Figure 6 : Spectrum Spreading/Dispreading in DS-CDMA
A radio channel is an adverse transmission channel. When digital signals transmitted over a radio channel, bit errors may occur in the transmission data flow due to various reasons, causing image jumps and disconnection at the receive end.
The step of channel coding can be used to process the data flow appropriately, so that the system can have error correction capability and anti-interference capability to certain extent, thus greatly avoiding bit errors in the code flow.
Therefore, channel coding aims at increasing data transmission efficiency by reducing bit error rate.
Ultimately, channel coding intends to increase the reliability of the channel, but it may reduce the transmission of useful information data.
Channel coding works by inserting some code elements, usually referred to as overhead, into the source data code flow, for error detection and correction at the receiving end.
This is like the transport of glasses.
To ensure that no glasses are broken during this process, we usually use foams or sponge to package them. However, such packaging reduces the total number of glasses.
Similarly, over a channel with fixed bandwidth, the total transmission code rate is fixed. As channel coding increases data amount, the useful information code rate is reduced. This is the cost.
The number of useful bits divided by the total number of bits derives the coding efficiency, which varies slightly from one coding mode to another.
The coding/decoding technology and interleaving technology can work together to increase the bit error performance.
Compared with the case without coding, the traditional convolution code can increase the bit error rate by two orders of magnitude, to 10-3 ~ 10-4, and the Turbo code can further increase the bit error rate to 10-6.
Because the Turbo code has a coding performance close to the limit of Shannon theorem, it is adopted as the data coding/decoding technology for 3G. The convolution code is mainly used for voice and signaling of low data rates.
Interleaving/deinterleaving is an important step of the combined channel error correction system. The actual errors in the channel are usually burst errors or both burst errors and random errors.
If burst errors are first discretized into random errors, which are then corrected, the system’s anti-interference performance can be improved.
The interleaver works to discretize long burst errors or multiple burst errors into random errors, that is, discretizing the errors.
The interleaving technology rearranges the coded signals by following certain rules.
After deinterleaving, burst errors are dispersed over time, making them similar to random errors that occur separately.
Spread Spectrum is an information transmission mode.
It modulates information signals with spreading code at sending end and enables spectrum width of information signals much wider than bandwidth for information transmission.
It dispreads at receiving end with same spreading code, to resume data of transmitted information.
Figure 6 shows basic operations of spectrum spread/dispread. Supposing subscriber data rate is R, subscriber data is 101101, and according to the rule that 1 is mapped as -1, 0 is mapped as +1, map subscriber data as -1+1-1-1+1-1 and time it with spreading code. Spreading code is 01101001 in this example. Time each subscriber data bit to this code series including 8 code chips. Concluded data rate after spread is 8 × R and is random, like spreading code. Its spread spectrum factors are 8.
Broadband signals after spread spectrum are transmitted to receiving end via radio channels.
Time code sequence with same spread spectrum code (dispreading code) when dispreading at receiving end to resume original subscriber data.
Spreading signal speed by 8 times factor may result in bandwidth spreading of subscriber data signals (therefore, CDMA system is often called spread spectrum system).
Dispreading resumes signal rate to original rate.
Figure 6 Spectrum Spreading/Dispreading in DS-CDMA
Distributing different spread spectrum to different subscriber can distinguish different subscriber, as shown in above sector.
Supposing that there are three subscribers and that signals they send are b1, b2 and b3, spread their signals with spreading code of c1, c2 and c3 and final sending signal is y=b1c1 + b2c2 + b3c3. Supposing that there is no interference in signal transmission, the receiving end:
- Gets signals after dispread with c1
- z1 = y * c1 = c1 * (b1c1 + b2c2 + b3c3) = b1 + (b2c2c1 + b3c3c1)
- Gets signals after dispread with c2
- z2 = y * c2 = c2 * (b1c1 + b2c2 + b3c3) = b2 + (b1c1c2 + b3c3c2)
- Gets signals after dispread with c3
- z3 = y * c3 = c3 * (b1c1+b2c2+b3c3) = b3 + (b1c1c3 + b2c2c3)
All parts in the brackets in above three formulas are interference of other subscriber signals to this signal. This interference can be absolutely avoided if using orthogonalized codes.
Orthogonalized code is the code that is 1 after timing itself and is 0 after timing other codes. So:
z1 = y * c1 = c1 * (b1c1 + b2c2 + b3c3) = b1 + (b2c2c1 + b3c3c1) = b1 + 0 + 0 = b1
z2 = y * c2 = c2 * (b1c1 + b2c2 + b3c3) = b2 + (b1c1c2 + b3c3c2) = b2 + 0 + 0 = b2
z3 = y * c3 = c3 * (b1c1 + b2c2 + b3c3) = b3 + (b1c1c3 + b2c2c3) = b3 + 0 + 0 = b3
Modulation is the process to use one signal (know as modulation signal) to control another signal of carrier (known as carrier signal), so that a characteristic parameter of the later changes with the former.
At the receiving end, the process to restore the original signal from the modulated signal is called demodulation.
During signal modulation, a high-frequency sine signal is often used as the carrier signal.
One sine signal involves three parameters: amplitude, frequency and phase.
Modulation of each of these three parameters is respectively called amplitude modulation, frequency modulation, and phase modulation.
In the WCDMA system, the modulation is Quaternary Phase Shift Keying (QPSK).
If High Speed Downlink Package Access (HSDPA) is used, the downlink modulation mode can also be 16QAM.
Modulating rate of WCDMA uplinks/downlinks are both 3.84 Mcps and modulate complex-valued code chip sequence generated by spread spectrum in QPSK mode.
Figure 7 shows uplink modulation and Figure 8 shows downlink modulation.
Figure 7 Uplink Modulation
Figure 8 Downlink Modulation