Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN---6

Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN
DIVERSITY OPTIONS
OFDMA PHY supports AAS and also a set of second-, third-, and fourth-order transmit diversity options.
With the AAS option, the system uses a multiple-antenna transmission to improve the coverage and capacity of the system while minimizing the probability of outage through transmit diversity, beam forming, and null steering.
Transmit diversity options consist of a comprehensive set of methods based on second- or fourth-order diversity in DL and second-order diversity in UL that can be flexibly chosen to tradeoff capacity and coverage. The set includes both closed- and open-loop options and also supports Spatial Multiplexing (SM) for maximum spectral efficiency.
Advanced Antenna Systems
Two optional AAS modes are supported in OFDMA PHY: Diversity-Map Scan and Direct Signaling Method. Diversity-Map Scan supports both diversity (FUSC and PUSC) and adjacent (AMC) subcarrier permutation options. The Direct Signaling Method supports adjacent subcarrier permutation with less overhead in control signaling.
We now discuss the Diversity-Map Scan option when applied to the AMC subcarrier allocation method.
Figure 7 shows the AAS Diversity Map Zone within a frame. The DL subframe includes a non-AAS section and an AAS section specified by information elements provided in the DL MAP.
Within the AAS zone, subchannel numbers 4 and N-4 (N is the index for the last logical subchannel) are allocated to the AAS DL MAP where AAS MAP allocations are specified for AAS users. The figure illustrates a four-antenna configuration where the AAS preamble and AAS DL MAPs structure are repeated four times to support the corresponding four groups of users.

Figure 7: AAS diversity MAP zone
click image for larger view
Within the AAS zone, the AAS BS specifies allocations to be used for SS Ranging. In TDD mode, the BS can extract the channel information required for beam forming from the Ranging Request messages received from the SS’s. In FDD mode, beam forming is done through the AAS Feedback Request and Response messages where channel response information along with mean Received Signal Strength Indicator (RSSI) and Carrier to Interference plus Noise Ratio (CINR) are reported back to the BS by the SS.
Transmit Diversity
OFDMA mode supports second-, third- and fourth-order transmit diversity options in DL and second-order transmit diversity in UL. All diversity options are applicable to both diversity and adjacent subcarrier permutations.
Space Time Coding (STC) based on Alamouti algorithm [19] and Frequency Hopping Diversity Code (FHDC) are two options for second-order diversity in DL. Although not specified by the standard, the number of receive antennas can be specified depending on the performance required.
Second-Order STC
Second-order STC in DL supports coding rates of 1 and 2 using the following two transmission format matrices.
Equation (4)
Equation (5)
Here Sk’s are OFDM symbols in the frequency domain right before IFFT operation.
The optional STC transmit diversity is also supported in UL using the transmission format matrix A of Equation (4). Matrix B of Equation (5) can be used by two SS’s in a collaborative special multiplexing mode.
Fourth-Order STC
The fourth-order transmit diversity in DL supports rates 1, 2, or 4 using the following transmission format matrices A, B, and C, respectively.
Equation (6)
Equation (7)
Equation (8)
Here, Sk’s are OFDM symbols in the frequency domain right before the IFFT operation.
Third-Order STC
The third-order transmit diversity in DL supports rates 1, 2, or 3 using the following transmission format matrices A, B, and C, respectively.
Equation (9)
Equation (10)
Equation (11)
In Equations (9) and (10), we have
Equation (12)
where θ = (tan-1 2)/2, Sκ = SκΙ + j • Sκφ, Sκ = Xκ • ejθ for k=1,2,…,8 and Xk‘s are OFDM symbols in the frequency domain right before the IFFT operation.
Precoding
A general KxL precoding matrix W is specified to be applied to the output x of any second-, third- or fourth-order diversity option mentioned earlier. This way an L th order output vector Z of the STC block is transformed into a final k th order vector for transmission on antennas.
Equation (13)
Precoding can be performed either in closed-loop or open-loop form. In the case of open-loop, the BS weights the transmission according to the channel measurement performed on the UL signal, where a reciprocity assumption can be made for a TDD mode, for example. In the case of closed-loop, BS uses the Channel Quality Indications feedback from the SS.