**6. Conclusions**

22 Recent Trends in Multiuser MIMO Communications

Mu-Threshold is closer to the Mu-Ideal.

the table.

**Channel**

**Table 6.** Multiuser diversity gain

overhead required for the CSI acquisition.

on performance when high data rates are employed.

performance throughput has been obtained for no more than *m* = 3 slots). As a result,

Another interesting observation is that the two practical schemes are closer to the ideal under worse channel conditions. In the case of *ChA*, for instance, the improvement margin is 26.8 % for Mu-Opportunistic and only 5.6 % for Mu-Threshold (less that 1 Mbps below the upper throughput bound). The gap between the achieved throughput and the ideal performance opens as the channel conditions improve and in the case of *ChD* both schemes have an improvement margin of more than 50 %. This occurs because the overhead information, consisting of control packets transmitted at the lowest rate, has a greater impact

Table 6 gives an estimation of the improvement achieved by exploiting the multiuser diversity. This gain is reflected in the increase of the average data transmission rate compared to the average user rate for each channel model. The average data transmission rate is calculated as the average of the rates employed for the transmission of all data frames. The average user rate is obtained by calculating the average value of the maximum rate at which a user can transmit, if the best beam (i.e., with the higher SNIR) for the particular user is selected. This value depends on the channel model and is indicated in the second column of

**Avg. User Avg. Tx Rate (Mbps)**

*ChA* 12 9.73 18.77 27.60 18.77 *ChB* 18 14.37 23.76 30.48 23.76 *ChC* 24 19.01 34.46 44.40 34.46 *ChD* 36 32.41 46.73 51.64 46.73

In the case of Mu-Basic, the average transmission rate is lower than the average user rate. This is a direct consequence of random scheduling and beam allocation: users may be selected for transmission when their channel quality is low, or they may receive increased interference from other simultaneous transmissions due to the suboptimal beam allocation. Mu-Opportunistic, on the other hand, exploits multiuser diversity by assigning the best user on every beam. As a result, most transmissions take place at rates above the average. In the case of *ChD*, for instance, the transmission rate is 46.7 Mbps whereas the average user rate is limited to 36 Mbps. It should be noted that Mu-Opportunistic yields the same average transmission rate as the Mu-Ideal scheme, since both schemes implement the same scheduling policy. Despite providing the same transmission rate, the throughput performance of Mu-Opportunistic is lower than the ideal, due to the additional control

Finally, the maximum transmission rate values are achieved by Mu-Threshold. At first glance, is seems puzzling to obtain rates above those of the Mu-Ideal scheme. Nevertheless, this can be explained with the help of the data presented in Table 4. By imposing a rate threshold, Mu-Threshold scheme controls the minimum rate that can be employed for transmission.

**Rate (Mbps) Mu-Basic Mu-Opport. Mu-Thres. Mu-Ideal**

This chapter has presented a novel approach for the integration of multiuser capabilities in IEEE 802.11n/ac based WLANs. On one hand, a low-complexity beamforming technique named MOB has been employed at the PHY layer. The main strength of MOB lies in the

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and the rate threshold parameters. In the case of Mu-Opportunistic scheme, the control overhead increases linearly with the user number and performance eventually drops. • Under harsh channels, the performance of the proposed multiuser schemes approaches the upper performance bound set by the ideal case of having perfect CSI knowledge with no additional overhead. On the other hand, under more favorable channel conditions, there is still a margin for potential improvement by exploiting multiuser transmissions.

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

This work has been funded by the Research Projects GREENET (PITN-GA-2010-264759), CO2GREEN (TEC2010-20823), Green-T (CP8-006) and GEOCOM (TEC2011-27723-C02-01).

1 Signal Theory and Communications Department of Technical University of Catalunya

[1] IEEE Standard for Information technology – Telecommunications and information exchange between systems – Local and metropolitan area networks – Specific requirements – Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications. *IEEE Std 802.11-2007 (Rev. of IEEE Std 802.11-1999)*,

[2] IEEE Standard for Information Technology – Telecommunications and information exchange between systems – Local and metropolitan area networks – Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications Amendment 5: Enhancements for Higher Throughput. *IEEE Std*

[3] G. Caire and S. Shamai. On the achievable throughput of a multiantenna Gaussian broadcast channel. *IEEE Transactions on Information Theory*, 49(7):1691–1706, July 2003.

[4] M.Z. Siam and M. Krunz. An overview of MIMO-oriented channel access in wireless networks [medium access control protocols for wireless LANs]. *IEEE Wireless*

[5] Per H. Lehne and Magne Pettersen. An Overview of Smart Antenna Technology for Mobile Communications Systems. *IEEE Communication Surveys &Tutorials*, 2(4):2–13, 4th

Elli Kartsakli1,<sup>⋆</sup>, Nizar Zorba1, Luis Alonso2 and Christos Verikoukis<sup>3</sup>

2 Electrical Engineering Department of The University of Jordan, Jordan

3 Telecommunications Technological Center of Catalunya (CTTC), Barcelona, Spain

<sup>⋆</sup> Address all correspondence to: ellik@tsc.upc.edu

*802.11n-2009*, pages c1–502, October 2009.

*Communications Magazine*, 15(1):63–69, February 2008.

**Acknowledgements**

**Author details**

(UPC), Barcelona, Spain

December 2007.

Quarter 1999.

**References**

**Figure 15.** Throughput performance comparison versus the number of users

fact that it only requires partial CSI information at the transmitter side, in the form of SNIR measurements acquired by the downlink users. Since the IEEE 802.11n/ac specifications support beamforming, MOB can be easily implemented with minor modifications in the beamforming steering matrices.

On the other hand, in order to exploit the potential of the MOB technique in a realistic scenario, it is necessary to design appropriate MAC layer mechanisms to handle multiuser transmissions. In this chapter, three MAC layer schemes have been proposed. The first scheme, Mu-Basic, implemented a simple random scheduling multiuser scheme, meant to serve as a performance reference. Then, two opportunistic schemes have been proposed, Mu-Opportunistic and Mu-Threshold, that enhance performance by extracting the multiuser diversity gain.

The performance evaluation of the proposed multiuser schemes has led to many interesting observations. The lessons learned can be employed to improve the proposed algorithms but also as more general guidelines in the design of multiuser MAC schemes. The more remarkable conclusions are summarized as follows:


and the rate threshold parameters. In the case of Mu-Opportunistic scheme, the control overhead increases linearly with the user number and performance eventually drops.

• Under harsh channels, the performance of the proposed multiuser schemes approaches the upper performance bound set by the ideal case of having perfect CSI knowledge with no additional overhead. On the other hand, under more favorable channel conditions, there is still a margin for potential improvement by exploiting multiuser transmissions.
