Transmit Diversity refers to the techniques used for transmission in the forward link that help in implementing the multicarrier approach of cdma2000. Transmit diversity can reduce the required Eb/N0 (or required transmit power per Channel) and thus enhance the system capacity.
1. Multicarrier approach
i. Coded information symbols are demultiplexed among multiple 1.25 MHz carriers: NX 1.25MHz where N =1, 3, 6, 9, 12
ii. Frequency diversity is equivalent to spreading the signal over the entire bandwidth.
iii. Both time and frequency diversity are captured by convolutional coder/ symbol repetition and interleaver.
iv. The rake receiver captures signal energy from all bands.
v. Each forward link Channel may be allocated an identical Walsh code on all carriers.
vi. Fast power control is used.
Fig11. Multicarrier approach in cdma2000
Transmit Diversity can be implemented in the following ways:
2. Multicarrier Transmit Diversity
i. Antenna diversity can be implemented in a multicarrier forward link with no impact on the subscriber terminal, where a subset of carriers is transmitted on each antenna.
ii. In a 3X 1.25 MHz multicarrier transmitter, the serial coded information symbols are divided into three parallel data streams, and each data stream is spread with a Walsh code and a long pseudo-random noise (PN) sequence at a rate of 1.2288 Mcps. At the output of the transmitter, there are three carriers: A, B, and C. Similarly for N=6, 9, 12.
iii. After processing the serial coded information symbols with parallel carriers, the multicarrier will be transmitted by a multi-antenna, called multi-carrier transmit diversity (MCTD).
iv. In the MCTD, the total carriers are divided into subsets, and then each subset of the carriers is transmitted on each antenna, where frequency filtering provides near-perfect orthogonality between antennas. This provides improved frequency diversity and hence increases forward link capacity.
Fig12. 3X 1.25MHz multicarrier transmitter
3. Direct-Spread Transmit Diversity
i. Orthogonal transmit diversity (OTD) may be used to provide transmit diversity for direct spread (N =1).
ii. Coded bits are split into two data streams and are transmitted via separate antennas.
iii. A different orthogonal code is used per antenna for spreading. This maintains the orthogonality between the two output streams, and hence self interference is eliminated in flat fading.
iv. By splitting the coded bits into two separate streams, the effective number of spreading codes per user is the same as the case without OTD.
v. To increase the number of Walsh codes, two information bits are mapped to a single QPSK symbol; instead of BPSK in which one bit represents one symbol. This increases the length of Walsh codes by a factor of 2.
vi. An auxiliary pilot is introduced for the additional antenna.
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