Interference-Free Transmission for X channels - Interference Cancellation Using Space-Time Processing and Precoding Design - page 92

Multi−User, 2 bits/sec/Hz

10 0

Beamforming using 256QAM
Interference alignment using QPSK
Our Scheme II using QPSK
Our Scheme I using QPSK

10 −1

10 −2

10 −3

10 −4

10 −5

0 5 10 15 20 25 30 35 40

Signal to Noise Ratio (dB)

Fig. 5.9 Simulation results for 2 users each with 8 transmit antennas and 2 receivers each with 6
receive antennas

schemes. Figure 5.8 shows the results when there are 2 transmitters each with 5
transmit antennas and 2 receivers each with 3 antennas. In this case, our proposed
Scheme I will not work. Instead, we can use Scheme II. Figure 5.8 shows that our
proposed scheme still performs better in terms of diversity and coding gain.
Now we assume that there are 2 transmitters each with 8 transmit antennas and 2
receivers each with 6 antennas. In this case, we can use both Scheme I and Scheme II.
However, as shown in Fig. 5.9 , Scheme I has better performance than Scheme II. The
reason is that when the number of receive antennas is the same, Scheme II requires
more transmit antennas compared with Scheme I. Therefore, when the number of
transmit antennas is also the same, Scheme I will have more degrees of freedom and
thus have better coding gain. In addition, by Fig. 5.9 , we can see that both Scheme I
and Scheme II can provide interference-free transmission and full diversity.

5.8 Conclusions

In this chapter, we propose a precoding and decoding scheme for X channels to
achieve interference-free transmission for each codeword with full diversity and low
decoding complexity. To the best of our knowledge, this is the first scheme to achieve
full diversity and interference cancellation simultaneously when all the users transmit