第4章-CDMA数字蜂窝移动通信系统.ppt

* 功率控制保证每个用户只用不高于最低功率需求值的发射功率，使给其他用户干扰最小。 通常所需Eb/N0 6dB 处理增益W/R在21dB 当信噪比为-15dB时，信号质量可接收。 * * * * * * * * * * * * * * * CDMA系统导频集 G网切换是载频之间的切换，c网是导频切换 导频集是指具有相同的频率但有不同的PN码相位的导频集合。 激活集：与正在联系的基站对应的导频集合。 候选集：当前不在有效集中，但是已有足够的强度表明与该导频对应基站的前向业务信道可以被成功解调的导频集合。 相邻集：当前不在有效集或候选集中但是有可能进入候选集的导频集合。 剩余集：其它导频集合。 T_ADD * * * * * * * * CDMA的英文全拼为Code Division Multiple Access（码分多址）。CDMA是一种采用先进无线扩频技术的数字通信多址接入方式。 CDMA多址实现是给每一个信号分配一个伪随机二进制序列进行扩频，不同信号的能量被分配到不同的伪随机序列；在接收机中，信号用相关器加以分离，这种相关器只接收选定的二进制序列并压缩其频谱。 * Following AMPS’ deployment, several new technologies have been introduced. Time Division Multiple Access (TDMA) is the technique used by the IS-54/IS-136 and the GSM standards. These technologies transmit phase modulated digital signals, and combine multiple users onto each individual radio signal using a scheme of repeating time slots. IS-54/IS-136 allows three users per 30 KHz. signal (six users likely later with a better vocoder), while GSM allows eight users per 200 KHz. signal (possibly 16 later with a better vocoder). To hear a TDMA user, the receiver must know what frequency is being used, and which time slots contain the user’s bits. Thus, users can be thought of as divided both within frequency and time, and a channel is defined as a frequency and a time slot. CDMA operates quite differently from the foregoing technologies. In CDMA, all users generally transmit simultaneously in a “herd” on a single frequency. Therefore, neither time nor frequency can be usefully exploited to recover the signal of one user. Instead, each user transmits large numbers of “chips” to represent individual bits of its message. These chips are generated using special coding patterns which are unique user-to-user. An unrelated user’s chip pattern appears to add up to nothing when interpreted using the chip pattern of the desired user. By integrating every users’ recovered chips over the entire period of one bit, the desired user’s bit value becomes apparent and all other signals an