WWiSE

The WWiSE consortium includes several well-known chipmakers: Airgo (the manufacturer of the first "pre-N" devices on the market), Broadcom, Conexant, and Texas Instruments. Motorola joined the consortium in February 2005, just as this book headed to press.

MAC Enhancements

As would be expected from a name touting spectral efficiency, WWiSE is more the more heavily weighted towards improving the MAC efficiency of the two proposals. To get to 100 Mbps net payload throughput, 12,000 bytes (960,000 bits) need to be transmitted in 960 microseconds. WWiSE's PHY specification has a 135 Mbps data rate in a basic two-antenna configuration with two data streams, which can move the data in 711 microseconds. The remaining 249 microseconds are used for preambles, framing, interframe spacing, and the single block acknowledgment.

Channels and radio modes

WWiSE uses both 20 MHz and 40 MHz channels. 40 MHz operation may be through a single 40 MHz channel, or through a 20 MHz channel pair in which both channels are used simultaneously for data transmission. One channel is designated as the primary channel, and operates normally. The secondary channel is used only for channel aggregation, and does not have stations associated on it. The secondary channel is used for "overflow" from the primary; carrier sensing functions are performed only on the primary channel.

Although the use of two channels is really a physical layer operation, there are some housekeeping functions performed by the MAC. A new information element, the Channel Set element, is sent in the primary channel Beacon frames so that stations are informed of the secondary channel in the pair. Access points also send Beacon frames on the secondary channel; unlike most Beacon operations, though, the purpose is to discourage clients from associating, or other devices from choosing that channel for operation. A secondary channel Beacon frame is very similar to the primary channel Beacon, but the only supported rate is a mandatory MIMO PHY rate. To further discourage use of the channel, it may also include the contention-free information element.

Protection

Like 802.11g, the new PHYs require enhanced protection mechanisms to avoid interfering with existing stations. Naturally, the protection mechanisms specified in 802.11g are adopted for operation of 2.4 GHz stations that may have to avoid interfering with older direct sequence or 802.11b equipment. When access points detect the presence of older equipment, it will trigger the use of RTS-CTS or CTS-to-self protection as described in Chapter 14.

However, additional protection may be required to avoid having a MIMO station transmit at a rate not understood by 802.11a or 802.11g equipment. The WWiSE proposal contains an OFDM protection scheme to allow MIMO stations to appropriately set the NAV on older OFDM stations. The protection mechanism is identical to the one described in Chapter 14, but it takes place using OFDM data rates.

Finally, the WWiSE proposal uses two bits in the ERP information element in Beacon frames to indicate whether OFDM protection is needed. In some cases, OFDM protection may be needed to assist an older 802.11g network, but no protection is needed for 802.11b stations. Access points monitor the radio link to determine if OFDM protection is needed. To assist stations using channel pairs, they also report on whether a secondary channel is in use.

Aggregation, bursting, and acknowledgment

The WWiSE proposal increases the maximum payload size from 2,304 bytes to over 8,000 bytes. Increasing the payload increases the payload-to-overhead and the ratio can increase efficiency if the larger frames or bursts can be delivered successfully.

Aggregation bundles multiple higher-level network protocol packets into a single frame. Each packet gets a subframe header with source and destination addresses, and a length to delimit the packet, as shown in Figure 15-2. Aggregation can only be used when the frames bundled together have the same value for the Address 1 field, which is the receiver of the frame. Frames from an access point in an infrastructure network use Address 1 as the destination, so access points can only aggregate frames bound for a single station. A station in an infrastructure network can, however, aggregate frames to multiple destinations. Station transmissions use the Address 1 field for the AP, since all frames must be processed by the AP prior to reaching the backbone network. Upon aggregation, the destination address is the "next hop" processing station, and the source is the creator of the frame. Upon deaggregation, the individual subframes will be processed according to the sub-frame headers. Due to the requirement that the receiver address must be the same, it is not possible to aggregate a mixture of unicast, broadcast, and multicast data. The proposal contains no rules about when to use aggregation.

Figure 15-2. Aggregation in WWiSE

Bursting is a related, but slightly different concept. Frame aggregation glues higher-layer protocol packets together for transmission in larger lumps. Bursting does the same at the physical layer. Once a station has invested a significant amount of protocol overhead to obtain control of the channel, it can just keep on transmitting. One of the advantages of using multiple physical frames, as opposed to higher-layer frames, is that each physical frame has its own source and destination. A frame burst can consist of traffic intended for a variety of different destination addresses. In a frame burst, there are two additional interframe spaces defined, the Zero Interframe Space (ZIFS) and the Reduced Interframe Space (RIFS). Successive frames that use the same transmit power may use the ZIFS for immediate transmission. If the transmit power is changed between frames, the RIFS may be used. The RIFS is shorter than other interframe spaces, though, so it allows a station to retain control of the channel. In Figure 15-3, the first frame cannot be aggregated, and is transmitted after the transmitter gains control of the channel. Once it has gained control, it can hold on as long as allowed. The second and third frames use the same transmission power, and so are transmitted after the zero interframe space. Additionally, they share the Address 1 field and are therefore bundled into an aggregate frame. For transmission of the next frame, power needs to be changed, requiring the use of the reduced interframe space. The fourth and fifth frames can be aggregated, and are transmitted as a single aggregate frame. When the queued data has been transmitted, the station relinquishes control of the channel.

Figure 15-3. Bursting in WWiSE

In the initial version of the 802.11 MAC, a positive acknowledgment was required for every unicast data frame. WWiSE lifts this restriction, and allows for a more flexible acknowledgment policy. In addition to the "normal" policy, frames can be transmitted without an acknowledgment requirement, or with block acknowledgments instead.

The WWiSE MIMO PHY

The WWiSE proposal is a slight evolution of 802.11a, using MIMO technology. The basic channel access mechanisms are retained, as is the OFDM encoding. At a high level, the WWiSE PHY is mainly devoted to assigning bits to different antennas.

Structure of an operating channel

Like 802.11a, the radio channel is divided into 0.3125 MHz subcarriers. As in the 802.11a channel subdivisions, a 20 MHz channel in the WWiSE proposal is divided into 56 subcarriers. 40 MHz channels, which are optional, are divided into 112 subcarriers. In addition to being optional, 40 MHz channels are only supported in the 5 GHz band because it is not possible to squeeze multiple 40 MHz channels into the ISM band. (And if you thought network layout was hard with three channels, wait until you try with two!) Figure 15-4 shows the structure of both the 20 MHz and 40 MHz operating channels in the WWiSE proposal.

As in 802.11a, subcarriers are set aside as pilots to monitor the performance of the radio link. Fewer pilot carriers are needed in a MIMO system because the pilot carriers run through as many receiver chains. A 20 MHz 802.11a channel uses four pilot

Figure 15-4. WWiSE pilot carrier structure

subcarriers. In the WWiSE proposal, a 20 MHz channel requires only two pilot carriers because each pilot is processed by two receiver chains, which has the same effect as four pilots processed by a single receiver chain. With fewer pilots, more subcarriers can be devoted to carrying data. 20 MHz WWiSE channels have 54 data subcarriers; 40 MHz channels have exactly twice as many at 108.

Modulation and encoding

The WWiSE proposal does not require new modulation rates. It uses 16-QAM (4 bit) and 64-QAM (6 bit) modulation extensively, but does not require finer-grained modulation constellations.

Coding is enhanced, however. A new convolutional code rate of 5/6 is added. Like the 2/3 and 3/4 code rates defined by 802.11a, the 5/6 code is defined by puncturing the output to obtain a higher code rate. WWiSE also defines the use of a low density parity check (LDPC) code.

Interleaver

In 802.11a, the interleaver is responsible for assigning bits to subcarriers. MIMO interleavers are more complex because they must assign bits to a spatial stream in addition to assigning bits to positions on the channel itself. The WWiSE interleaver takes bits from the forward error coder and cycles through each spatial stream. The first bit is assigned to the first spatial stream, the second bit is assigned to the second spatial stream, and so on. The interleaver is also responsible for scrambling the encoded bits within each spatial stream.

Space-time block coding

In most cases, an antenna will be used for each spatial stream. However, there may be cases when the number of antennas is greater than the number of spatial streams. If, for example, most APs wind up using three antennas while clients only use two, there is an "extra" transmit antenna, and the two spatial data streams need to be assigned to the three antennas. Transmitting a single spatial stream across multiple antennas is called space-time block coding (STBC).

The basic rule for splitting a spatial stream across multiple antennas is to transmit two related streams on different antennas. As discussed in Chapter 13 on 802.11a, the radio wave is composed of in-phase and quadrature components, where the quadrature wave is a quarter-cycle out of phase with the in-phase component. Phase shifts are represented mathematically by the imaginary part of the complex number in the constellation. The complex conjugate of a complex number has the same real part, but flips the sign on the imaginary part. Physically, the radio wave from the complex conjugate will have the same in-phase component, but the quadrature component will have the oppose phase shift. When there are extra antennas, the WWiSE proposal mandates that a spatial stream and its complex conjugate are transmitted on an antenna pair. Table 15-1 reviews the rules. The rules for splitting spatial streams are independent of the channel bandwidth, although 40 MHz spatial streams will carry more bits.

Table 15-1. WWiSE encoding rules when antennas outnumber spatial streams

Transmit antennas	Spatial streams	First spatial stream	Second spatial stream	Third spatial stream
2	1	Coded across antennas 1 and 2	N/A	N/A
3	2	Coded across antennas 1 and 2	Transmitted normally on third antenna	N/A
4	2	Coded across antennas 1 and 2	Coded across antennas 3 and 4	N/A
4	3	Coded across antennas 1 and 2	Third antenna	Fourth antenna

 

Modulation rates

There are 24 data rates defined by the WWiSE PHY, with 49 different modulation options. Rather than take up a great deal of space in a table, here is a basic formula for the data rates:

Data rate (Mbps) = 0.0675 x channel bandwidth x number of spatial streams x coded bits per subcarrier x code rate

Channel bandwidth

Either 20 for 20 MHz channels, or 40 for 40 MHz channels or channel pairs.

Number of spatial streams

The number of spatial streams can be equal to 1, 2, 3, or 4. It must be less than or equal to the number of transmission antennas. Support for at least two spatial streams is mandatory.

Coded bits per subcarrier

In most cases, this will either be 6 for 64-QAM or 4 for 16-QAM. BPSK (1 coded bit per subcarrier) and QPSK (2 coded bits per subcarrier) are only supported in the 20 MHz channel mode with one spatial stream.

Code rate

The code rate may be 1/2 or 3/4 when used with 16-QAM, and 2/3, 3/4, or 5/6 when used with 64-QAM.

There may be multiple ways to get to the same data rate. As an example, there are four ways to get 108 Mbps:

• Four spatial streams in 20 MHz channels, using 16-QAM with R=1/2.
• Two spatial streams in 20 MHz channels, using 64-QAM with R=2/3.
• One spatial stream in a 40 MHz channel, using 64-QAM with R=2/3.
• Two spatial streams in 40 MHz channels, using 16-QAM with R=1/2.

In a basic mode with a single spatial stream, channel capacity is slightly higher than with 802.11a because fewer pilot carriers are used. Single-channel modulation tops out at 60.75 Mbps, rather than the 54 Mbps in 802.11a. By using all the highest throughput parameters (four 40 MHz spatial streams, with 64-QAM and a 5/6 code), the WWiSE proposal has a maximum throughput of 540 Mbps.

MIMO and transmission modes

Previous 802.11 PHY specifications had fairly simple transmission modes. The WWiSE proposal has 14 transmission modes, depending on 3 items:

• The number of transmit antennas, noted by xTX, where x is the number of transmit antennas. It ranges from 1 to 4, although a single antenna is only supported for 40 MHz channels. All 20 MHz channels must use at least two transmit antennas, though they may have only one spatial stream.
• Whether the frame is used in a greenfield (GF) or mixed mode (MM) environment. Mixed mode transmissions use physical headers that are backwards-compatible with other OFDM PHYs, while greenfield transmissions use a faster physical header.
• The channel bandwidth, which may be 20 MHz or 40 MHz.

Table 15-2 shows the resulting 14 transmission modes. There are several physical layer encodings defined for each of these modes, and they will be discussed in the PLCP section. The number of active antennas is only loosely related to the number of spatial streams. A system operating in the 4TX40MM mode has four transmit antennas, but it may have two or three spatial streams.

Table 15-2. WWiSE transmission modes

	20 MHx channels	40 MHz channels
Greenfield	2TX20GF 3TX20GF 4TX20GF	1TX40GF 2TX40GF 3TX40GF 4TX40GF
Mixed mode	2TX20MM 3TX20MM 4TX20MM	1TX40MM 2TX40MM 3TX40MM 4TX40MM

 

WWiSE PLCP

The PLCP must operate in two modes. In Greenfield mode, it operates without using backwards-compatible physical headers. Greenfield access is simpler: it can operate without backwards compatibility. As a starting point, consider Figure 15-5; it shows the PLCP encapsulation in the 1TX40GF, 2TX20GF, and 2TX40GF modes.

Figure 15-5. Greenfield 1TX40 and 2TX20/2TX40 modes

The fields in the frame are similar in name and purpose to all of the other PLCP frames discussed in this book.

MIMO-OFDM PLCP Preamble

The preamble consists of well-known bit sequences to help receivers lock on to the signal. Depending on the transmission mode, the preamble may be split into multiple parts. It generally consists of both short and long training sequences. In the WWiSE proposal, the same preamble is transmitted on all the antennas, but with small time shifts relative to the others. Figure 15-5 shows the training sequences used by two-antenna transmission modes. Although the training sequences consist of different bits, the shift is the same. Naturally, the single antenna 40 MHz mode would only have one active antenna transmitting a preamble.

SIGNAL-N

The SIGNAL-N field contains information that helps to decode the data stream. It is always sent using QPSK, R=1/2, and is not scrambled. It contains information on the number of spatial streams, channel bandwidth, modulation, and coding, and a CRC. More detail on the SIGNAL-N field follows this section.

SERVICE

The SERVICE field is identical to its usage in 802.11a. Unlike the other components of the PLCP header, it is transmitted in the Data field of the physical protocol unit at the data rate of the embedded MAC frame. The first eight bits are set to 0. As with the other physical layers, MAC frames are scrambled before transmission; the first six bits are set to 0 to initialize the scrambler. The remaining nine bits are reserved and must set to 0 until they are adopted for future use.

Data

The final field is a sequence of four microsecond symbols that carry the data. Data bits have six zero tail bits to ramp down the error correcting code, and as many pad bits as are required to have an even symbol block size.

The SIGNAL-N field

The SIGNAL-N field is used in all transmission modes. It has information to recover the bit stream from the data symbols. The SIGNAL-N field is shown in Figure 15-6.

Figure 15-6. WWiSE SIGNAL-N field

 

CONFIG

Six fields are grouped into the Configuration subfield.

 

 

 

 

 

 

NSS (number of spatial streams)

Three bits are used to indicate how many spatial streams are used. The value is zero-based, so it ranges from zero to three.

NTX (number of transmission antennas)

Three bits are used to indicate how many antennas are used to carry the number of spatial streams. The value is zero-based, so it ranges from zero to three.

BW (bandwidth)

Two bits carry the channel bandwidth. 20 MHz is represented by zero, and 40 MHz is represented by one.

CR (code rate)

Three bits indicate the code rate. 1/2 is zero, 2/3 is one, 3/4 is two, and 5/6 is three.

CT (code type)

Two bits indicate the type of code. Zero is a convolutional code, and one is the optional LDPC.

CON (constellation type)

Three bits indicate the type of constellation: zero for BPSK, one for QPSK, two for 16-QAM, and three for 64-QAM.

LENGTH

A 13-bit identifier for the number of bytes in the payload of the physical frame. It ranges from zero to 8,191.

LPI (Last PSDU indicator)

When multiple physical frames are sent in a burst, the LPI bit is set on the last one to notify other stations that the burst is coming to an end.

CRC

The CRC is calculated over all the fields except for the CRC and the tail bits.

Tail

Six bits are used as tail bits to ramp down the convolutional coder.

In the other transfer modes, shown in Figure 15-7, the preamble is split into chunks. In between the chunks, there may be Signal fields. SIGNAL-N fields are defined by the 802.11n proposal and are only decoded by 802.11n stations; the SIGNAL-MM field is used to retain backwards compatibility in a mixed mode with older OFDM stations. It is identical to the Signal field used by 802.11a, and is shown in Figure 13-16.

Figure 15-7. PLCP frame format for other transfer modes

 

WWiSE PMD

Figure 15-8 shows the basic layout of the WWiSE transmitter. It is essentially the same as the 802.11a transceiver, but it has multiple transmit chains. The interleaver is responsible for dividing coded bits among the different transmit chains and spatial streams.

Figure 15-8. WWiSE transceiver

Sensitivity is specified by the proposal, and it is identical to what is required of 802.11a receivers. Table 15-3 shows the required sensitivity. The proposal does not have any adjacent channel rejection requirements.

Table 15-3. WWiSE receiver sensitivity

Constellation	Rate	Sensitivity (dBm)	802.11a Sensitivity (dBm), for reference
BPSK	1/2	-82	-82
BPSK	3/4	-81	-81
QPSK	1/2	-79	-79
QPSK	3/4	-77	-77
16-QAM	1/2	-74	-74
16-QAM	3/4	-70	-70
64-QAM	2/3	-66	-66
64-QAM	3/4	-65	-65
64-QAM	5/6	-64	N/A

 

Characteristics of the WWiSE PHY

Parameters specific to the WWiSE PHY are listed in Table 15-4. Like the other physical layers, it also incorporates a number of parameters to adjust for the delay in various processing stages in the electronics.

Table 15-4. WWiSE MIMO PHY parameters

Parameter	Value	Notes
Maximum MAC frame length	8,191 bytes
Slot time	9 ms
SIFS time	16 ms	The SIFS is used to derive the value of the other interframe spaces (DIFS, PIFS, and EIFS).
RIFS time	2 ms
Contention window size	15 to 1,023 slots
Preamble duration	16 ms
PLCP header duration	4 ms
Receiver sensitivity	-64 to -82 dBm	Depends on speed of data transmission