Interference-Free Transmission for X channels - Interference Cancellation Using Space-Time Processing and Precoding Design - page 91

Multi−User, 2 bits/sec/Hz

10 0

Beamforming using 256QAM
Interference alignment using QPSK
Our scheme using QPSK

10 −1

10 −2

10 −3

10 −4

10 −5

10 −6

10 15 20 25 30 35 40

Signal to Noise Ratio (dB)

Fig. 5.8 Simulation results for 2 users each with 5 transmit antennas and 2 receivers each with 3
receive antennas

Scheme I will have more degrees of freedom to obtain better coding gain. Therefore,
when M J r , we will choose Scheme I and the required number of transmit

antennas in order to achieve interference-free transmission and full diversity for each
transmitter is N

≥ J t ·

≥ J t · ( 2 · J r − 1 ) .

5.7 Simulation Results

In this section, we provide simulation results to evaluate the performance of the
proposed scheme. First, we assume there are 2 transmitters each with 6 transmit
antennas and 2 receivers each with 4 antennas. Then we can use Scheme I to design
precoding and decoding scheme. Figure 5.7 presents simulation results using QPSK.
We compare the performance of our scheme with that of two other scenarios. In the
first scenario, we assume that at each time slot, only one transmitter sends signals to
one receiver using beamforming. 256-QAM is used to have the same bit-rate. In the
second scenario, at each time slot, each transmitter adopts the interference alignment
strategy that only guarantees all the interference at any receiver are aligned along
the same direction. The results show that our proposed scheme can achieve full
diversity. In comparison, simple interference alignment can only achieve diversity
one. Also our proposed scheme has better coding gain compared with the other two