**4. Multiuser MAC schemes**

8 Recent Trends in Multiuser MIMO Communications

Access Point (AP)

orthonormal beams

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

nt x 1 MISO Scenario (AP with nt antennas and single antenna users)

(a) MOB Step 1: The AP generates *nt* random

Access Point (AP)

> **User Selection** Beam 1: User n1 Beam 2: User n3

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. 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.

*n1*

*n2*

Access Point (AP)

*n1*

*n2*

(beam 1, SNIRn3\_1) (beam 2, SNIRn3\_2)

(b) MOB Step 2: Users measure the SNIR on each beam and feed back their best value

*n1*

*n2*

Beam 2 Beam 1

*n3*

(beam 1, SNIRn2\_1) (beam 2, SNIRn2\_2)

(beam 1, SNIRn1\_1) (beam 2, SNIRn1\_2)

*n3*

*n3*

DATA (*n1*)

DATA (*n3*)

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

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

Beam 2

Beam 1

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.

Since the proposed MAC schemes aim to support the MOB transmission technique, they provide a common set of functions, graphically shown in Figure 4. These functions provide a practical MAC layer implementation to complement the three steps of the MOB scheme, namely the generation of the orthonormal beams, the acquisition of CSI feedback and the multiuser downlink transmission. In continuation, it is convenient to first present the common framework that applies to the three proposed schemes before proceeding with their detailed description that will focus on their differences in terms of complexity and efficiency. 10.5772/57129

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included in the RTS is a point of differentiation between the proposed schemes and

2. It acts as a sounding frame that will enable the receiving users to measure the SNIR on each of the *nt* generated beams. For this reason, the PHY layer preamble of the RTS contains a number of HT-LTFs (High-Throughput Long Training Fields), as defined in IEEE 802.11n standard. Apart from the training fields, the main body of the RTS frame

The structure of the modified RTS frame is shown in Figure 5. The length of the PHY layer preamble of the RTS frame is determined by the number *nt* of spatial streams (i.e., orthonormal beams and subsequently antennas). For a single-antenna transmission, a PHY layer header of 28 *µ*s is introduced, whereas for every additional spatial stream an extra HT-LTF of 4 *µ*s is required. The description of the PHY header fields is given in Table 1 and more details can be found in the IEEE 802.11n specification [2].<sup>1</sup> The length of the MAC header mainly depends on the receiver address field. When a single receiver address is employed, the MAC header has a length of 20 bytes. Nevertheless, some of the proposed MAC schemes include multiple destinations in this address field, as it will be

RTS

Control

8 μs 8 μs 8 μs 4 μs 4 μs 4 2 bytes 2 bytes ≥ 6 bytes 6 bytes bytes

HT-GF-STF High-Throughput (HT) Greenfield Short Training Field

• *The transmission of a CTS frame by the downlink users*. Once the users receive the RTS frame and estimate their channel quality, they reply with a CTS frame that, unless otherwise stated, contains the best measured SNIR value and an integer identifier that corresponds to the respective beam. The structure of the modified CTS frame is shown in Figure 6. Assuming single-antenna users, a 28 *µ*s PHY layer preamble is required,

<sup>1</sup> The PHY layer header structure presented in this section has been based on the IEEE 802.11n greenfield operation mode meant for IEEE 802.11n-only compatible stations. If compatibility with legacy devices is desired, the PHY layer

headers should be modified accordingly, as indicated in Clause 20 of the IEEE 802.11n specification [2].

LTF FCS Frame

Rx Address

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

MAC Header ≥ 20 bytes

Tx

Duration / ID

Address ...

HT-

Number of spatial

will be discussed later in this section.

further clarified later.

HT-GF-

**Figure 5.** Structure of the modified RTS frame

STF HT-LTF HT-SIG HT-

(24 + 4∙*nt*) μs

**Element Description**

HT-SIG HT SIGNAL Field HT-LTF HT Long Training Field

**Table 1.** Elements of the PHY layer header for the Multiuser MAC schemes

LTF

streams (beams) PHY Header

HT-LTF1 First HT Long Training Field

is transmitted conventionally (i.e., on a single beam).

**Figure 4.** MAC layer functions to support the MOB transmission technique

As illustrated in Figure 4, the common functions provided by the MAC layer schemes are:

	- 1. It is a call for participation in the downlink phase that may be addressed to a subset or to all the associated users (i.e., multicast or broadcast). The employed receiver address

included in the RTS is a point of differentiation between the proposed schemes and will be discussed later in this section.

2. It acts as a sounding frame that will enable the receiving users to measure the SNIR on each of the *nt* generated beams. For this reason, the PHY layer preamble of the RTS contains a number of HT-LTFs (High-Throughput Long Training Fields), as defined in IEEE 802.11n standard. Apart from the training fields, the main body of the RTS frame is transmitted conventionally (i.e., on a single beam).

The structure of the modified RTS frame is shown in Figure 5. The length of the PHY layer preamble of the RTS frame is determined by the number *nt* of spatial streams (i.e., orthonormal beams and subsequently antennas). For a single-antenna transmission, a PHY layer header of 28 *µ*s is introduced, whereas for every additional spatial stream an extra HT-LTF of 4 *µ*s is required. The description of the PHY header fields is given in Table 1 and more details can be found in the IEEE 802.11n specification [2].<sup>1</sup> The length of the MAC header mainly depends on the receiver address field. When a single receiver address is employed, the MAC header has a length of 20 bytes. Nevertheless, some of the proposed MAC schemes include multiple destinations in this address field, as it will be further clarified later.

**Figure 5.** Structure of the modified RTS frame

10 Recent Trends in Multiuser MIMO Communications

MOB steps at the PHY layer

AP generates *nt* orthonormal beams

Users return the highest measured SNIR

AP maps best users on beams and transmits data

phase.

time axis

**Figure 4.** MAC layer functions to support the MOB transmission technique

Since the proposed MAC schemes aim to support the MOB transmission technique, they provide a common set of functions, graphically shown in Figure 4. These functions provide a practical MAC layer implementation to complement the three steps of the MOB scheme, namely the generation of the orthonormal beams, the acquisition of CSI feedback and the multiuser downlink transmission. In continuation, it is convenient to first present the common framework that applies to the three proposed schemes before proceeding with their detailed description that will focus on their differences in terms of complexity and efficiency.

> MAC layer functions to support MOB

AP gains access to the medium and initiates downlink phase

AP transmits a multiuser RTS with the required training fields

Users transmit CTS with measured SNIR

AP transmits multiple downlink data packets to the selected users

Users transmit ACK for correctly received data frames

As illustrated in Figure 4, the common functions provided by the MAC layer schemes are:

• *The initiation of the downlink phase*. The proposed multiuser schemes constitute a downlink phase that is always initiated by the AP, so for the sake of simplicity the backoff mechanism defined in the IEEE 802.11 specification is not employed in this study. Generally, in a scenario with both uplink and downlink transmissions, the AP would have to follow the backoff rules to gain access to the medium before initiating the downlink

• *The generation of a multiuser RTS frame*. The initiation of the downlink phase is marked by

1. It is a call for participation in the downlink phase that may be addressed to a subset or to all the associated users (i.e., multicast or broadcast). The employed receiver address

the transmission of a modified RTS frame that basically serves two purposes:

MAC design issues

Poll all users or just a subset? (i.e., broadcast or multicast RTS?)

All or some users reply with a CTS?

How are multiple CTS transmissions handled?


**Table 1.** Elements of the PHY layer header for the Multiuser MAC schemes

• *The transmission of a CTS frame by the downlink users*. Once the users receive the RTS frame and estimate their channel quality, they reply with a CTS frame that, unless otherwise stated, contains the best measured SNIR value and an integer identifier that corresponds to the respective beam. The structure of the modified CTS frame is shown in Figure 6. Assuming single-antenna users, a 28 *µ*s PHY layer preamble is required,

<sup>1</sup> The PHY layer header structure presented in this section has been based on the IEEE 802.11n greenfield operation mode meant for IEEE 802.11n-only compatible stations. If compatibility with legacy devices is desired, the PHY layer headers should be modified accordingly, as indicated in Clause 20 of the IEEE 802.11n specification [2].

whereas the MAC header complies with the IEEE 802.11n specification, with the addition of an extra 1-byte field that contains the CSI information (i.e., the SNIR and the beam identifier).<sup>2</sup> Two design issues arise at this point. The first is whether a CTS should be transmitted by every polled user, or a limit should be posed to the number of CTS replies, for example by filtering out users with very bad channel conditions. The second issue concerns the transmission order of the CTS frames by the multiple users which can be either deterministic, thus collision-free, or random (probabilistic) that will likely result to collisions among simultaneously transmitted CTS. These two issues will be handled in different ways by the proposed MAC schemes, as it will be discussed later.

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beginning of the transmission sequence, the AP randomly selects *nt* users from the downlink message buffer and transmits a multidestination RTS frame that includes the respective *nt*

The figure focuses on the receiver address field, since the remaining part of the RTS frame follows the structure indicated previously (Figure 5). The order in which the addresses are listed serves two purposes. First, it indicates the order in which CTS frames are to be sent in order to avoid collisions. Second, the address list is used to implicitly map the polled users to the beams. The users that receive the RTS frame check whether their address is in the list and wait for a predefined time before sending a CTS, which includes the SNIR measurement that corresponds to the assigned beam.<sup>3</sup> Note that in this case, the users do not reply with

> Rx Address

Rx Address (beam 2)

6 bytes 6 bytes 6 bytes

... Rx Address (beam *nt*)

(STA3 @ beam 1, STA1 @ beam 2)

ACK

Data Phase ACK Phase

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

ACK

Mu-Basic RTS

the best SNIR value since the beam assignment is predefined by the AP.

Rx Address (beam 1)

CTS Phase (Collision-free)

CTS

DIFS Multiuser MPDU

NAV (RTS) NAV (MPDU) Other

Mu-Basic Downlink Transmission Sequence

The AP proceeds to the simultaneous transmission of the *nt* data packets after selecting the transmission rate for each beam, according to the corresponding SNIR measurement that indicates the link quality. The users acknowledge the data reception by sequentially sending

<sup>3</sup> Since each CTS slot is of a fixed duration (i.e., a SIFS time and the time required for the transmission of the 15 byte CTS with the minimum available transmission rate) and assuming negligible propagation delays, each user can

CTS

SIFS

receiver addresses, as illustrated in Figure 7.

**Figure 7.** The modified RTS frame for the Mu-Basic scheme

Multidest. RTS Phase

> RTS (STA3, STA1)

**Figure 8.** Transmission sequence example for the Mu-Basic scheme

determine when to initiate the CTS transmission.

AP

STA1

STA2

STA3

STAN …

STA

**Figure 6.** Structure of the modified CTS frame, including CSI feedback


In the remaining part of this section, the three proposed MAC layer schemes will be described in detail.

### **4.1. Mu-Basic scheme**

The first and simplest scheme is called *Mu-Basic* and is a straightforward adaptation of the IEEE 802.11 mechanism to support downlink multiuser transmission. This scheme is based on the principle that at most *nt* users can be served simultaneously by an AP equipped with *nt* transmitting antennas that generate an equal number of orthogonal beams. Hence, in the

<sup>2</sup> In this work, it has been assumed that a SNIR quantization scheme has been employed so that the CSI field can be sufficiently represented by 1 byte.

beginning of the transmission sequence, the AP randomly selects *nt* users from the downlink message buffer and transmits a multidestination RTS frame that includes the respective *nt* receiver addresses, as illustrated in Figure 7.

The figure focuses on the receiver address field, since the remaining part of the RTS frame follows the structure indicated previously (Figure 5). The order in which the addresses are listed serves two purposes. First, it indicates the order in which CTS frames are to be sent in order to avoid collisions. Second, the address list is used to implicitly map the polled users to the beams. The users that receive the RTS frame check whether their address is in the list and wait for a predefined time before sending a CTS, which includes the SNIR measurement that corresponds to the assigned beam.<sup>3</sup> Note that in this case, the users do not reply with the best SNIR value since the beam assignment is predefined by the AP.

**Figure 7.** The modified RTS frame for the Mu-Basic scheme

12 Recent Trends in Multiuser MIMO Communications

PHY Header 28 μs

STF HT-LTF HT-SIG HT-

8 μs 8 μs 8 μs 4 μs

**Figure 6.** Structure of the modified CTS frame, including CSI feedback

HT-GF-

by the measured SNIR.

**4.1. Mu-Basic scheme**

sufficiently represented by 1 byte.

in detail.

whereas the MAC header complies with the IEEE 802.11n specification, with the addition of an extra 1-byte field that contains the CSI information (i.e., the SNIR and the beam identifier).<sup>2</sup> Two design issues arise at this point. The first is whether a CTS should be transmitted by every polled user, or a limit should be posed to the number of CTS replies, for example by filtering out users with very bad channel conditions. The second issue concerns the transmission order of the CTS frames by the multiple users which can be either deterministic, thus collision-free, or random (probabilistic) that will likely result to collisions among simultaneously transmitted CTS. These two issues will be handled in

Control

• *The transmission of multiuser data frames by the AP*. Once the AP collects the feedback information included in the CTS frames it assigns the user with the highest measured SNIR on each beam (at most one user per beam) and transmits a maximum of *nt* data packets simultaneously. The data packets employ the channel over the same time, frequency and code but are transmitted over different beams. This can be supported by the IEEE 802.11n standard, by exploiting the multiplexing capabilities of multi-antenna systems. This is actually an important shift from current systems where the simultaneous transmission of multiple packets in the same medium leads to collision and packet loss. Link adaptation is also employed and the transmission rate on each beam is determined

• *The transmission of ACK frames*. The users signal the correct reception of a data frame by transmitting an ACK. In the proposed schemes, the multiple (up to *nt*) ACK frames are

In the remaining part of this section, the three proposed MAC layer schemes will be described

The first and simplest scheme is called *Mu-Basic* and is a straightforward adaptation of the IEEE 802.11 mechanism to support downlink multiuser transmission. This scheme is based on the principle that at most *nt* users can be served simultaneously by an AP equipped with *nt* transmitting antennas that generate an equal number of orthogonal beams. Hence, in the

<sup>2</sup> In this work, it has been assumed that a SNIR quantization scheme has been employed so that the CSI field can be

transmitted sequentially, following the mapping of the users onto the beams.

MAC Header 15 bytes

FCS Frame

Rx Address 2 bytes 2 bytes 6 bytes 4 bytes

CSI 1 byte

Duration / ID

different ways by the proposed MAC schemes, as it will be discussed later.

CTS

LTF

**Figure 8.** Transmission sequence example for the Mu-Basic scheme

The AP proceeds to the simultaneous transmission of the *nt* data packets after selecting the transmission rate for each beam, according to the corresponding SNIR measurement that indicates the link quality. The users acknowledge the data reception by sequentially sending

<sup>3</sup> Since each CTS slot is of a fixed duration (i.e., a SIFS time and the time required for the transmission of the 15 byte CTS with the minimum available transmission rate) and assuming negligible propagation delays, each user can determine when to initiate the CTS transmission.

an ACK frame. An example of the transmission sequence according to the Mu-Basic scheme is given in Figure 8. In this example, there are *nt* = 2 antennas at the AP, so two users are randomly selected for transmission (STA3 and STA1).

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Data Phase ACK Phase

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

ACK

ACK

Multiuser MPDU (STA2 @ beam 1, STAN @ beam 2)

AP

DIFS RTS (STA1, STA2,…,STAN )

Multidest. RTS Phase

CTS Phase (Collision-free)

CTS

SIFS

**Figure 10.** Transmission sequence example for the Mu-Opportunistic scheme

CTS

NAV (RTS) NAV (MPDU) Other

...

Mu-Opportunistic Downlink Transmission Sequence

The users measure the SNIR on all the beams and include the maximum SNIR value in the CTS, along with an integer identifier of the beam that yielded that value. As before, CTS packets are transmitted in a collision-free manner, following the order of the address list in the RTS. After receiving all the feedback, the AP assigns each beam to the user with the highest SNIR and proceeds to the downlink data transmission. If a beam is not selected by any user then it is not used for transmission, even though this is not likely to happen very often for a large number of active users and a time-varying channel. Correct data reception is marked by the transmission of ACK frames that are sent sequentially, according to the beam allocation order (the user served on the first beam replies first, and so on). An example of the transmission sequence according to the Mu-Opportunistic scheme is given in Figure 10. In this example, there are *nt* = 2 antennas at the AP and *N* users with available data. The AP receives *N* CTS frames and then selects the best set of users (STA2 and STA*N*, in the example)

The Mu-Opportunistic fully exploits multiuser diversity since it opportunistically schedules users with good channel conditions and with low mutual interference (i.e., users with high SNIR values measured on different beams). The weakness of this scheme is that it introduces significant overhead, mainly due to the long CTS phase, and the trade-off between overhead

The Multiuser Threshold-Selective algorithm (*Mu-Threshold)* is the third proposed multiuser MAC layer scheme. It maintains the opportunistic scheduling policy of selecting a set of users with high rates and low mutual interference but also aims to limit the additional control overhead. In order to achieve these objectives, it introduces two major changes with respect

• Instead of the deterministic, collision-free CTS transmissions, Mu-Threshold introduces a CTS contention phase during which users compete with each other within a predefined number of slots. Generally, even though collisions among CTS frames are likely to occur, the number of slots is smaller than the total number of users, thus reducing the length of

and efficiency becomes critical, especially as the number of users *N* grows.

CTS

STA1 STA2

STAN …

STA

for the downlink data transmission.

**4.3. Mu-Threshold scheme**

to the Mu-Opportunistic scheme:

the CTS phase.

STA3

To avoid collisions by users that do not participate in the process, the IEEE 802.11 NAV mechanism can be employed. For this reason, the time from the transmission of the RTS until the end of the CTS phase is marked in the duration field of the RTS frame (Figure 5). The remaining time of the frame sequence, from the end of the CTS phase until the transmission of the last ACK, is indicated in the respective duration field of the data packet MAC header. Hence, non-participating users can set their NAV timer upon the RTS reception and can later update it when the header of a data packet is decoded.

Mu-Basic is easy to implement since it is a simple polling scheme initiated by the AP. Its performance will serve as a benchmark for the evaluation of the two more advanced multiuser schemes that will be presented next. In the considered case the destination users are randomly selected, however different criteria could also be applied to prioritize users with specific demands (e.g., with delay sensitive applications). Mu-Basic requires some additional overhead in the RTS frame as multiple receiver addresses must be included, but has the shortest possible CTS phase, since the number of received CTS frames is equal to the *nt* served users (it would not make sense to receive feedback from less than *nt* users if all the parallel streams were to be employed). On the other hand, multiuser diversity is not exploited since the users are scheduled without any consideration of their channel quality. Thus, the user selection and the beam assignment processes are not optimally done. As a result, the interference among the scheduled set of users may be high, leading to transmissions at low data rates (i.e. interference controlled system).
