**1. Introduction**

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The spatial dimension surfacing from the usage of multiple antennas promises improved reliability, higher spectral efficiency [24], and the spatial separation of users [6]. This spatial dimension (MIMO) is particularly beneficial for precoding in the downlink of multi-user cellular systems (broadcast channel), where these spatial degrees of freedom at the transmitter can be used to transmit data to multiple users simultaneously. This is achieved by creating independent parallel channels to the users (canceling multi-user interference) and the users subsequently employ simplified single-user receiver structures. However, the transformation of cross-coupled channels into parallel non-interacting channels necessitates perfect channel state information at the transmitter (CSIT) whose acquisition in a practical system, in particular frequency division duplex (FDD) system, is far from realizable. The complexity associated with the feedback overhead coupled with the low rate feedback channels are the major impediments in CSIT acquisition. This leads to the precoding strategies based on the partial or quantized CSIT [15], which limit the gains of multi-user MIMO.

On the design of feedback, there is stark contrast between the theoretically established results and current standards. Theory has established that the amount of CSIT feedback in a downlink system needs to grow in proportion to the SNR [11] and otherwise the degrees of freedom are lost. However to avoid the burden of feedback and due to complexity constraints, the modern wireless systems have been restricted to fixed rate feedback schemes. With such premises, LTE and LTE-Advanced have focused on the structured precoder codebook based approach [17, 19] by using a small number of feedback bits. These LTE precoders are characterized by low-resolution and are further based on the principle of equal gain transmission (EGT). These precoders when employed for the multi-user MIMO mode of transmission are unable to cancel the multi-user interference thereby increasing the sub-optimality of conventional single-user detection. This fixed low-level quantization of LTE codebook, therefore, eclipses most of the benefits of multi-user MIMO and raises questions

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©2012 Ghaffar, Knopp, Kaltenberger, licensee InTech. This is an open access chapter distributed under the

about the feasibility of this mode of transmission [22, page 244]. This strong perception is based on the fact that users can not cooperate in multi-user scenario and further on the assumption that users employ simple single-user receivers.

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to the rank of the channel matrix (maximum 4). In this chapter, we restrict ourselves to the baseline configuration with the eNodeB (LTE notation for the base station) equipped with 2 antennas while we consider single and dual antenna user equipments (UEs). Physical layer technology employed for the downlink in LTE is OFDMA combined with bit interleaved coded modulation (BICM) [4]. Several different transmission bandwidths are possible, ranging from 1.08 MHz to 19.8 MHz with the constraint of being a multiple of 180 kHz. Resource Blocks (RBs) are defined as groups of 12 consecutive resource elements (REs - LTE notation for the subcarriers) with a bandwidth of 180 kHz thereby leading to the constant RE spacing of 15 kHz. Approximately 4 RBs form a subband and the feedback is generally done on subband basis. Seven operation modes are specified in the downlink of LTE, however, we

Multi-user MIMO in LTE and LTE-Advanced - Receiver Structure and Precoding Design

• **Transmission mode 2.** Fall-back transmit diversity. Transmission rank is 1, i.e. one codeword is transmitted by the eNodeB. Employs Alamouti space-time or

• **Transmission mode 4.** Closed-loop spatial multiplexing. Transmission rank is 2, i.e. two codewords are transmitted by the eNodeB to the UE in the single-user MIMO mode. UEs

• **Transmission mode 5.** Multi-user MIMO mode. Supports only rank-1 transmission, i.e.

• **Transmission mode 6.** Closed-loop precoding for rank-1 transmission, i.e. one codeword

In the case of transmit diversity and closed-loop precoding, one codeword (data stream) is transmitted to each UE using Alamouti code in the former case and LTE precoders in the latter case. Time-frequency resources are orthogonal to the different UEs in these modes thereby avoiding interference in the system. However, in the multi-user MIMO mode, parallel codewords are transmitted simultaneously, one for each UE, sharing the same time-frequency resources. Note that LTE restricts the transmission of one codeword to each UE in the

For closed-loop transmission modes (mode 4, 5 and 6), precoding mechanisms are employed at the transmit side with the objective of maximizing throughput. The precoding is selected and applied by the eNodeB to the data transmission to a target UE based on the channel feedback received from that UE. This feedback includes a precoding matrix indicator (PMI), a channel rank indicator (RI) and a channel quality indicator (CQI). PMI is an index in the codebook for the preferred precoder to be used by the eNodeB. The granularity for the computation and signaling of the precoding index can range from a couple of RBs to the full bandwidth. For transmission mode 5, the eNodeB selects the precoding matrix to induce high orthogonality between the codewords so that the interference between UEs is minimized. In transmission modes 4 and 6, the eNodeB selects the precoding vector/matrix such that codewords are transmitted to the corresponding UEs with maximum throughput. In order to avoid excessive downlink signaling, transmission mode for each UE is configured semi-statically via higher layer signaling, i.e. it is not allowed for a UE to be scheduled in one subframe in the multi-user MIMO mode and in the next subframe in the single-user MIMO mode. For transmission modes 4, 5 and 6, low-resolution precoders are employed which are based on the principle of EGT. For the case of eNodeB with two antennas, LTE

shall focus on the following four modes:

need to have minimum of two antennas.

for the UE in the single-user MIMO mode.

space-frequency codes [1].

one codeword for each UE.

multi-user MIMO mode.

In this chapter, we focus on a new paradigm of multi-user MIMO where users exploit the discrete structure of interference, instead of ignoring it or assuming it to be Gaussian and merging it in noise. We compare the two strategies of interference exploitation and interference cancellation in multi-user scenario. For the former, we look at low complexity multi-user detectors. Though multi-user detection has been extensively investigated in the literature for the uplink (multiple access channel), its related complexity has so far prohibited its employment in the downlink (broadcast channel). For the multiple access channel, several multi-user detection techniques exist in the literature starting from the optimal multi-user receivers [25] to their near-optimal reduced complexity counterparts (sphere decoders [3]). The complexity associated with these techniques led to the investigation of low-complexity solutions as sub-optimal linear multi-user receivers [20], iterative multi-user receivers [26, 28], and decision-feedback receivers [5, 12]. Since in practice, most wireless systems employ error control coding combined with the interleaving , recent work in this area has addressed multi-user detection for coded systems based on soft decisions [13, 23]. We focus in this chapter on a low-complexity interference-aware receiver structure which not only reduces one complex dimension of the system but is also characterized by exploiting the interference structure in the detection process. Considering this receiver structure, we investigate the effectiveness of the low-resolution LTE precoders for the multi-user MIMO mode and show that multi-user MIMO can bring significant gains in future wireless systems if the users resort to intelligent interference-aware detection as compared to the sub-optimal single-user detection.

In an effort of bridging the gap between the theoretical and practical gains of multi-user MIMO, this chapter investigates the structure of LTE codebook by analyzing the pairwise error probability (PEP) expressions. The analysis shows that LTE precoders suffer from the loss of diversity when being employed in multi-user MIMO transmission mode but no such loss is observed in single-user MIMO mode. Based on this analysis, a new codebook design is proposed and it is shown that with a nominal increase in the feedback, the performance of multi-user MIMO improves to within 1.5 dB from the lower bound (single-user MIMO). To verify the proposed codebook design, widely studied Gaussian random codebooks [11], [2] are considered for comparison. Note that though the overall discussion in this chapter has generally been on LTE and LTE-Advanced framework, the proposed feedback and precoding design can serve as a guideline for multi-user MIMO modes in any other modern wireless system which employs limited feedback schemes for CSIT acquisition.
