 retransmission requests and the QoS levels of MTs' services. The advantage of this scheme is that we can get lower frame delay with a small number of users' access requests, even if the user number increases and collisions arise; the frame delay is also controlled in an acceptable range. The dynamic allocation scheme is that BS increases the number of random access channel (RACH) subchannels of the next MAC frame if collided RACH subchan- nels exist in the current frame and decreases it with the proper weighting factor in the case of successful attempts. Thus, it can dynamically increase the subchannels' number of random access channels in the case of a large number of access attempts and decrease the subchannels when there are fewer access requests. The proposed scheme, based on results of access attempts, can get higher throughput compared to fixed allocation schemes. The scheme is also adoptable into this system. 2.2.2.2.9 Link Performance of FuTURE TDD System In the simulation, 3.5-GHz carrier frequency and 20-MHz system bandwidth are considered. The bandwidth of each subcarrier is 19.5 kHz and 832 subcarriers are employed to transmit information. In order to provide a guard band for D/A con- version, nulls are placed at the end of the spectrum. To implement OFDM modula- tion/demodulation, 1,024-point inverse fast Fourier transform (IFFT)/FFT is used. Some of the service-specific parameters are presented in Table 2.2. The simulation channel is a six-path Rayleigh fading channel with an exponential power-delay profile defined by FuTURE. The maximum delay spread is 10 us. Table 2.2 Parameters for Different Services Data Rate 30 Mb/s 50 Mb/s 100 Mb/s Modulation 16 QAM 16 QAM/64 QAM Channel coding Turbo Turbo Turbo Coding rate Antennas at AP Antennas at MT © 2009 by Taylor & Francis Group, LLC Evolution from TD-SCDMA to FuTURE 120 kmph_30 Mbps 250 kmph_30 Mbps 60 kmph_50 Mbps 250 kmph_50 Mbps 5 kmph_100 Mbps 250 kmph_100 Mbps Eb/NO(dB) Figure 2.9 Performance of various data services (outdoors, 250 km/h). Simulat