In this series:

Parts 1 and 2 of our series on multi-user MIMO discussed its advantages. However, the link and system-level performance gains come at the expense of not knowing the channel state information at the transmitter, as well as extra overhead incurred by having the user equipment feeding back this information. Several works have addressed limited feedback

[1-2] but it is still a concern when implementing MU-MIMO concepts into existing standards like LTE [4].

MU-MIMO and carrier aggregation have been widely perceived as the primary means to dramatically improve mobile broadband services and to support wider transmission bandwidths. MU-MIMO is already supported in LTE Release 8 via transmission mode 5 (TM5) (wherein eNB employs two antennas). In theory MU-MIMO provides throughput gains that scale linearly with the number of antennas at the eNB [2]. However, several problems such as the residual multi-user interference seen at the user equipment (UE) need to be addressed [3] in the case of TM5.

Compared to single-user MIMO, the spatial separation between users is an advantage as the channels of these users tend to be less correlated. However, given the limited codebook size, the inter-user interference experienced by the UEs cannot be jointly processed at the receive side. Thus, no efficient interference mitigation is possible [3-4]. Without adequate interference mitigation, the benefits of MU-MIMO cannot be achieved. As we described in previous blog posts, interference management techniques like zero-forcing precoding [2] can be carried out by the eNB if the downlink channel estimates are available. Unfortunately, obtaining the required channel state information estimates at the eNB require a substantial amount of feedback bandwidth in the uplink. This led LTE Release 8 to adopt a simple codebook-based precoding scheme in TM5. While it can reduce the inter-user interference, there will still be some amount of interference experienced by the users. Therefore, to achieve the MU-MIMO gains in LTE systems, the UEs need to implement efficient interference cancelation techniques as proposed in [3] and investigated in the Spectrum Aggregation and Multi-user MIMO: Real-World Impact (SAMURAI) project [4].

The SAMURAI project focuses on the practical aspects of carrier aggregation and MU-MIMO techniques in LTE and LTE-Advanced systems. One outcome from this project is related to system level gain. It has been shown that a gain of up to 20% in cell throughput is expected when using an interference-aware receiver [3], at the expense of some loss in cell edge throughput performance. This has also been reported in Figure 11.8 in [5] as the trade-off between throughput and coverage when using beamforming.

An example on what one would expect from the use of an interference aware receiver is depicted below. In this case, a downlink fast fading channel with the dual-antenna eNodeB and two single-antenna UEs with a 3GPP LTE rate 1/3 turbo code is used with different puncturing patterns. In Figure 1, "IA Rx" and "SU Rx" represent the respective low-complexity interference-aware receiver and the single-user receiver [3]. In this test scenario, the sum rates are fixed (i.e. if two users are served with QPSK with rate 1/2 in the multi-user mode, then one user is served with QAM16 with rate 1/2 in the single-user mode, thereby equating the sum rate in both cases to 2 bps/Hz) [3].

MU MIMO performance in fading channel with the dual-antenna eNodeB and two single-antenna UEs

Figure 1. MU MIMO performance in fading channel with the dual-antenna eNodeB and two single-antenna UEs [3].

For more details, we refer the reader to [3-4].

References

[1] M. Ahmed Ouameur "MU-MIMO part 1"
[2] M. Ahmed Ouameur "MU-MIMO part 2"
[3] R Ghaffar and R. Knopp, " Interference-aware receiver structure for multiuser MIMO and LTE," EURASIP Journal on Wireless Communications and Networking 2011, 2011:40.
[4] B. BADIC, A. F. CATTONI, M. DIEUDONNE, J. DUPLICY, P. FAZEKAS, F. KALTENBERGER, I. Z. KOVÁCS, G. VIVIER, " Advances in Carrier Aggregation and Multi- User MIMO for LTE-Advanced: Outcomes from SAMURAI project," xxx.
[5] S. Sisia, I. Toufik and M. Baker, "LTE – The UMTS Long Term Evolution: From Theory to Practice," August 29, 2nd edition.