**5. Communication schemes for MU-MIMO systems**

Communication schemes for MU-MIMO systems include both uplink MU-MIMO (UL-MU-MIMO) and downlink MU-MIMO (DL-MU-MIMO). Case of uplink communication, users transmit signals to the base station. However, in the case of downlink communication, base station transmits signals to users. A representation of these systems is depicted in Figure 6. We assume that the base station is equipped with *N* antennas. Case of DL-MU-MIMO, the base station attempts to transmit signals to *K* users *U*1,. . . ,*UK* which are respectively equipped with antennas of numbers *M*1,..., *MK*.

For notations, if antenna *k* acts like a receiving antenna, it is denoted by *Rxk*. Otherwise, it is denoted by *Txk*.

**Figure 6.** MU-MIMO communication models: UL-MU-MIMO and DL-MU-MIMO

#### **5.1. UL-MU-MIMO**

8 Recent Trends in Multiuser MIMO Communications

*TxN*

. . .

BS

**Figure 5.** MU-MIMO configurations

MU-MIMO provides better performances.

**Table 1.** Comparison between SU-MIMO and MU-MIMO systems

*Rx*<sup>1</sup>

*Tx*<sup>1</sup>

*BS*<sup>1</sup>

*BS*<sup>2</sup>

*TxN*

. . .

*Tx*<sup>1</sup>

*TxN*

*U*<sup>1</sup>

✿✸

✒

③✿

✸

. . . *Rx*<sup>1</sup>

*U*<sup>1</sup>

✿✸

✒

✣✻

③✿

✸

✒

*RxM*<sup>1</sup>

. . .

*Rx*<sup>1</sup>

*UK*

❘

✲

✯

✇ �⑦

�

✲

(b) MU-MIMO

*Rx*<sup>1</sup>

*RxM*<sup>1</sup>

. . .

*Rx*<sup>1</sup>

*UK*

❘

�✣✻ ✒

�

✲

✯

✇

⑦

✲

. . .

*RxMK*

. . . . . .

*RxMK*

. . .

*U*

✿✸

③✿

. . .

*Tx*<sup>1</sup>

*BS*<sup>1</sup>

❃

*BS*<sup>2</sup>

❘

**Feature MU-MIMO SU-MIMO**

CSI Perfect CSI is required No CSI

**5. Communication schemes for MU-MIMO systems**

Advantage Multiplexing gain No interference

*TxN*

. . .

*Tx*<sup>1</sup>

*TxN*

(c) MU-MIMO with cooperation

order to achieve high throughput and to improve the multiplexing gain [16]. Finally, the performances of MU-MIMO and SU-MIMO systems in terms of throughput depend on the SNR level. In fact, at low SNRs, SU-MIMO performs better. However, at high SNRs level,

Main aspect BS communicates with multiple users BS communicates with a single user Purpose MIMO capacity gain Data rate increasing for single user

Communication schemes for MU-MIMO systems include both uplink MU-MIMO (UL-MU-MIMO) and downlink MU-MIMO (DL-MU-MIMO). Case of uplink communication,

Throughput Higher throughput at high SNR Higher throughput at low SNR

. . .

*Tx*<sup>1</sup> *RxM*

(a) SU-MIMO

between BSs Cooperation

> Let *Xk*(*Mk* × <sup>1</sup>), the transmit signal vector of user *Uk*; *<sup>k</sup>* = 1, . . . , *<sup>K</sup>*. We assume that data streams associated to user *Uk*; *k* = 1, . . . , *K* are zero mean white random vectors where :

$$\mathbb{E}\{X\_k X\_k^\*\} = I\_{M\_k};\ k = 1, \dots, K \tag{7}$$

E denotes the expected value operator.

The complex channel matrix relating user *Uk*; *k* = 1, . . . , *K* to the base station, *Hk* is of dimension (*<sup>N</sup>* × *Mk*). In presence of additive noise signal *<sup>b</sup>*(*<sup>N</sup>* × <sup>1</sup>), the received signal vector at the base station, *y*(*N* × 1) is expressed in the slow fading model by:

$$y = \sum\_{k=1}^{K} H\_k \cdot X\_k + b \tag{8}$$

The noise signal vector is a zero mean white Gaussian variable with variance *σ*<sup>2</sup> *<sup>b</sup>* . The uplink scenario should satisfy two constraints:

• It should be as many receive antennas at the base station as the total number of users antennas.

• Each user should have as many transmit antennas as the number of data streams.

In Figure 7, the block diagram for the UL-MU-MIMO includes a joint linear precoder and decoder. Linear precoders associated to users *U*1,..., *UK* will be respectively denoted by

**Figure 7.** Block diagram for the UL-MU-MIMO with coding techniques: *N* antenna BS and *K* multiple antenna users

*F*1,..., *FK*. The received signal vector at the BS is then expressed as :

$$y = \sum\_{k=1}^{K} H\_k \cdot F\_k \cdot X\_k + b \tag{9}$$

10.5772/57133

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http://dx.doi.org/10.5772/57133

*Bk*; *<sup>k</sup>* = 1, ..., *<sup>K</sup>* is an additive noise signal vector of size (*Mk* × <sup>1</sup>). Equation (11) could be

The second term of the sum in equation (13) represents the interference signal coming from multiple users. Processing techniques such as beamforming should be introduced in the block diagram of the MU-MIMO system for mitigating the effect of users interference and

MU-MIMO technology finds its applications in many areas and is nowadays exploited in

**3GPP LTE:** 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) is one of the next generation cellular networks which exploit the MU-MIMO technology. Thanks to this technology, available radio spectrum 3GPP LTE networks could achieve higher

**Release 8 of LTE:** The first release of LTE (Release 8) was commercially deployed in 2009. Release 8 has introduced SU-MIMO scheme in the communication system model. This release only uses one transmission mode (Transmission mode 5) which has been defined for MU-MIMO systems. Transmission mode 5 supports rank 1 transmission for two User Equipments (UEs). In order to achieve the performances of MU-MIMO systems, feedback parameters such as the channel Rank Indicator (RI) and the Channel Quality Indicator

**Release 9 of LTE:** The second release of LTE (Release 9) provides enhancements to Release 8. The LTE Release 9 supports transmission mode 8 and includes both SU-MIMO and

**LTE advanced:** Other progress in LTE MIMO systems have been obtained through LTE advanced. The performed mode is the transmission mode 9. This mode allows for a

**Multiple-cell networks:** MU-MIMO systems have received wide spread success in wireless networks. Examples of applications include the multiple-cell networks [21] with multiple access channels where possible coordination among base stations is established. Figure 8 shows a MU-MIMO coordinated network in a cellular network. Three classes of cells are

The coordination between cells is performed by the Central Station (CS). The aim of this coordination is to mitigate the effect of inter-cells interference. Coding techniques should

= *Hk* · *Xk* +

improving the performances of the communication system.

many evolving technologies wich are described in the following.

spectral efficiencies than existing 3G networks [18][19].

possible switch between SU-MIMO and MU-MIMO.

be employed in order to mitigate the effect of interfering cells.

presented. These cells are referred as :

(CQI)/Precoding Matrix Indicator (PMI) feedback [17] are required.

*K* ∑ *j*�=*k*

*Yk* = *Hk* · *<sup>x</sup>* + *Bk* (12)

(*Hk* · *Xj*) + *Bk* ; *<sup>k</sup>* = 1, . . . , *<sup>K</sup>* (13)

Multi User MIMO Communication: Basic Aspects, Benefits and Challenges

also written:

**6. Fields of application**

MU-MIMO schemes.

• Coordinated cells • Central cell • Interfering cells

An estimate of the transmitted signal vectors denoted by *Yk*; *k* = 1, . . . , *K* are obtained by using the linear decoders *G*1,..., *GK*. The decoding process is such that :

$$\mathbf{Y}\_k = \mathbf{G}\_k \cdot \mathbf{y}$$

#### **5.2. DL-MU-MIMO**

DL-MU-MIMO communication model assumes that *K* users are simultaneously receiving signals from the base station. The transmitted signal vector *x*(*N* × 1) is expressed as the sum of signals intended to users *U*1,..., *UK*:

$$\mathbf{x} = \sum\_{k=1}^{K} \mathbf{X}\_k \tag{10}$$

The channel matrix between user *Uk*; *<sup>k</sup>* = 1, . . . , *<sup>K</sup>* and the base station is denoted by *Hk*(*Mk* × *<sup>N</sup>*). At each user, received signal vector of dimension (*Mk* × <sup>1</sup>); *<sup>k</sup>* = 1, . . . , *<sup>K</sup>* is given by:

$$Y\_k = H\_k \cdot \mathbf{x} + B\_k \qquad ; \quad k = 1, \ldots, K \tag{11}$$

*Bk*; *<sup>k</sup>* = 1, ..., *<sup>K</sup>* is an additive noise signal vector of size (*Mk* × <sup>1</sup>). Equation (11) could be also written:

$$Y\_k = H\_k \cdot \mathfrak{x} + B\_k \tag{12}$$

$$\mathbf{x} = H\_k \cdot \mathbf{X}\_k + \sum\_{j \neq k}^{K} (H\_k \cdot \mathbf{X}\_j) + B\_k \qquad ; \quad k = 1, \dots, K \tag{13}$$

The second term of the sum in equation (13) represents the interference signal coming from multiple users. Processing techniques such as beamforming should be introduced in the block diagram of the MU-MIMO system for mitigating the effect of users interference and improving the performances of the communication system.
