  We compare four open-loop transmit diversity schemes in a coded Orthogonal
Frequency Division Multiplexing (OFDM) system with four transmit antennas,
namely cyclic delay diversity (CDD), Space-Time Block Code (STBC, Alamouti code
is used) with CDD, Quasi-Orthogonal STBC (QO-STBC) and
Minimum-Decoding-Complexity QOSTBC (MDC-QOSTBC). We show that in a coded system
with low code rate, a scheme with spatial transmit diversity of second order
can achieve similar performance to that with spatial transmit diversity of
fourth order due to the additional diversity provided by the phase shift
diversity with channel coding. In addition, we also compare the decoding
complexity and other features of the above four mentioned schemes, such as the
requirement for the training signals, hybrid automatic retransmission request
(HARQ), etc. The discussions in this paper can be readily applied to future
wireless communication systems, such as mobile systems beyond 3G, IEEE 802.11
wireless LAN, or IEEE 802.16 WiMAX, that employ more than two transmit antennas
and OFDM.
