructed using 12-tone STFs and 6-tone STFs enabling robust detection, frequency and time synchronization and channel estimation as described with reference toFIG.10B. The preamble ofFIG.10Benables estimation of frequency offsets of up to ±2Δf.

FIG.10Bshows a preamble having two 12-tone STFs, two 6-tone STFs and two LTFs. The preamble1010comprises STFs10111-2, STFs10121-2and LTFs10131-2. STFs10111-2are each 6-tone STFs and have a duration of 40 μs. STFs10111-2may be used for AGC, frequency offset estimation, and coarse timing. STFs10121-2are each 12-tone STFs and have a duration of 40 μs. STFs10121-2may be used for fine frequency offset estimation, fine timing estimation, and channel estimation. LTFs10131-2are each 26-tone LTFs and have a duration of 40 μs. LTFs10131-2may be used for channel estimation.

In an embodiment, the transmission of a SIG field or data may be repeated in order to achieve coding and diversity gain. Repetition may be performed on a block-by-block basis (i.e., block-wise) or on a bit-by-bit basis (i.e., bit-wise). Additionally, the data or the SIG field may be scrambled, error encoded, interleaved, and mapped to a modulation scheme before transmission.

To increase coding and diversity gain, bit-wise repetition may be performed after error correction encoding and before interleaving as described with reference toFIG.11A.

FIG.11Ashows an example of bit-wise repetition performed after forward error correction (FEC) encoding. InFIG.11A, a SIG field or data is scrambled by a scrambler1101and FEC encoded by FEC encoder1102. Bit-wise repetition1103is then performed on the output of FEC encoder1102. After bit-wise repetition1103, an interleaver1104is applied. The interleaver1104may have any number of columns (for example, eight columns). After interleaving, a mapper1105for any modulation scheme, such as BPSK, is applied and modulated data may be transmitted.

In an alternative embodiment, bit-wise repetition may be performed before FEC encoding.FIG.11Bshows bit-wise repetition performed before FEC encoding.

In another alternative embodiment, bit-wise repetition may be performed before FEC encoding and repeated bits may be separately FEC encoded and interleaved, as described with reference toFIG.11C.

FIG.11Cshows bit-wise repetition performed before FEC encoding. InFIG.11C, a SIG field or data is scrambled by a scrambler1101and bit-wise repetition1103is then performed on the output of the scrambler1101. The output of bit-wise repetition1103is then separately FEC encoded and interleaved, whereby FEC encoder11031and interleaver11041are applied to a first output of the bit-wise repeater1102, and FEC encoder11032and interleaver11042are applied to a second output of the bit-wise repeater1102. Mapper1105is then applied to the output of both interleaver11041and interleaver11042, and modulated data may be transmitted.

In another embodiment, block-wise repetition may be performed in place of bit-wise repetition. Block-wise repetition may be performed after FEC encoding, as described with reference toFIG.12.

FIG.12shows an example of block-wise repetition performed after FEC encoding. InFIG.12, a scrambler1201is applied to a SIG field or data. The output of the scrambler1201is then FEC encoded by FEC encoder1202. The output of FEC encoder1202is then separately block-wise encoded and interleaved, whereby block-wise encoder12031and interleaver12041are applied to a first output of the FEC encoder1202, and block-wise encoder12032and interleaver12042are applied to a second output of the FEC encoder1202. The outputs of interleavers12041and12042are then combined and provided to mapper1205for modulation mapping, and mapped data may be transmitted.

The use of repetition may be implicitly indicated when bandwidth selection is indicated. For example, if a 1 MHz or a 2 MHz bandwidth is indicated, then the use of repetition is implicitly indicated, and vice-versa. Further, in IEEE 802.11 communication, the use of repetition may be indicated by an RXVECTOR. An RXVECTOR may have a list of parameters that a physical layer (PHY) provides to a local MAC entity. For example, the RXVECTOR may indicate the use of either one of a 1, 2, 4, or 8 MHz bandwidth, or the use of one of a 2, 4, 8, or 16 MHz bandwidth. Further, repetition may be used only when the RXVECTOR indicates the use of either one of a 1, 2, 4, or 8 MHz bandwidth.

In another embodiment, transmit antenna diversity with cyclic shifting may be employed for transmitting a SIG field or data to increase frequency diversity, as described with reference toFIG.13. Further, to achieve repetition gain, the SIG field or data may be bit-wise or block-wise repeated, as described with reference toFIGS.11A-Cand12, prior to being transmitted using antenna diversity and cyclic shifting.

FIG.13shows transmission of a SIG field or data using antenna diversity and cyclic shifts. A SIG field or data that is an output ofFIG.11A-Cor12(i.e., a SIG field or data that is scrambled, FEC-encoded, repeated, interleaved, or mapped to a modulation scheme) is provided to an IFFT1301. The output of the IFFT1301is provided to each of four paths. In a first path, a short GI1303is inserted, in a second path a first cyclic shift13021and a short GI1303are inserted, in a third path a second cyclic shift13022and a short GI1303are inserted, and in a fourth path a third cyclic shift13023and a short GI1303are inserted. After cyclic shift and short GI insertion, data is transmitted using antennas1304for each path. It is noted that repetition and frequency diversity gain are achieved because each path receives from the IFFT1301a SIG field, a repeated SIG field, or a combination of a SIG field and a repeated SIG field.

In an embodiment, a power headroom report may be requested from a receiver, for example, using a power headroom request, and a power headroom report may be provided by a receiver. If there are multiple receivers in a serving area, a power headroom request may be sent to the multiple receivers, for example, using a power headroom poll.

A power headroom response may be transmitted using round robin reports or simultaneous reporting using orthogonal reporting, such as orthogonal data signatures. Further, a received signal strength indication (RSSI) or a received channel power indication (RCPI) may be measured based on a preamble or a data transmission, and if the RSSI or RCPI changes from a previous measurement, a correction in transmit power may be determined taking into account the power headroom of multiple receivers.

A CRC code for a SIG field of a preamble may be punctured in order to reduce a number of parity bits. By optimally puncturing a CRC code, a shorter code may be generated. For example, an 8-bit CRC code may be punctured to generate a 4-bit CRC code.

In an example, an 8-bit CRC may be generated using the polynomial x8+x2+x+1 for a 26-bit SIG field. A resultant parity check matrix for the CRC code may be described as H=[P I8], where P is an 8×26 matrix and I8is an 8×8 identity matrix. In order to reduce the number of parity bits from 8 to 4, 4 rows may be removed from matrix P to get a 4×26 matrix denoted as P1. A new parity check matrix may be used, where the parity check matrix is H1=[P1 I4], P1 is a 4×26 matrix, and I4is a 4×4 identity matrix.

In order to avoid a minimum Hamming distance of 1, a requirement may be imposed that no column of the parity check matrix may be composed of all zeros. Therefore, when 4 rows are removed in the example above, it is desirable for the matrix P1 not to have an all-zero column.

The Hamming Weight (HW) distribution of a code may be derived from the parity check matrix as follows: the number of codewords with HW=i is the number of combinations of i columns of the parity check matrix, such that a linear combination is equal to a zero vector.

For an 8-bit code, there are 70 combinations of 4-bit puncturing patterns, (i.e., there are 70 ways to puncture an 8-bit code to a 4-bit code). Table 3 shows a HW distribution for 26 information bits when a 4-bit CRC is derived by puncturing from an 8-bit CRC. Table 4, on the other hand, shows a HW distribution for 38 information bits when a 4-bit CRC is derived by puncturing from an 8-bit CRC. In Tables 3 and 4, the HW distribution is shown for HW=1, 2, 3, 4, and 5. Additionally, c0represents the most significant bit (MSB) that is punctured and c7represents the least significant bit (LSB) that is punctured.

Table 3 shows a HW distribution when a 4-bit CRC is derived from an 8-bit CRC for 26 information bits.

Table 4 shows a HW distribution when a 4-bit CRC is derived from an 8-bit CRC for 38 information bits.

As shown in Tables 3 and 4, puncturing bits c7c6c5c4(i.e., the four LSBs) or bits c3c2c1c0(i.e., the four MSBs) does not yield results as good as other alternatives, such as puncturing bits c5c4c2c0, since both c7c6c5c4and c3c2c1c0have a minimum hamming distance of 1. Further, any of the puncturing combinations in Tables 3 and 4 may be chosen for a code with a minimum distance 2 to detect all single bit-error patterns.

It is worth noting that a puncturing combination such as bits c5c4c1c0shown in Table 3 yields a probability of false positives that is close to that of an optimal 4-bit CRC generated using the polynomial x4+x+1.

In an embodiment, the type of modulation used for SIG field symbols may be used to signal information, such as information as to whether beamforming is utilized, for example, a SIG field may comprise any number of OFDM symbols, such as 5 OFDM symbols or 6 OFDM symbols. Further, the 6 OFDM symbols may be modulated using quadrature binary phase shift keying (QBPSK) or binary phase shift keying (BPSK). It is worth noting that according to QBPSK, a symbol is modulated using +/−j, whereas according to BPSK, a symbol is modulated using +/−1.

The modulation scheme of any one of the OFDM symbols may be used to signal information. For example, the modulation scheme of the first OFDM symbol may be used to signal that beamforming is utilized, whereby if the first OFDM symbol is modulation using BPSK, then the use of beamforming is not indicated, whereas if the first OFDM symbol is modulation using QBPSK, then the use of beamforming is indicated.

Further, the modulation scheme of the OFDM symbols of the SIG field may be used in conjunction with a CRC attachment or FEC encoding to increase the number of information bits signaled using the SIG field.

FIG.14Ashows an example of coding of SIG field bits. InFIG.14A, X bits of a SIG field are provided to a CRC attachment unit1401. The CRC attachment unit1401generates A CRC bits. The X+A SIG field bits and CRC attachment bits are provided to an FEC encoder1402to generate an output. The output of the FEC encoder1402is then provided to a repeater, interleaver, and mapper unit1403, and the encoded and modulated X+A field bits and CRC attachment bits may be transmitted.

The number of bits signaled using the SIG field may be increased from X to X+Y while protecting the additional Y SIG fields using CRC encoding as described with reference toFIG.14B.

FIG.14Bshows an example of coding of SIG field bits and additional bits. InFIG.14B, X SIG field bits and Y additional SIG field bits are provided to a CRC attachment unit1401. The CRC attachment unit1401generates A CRC attachment bits based on both the X SIG field bits and Y additional SIG field bits, and provides an output of X+Y+A bits. The X+A bits pertaining to the SIG field and CRC attachment are provided to an FEC encoder1402to generate an output. The output of the FEC encoder1402is provided to a repeater, interleaver, and mapper unit1403in the same manner as described with reference toFIG.14A.

However, the Y additional bits may not be transmitted in the same manner as the X+A bits pertaining to the SIG field and CRC attachment. Instead, the Y additional bits may be provided to a SIG symbol modulation unit1404, which modulates the Y bits using the SIG field OFDM symbols as described above. For example, if the Y bits are [0 0 1 1 1], then five SIG field OFDM symbols may be modulated as [QQBBB] to indicate the Y bits, where a QBPSK modulation of a SIG field symbol indicates a zero bit and a BPSK modulation indicates a 1 bit.

As described with reference toFIG.14B, the Y additional bits are protected using the A CRC attachment bits. Further, no additional overhead is needed for signaling the Y additional bits. Further, in addition to CRC attachment, the Y additional bits may also be protected by an FEC code, as described with reference toFIG.14C.

FIG.14Cshows an example of coding of SIG field bits and additional bits. InFIG.14C, X SIG field bits and Y additional bits are provided to a CRC attachment unit1401. The CRC attachment unit1401generates A CRC bits and provides an output of X+Y+A bits to the FEC encoder1402, which generates an output. A first portion of the output of the FEC encoder1402is provided to a repeater, interleaver, and mapper unit1403and a second portion of the output of the FEC encoder1402(for example, 5 or 6 bits) is provided to a SIG symbol modulation unit1404, which modulates the Y bits using the SIG field OFDM symbols as described above.

Further, the modulation scheme of the OFDM symbols of the SIG field may be used to introduce more CRC attachment bits in order to improve the robustness of the CRC attachment. For example, the CRC attachment may be increased from A bits to A+B bits. Further, the additional B CRCs bits may signaled using the modulation of the OFDM symbols. Further, the additional B CRC bits may be further coded using any coding scheme as described with reference toFIG.14D.

FIG.14Dshows an example of coding of SIG field bits and additional CRC bits. InFIG.14D, X SIG field bits are provided to a CRC attachment unit1401. The CRC attachment unit1401generates a CRC attachment of A+B bits (i.e., the CRC attachment unit1401generates B additional attachment bits than shown inFIGS.14A-C). The X SIG field bits and the A CRC attachment bits are provided to FEC encoder1402to generate an output. The output of the FEC encoder1402is provided to a repeater, interleaver, and mapper unit1403, and may then be transmitted. On the other hand, the B CRC attachment bits are provided to a (B,Z) encoder1405. The (B,Z) encoder1405encodes the B CRC attachment bits using Z bits, (for example, the (B,Z) encoder may encode B=4 bits using Z=6 bits). The Z bits are provided to a SIG symbol modulation unit1404which modulates the Z bits using the SIG field OFDM symbols as described above.

In an embodiment, a cyclic shift may be applied to Greenfield preamble, such as Greenfield preamble400. The cyclic shift may prevent unintentional beamforming when identical signals are transmitted on different spatial streams. The cyclic shift may be similar to a cyclic shift used in a subsequent data transmission particularly in the case where the same data transmission is performed over multiple antennas. However, the values of the cyclic shift used in the Greenfield preamble and a subsequent data transmission may be different for different antennas.