Performance Analysis of VBLAST-Enhanced 802.11a PHY

The main idea of the BLAST architecture is to split the information bit stream into several sub-streams of equal length (called layers) and transmit them in parallel using a set of transmit antennas (the number of transmit antennas equals the number of sub-streams) at the same time and frequency. VBLAST (vertical BLAST) is known as a simplified version of the BLAST algorithm [31]. It is capable of achieving high spectral efficiency while being relatively simple to implement [32]. The layers are vertically arranged and each of them is transmitted over one particular antenna as shown in Figure 4.5. We use the same MIMO-OFDM system as in Section 4.1.2. The received signal vector in subband k can be written as

(4.17)

superposition of all transmitted symbols scaled by the channel gain and corrupted by AWGN. Assume the receiver uses a ML detection criterion based on perfect channel knowledge. The estimated symbol vector in the subband k is

2

where the minimization is performed over all admissible vectors S_k. An error occurs when the receiver mistakes a transmitted vector for another vector from the set of possible vectors. The probability that the receiver mistakes the transmitted vector for another vector

i

Sk j

S , given knowledge of the channel realization at the receiver (also k

referred to as the pairwise error probability), is [33][34]

( ) ( )

is the squared minimum distance of the separation of the vector constellation points at the receiver. Using the Rayleigh-Rits criterion, we can bound by squared minimum distance of the transmit scalar constellation. Therefore, we can get the upper bound of the instantaneous SER for VBLAST transmission

2

With a Gray coding, BER can be approximated by

,

1

b VBLAST s VBLAST

P P

≈ N ⋅ _, (4.22)

where N is the bit numbers modulated into one symbol. Figure 4.6 shows the performance of the QPSK modulated VBLAST system compared with the derived upper bound over the flat Rayleigh fading channel

matrix . The result confirms the

performance analysis in this section. From (4.22), we can obtain the instantaneous error performance of different PHY modes with the VBLAST-enhanced 802.11a system using the formulas in Section 3.3. Figure 4.7 illustrates the obtained results with the same channel matrix as before.

0.0079513 0.69987 0.56974 0.20471

4.2 Link Adaptation Scheme

In (4.16), the instantaneous received SNR of the STBC-enhanced 802.11a system is governed by the squared Frobenius norm of the channel matrix H_k, that is to say the

squared Frobenius norm of channel matrix H_k determines the performance of the STBC-enhanced 802.11a system. On the other hand, (4.21) shows that a larger value of

guarantees a smaller error probability of the VBLAST-enhanced 802.11a system. The performance of the VBLAST-enhanced 802.11a is strongly linked to the smallest singular value of the channel matrix H

,min

λk

k. Therefore, the complete channel information is required to evaluate the performances for both the STBC and VBALST systems for selecting these two schemes. We adopt the RTS/CTS exchange to obtain the channel information and perform LA. The receiver estimates the channel

information while receiving the RTS frame, then it uses this information to computes the effective goodput defined in Chapter 3, the metric for selecting the appropriate transmission scheme, and feeds the decision back to the transmitter via the CTS frame.

For this purpose, we modified the algorithm proposed by [8]. In [8], instead of carrying the duration of the reservation, the frames carry the data rate and data frame length. We add into one more parameter, the MIMO scheme. This modification serves the dual purpose of providing a mechanism by which the receiver can communicate the chosen transmission strategy to the transmitter, while still providing neighboring nodes with enough information to calculate the duration of the requested reservation.

The detailed algorithm is as follows.

At the first, the transmitter chooses the transmission strategy as the lowest rate PHY mode, i.e. STBC with the BPSK modulation and rate-1/2 convolutional coding, and then stores the corresponding parameters into the RTS frame. This ensures the reservation period is the most robust. The neighboring nodes hearing the RTS frame calculate the duration of the requested reservation, D_rts, using the information carried in the RTS frame and update its NAV. The receiver uses the RTS frame to estimate the channel information and selects the appropriate transmission strategy based on this estimation. Then, it transmits the CTS frame with the selected MIMO scheme, data rate, and data frame length back to the transmitter. The neighboring nodes hearing the CTS frame calculate the duration of the requested reservation, D_cts, using the information carried in the CTS frame and update its NAV to account for the difference between D_rts and D_cts. Finally, the transmitter responds to the receipt of the CTS by transmitting the data frame with the transmission strategy chosen by the receiver.

In the instance that the transmission strategies chosen by the transmitter and

receiver are different, the reservation duration, D_rts, calculated by the information

carried in the RTS frame is no longer valid. Thus, we refer to D_rts as a tentative reservation. A tentative reservation serves only to inform neighboring nodes that a reservation has been requested but the duration of the final reservation may differ. The tentative reservation effectively serves as a placeholder, denying any later requests that would conflict with it, until either a new reservation is received (D_cts) or it is confirmed as the final reservation. Final reservations are confirmed by the presence of a special subheader, called the reservation subheader (RSH), in the MAC header of the data frame. RSH consists of a subset of the header fields that are already present in the IEEE 802.11 data frame, plus a check sequence that serves to protect the subheader.

The fields in RSH consist of those needed to update the NAV, and essentially preserve the same fields present in an RTS frame. Furthermore, the fields (minus the check sequence) still retain the same functionality that they have in a standard IEEE 802.11 header. The functionality of RSH is as follows: when transmitter sends the data frame with the special MAC header containing RSH. The nodes out of the CTS transmission range can use the information carried in RSH to calculate the final reservation, D_rsh,

to update the NAV to account for the difference between D_rts and D_rsh.

Note that, for the neighboring nodes to update their NAV correctly, they must know what contribution D_rts has made to their NAV. This can be done by maintaining a list of the end times of each tentative reservation, indexed according to the (transmitter, receiver) pair. Thus, when an update is required, a node can use the list to determine if the difference in the reservation will require a change in the NAV.