**3.2. MIMO/MISO channel**

With the use of OFDM, the frequency selective MIMO/MISO channel is transformed into a number of frequency flat channels. In particular, a block-fading model is considered <sup>116</sup> Recent Trends in Multi-user MIMO Communications Multiuser MAC Schemes for High-Throughput IEEE 802.11n/ac WLANs 7 10.5772/57129 Multiuser MAC Schemes for High-Throughput IEEE 802.11n/ac WLANs http://dx.doi.org/10.5772/57129 117

**Figure 2.** Scenario setup

6 Recent Trends in Multiuser MIMO Communications

**3. System setup and multiuser physical layer**

modifications that will be explained in this section.

**3.1. Problem statement and system setup**

sophisticated scheduling policy.

multiple-antenna users.

following sections.

**3.2. MIMO/MISO channel**

interesting open issues.

among the different access categories, and more generally, user and resource allocation issues and rate adaptation are not explicitly defined in the IEEE 802.11ac draft standard and pose

This section discusses the problem statement and describes the considered setup. In continuation, a brief description of the channel model is given and the multiuser transmission technique used at the PHY layer is explained. In general, the IEEE 802.11n MIMO specification with OFDM has been considered as the base for the PHY layer, with some

As indicated in the state of the art, it can generally be said that most contributions on multiuser transmission schemes focus on particular aspects of the problem and simplify the rest. Usually, when the focus is laid on the PHY layer transmission techniques, practical mechanisms for the channel access and the feedback acquisition are not considered, whereas multiuser MAC schemes often fail to consider PHY layer implementation issues. For example, some schemes optimize resource allocation but ignore feedback mechanisms and others minimize the required feedback but assume a dedicated control channel and a less

This chapter will introduce a multiuser MAC mechanism that handles in a joint manner the processes of channel access, scheduling, channel estimation and feedback acquisition, in conjunction with a low-complexity beamforming technique at the PHY layer. The proposed schemes have been designed in the context of a downlink communication channel in an infrastructure WLAN in which multiple antennas are available at the transmitter side. Without loss of generality, a MISO scenario with single-antenna users has been considered, even though the presented analysis can be also applied to MIMO systems with

The considered setup is illustrated in Figure 2. The proposed schemes can be considered as a downlink transmission phase, initiated by an Access Point (AP) equipped with *nt* antennas (*nt* ≥ 2) in a system with *N* single-antenna users. By exploiting the MIMO/MISO spatial signal processing capabilities and employing an appropriate transmission technique, the AP can serve up to *nt* users at the same frequency and time. Nevertheless, in order to extract multiuser diversity gain, the pool of served users should exceed the number of transmitting antennas (i.e., *N* > *nt*). Transmitting multiple downlink packets simultaneously, however, is feasible only when there is no interference among the selected users, or in a more realistic case, when the interference is relatively low. Hence, the AP must have some knowledge of the channel to select the most appropriate set of users for each transmission. These issues must be handled by the MAC layer in a practical way, as it will be described in detail in the

With the use of OFDM, the frequency selective MIMO/MISO channel is transformed into a number of frequency flat channels. In particular, a block-fading model is considered for the channel which remains constant during the coherence time and changes between consecutive time intervals with independent and identically distributed complex Gaussian entries ∼ CN (0, 1). This model represents the IEEE 802.11n channel model B in NLOS conditions [29], assuming that there are no time correlations among the different blocks and that the channel impulse response changes at a much slower rate than the transmitted baseband signal.

In the considered MISO downlink scenario, the channel between the AP that is equipped with *nt* antennas and the *i*th single-antenna user (out of *N* total users with *N* > *nt*) is described by a 1 × *nt* complex channel matrix **<sup>h</sup>***i*(*t*). Let **<sup>x</sup>**(*t*) be the *nt* × 1 vector with the transmitted signal to all the selected users in a particular transmission sequence and *yi*(*t*). Then, the received signal for the *i th* user can be expressed as

$$y\_i(t) = \mathbf{h}\_i(t)\mathbf{x}(t) + z\_i(t) \tag{1}$$

where *zi*(*t*) is an additive Gaussian complex noise component with zero mean and *<sup>E</sup>*{|*zi*| 2 } = *σ*<sup>2</sup> is the noise variance. The transmitted signal **x**(*t*) encloses the independent data symbols *si*(*t*) to all the selected users with *<sup>E</sup>*{|*si*| 2 } = 1. A total transmitted power constraint *Pt* = 1 is considered and for ease of notation, time index is dropped whenever possible.

#### **3.3. Multibeam Opportunistic Beamforming (MOB)**

Multibeam Opportunistic Beamforming (MOB) is a low-complexity transmission technique for multiple-antenna broadcast channels [30]. MOB requires the presence of multiple antennas at the transmitter side and one or more antennas at each receiving user, meaning that it can be applied to MISO or MIMO scenarios. Its goal is to exploit multiuser diversity by finding a set of orthogonal users that can be simultaneously served on orthogonal beams, while maintaining the interference low. The key advantage of this transmission scheme is that it only requires partial Channel State Information (CSI) at the transmitter side in terms of the user received SNIR, making it very suitable for multiuser downlink communications.

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(3)

**b***<sup>k</sup> sk* (2)

Multiuser MAC Schemes for High-Throughput IEEE 802.11n/ac WLANs

time. In the best case where *nt* users are selected for downlink transmission, the transmitted

where *sk* are the data symbols that correspond to the *k*th selected user, **b***<sup>k</sup>* is the assigned

Although the beams are orthogonally generated, some of this orthogonality is lost in the propagation channel [30]. Consequently, some interference is generated by each beam on non-intended users. The SNIR formulation for the *k*th user that is served by the *v*th beam is

> 1 *nt* <sup>|</sup>**h***k***b***k*<sup>|</sup>

*<sup>σ</sup>*<sup>2</sup> <sup>+</sup> *nt* ∑ *u*�=*v* 1 *nt* <sup>|</sup>**h***k***b***u*<sup>|</sup>

where a uniform power allocation is considered. The numerator is the received power from the desired beam, while the denominator represents the noise plus the interference power

As the number of users *N* grows, the AP can search for users in a larger pool, thus increasing the probability of finding a set of *nt* users that do not interfere a lot among themselves [30]. Obviously, having *N* ≈ *nt* results in an interference limited system, but for more practical values, such as *nt* = 2 transmit antennas and *N* ≥ 10 users, this scheme is efficient and has been shown to obtain higher performance with respect to single user opportunistic

The IEEE 802.11n PHY layer specification does not contemplate multiuser transmissions, even though it supports beamforming as a means to achieve higher data rates in point-to-point communications. Since the MOB scheme is practically a random beamforming transmission technique, it can be easily implemented within the standard without any further requirements in terms of hardware. The only necessary modification is to set accordingly the values of the beamforming steering matrices defined in the standard in order to form the

The MOB technique is a low-complexity transmission scheme that can be easily implemented at the PHY layer to provide multiuser downlink communications. In a practical system, however, the beamforming scheme must be accompanied by a set of MAC layer functions to collect the necessary feedback information and handle the additional challenges that stem from simultaneous multiuser transmissions. This section will present three MAC layer schemes that modify the IEEE 802.11n MAC protocol to account for the demands and restrictions of the MOB technique. The required modifications are easy to implement within the IEEE 802.11n/ac standards and are backward compatible with the legacy single user

transmission, in the sense that MOB and legacy users can coexist in the system.

2

2

*nt* ∑ *k*=1

**x** = 1 *nt*

*SNIRk*,*<sup>v</sup>* =

unit-power beam and the square root term is employed for total power constraint.

signal **x** can be expressed as

from the other beams.

beamforming [31], [32].

random orthonormal beams.

**4. Multiuser MAC schemes**

The main steps of MOB are illustrated in Figure 3. It should be mentioned that these steps describe the main concept behind the MOB scheme without entering into implementation details. These will be more thoroughly addressed in Section 4 where the description of the proposed multiuser MAC schemes will take place. At the beginning of each transmission sequence, the AP forms *nt* random orthogonal beams, equal to the number of its transmitting antennas (plot (a)). The users measure the SNIR related to each beam, select the highest measured SNIR value to the AP (plot (b)). In turn, the AP selects the best user for each beam and initiates the downlink data transmission (plot (c)). The scheme presented in [30] involves the opportunistic transmission by the users with the highest instantaneous SNIR for each beam, although MOB can also be combined with different scheduling policies.

(a) MOB Step 1: The AP generates *nt* random orthonormal beams (b) MOB Step 2: Users measure the SNIR on each beam and feed back their best value

(c) MOB Step 3: The AP maps best users on beams and begins downlink transmission

**Figure 3.** Basic steps of MOB transmission technique

Through this low-complexity processing based on the instantaneous SNIR values, the MOB scheme achieves a high system sum rate by spatially multiplexing several users at the same time. In the best case where *nt* users are selected for downlink transmission, the transmitted signal **x** can be expressed as

$$\mathbf{x} = \sqrt{\frac{1}{n\_l}} \sum\_{k=1}^{n\_l} \mathbf{b}\_k \,\mathrm{s}\_k \tag{2}$$

where *sk* are the data symbols that correspond to the *k*th selected user, **b***<sup>k</sup>* is the assigned unit-power beam and the square root term is employed for total power constraint.

Although the beams are orthogonally generated, some of this orthogonality is lost in the propagation channel [30]. Consequently, some interference is generated by each beam on non-intended users. The SNIR formulation for the *k*th user that is served by the *v*th beam is

$$SNR\_{k, \upsilon} = \frac{\frac{1}{n\_l} |\mathbf{h}\_k \mathbf{b}\_k|^2}{\sigma^2 + \sum\_{\boldsymbol{\mu} \neq \upsilon}^{n\_l} \frac{1}{n\_l} |\mathbf{h}\_k \mathbf{b}\_\boldsymbol{\mu}|^2} \tag{3}$$

where a uniform power allocation is considered. The numerator is the received power from the desired beam, while the denominator represents the noise plus the interference power from the other beams.

As the number of users *N* grows, the AP can search for users in a larger pool, thus increasing the probability of finding a set of *nt* users that do not interfere a lot among themselves [30]. Obviously, having *N* ≈ *nt* results in an interference limited system, but for more practical values, such as *nt* = 2 transmit antennas and *N* ≥ 10 users, this scheme is efficient and has been shown to obtain higher performance with respect to single user opportunistic beamforming [31], [32].

The IEEE 802.11n PHY layer specification does not contemplate multiuser transmissions, even though it supports beamforming as a means to achieve higher data rates in point-to-point communications. Since the MOB scheme is practically a random beamforming transmission technique, it can be easily implemented within the standard without any further requirements in terms of hardware. The only necessary modification is to set accordingly the values of the beamforming steering matrices defined in the standard in order to form the random orthonormal beams.
