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Tham khảo tài liệu 'wimax, new developments 2011 part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 74 WIMAX New Developments electronics and the antenna. The importance of the choice of architecture has been demonstrated as have the impacts of key elements such as frequency synthesizers power amplifiers and emission filters. This section points out the considerations to add for a global design of such a transceiver regarding the performance of Digital to Analogue Converters DACs and antennas at these frequencies. 6.1 Digital and Analogue The DAC enables baseband signal generation after the shaping filter. It should have low distortion sufficient bandwidth and low consumption. DACs are used in conventional architecture for I and Q paths generation and in polar architecture for phase I and Q and envelope paths. In polar architecture there is one DAC more and the required bandwidth is extended due to the non-linear processing when generating the phase and the magnitude envelope of the signal. Also the coding of the envelope is an additional restriction in terms of speed for the SA. As the Signal to Noise Ratio SNR of the signal is admitted to grow with the number of bits and bandwidth these specifications are mandatory limiting factors. Nowadays some converters work in the range of several bits near a GHz and around 12 to 20 bits near a MHz. Due to the conclusions of previous sections the example presented here is the simulation of a polar architecture for an OFDM signal with 64 sub-carriers typically IEEE802.11a . The symbol rate is 20 MHz and the carrier frequency is 5.2 GHz but can be shifted to 3.7 GHz without altering the observations because the DAC influences are introduced on the baseband processing. Figure 21 presents the emitted spectrum in an ideal polar EER transmitter simulation with limitation of the bandwidth on the envelope and phase signals. Limits are three times the symbol rate for the envelope and seven times for the phase ones. The mask of the IEEE 802.11a standard is added on the same figure and it is noticeable that the emitted spectrum is .
