The use of offset QPSK in a MIMO channel with space-time coding has been demonstrated using OQPSK with the 2 x 1 Alamouti space-time code. It was shown that the desirable orthogonalization properties of the Alamouti space-time code with QPSK do not carry over to the offset case. Unshaped OQPSK and shaped OQPSK were considered. Shaped OQPSK (defined as a CPM) was recast in terms of a memoryless waveform mapper to examine it in the same context as unshaped OQPSK. The basic detector structures for both unshaped and shaped OQPSK are essentially the same, as illustrated in Figure 3. Each detector samples a detection filter output at the bit rate then uses a trellis to account for the memory created by the offset nature of the modulated waveform. The case of differential delays using Alamouti encoded SOQPSK was also explored. The basic detector structure was modified as summarized in Figure 3 (c). The effect on bit error rate performance due to channel gain phase difference, differential delay, timing error, and phase error are explored via simulation and summarized in Figures 12, 14-17. In conclusion, we demonstrated that OQPSK can perform quite well in a space-time coded MIMO channel, although the complexity is significantly higher than when non-offset QPSK is used.

Inspec keywords: orthogonal codes; MIMO communication; error statistics; space-time codes; quadrature phase shift keying

Other keywords: bit error rate performance; channel gain phase difference; phase error; unshaped OQPSK; orthogonalization properties; timing error; MIMO communications; detection filter output; Alamouti space-time code; BER; differential delays; shaped OQPSK; detector structure; offset modulations; non-offset QPSK

Subjects: Radio links and equipment; Other topics in statistics; Codes