## 5 Results  
HIPERLAN/2 system shown in Figure 2 is tested when the 16 QAM modulation is used. Figure 6 shows the symbols scattering for different stages: (a) at the transmitter side, (b) inside the channel, and (c) at the receiver side. It can be noticed that the channel has a significant effect on the transmitted symbols. The bit error rate of the HIPERLAN/2 system shown in Figure 3 utilizing different modulation schemes including 4-, 16-, 32-, and 64-QAM, in addition to the using the two types of Alamouti technique in terms of the number of antennas used at the transmit and the receive side is shown in Figures 7 and 8. The HIPERLAN/2 performance when the user has two transmit antennas and one receive antenna and when each one has two antennas for the transmit and two for the receive, it can be noticed that, as the modulation complexity increases, the required SNR should be increased to achieve the same BER. In addition, the use of additional transmit antenna helps in achieving the same BER with lower SNR. Figure 6: Symbols scattering at (**a**) the transmitter, (**b**) the channel, and (**c**) the receiver. After that, a series of tests has been applied to the system to include the Alamouti technique as shown in Figure 3, where each user has two transmit antennas and one receiver antenna which are closely located, whereas in the other case, each one has two transmit antennas and two receiver antennas. The bit error rate of both users is tested depending on three parameters, which are: the transmitted power, the received power, and the feedback accuracy, all these parameters are tested with different modulation schemes. Figure 9, Figure 10, Figure 11 show the bit error rate in case the transmitted power and the received power are constant. It can be noticed that the feedback accuracy has a significant effect on the performance, as the feedback accuracy increases, the performance will be enhanced. At the same time, the feedback accuracy should be increased in the case of using a higher modulation scheme to improve the overall system performance. Figure 8: HIPERLAN/2 performance when nTx = 2 and nRx = 2 for different modulation schemes. Figure 7: HIPERLAN/2 performance when nTx = 2 and nRx = 1 for different modulation schemes. Figure 9: Modified HIPERLAN/2 performance, 16 QAM, when feedback accuracy is varying (**a**) nTX = 2, nRx = 1, and (**b**) nTx = 2, nRx = 2. Figure 10: Modified HIPERLAN/2 performance, 32 QAM, when feedback accuracy is varying (**a**) nTx = 2, nRx = 1, and (**b**) nTx = 2, nRx = 2. On the other hand, HIPERLAN/2 system with Alamouti was also tested when the transmitted power and the feedback accuracy are constant and only the received power is varying, as shown in Figure 12, Figure 13, Figure 14. It is clear that the overall performance will be enhanced as the received power increased for all modulation schemes. Moreover, the increase of transmitted antennas helps in improving system performance. Figure 11: Modified HIPERLAN/2 performance, 64 QAM, when feedback accuracy is varying (**a**) nTx = 2, nRx = 1, and (**b**) nTx = 2, nRx = 2. Figure 12: Modified HIPERLAN/2 performance, 16 QAM, when received power is varying (**a**) nTx = 2, nRx = 1, and (**b**) nTx = 2, nRx = 2. Figure 13: Modified HIPERLAN/2 performance, 32 QAM, when received power is varying (**a**) nTx = 2, nRx = 1, and (**b**) nTx = 2, nRx = 2. Finally, the proposed system was tested when the received power and the feedback accuracy are constant, and only the transmitted power is varying for different cases, as shown in Figure 15, Figure 16, Figure 17. It is obvious that the transmitted power affects the system performance, whereas at 50 dBm transmitted power, the system was not able to recover the original signal even when there is a delay between the received power and the feedback accuracy. Figure 8:0 Modified HIPERLN/2 performance, 64 QAM, when received power is varying (**a**) nTx = 2, nRx = 1, and (**b**) nTx = 2, nRx = 2.  
a high SNR. However, as the transmitted power decreases, the overall performance is improved.