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Can thiệp hủy thu MC-CDMA sử dụng hủy bỏ sự can thiệp khai thác LLRs xuất phát để phát hiện SingleUser trong từng giai đoạn phát hiện, trong giai đoạn thứ hai và tiếp tục hạn đại diện cho sự can thiệp truy cập nhiều trong LLRs khoảng có thể được thiết lập để không. 2.1.8 | 72 MC-CDMA and MC-DS-CDMA Interference Cancellation MC-CDMA receivers using interference cancellation exploit the LLRs derived for singleuser detection in each detection stage where in the second and further stages the term representing the multiple access interference in the LLRs can approximately be set to zero. 2.1.8 Flexibility in System Design The MC-CDMA signal structure introduced in Section 2.1.1 enables the realization of powerful receivers with low complexity due to the avoidance of ISI and ICI in the detection process. Moreover the spreading code length L has not necessarily to be equal to the number of sub-carriers Nc in an MC-CDMA system which enables a flexible system design and can further reduce the complexity of the receiver. The three MC-CDMA system modifications presented in the following are referred to as M-Modification Q-Modification and M Q-Modification 15 16 23 . These modifications can be applied in the up- and in the downlink of a mobile radio system. 2.1.8.1 Parallel Data Symbols M -Modification As depicted in Figure 2-11 the M-Modification increases the number of sub-carriers Nc while maintaining constant the overall bandwidth B the spreading code length L and the maximum number of active users K. The OFDM symbol duration increases and the loss in spectral efficiency due to the guard interval decreases. Moreover the tighter sub-carrier spacing enables one to guarantee flat fading per sub-channel in propagation scenarios with small coherence bandwidth. With the M-Modification each user transmits simultaneously M 1 data symbols per OFDM symbol. The total number of sub-carriers of the modified MC-CDMA system is Nc ML. 2.86 1st data symbol per user Mth data symbol per user Figure 2-11 M-Modification MC-CDMA 73 Each user exploits the total of Nc sub-carriers for data transmission. The OFDM symbol duration including the guard interval increases to r Tg MLT c 2.87 where it can be observed that the loss in spectral efficiency due to the guard .
