Interference Cancellation Using Space-Time Processing and Precoding Design by Feng Li

Author:Feng Li
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

(3.62)

where

(3.63)

(3.64)

Using the techniques in Sect. 3.3, we show

(3.65)

where

(3.66)

It is easy to see that full diversity for User 1 is achieved. Similarly, we can prove that we can achieve full diversity for other users.

3.5 Simulation Results

In this section, we provide simulation results that confirm our analysis in the previous sections. The performance of our proposed scheme is shown in Figs. 3.3, 3.4, 3.5, 3.6 and 3.7. In Fig. 3.3, we consider 4 users each equipped with 4 transmit antennas and a receiver with 4 receive antennas. We compare our scheme using QPSK and Equations (3.28), (3.29) with the scheme proposed in [4] for the same configuration without channel information at the transmitter. With 4 receive antennas, the multi-user detection (MUD) method offered in [4] cancels the interference and provides a diversity of 4. Our scheme also cancels the interference completely but provides a diversity of 16 by utilizing the channel information at the transmitter. We also compare our scheme with two other schemes that can realize interference cancellation and full diversity as well. In both of these two schemes, we assume there are 4 users each with 4 transmit antennas and one receiver with 4 receive antennas. The first scheme is to let only one user transmit using single beamforming at each time slot [5]. In order to have the same bit-rate, 256-QAM is used. The second scheme is to let the first 2 users transmit using the scheme for 2 users in [2] at the first 4 time slots and the second 2 users transmit at the second 4 time slots. Similarly, in order to have the same bit-rate, 16-QAM is used. The results, in Fig. 3.3, show that our scheme outperforms both of these two schemes, by 13 dB and 8 dB, respectively. Finally, we compare our scheme with the following TDMA multiple beamforming scheme. At each time slot, only one user transmits using multiple beamforming, i.e., sends 4 symbols along the 4 eigenvectors of the channel matrix [5]. QPSK is used to match the rate. From the simulation, our scheme outperforms this scheme whose diversity is only 1.

In addition, in Fig. 3.4, we compare our scheme with all mentioned schemes when the rate is changed to adapt with the received SNR. In the simulation, the single beamforming scheme switches between 16-QAM and 256-QAM. The multiple beamforming scheme, the MUD scheme in [4] and our scheme all switch between BPSK and QPSK. The interference cancellation scheme in [2] switches between QPSK and 16-QAM. The threshold to switch between the two rates is properly chosen such that the two constellations are used with the same probability in each scheme. So the rate of all these schemes is 1.5 bits/sec/Hz. We have also provided a “fixed rate" set of simulation results. In all cases, for 1.5 bits/sec/Hz, what we mean by “fixed rate" is the average between the performance of two fixed-rate systems using BPSK and QPSK. From Fig. 3.4, we can see that adapting the rate can improve the performance compared with using a fixed rate.

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