**4.2. Mu-Opportunistic scheme**

In an effort to exploit multiuser diversity and transmit opportunistically to the best set of users, according to the principles of the MOB transmission technique, the *Mu-opportunistic* scheme has been proposed. This scheme provides a mechanism for the AP to acquire the CSI of all users before reaching a scheduling decision, in order to optimize user selection and beam allocation. To this end, in the beginning of the transmission sequence, the AP polls all users with available data for downlink transmission. For the sake of simplicity, it will be assumed that the system is under saturation and there is always downlink traffic for each of the *N* system users.4 Hence, the AP transmits a multidestination RTS frame that includes the *N* receiver addresses of all the network, as illustrated in Figure 9.

**Figure 9.** The modified RTS frame for the Mu-Opportunistic scheme

<sup>4</sup> The non saturation case will be examined later in this chapter.

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

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) for the downlink data transmission.

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

### **4.3. Mu-Threshold scheme**

14 Recent Trends in Multiuser MIMO Communications

randomly selected for transmission (STA3 and STA1).

update it when the header of a data packet is decoded.

transmissions at low data rates (i.e. interference controlled system).

the *N* receiver addresses of all the network, as illustrated in Figure 9.

(user 1)

**Figure 9.** The modified RTS frame for the Mu-Opportunistic scheme

<sup>4</sup> The non saturation case will be examined later in this chapter.

**4.2. Mu-Opportunistic scheme**

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

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

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

In an effort to exploit multiuser diversity and transmit opportunistically to the best set of users, according to the principles of the MOB transmission technique, the *Mu-opportunistic* scheme has been proposed. This scheme provides a mechanism for the AP to acquire the CSI of all users before reaching a scheduling decision, in order to optimize user selection and beam allocation. To this end, in the beginning of the transmission sequence, the AP polls all users with available data for downlink transmission. For the sake of simplicity, it will be assumed that the system is under saturation and there is always downlink traffic for each of the *N* system users.4 Hence, the AP transmits a multidestination RTS frame that includes

> Rx Address

Rx Address (user 2)

6 bytes 6 bytes 6 bytes

Rx Address (user *N*)

Mu-Opportunistic RTS

... Rx Address

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 to the Mu-Opportunistic scheme:

• 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 the CTS phase.

• In order to reduce the CTS collision probability, the algorithm imposes a SNIR threshold so that only users with a relatively good channel are allowed to participate in the feedback process. Even though the idea of threshold application is not new, the novelty lies in the inclusion of this concept in a feasible MAC scheme for a multiuser MIMO scenario.

10.5772/57129

127

Data Phase ACK Phase

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

ACK

ACK

http://dx.doi.org/10.5772/57129

Multiuser MPDU (STAN @ beam 1, STA3 @ beam 2)

AP

DIFS

Multicast RTS Phase RTS (multicast address )

CTS Contention Phase

Slot 1 Slot 2 ... Slot *m*

SIFS

**Figure 12.** Transmission sequence example for the Mu-Threshold scheme

CTS CTS

NAV (RTS) NAV (MPDU) Other

CTS ...

Mu-Threshold Downlink Transmission Sequence

the feedback information collected by the received CTS frames and transmits a maximum of *nt* data packets simultaneously. Note that, unlike the contention phase where collisions among CTS frames can occur, the transmission of data is collision-free. Finally, the users acknowledge the data reception by sequentially sending an ACK frame, following the order

An example of the transmission sequence according to the Mu-Threshold scheme is given in Figure 12. In this example, there are *nt* = 2 antennas at the AP and *N* users with available data that compete in *m* CTS slots (with *m* < *N* in general). Some users may select the same slot and collide (e.g., STA1 and STA2), others may transmit a CTS successfully (e.g. STA3 and STA*N*) and finally a number of users will refrain from this phase due to their unfavorable

An important decision is the selection of the SNIR threshold that serves two purposes: it reduces the number of contending users, thus decreasing the probability of CTS collisions, and it filters out those users with harsh channel condition, resulting to transmissions with higher data rates. Nevertheless, selecting a high threshold could cause adverse effects such as starvation if the majority of users experience low link quality. The threshold is determined by the AP and it is made known to the users during an initial association phase (alternatively, it could be included in the RTS packet, thus increasing its size by a few overhead bits). It is also possible to design a dynamic scheme that will adapt the threshold value at runtime

The number of the CTS contention slots *m* is another important parameter that depends on the number of participating users which, in turn, is determined by the total number of users *N*, their channel condition and the selected threshold. An interesting observation is that, since the duration of each CTS slot is fixed, the duration field of the RTS packet (that indicates the length of the CTS phase) implicitly reveals the number of contention slots *m*. Therefore, the AP can let the users know the value of *m* without requiring an additional

CTS

STA1 STA2

STAN …

STA

channel conditions.

control field.

of the user mapping onto the beams.

depending on measured channel statistics.

STA3

The frame exchange sequence of the Mu-Threshold scheme is initiated with the broadcast transmission of an RTS by the AP. The advantage of this configuration is that it calls all the users to participate in the CTS contention phase by employing a single 6-byte destination address instead of a long address list, as shown in Figure 11. Without doubt, this setup is meaningful under a saturation scenario in which the AP has always packets to transmit to all the associated users. This consideration is made to facilitate the evaluation of the full potential of the Mu-Threshold scheme, given that opportunistic downlink schemes are mostly needed under high-traffic conditions. In non-saturation conditions, the Mu-Threshold scheme could be applied with a minor modification. In this case, the AP would have to periodically set up multicast groups with the subset of active users (i.e., those who are waiting to receive downlink data) and use a multicast instead of a broadcast address.

After the RTS transmission, a CTS contention phase of *m* slots is initiated, with *m* being a system parameter subject to optimization. The slots have a predefined length, equal to a SIFS duration plus the time required for the transmission of the 15 byte CTS with the minimum available transmission rate. Depending on whether the maximum SNIR measured by a user is above or below the threshold, the user is either allowed to participate in this phase, or forced to remain silent until the beginning of a new frame sequence. Those allowed to participate select randomly a slot with equal probability and transmit a CTS containing the maximum measured SNIR and the corresponding beam identifier. Whenever multiple users select the same slot a collision occurs and the involved CTS frames are considered lost (the capture effect is not considered, even though it could increase the effectiveness of the proposed scheme). A slot can also remain empty if no user selects it for transmission.

**Figure 11.** The modified RTS frame for the Mu-Threshold scheme

The next stage of the algorithm depends on the outcome of the contention phase. If no CTS has been correctly received (due to either collisions or lack of user participation because of the SNIR threshold value) no data is transmitted and a new contention phase is initiated.<sup>5</sup> User synchronization has been assumed, so that a collision in the *m*th slot only affects the involved CTS packets and does not have any effect on transmissions in the remaining slots of the contention phase. Thus, if at least one CTS is received, transmission of downlink data packets can take place. As before, the AP assigns the best user on each beam, based on

<sup>5</sup> Different policies could be implemented to avoid the presence of empty frames (e.g., transmission to a randomly selected user or to a user with a long waiting time using a basic rate) but will not be considered in this work.

**Figure 12.** Transmission sequence example for the Mu-Threshold scheme

16 Recent Trends in Multiuser MIMO Communications

• In order to reduce the CTS collision probability, the algorithm imposes a SNIR threshold so that only users with a relatively good channel are allowed to participate in the feedback process. Even though the idea of threshold application is not new, the novelty lies in the inclusion of this concept in a feasible MAC scheme for a multiuser MIMO scenario.

The frame exchange sequence of the Mu-Threshold scheme is initiated with the broadcast transmission of an RTS by the AP. The advantage of this configuration is that it calls all the users to participate in the CTS contention phase by employing a single 6-byte destination address instead of a long address list, as shown in Figure 11. Without doubt, this setup is meaningful under a saturation scenario in which the AP has always packets to transmit to all the associated users. This consideration is made to facilitate the evaluation of the full potential of the Mu-Threshold scheme, given that opportunistic downlink schemes are mostly needed under high-traffic conditions. In non-saturation conditions, the Mu-Threshold scheme could be applied with a minor modification. In this case, the AP would have to periodically set up multicast groups with the subset of active users (i.e., those who are waiting to receive downlink data) and use a multicast instead of a broadcast address.

After the RTS transmission, a CTS contention phase of *m* slots is initiated, with *m* being a system parameter subject to optimization. The slots have a predefined length, equal to a SIFS duration plus the time required for the transmission of the 15 byte CTS with the minimum available transmission rate. Depending on whether the maximum SNIR measured by a user is above or below the threshold, the user is either allowed to participate in this phase, or forced to remain silent until the beginning of a new frame sequence. Those allowed to participate select randomly a slot with equal probability and transmit a CTS containing the maximum measured SNIR and the corresponding beam identifier. Whenever multiple users select the same slot a collision occurs and the involved CTS frames are considered lost (the capture effect is not considered, even though it could increase the effectiveness of the proposed scheme). A slot can also remain empty if no user selects it for transmission.

> Rx Address

6 bytes Rx Broadcast Address

The next stage of the algorithm depends on the outcome of the contention phase. If no CTS has been correctly received (due to either collisions or lack of user participation because of the SNIR threshold value) no data is transmitted and a new contention phase is initiated.<sup>5</sup> User synchronization has been assumed, so that a collision in the *m*th slot only affects the involved CTS packets and does not have any effect on transmissions in the remaining slots of the contention phase. Thus, if at least one CTS is received, transmission of downlink data packets can take place. As before, the AP assigns the best user on each beam, based on

<sup>5</sup> Different policies could be implemented to avoid the presence of empty frames (e.g., transmission to a randomly selected user or to a user with a long waiting time using a basic rate) but will not be considered in this work.

**Figure 11.** The modified RTS frame for the Mu-Threshold scheme

Mu-Threshold RTS

the feedback information collected by the received CTS frames and transmits a maximum of *nt* data packets simultaneously. Note that, unlike the contention phase where collisions among CTS frames can occur, the transmission of data is collision-free. Finally, the users acknowledge the data reception by sequentially sending an ACK frame, following the order of the user mapping onto the beams.

An example of the transmission sequence according to the Mu-Threshold scheme is given in Figure 12. In this example, there are *nt* = 2 antennas at the AP and *N* users with available data that compete in *m* CTS slots (with *m* < *N* in general). Some users may select the same slot and collide (e.g., STA1 and STA2), others may transmit a CTS successfully (e.g. STA3 and STA*N*) and finally a number of users will refrain from this phase due to their unfavorable channel conditions.

An important decision is the selection of the SNIR threshold that serves two purposes: it reduces the number of contending users, thus decreasing the probability of CTS collisions, and it filters out those users with harsh channel condition, resulting to transmissions with higher data rates. Nevertheless, selecting a high threshold could cause adverse effects such as starvation if the majority of users experience low link quality. The threshold is determined by the AP and it is made known to the users during an initial association phase (alternatively, it could be included in the RTS packet, thus increasing its size by a few overhead bits). It is also possible to design a dynamic scheme that will adapt the threshold value at runtime depending on measured channel statistics.

The number of the CTS contention slots *m* is another important parameter that depends on the number of participating users which, in turn, is determined by the total number of users *N*, their channel condition and the selected threshold. An interesting observation is that, since the duration of each CTS slot is fixed, the duration field of the RTS packet (that indicates the length of the CTS phase) implicitly reveals the number of contention slots *m*. Therefore, the AP can let the users know the value of *m* without requiring an additional control field.
