tailieunhanh - CDMA truy cập và chuyển mạch P10

Optimization Techniques for ‘Pseudo-Orthogonal’ CDMA Overview The CDMA systems presented in the previous chapters were mainly based on the synchronous or orthogonal approach. As we have discussed, orthogonal CDMA achieves maximum capacity, but it requires synchronization of all transmitting users in a multipoint-to-point access network. Such a synchronization, however, may not always be possible in a high mobility environment. In such an enviroment, we use ‘Pseudo-Orthogonal’ (PO) CDMA. | CDMA Access and Switching For Terrestrial and Satellite Networks Diakoumis Gerakoulis Evaggelos Geraniotis Copyright 2001 John Wiley Sons Ltd ISBNs 0-471-49184-5 Hardback 0-470-84169-9 Electronic 10 Optimization Techniques for Pseudo-Orthogonal CDMA Overview The CDMA systems presented in the previous chapters were mainly based on the synchronous or orthogonal approach. As we have discussed orthogonal CDMA achieves maximum capacity but it requires synchronization of all transmitting usess in a multipoint-to-point access network. Such a synchronization however may not always be possible in a high mobility environment. In such an enviroment we use Pseudo-Orthogonal PO CDMA. In the PO-CDMA capacity users CDMA-band is limited by interference resulting from the nee of impefeectly orhiogonal codes PN-codes see Chapter 2 to separate the users. Thus power leakage occurs between the signals of different users. In this chapter we present two techniques which are used to optimize the performance or maximize the capacity of a PO-CDMA for terrestrial mobile or satellite networks in uplink transmission. These techniques are 1 adaptive power control and 2 multi-user detection. Power control is used to mitigate the near-far problem which appears at the PO-CDMA receiver. That is the power leakage to the signal of far user from the signal of a near user may be so severe that reception by the far-user may not be possible. A power control mechanism adjusts the transmit power of each user so that the received signal power of each user is approximately the same. Such a power control mechanism is presented in Section . Another more advanced technique that a PO-CDMA receiver may use to optimize performance is interference cancelation or multi-user detection. In Section we present a survey of multi-user detection methods that appears in the literature and we propose a new one based on minimum mean square error estimation and iterative decoding. Adaptive Power Control .