**2.1. PHY layer techniques for multiple antenna systems**

In recent years, the technology of smart antennas has been widely investigated in an effort to increase the capacity of wireless networks. A smart antenna system combines multiple spatially distributed antenna elements with intelligent signal processing algorithms that adjust the antenna radiation pattern in order to achieve some desired objective. Smart antennas can be classified into three categories according to their level of intelligence [5]:


MIMO systems employ smart antenna technology with a high level of intelligence, aiming to improve transmission rates and enhance reliability and robustness. The term MIMO implies the availability of at least two antennas at each end of the communication link. When multiple antennas are employed only at the transmitter side the system is known as Multiple-Input Single-Output (MISO), whereas Single-Input Multiple-Output (SIMO) systems imply a single transmitting and multiple receiving antennas.

2 Recent Trends in Multiuser MIMO Communications

standard are also key issue.

**2. State of the art**

chapter with some general conclusions.

**2.1. PHY layer techniques for multiple antenna systems**

and steer a beam towards that direction to enhance reception.

towards specific, predefined directions.

out interference sources.

barrier. The new draft standard contemplates, among other things, multiuser transmissions

Motivated by this open line of work, this chapter is dedicated to the investigation of solutions for the incorporation of multiuser capabilities in IEEE 802.11n/ac WLAN systems by using Cross-Layer (CL) information, while maintaining backward compatibility with the standard. The main contribution is the design of a number of opportunistic channel-aware multiple antenna MAC schemes that handle multiuser downlink transmissions and explore the advantages that can be gained by exploiting multiuser diversity. Backward compatibility with the existing 802.11n standard and possible integration within the new 802.11ac draft

The remaining part of this chapter is divided into six sections. Section 2 provides an overview of the state of the art on multiuser MAC layer protocols, a review of the multiuser mechanism included in the 802.11ac draft standard and the additional challenges that arise from simultaneous transmissions. Section 3 discusses the problem statement and presents the Employed beamforming transmission technique. The description of the proposed multiuser MAC schemes is given in Section 4. Section 5 provides the performance evaluation of the proposed schemes and discusses the obtained trade-offs and finally, Section 6 closes the

Multiuser transmissions require the use of multiple antenna transmission techniques and advanced signal processing at the Physical (PHY) layer, as well as more complex MAC layer schemes. An overview of the most representative transmission techniques for smart antennas and MIMO systems is given in Section 2.1, whereas the available multiuser MAC schemes are presented in Section 2.2. Finally, a brief description of the multiuser capabilities of the IEEE 802.11ac draft standard and the relevant open issues are presented in Section 2.3.

In recent years, the technology of smart antennas has been widely investigated in an effort to increase the capacity of wireless networks. A smart antenna system combines multiple spatially distributed antenna elements with intelligent signal processing algorithms that adjust the antenna radiation pattern in order to achieve some desired objective. Smart antennas can be classified into three categories according to their level of intelligence [5]:

• The *switched beam antennas* have the lowest intelligence and can employ beamforming

• The *dynamically phased antennas* can determine the direction of arrival of a received signal

• Finally, the *adaptive array antennas* can additionally adjust their radiation pattern to null

MIMO systems employ smart antenna technology with a high level of intelligence, aiming to improve transmission rates and enhance reliability and robustness. The term MIMO implies the availability of at least two antennas at each end of the communication link. When multiple antennas are employed only at the transmitter side the system is known

in the downlink but leaves many open issues, especially on multiuser scheduling.

There are several techniques for the exploitation of multiple antennas at the PHY layer, schematically illustrated in Figure 1. A brief description of each technique will be given next, but more detailed explanation can be found in [6] and [7].

The most conventional techniques are beamforming and interference suppression, shown in Figure 1a. By means of beamforming, the received signal strength of a point-to-point communication link is increased thus resulting to higher supported data rates and extended coverage range. Interference suppression is achieved by steering the nulls of the antenna radiation pattern towards specific directions. This technique can be employed to reduce the interference produced by the transmitter but also to limit the received interference by other systems. As a result the link reliability and the spectral efficiency of the system are enhanced.

Another very efficient technique is spatial diversity that effectively mitigates multi-path fading and therefore provides increased robustness against errors (Figure 1b). Depending on whether the multiple antenna elements are placed at the receiver (SIMO) or the transmitter (MISO), the spatial diversity schemes can be classified as receive and transmit diversity, respectively. In receive-diversity schemes, independently faded copies (due to different propagation paths) of the same signal arrive at each antenna element of the receiver and are appropriately combined or selected to enhance reception [8]. In transmit-diversity schemes the same signal is transmitted over multiple antennas after some processing has taken place to ensure that the received multiple copies of the signal will be successfully separated by the receiver [9][10][11]. Clearly, in MIMO systems, joint receive and transmit diversity schemes can be implemented.

A very powerful transmit-diversity technique that achieves both diversity and coding gain is the Space-Time Coding (STC) that involves signal coding over space (multiple antennas) and time (multiple symbol times). There are two main approaches to STC design, the Space-Time Trellis Coding (STTC) [12] and the Space-Time Block Coding (STBC) [13][14]. STTC provides considerable coding and diversity gains with the cost of high decoding complexity. On the other hand, STBC is less efficient since it mainly offers diversity gain (and minimal or zero coding gain) but has the significant property of using linear decoding at the receiver.

Another PHY layer technique is spatial multiplexing (Figure 1c), according to which multiple independent data streams are simultaneously transmitted in the same frequency spectrum using multiple antennas. The receiver manages to extract the data streams from the received signal by employing spatial processing techniques that exploit multi-path fading. As a result, the throughput performance is increased. A very popular and spectral efficient spatial multiplexing scheme is V-BLAST (Vertical- Bell Laboratories Layered Space Time) [15]. As far as point-to-multipoint links are concerned, the spatial multiplexing of signals known as SDMA allows multiple simultaneous transmissions in the same frequency, thus multiplying the capacity of the system [16].

Summarizing, the main PHY layer techniques that are available in multiple antenna systems are beam-forming, interference cancellation, spatial diversity and spatial multiplexing. These techniques can be used separately or in combination, to obtain the desired effect. Finally, it has been demonstrated that there is a fundamental trade-off between diversity gain and spatial multiplexing gain that reflects to a design decision in favor of increased reliability or throughput, respectively [17][18].

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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

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

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.

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

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

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

average 4.4 slots to find the best two users in the system [25].

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

first quarter of 2014.

spatial streams per user).

(a) Beamforming and Interference Cancellation

**Figure 1.** Multiple antenna transmission techniques
