**5. Performance analysis**

12 Recent Trends in Multiuser MIMO Communications

2

4

6

bps/Hz

quadrants.

8

10

12

0 10 20 30 40 50

QAM64

No Scheduling −SU Rx LTE Precoders − SU Rx LTE Precoders − IA Rx MF EGT Precoders − IA Rx MF Precoders − IA Rx

SNR

**Figure 3.** Sum rates of different transmission schemes for the downlink channel with dual-antenna eNodeB and 2 single-antenna UEs. 'No Scheduling - SU Rx' indicates the case once the eNodeB uses the LTE precoders without employing the geometric scheduling strategy. In all other cases, the eNodeB employs the geometric scheduling strategy along with the LTE precoders, MF EGT precoders and MF precoders. SU Rx indicates the cases when UEs employ single-user detection while IA Rx indicates the cases when UEs resort to the intelligent detection by employing the low-complexity interference-aware receivers.

The geometric scheduling algorithm ensures that the eNodeB chooses the second UE to be

Fig. 3 shows the sum rates of a broadcast channel with the dual-antenna eNodeB and 2

are the unquantized precoders given in (20) and (21) respectively while LTE precoders are the quantized precoders given in (1). The sum rates of unquantized precoders along with those of LTE quantized precoders are shown for the case of single-user receivers and for the case of low-complexity interference-aware receivers. The results show that under the proposed transmission strategy, the sum rate can be significantly improved (unbounded in SNR) if the low-complexity interference-aware receivers are used as compared to the case when the UEs resort to sub-optimal single-user detection where rates are bounded (in SNR). The behavior of single-user detection is attributed to the fact that this detection strategy considers interference as noise so the SINR is low once no geometric scheduling has been employed by the eNodeB while the SINR improves due to the reduction of interference once geometric scheduling is employed. However the rates remain bounded in the SNR if the UEs resort to the single-user detection which is due to the fact that increasing the SNR (transmit SNR) also increases the interference strength thereby bounding the SINR at high values of the transmit SNR. On the other hand, there is significant improvement in the sum rate once UEs resort to intelligent detection by employing the low-complexity interference-aware receivers. In this case, the sum rate is unbounded if the rate (constellation size) of each UE is adapted with the SNR. Note that the quantized CSIT (LTE precoders) appears to have no effect at high SNR once UEs resort to intelligent interference-aware detection. This behavior is because the

single-antenna UEs for QAM64 alphabets. SNR is the transmit SNR, i.e. *<sup>σ</sup>*<sup>2</sup>

<sup>1</sup> and **<sup>h</sup>**†

<sup>1</sup> <sup>=</sup> *<sup>σ</sup>*<sup>2</sup>

<sup>2</sup> lie in the opposite

<sup>1</sup> **<sup>p</sup>**<sup>1</sup><sup>2</sup> +*σ*<sup>2</sup> <sup>2</sup> **<sup>p</sup>**<sup>2</sup><sup>2</sup> *N*0

<sup>2</sup> . MF and MF EGT precoders

served on the same RE as the first UE such that their channels **h**†

whereas the two UEs have equal power distribution, i.e. *σ*<sup>2</sup>

We now focus on the EGT characteristic of the LTE precoders and carry out the performance analysis of the EGT in single-user and multi-user MIMO systems. We restrict to the case of single-antenna UEs while the eNodeB has two antennas.
