**5. Performance evaluation**

#### **5.1. Simulation setup**

This section will focus on the performance evaluation of the proposed multiuser schemes. Simulation results have been obtained with the help of a custom-made link layer simulation tool implemented in C++. Theoretical analysis of the proposed schemes has also been derived and more details can be found in [33].

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instance will oscillate around the mean value (with the same variance for all users), through

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

**Parameter Value** Number of antennas (AP) *nt* = 2 Number of antennas (Users) *nr* = 1 Downlink Users *N* = 10 SIFS 16 *µs* aSlotTime 9 *µs* PHY Header (AP) 28 *µs* PHY Header (Users) 32 *µs* MAC Header 40 bytes RTS (Mu-Basic) 14 + 6 · *nt* bytes RTS (Mu-Opportunistic) 14 + 6 · *N* bytes

RTS (Mu-Threshold) 20 bytes CTS 15 bytes DATA 2312 bytes ACK 14 bytes Bandwidth 20 MHz

Saturated traffic conditions have been considered, with a constant flow of downlink traffic for all users always available at the buffers of the AP. The rationale behind this assumption has been to evaluate the maximum gain that can be extracted from downlink transmissions, which requires the system to operate under heavy traffic load. Unless otherwise stated, the size of the data packets has been fixed to 2312 bytes. All control frames are transmitted at the lowest rate (i.e., at 6 Mbps) to ensure correct reception. The IEEE 802.11n frame format has been adopted at the MAC layer, with the modifications proposed in Section 4 for each

This section compares the performance achieved by the proposed multiuser MAC schemes, Mu-Basic, Mu-Opportunistic and Mu-Threshold. Figures 13 and 14 plot the throughput and mean total delay performance for the four channel models, *N* = 10 users and a packet size of *L* = 2312 bytes. The performance of the non-realistic Mu-Ideal scheme is also depicted, as a reference of the upper bound that corresponds to the considered scenarios. Mu-Ideal is an ideal opportunistic multiuser scheme in which the users with the highest SNIR values are scheduled on each beam. In other words, the same scheduling objective as in the Mu-Opportunistic scheme (Section 4.2) is targeted. The difference is that, in the Mu-Ideal scheme it has been assumed that the AP has a perfect knowledge of the channel condition and can select the best set of users without any additional overhead. Clearly, this scheme is not practical, since some mechanism for the CSI acquisition must be available at the AP.

multiuser scheme. A summary of the simulation parameters is given in Table 3.

**5.2. Performance comparison of the multiuser schemes**

the block fading channel defined in Section 3.2.

**Table 3.** Simulation parameters

The simulation setup considers an infrastructure downlink network that consists of an AP with *nt* = 2 transmitting antennas and *N* = 10 single-antenna users (MISO scenario). An ideal Adaptive Modulation and Coding (AMC) that ensures error-free data transmission has been assumed at the PHY layer, given that the rate for each transmission is selected according to the link quality, as expressed by the SNIR.

A channel model that represents the IEEE 802.11n channel model B in Non-line-of-sight (NLOS) conditions has been considered [29]. As mentioned in Section 3.2, a block-fading model with independent and identically distributed complex Gaussian entries ∼ CN (0, 1) has been considered, with a noise variance of 0.1.<sup>6</sup> Each block corresponds to the duration of a frame sequence and no correlations have been assumed among the different blocks. This model has been employed to generate a SNIR matrix that represents the channel condition of each user on a frame-by-frame basis. The SNIR limits employed to determine the available transmission rate of each user are given in Table 2 [34].


#### **Table 2.** SNIR thresholds

Four different scenarios have been considered, characterized by four channel implementations (i.e., different SNIR matrices) denoted by *ChA*, *ChB*,*ChC* and *ChD*. The average link quality varies for each channel, with *ChA* corresponding to the most unfavorable conditions and *ChD* representing a channel with high quality links. For reference, the average user SNIR for channels *ChA* to *ChD* is 15dB, 17dB, 20dB and 25dB, respectively. According to Table 2, the average user rate for each scenario will be 12, 18, 24, and 36 Mbps, respectively. Since the channel realizations are random, the available rate for each user at every time

<sup>6</sup> Without loss of generality, a relatively low noise variance has been used. Higher values would lead to different numerical results but without affecting the behavior of the evaluated MAC schemes.


instance will oscillate around the mean value (with the same variance for all users), through the block fading channel defined in Section 3.2.

**Table 3.** Simulation parameters

18 Recent Trends in Multiuser MIMO Communications

and more details can be found in [33].

to the link quality, as expressed by the SNIR.

transmission rate of each user are given in Table 2 [34].

This section will focus on the performance evaluation of the proposed multiuser schemes. Simulation results have been obtained with the help of a custom-made link layer simulation tool implemented in C++. Theoretical analysis of the proposed schemes has also been derived

The simulation setup considers an infrastructure downlink network that consists of an AP with *nt* = 2 transmitting antennas and *N* = 10 single-antenna users (MISO scenario). An ideal Adaptive Modulation and Coding (AMC) that ensures error-free data transmission has been assumed at the PHY layer, given that the rate for each transmission is selected according

A channel model that represents the IEEE 802.11n channel model B in Non-line-of-sight (NLOS) conditions has been considered [29]. As mentioned in Section 3.2, a block-fading model with independent and identically distributed complex Gaussian entries ∼ CN (0, 1) has been considered, with a noise variance of 0.1.<sup>6</sup> Each block corresponds to the duration of a frame sequence and no correlations have been assumed among the different blocks. This model has been employed to generate a SNIR matrix that represents the channel condition of each user on a frame-by-frame basis. The SNIR limits employed to determine the available

**Rate (Mbps) SNIR (dB)**

 -8 to 12.5 12.5 to 14 14 to 16.5 16.5 to 19 19 to 22.5 22.5 to 26 48 26 to 28 54 >28

0 (no transmission) ≤-8

Four different scenarios have been considered, characterized by four channel implementations (i.e., different SNIR matrices) denoted by *ChA*, *ChB*,*ChC* and *ChD*. The average link quality varies for each channel, with *ChA* corresponding to the most unfavorable conditions and *ChD* representing a channel with high quality links. For reference, the average user SNIR for channels *ChA* to *ChD* is 15dB, 17dB, 20dB and 25dB, respectively. According to Table 2, the average user rate for each scenario will be 12, 18, 24, and 36 Mbps, respectively. Since the channel realizations are random, the available rate for each user at every time <sup>6</sup> Without loss of generality, a relatively low noise variance has been used. Higher values would lead to different

numerical results but without affecting the behavior of the evaluated MAC schemes.

**5. Performance evaluation**

**5.1. Simulation setup**

**Table 2.** SNIR thresholds

Saturated traffic conditions have been considered, with a constant flow of downlink traffic for all users always available at the buffers of the AP. The rationale behind this assumption has been to evaluate the maximum gain that can be extracted from downlink transmissions, which requires the system to operate under heavy traffic load. Unless otherwise stated, the size of the data packets has been fixed to 2312 bytes. All control frames are transmitted at the lowest rate (i.e., at 6 Mbps) to ensure correct reception. The IEEE 802.11n frame format has been adopted at the MAC layer, with the modifications proposed in Section 4 for each multiuser scheme. A summary of the simulation parameters is given in Table 3.
