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.

圖5.3顯示如何以一種循環復用的方式把時隙分配給用戶，每個用戶每幀分得一個時隙。

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.

待發送的輸入數據在前一幀期間被緩存，在分配給該信道的時隙中以較高速率爆發式發送出去。

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.

系統的處理增益是指擴頻系統通過擴頻和反擴頻的性質所表現出來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.

這一處理過程使每一用戶的信號與所有其他用戶的信號正交，因而相互之間沒有干擾。

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.

如果一個用戶使用的Walsh碼在時間上相對于其他所有Walsh碼移動了超過約十分之一的碼片周期，就失去了正交性，導致用戶間干擾。

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.

由于用戶之間不可避免的定時偏差，Walsh碼幾乎沒用，因為它們之間不再正交。

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.

反向鏈接是非正交的，導致用戶間的嚴重干擾。由于這一原因，反向信道限制了系統的容量。

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（正交頻分復用）與單載波方案相比的主要優點是不需要復雜的均衡濾波器就能應對嚴重的信道問題，如：在長銅線中的高頻衰減、窄帶干擾以及由于多路徑而引起的頻率選擇性衰落。

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

正交性也使OFDM的頻譜利用率到接近于Nyquist頻率。

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

OFDM信號一般具有“白的”頻譜，使之在與其他用戶使用同一信道的情況下具有良好的抗電磁干擾性質。

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.

當只有多普勒頻移時可以用接收機來補償，當多普勒頻移和多路徑結合在一起時，情況就變得更糟，因為反射會出現在不同的頻率偏移上，這種偏移很難校正。

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的一個關鍵的特性是因為低符號速率調制方案（也就是與信道時間特性相比，符號的持續時間相對較長）很少受到由多徑引起的符號間干擾的影響，并行地傳輸許多低速率數據流要比傳輸一個高速率數據流有利。

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.

在保護間隔中傳輸的循環前綴是由復制到保護間隔中的OFDM符號的尾部組成的，保護間隔是在OFDM符號之前傳輸的。

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，當一部分信道帶寬衰減時，頻率交織將確保由帶寬衰減部分的那些子載波產生的比特誤差會分散在整個比特流上而不是集中起來。