Notes on the Text

Part I

1. Figure 5.3 shows how the time slots are provided to users in a round robin fashion, with each user being allotted one time slot per frame.

图5.3显示如何以一种循环复用的方式把时隙分配给用户，每个用户每帧分得一个时隙。

2. The input data to be transmitted is buffered over the previous frame and burst transmitted at a higher rate during the time slot for the channel.

待发送的输入数据在前一帧期间被缓存，在分配给该信道的时隙中以较高速率爆发式发送出去。

3. The process gain of a system indicates the gain or signal to noise improvement exhibited by a spread spectrum system by the nature of the spreading and despreading process.

系统的处理增益是指扩频系统通过扩频和反扩频的性质所表现出来的增益或信噪比的提高。

4. By doing this the signals from each user is orthogonal to every other user, resulting in no interference between the signals.

这一处理过程使每一用户的信号与所有其他用户的信号正交，因而相互之间没有干扰。

5. If the Walsh code used by one user is shifted in time by more than about 1/10 of a chip period with respect to all the other Walsh codes, it loses its orthogonal nature, resulting in inter-user interference.

如果一个用户使用的Walsh码在时间上相对于其他所有Walsh码移动了超过约十分之一的码片周期，就失去了正交性，导致用户间干扰。

6. Due to the unavoidable timing errors between the users, there is little point in using Walsh codes as they will no longer be orthogonal.

由于用户之间不可避免的定时偏差，Walsh码几乎没用，因为它们之间不再正交。

7. The reverse link is not orthogonal, resulting in significant inter-user interference. For this reason the reverse channel sets the capacity of the system.

反向链接是非正交的，导致用户间的严重干扰。由于这一原因，反向信道限制了系统的容量。

Part Ⅱ

1. The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions—for example, attenuation of high frequencies at a long copper wire, narrowband interference and frequency-selective fading due to multipath—without complex equalization filters.

OFDM（正交频分复用）与单载波方案相比的主要优点是不需要复杂的均衡滤波器就能应对严重的信道问题，如：在长铜线中的高频衰减、窄带干扰以及由于多路径而引起的频率选择性衰落。

2. The orthogonality also allows high spectral efficiency, near the Nyquist rate.

正交性也使OFDM的频谱利用率到接近于Nyquist频率。

3. OFDM generally has a nearly “white” spectrum, giving it benign electromagnetic interference properties with respect to other co-channel users.

OFDM信号一般具有“白的”频谱，使之在与其他用户使用同一信道的情况下具有良好的抗电磁干扰性质。

4. Whilst Doppler shift alone may be compensated for by the receiver, the situation is worsened when combined with multipath, as reflections will appear at various frequency offsets, which is much harder to correct.

当只有多普勒频移时可以用接收机来补偿，当多普勒频移和多路径结合在一起时，情况就变得更糟，因为反射会出现在不同的频率偏移上，这种偏移很难校正。

5. One key principle of OFDM is that since low symbol rate modulation schemes (i.e. where the symbols are relatively long compared to the channel time characteristics) suffer less from intersymbol interference caused by multipath, it is advantageous to transmit a number of low-rate streams in parallel instead of a single high-rate stream.

OFDM的一个关键的特性是因为低符号速率调制方案（也就是与信道时间特性相比，符号的持续时间相对较长）很少受到由多径引起的符号间干扰的影响，并行地传输许多低速率数据流要比传输一个高速率数据流有利。

6. The cyclic prefix, which is transmitted during the guard interval, consists of the end of the OFDM symbol copied into the guard interval, and the guard interval is transmitted followed by the OFDM symbol.

在保护间隔中传输的循环前缀是由复制到保护间隔中的OFDM符号的尾部组成的，保护间隔是在OFDM符号之前传输的。

7. For example, when a part of the channel bandwidth is faded, frequency interleaving ensures that the bit errors that would result from those subcarriers in the faded part of the bandwidth are spread out in the bit-stream rather than being concentrated.

例如，当一部分信道带宽衰减时，频率交织将确保由带宽衰减部分的那些子载波产生的比特误差会分散在整个比特流上而不是集中起来。