Multi-User Detection (MUD)

Multi-User Detection (MUD) is through the elimination of inter-cell interference to improve performance, increase system capacity. Actual capacity depends on the algorithm to increase the effectiveness of the wireless environment and system load. In addition to system improvements, but also can effectively alleviate the near-far effect. 
History 
1979 K. Schneider proposed multi-user detection (MultipleUser Detection, MUD) concept has been through 20 years of development, especially in 1986, Verdu optimum multiuser detection algorithm proposed after the multi-user detection techniques for wireless communication of the most important academic research focus. Multi-user detection in CDMA systems definition; joint consideration while taking up a channel for all users or certain users, eliminate or reduce any other user of the user impact, and simultaneous detection of all users or certain user information a signal detection method. Best multi-user detector is the maximum likelihood sequence estimation (MLSE) detector, the detector will have the greatest likelihood sequence b, based on signals from the receiver r (t) to find the send sequence b, b, the probability of making the greatest technology. In DS / CDMA system MLSE detector can Vitezbi algorithm. Although the optimal detector has an excellent performance, but the algorithm complexity is too high, difficult to achieve. 
Origin 
In the CDMA cellular mobile communications, interference is probably divided into three types: additive white noise, multipath interference and multi-user multi-access interference between. Because communication in the same small range of users while not one but multiple, multiple users in CDMA occupy the same slot, the same frequency, when the large number of users to communicate simultaneously, the multiple access interference as the main interference . CDMA system is a multi-input multiple-output (MIMO) system, using the traditional single-input single-output (SISO) detection methods, such as the matched filter, can not take full advantage of information between users, while the multiple access interference is Gaussian white noise that . Therefore, multiple access interference not only seriously affect the system's immunity, but also strictly limits the system capacity increase. 
In multipath fading environment, due to differences between users with the orthogonal spreading code is often difficult to maintain, resulting in interference between multiple users, and limits system capacity improvement. 
Solve the above problems is an effective method is to use multi-user detection (MUD). As the channel non-orthogonal and different users of non-orthogonal spreading the word Ma, leading to the existence of mutual interference between users, the role of multi-user detection is to remove interference between multiple users. Is based on multi-user detection algorithm, after the non-orthogonal channels and non-orthogonal spreading code word, sentence boundaries to redefine the user, in this new line, you can achieve better decision results to In addition to the mutual interference between users. 
Feature 
Advantages of multi-user detection techniques mainly in the following areas: 
1, to improve the CDMA system capacity, increase the number of users. Number of users increases, means higher efficiency of the wireless spectrum. 
2, lower CDMA user equipment (UE) transmit power, increase the UE standby and talk time. On the other hand, the performance of the RF part of the UE in order to reduce the cost and failure rate. 
3, reducing the radio frequency radiation on the user's physiological and psychological effects of mobile communication devices on the environmental impact of more green. 
4, increase communication distance, increasing the cell's coverage area, reducing the overall cost base. 
The main advantage of multi-user detection can be effectively reduced and the elimination of multipath interference, multiple access interference and near-far effect; simplified power control; orthogonal spreading codes to reduce the cross-correlation is not perfect by the negative impact; to improve system performance, increase system capacity, increase cell coverage. 
The main disadvantage of multi-user detection is greatly increased equipment complexity; increased system delay; channel estimation by non-stop to get the main features of the user spreading code parameters, channel estimation accuracy directly affect the performance of multi-user detection.