4.5 Multi-User Detection Technology

Multi-user detection (MUD) technology can improve the system performance and increase the system capacity by canceling the inter-cell interference. In addition, MUD technology can effectively release the near/far effect in direct sequence spread spectrum CDMA system.
As the channels are non-orthogonal and different users’ spread spectrum codes are non-orthogonal, there is mutual interference between users. MUD is to cancel the mutual interference between multi users. Generally speaking, for uplink MUD, only the inter-cell interference can be canceled, while the intra-cell interference is difficult to cancel due to lack of necessary information (such as the detailed information of users in adjacent cells). For downlink MUD, only the interference in common channel (such as pilot frequency and broadcast channel) can be canceled.
Each user transmits data bit  b1,b2,…, bN, frequency is spread by spread spectrum code word via non-orthogonal fading channel in the space, and noise n(t) is added, then the user signals received at the receive end is correlated to synchronous spread spectrum code word and the correlation is composed of multiplier and integral cleaner. The interference between users is removed with MUD algorithm from the de-spread result, thus obtaining the estimated value of users’ signal b1,b2,…, bN.
It is observed from the following figure that the performance of MUD depends on synchronized spread spectrum code word tracing of the correlator, detection performance of each user signal, relative energy and the accuracy of channel estimation.

According to the uplink MUD, only the intra-cell interference can be canceled. Provided that the inter-cell interference energy was f times of the intra-cell interference energy, the capacity in the cell would increase by (1+f)/f without intra-cell interference. According to the rule that the transmit power attenuates linearly by 4 powers of distance, the inter-cell interference is 55% of intra-cell interference. Therefore, MUD would ideally reduce 2.8 times of interference. However in practice, the validity of MUD is below 100%. The validity of MUD depends on the detection methods, traditional receiver estimation accuracy and intra-cell user service model. For example, if there are some high-speed data users in the cell, MUD is adopted to cancel the interference power caused by these high-speed data users, which obviously can be more effective to increase system capacity.
But the shortcoming of it is that noise would be increased and demodulation signal would be greatly delayed. 
Interference cancellation is to estimate interference from different users and multiple paths and then cancel the interference from the receive signals. Serial interference cancellation (SIC) is to gradually cancel the interference caused by the biggest user, while parallel interference cancellation (PIC) is to simultaneously cancel the interference cause by other users.

PIC is, for each user, to cancel signal energy caused by other users at every level of interference cancellation and demodulate it. The interference caused by other users can be basically canceled after 3 ~ 5 times of such interference cancellation. Note that at every level of interference cancellation, not all signal energy caused by other users can be canceled. We just multiply it by a relatively small coefficient to avoid the growing error in the traditional receive detection. The advantage of PIC is that it can easily implement multi-user interference cancellation and the delay of it is better than that of SIC.
As far as WCDMA uplink MUD is concerned, the present ideal technology is PIC, because it demands just 3 to 5 times of resources traditional receiver demands, and the data path delay is also small.

As far as WCDMA downlink MUD is concerned, it focuses on canceling the interference of downlink common pilot channel, shared channel and broadcast channel and the interference of common channels in a co-frequency adjacent base station.

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