**2.2. MAC protocols for multiuser transmissions**

An overview of the most representative examples of multiuser scheduling and resource allocation can be found in [19]. The authors stress that selecting the best user subset for each transmission is the key to achieving multiuser diversity, but also point out several practical issues that must be taken care of, including the need for feedback acquisition on the channel state.

A significant number of contributions has been dedicated to the development of user selection and scheduling algorithms in the context of multiple antenna systems. An early work proposes the first-fit algorithm, a sub-optimum but less complex scheduling method that selects sets of packets that can be transmitted simultaneously [20]. However, one of the basic assumptions of this work is that the channel between the base station and the users is quasi-static and is considered known by the base-station, whereas scenarios with varying channel conditions are left for future consideration. In [21] the authors propose a SDMA/TDMA scheduler that assigns packets to time slots depending on their Quality of Service (QoS) requirements. Multiple packets can be spatially multiplexed in the same slot if they satisfy a Signal-to-Noise-and-Interference Ratio (SNIR) constraint. Again, this work mainly focuses on the scheduling policy and assumes that the spatial signature and QoS requirements for each packet are acquired during an initial admission phase.

Nevertheless, in realistic scenarios the channel condition cannot be considered known and a feedback mechanism must be established. Naturally, there is a trade-off between the potential performance enhancement when the channel is known and the reduced efficiency due to the introduced control overhead required for the feedback mechanism. One way to decrease feedback is by applying a threshold to exclude users with poor channel conditions from gaining access to the channel. This idea has been extensively studied in [22]. This work offers some guidelines for the threshold selection but it does not consider a specific multiple access scheme, nor the implementation of an actual feedback acquisition mechanism. In a different approach, binary feedback (1 or 0) is used by users to express whether they satisfy the threshold condition [23]. The idea is effective but assumes the presence of a dedicated low bit rate feedback channel, which is not the case in IEEE 802.11 based WLANs. Finally, another proposal combines the principle of splitting algorithms with threshold selection to determine the user with the best channel in less than three slots on average [24]. This work has been extended to provide detection of multiple users with good channel and needs on average 4.4 slots to find the best two users in the system [25].

4 Recent Trends in Multiuser MIMO Communications

transmitted data stream SP

**Transmit Diversity**

**Receive Diversity**

**Space Time Block Code (STBC)**

(b) Spatial Diversity Techniques

**2.2. MAC protocols for multiuser transmissions**

data stream SP SP

data stream SP SP

data stream SP SP

s1 -s2\* s2 s1\*

**Figure 1.** Multiple antenna transmission techniques

SP = Signal Processing

throughput, respectively [17][18].

transmitted

transmitted data stream

transmitted data stream

state.

spatial multiplexing gain that reflects to a design decision in favor of increased reliability or

null steering

(a) Beamforming and Interference Cancellation

transmitted data stream 1

transmitted data stream 2

transmitted data stream 1

SP

SP

transmitted data stream 2

An overview of the most representative examples of multiuser scheduling and resource allocation can be found in [19]. The authors stress that selecting the best user subset for each transmission is the key to achieving multiuser diversity, but also point out several practical issues that must be taken care of, including the need for feedback acquisition on the channel

A significant number of contributions has been dedicated to the development of user selection and scheduling algorithms in the context of multiple antenna systems. An early work proposes the first-fit algorithm, a sub-optimum but less complex scheduling method that selects sets of packets that can be transmitted simultaneously [20]. However, one of the basic assumptions of this work is that the channel between the base station and the users is quasi-static and is considered known by the base-station, whereas scenarios with varying channel conditions are left for future consideration. In [21] the authors propose a SDMA/TDMA scheduler that assigns packets to time slots depending on their Quality of Service (QoS) requirements. Multiple packets can be spatially multiplexed in the same slot if they satisfy a Signal-to-Noise-and-Interference Ratio (SNIR) constraint. Again, this work mainly focuses on the scheduling policy and assumes that the spatial signature and QoS

requirements for each packet are acquired during an initial admission phase.

received data stream

received

received

beamforming

received data stream

SP

SP

interfering user

**Spatial Multiplexing**

SP SP

**SDMA**

(c) Spatial Multiplexing Techniques

received data stream 1

received data stream 2

received data stream 1 received data stream 2

SP

Finally, there are some contributions that aim to include multiuser MAC schemes for IEEE 802.11 based systems. One example is the Multi-User Distributed Coordination Function (MU-DCF), presented in [26], that uses a four-way handshake that begins with a polling multiuser RTS frame. However there are several issues, mostly regarding the PHY layer implementation, that are not considered. A mathematical model for a downlink multiuser scheme for IEEE 802.11 is given in [27]. They show that performance can be improved by exploiting spatial multiplexing and conclude that there is still a need to design a modified MAC to support multiple transmissions and perform a good channel estimation mechanism.

### **2.3. Multiuser transmissions in the IEEE 802.11ac draft standard**

In an effort to obtain WLAN throughputs beyond the gigabit per second barrier, a new draft standard, the IEEE 802.11 ac, is being developed, to extend the 802.11n capabilities in the 5 GHz band. The main target of the IEEE 802.11ac draft standard is to provide high aggregate throughput beyond 1 Gbps. The task group is currently in the process of developing the draft 7.0 version of the standard, with the final approval of the amendment expected towards the first quarter of 2014.

An important innovative feature of IEEE 802.11ac is the support of point-to-multipoint transmissions that are possible thanks to the multiuser capability of MIMO systems. In other words, a MU-MIMO capable device can transmit multiple packets simultaneously to multiple destinations. A maximum number of four users can be simultaneously supported and up to eight spatial streams can be employed for transmissions (with a maximum of four spatial streams per user).

The standard is not yet in its final form, but the most prevailing approach so far for the scheduling of multiple data frames is presented in [28]. The authors propose some modifications to the IEEE 802.11 backoff procedure and introduce new mode known as sharing of the transmission opportunity limit (TXOP). The main idea is that when a station gains access to the channel, it may be allowed to transmit simultaneously multiple packets that may belong to different access categories (i.e., traffic priorities), something that was not permitted in previous versions of IEEE 802.11. However, the exact rules of packet selection 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 interesting open issues.

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2 } =

Maximum set of users simultaneously served by the AP

User *nt* 

...

Access Point (AP)

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

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*).

*th* user can be expressed as

where *zi*(*t*) is an additive Gaussian complex noise component with zero mean and *<sup>E</sup>*{|*zi*|

2

is considered and for ease of notation, time index is dropped whenever possible.

*σ*<sup>2</sup> is the noise variance. The transmitted signal **x**(*t*) encloses the independent data symbols

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

MAC queues

...

Matrices *<sup>N</sup>* downlink

*nt a*ntennas

MAC scheduler

User 1

User 1 User 2

User *N*

**Figure 2.** Scenario setup

baseband signal.

Then, the received signal for the *i*

*si*(*t*) to all the selected users with *<sup>E</sup>*{|*si*|

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

User *2*

User *nt +*1

...

...

X Beamforming Steering User *N*

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

*yi*(*t*) = **h***i*(*t*)**x**(*t*) + *zi*(*t*) (1)

} = 1. A total transmitted power constraint *Pt* = 1
