WCDMA ( Wideband CDMA) is CDMA radio communication mode based on direct spread-spectrum technology.
WCDMA has an obvious advantage over GSM and IS-95 in subscriber capacity and radio transmission performance, for it adopts a series of key technologies.
WCDMA bears following two meanings literally:
- WCDMA adopts CDMA communication technology.
CDMA technology is the most advanced communication technology in the world at present. It takes advantage of different codes to divide different channel and then distinguish different subscriber.
- WCDMA adopts wider spectrum
Narrowband power signals are sent out after being spread as broadband signals (spread-spectrum) with WCDMA technology.
Broadband signals have stronger anti-interference ability than narrow band signals. Wider bandwidth realizes RAKE receiving at subscriber end and increases communication quality.
Figure 5 shows WCDMA communication.
Bandwidth of original signals increases and power density decreases after spread-spectrum.
Signals meet with noise during the transmission.
Power density of the noise decreases after the dispreading, for spectrum dispreading is the same as spectrum spreading.
However, power density of original signals is much larger than that of noise (that is, signal-to-noise ratio is high) and it is easy to resume.
Figure 5 WCDMA Communication Principle
WCDMA adopts such advanced technologies as soft handover, diversity and power control to enlarge system capacity and increase communication quality greatly.
Mobile communication systems can be classified in multiple ways. For example, there are analog and digital by the nature of the signals; FM, PM, and AM by the modulation mode; and FDMA, TDMA and CDMA by the multiple access mode.
CDMA (Code Division Multiple Access) is a new while mature wireless technology developed from the spread spectrum communication technology, a branch of the digital technology.
Currently, the GSM mobile telephone networks of China Unicom and China Mobile are built with the combination of FDMA and TDMA.
GSM has tremendous advantages over the analog mobile telephone system. However, its spectrum efficiency is only three times of the analog system.
With a limited capacity, it has difficulty in offering voice quality equivalent to wired telephone. TDMA terminals support an access rate of only 9.6 kbps.
The TDMA system does not support soft handover, so calls may easily be dropped, affecting the service quality.
Therefore, TDMA is not the best technology for modern cellular mobile communication.
On the other hand, CDMA fully meets the requirements of modern mobile communication networks for large capacity, high quality, and integrated services, so it is well received by increasingly more operators and users.
CDMA emerges from the needs for wireless communications of higher quality.
In the CDMA communication system, the signals used by different users for information transmission are distinguished not by frequencies or timeslots, but by different code sequences.
CDMA allocates one pseudo random binary sequence for each signal for frequency spreading, and different signals are allocated with different pseudo random binary sequences.
In the receiver, correlators are used to separate the signals. The correlator of each user only receives the specified binary sequences and compresses their frequency spectrums, while ignoring all the other signals.
The code division multiple access concept of CDMA can be illustrated with a party of many persons.
At the party, many users talk at the same time in a room, and every conversation in the room is in a language you do not understand.
From your perspective, all these conversations sound like noise.
If you know these “codes”, that is, relevant languages, you can ignore the conversations you do not want to hear, and focus on only these you are interested in.
The CDMA system filters the traffic in a similar way. However, even if you understand all the languages used, you do not necessarily hear clearly all the conversations you are interested in.
In this case, you can tell the speakers to speak louder, and/or ask others to lower their voices. This is similar to the power control in the CDMA system.
In the frequency domain or time domain, multiple CDMA signals overlap. The receiver can sort out the signals that use the preset code pattern from multiple CDMA signals by using correlators.
Other signals using different code patterns are not demodulated, since their code patterns are different from those generated locally at the receiver.
One of the basic technologies of CDMA is spectrum spreading. CDMA is a multiple access technology featuring high confidentiality.
It was first developed in the Second World War to prevent interference from the enemies.
CDMA found wide application in anti-interference military communications during the war.
After 1960’s, it had been used in military satellite communication. Later, it was developed by Qualcomm into a commercial mobile communication technology.
After the first CDMA system was put into operation for commercial purpose in 1995, the technical advantages of the CDMA in theory were tested in practice, so it has seen rapid application in North America, South America and Asia.
In many countries and regions in the world, including China, Hong Kong, South Korea, Japan, and USA, CDMA is the major mobile communication technology used.
CDMA is superior to TDMA in system capacity, anti-interference, communication quality, and confidentiality, so IMT-2000 (3G) launched by ITU and subsequent standards all employ CDMA.
The basic characteristic of spread spectrum communication is that it uses a bandwidth for information transmission much wider than that of the information itself.
In other words, the data for transmission with certain signal bandwidth is modulated with high-speed pseudo random codes having a bandwidth wider than the signal bandwidth.
Thus, the bandwidth of the original data signals is spread, before the signals are transmitted following carrier modulation.
The receiving end uses exactly the same pseudo random codes to process the received bandwidth signals, converting the broadband signals into the original narrowband signals, that is, despreading, thus achieving information communication.
In addition, spread spectrum communication also has the following characteristics:
- It is a digital transmission mode.
- Bandwidth spreading is implemented by modulating the transmitted information with a function (spread spectrum function) irrelevant to the transmitted information.
- At the receiving end, the same spread spectrum function is used to demodulate the spread spectrum signals, restoring the transmitted information.
C.E. Shannon found the channel capacity formula in his research in information theory, as below:
C = W × Log2 (1+S/N)
C – Information transmission rate
S – Available signal power
W – Bandwidth of the line
N – Noise power
As can be seen from the formula:
To increase C, you can either increase W or increase S/N. In other words, when C is constant, W and S/N are interchangeable, where the increase of W reduces the requirement for S/N.
When the bandwidth increases to a certain level, the S/N is allowed to further decrease, making it possible for the useful signal power to decrease to a level close to the noise power or even inundated in the noise.
Spread spectrum communication uses the bandwidth transmission technology to obtain the benefit in S/N, which is the basic idea and theoretical basis of spread spectrum communication.
Spread spectrum communication has many outstanding performances insuperable by narrowband communication, enabling it to find wide application rapidly in various public and private communication networks.
Its advantages are outlined as below:
- Powerful anti-interference and low bit error rate
The spread spectrum communication system spreads the signal spectrum at the transmitting end and restores the original information at the receiving end, producing spread gains, thus greatly increasing the anti-jamming margin. Depending on the spread spectrum gains, signals can be extracted from noise even when the S/N is negative.
In the current commercial communication system, spread spectrum communication is the only communication mode that can work in the negative S/N environment.
- Easy same frequency use for higher radio spectrum utilization
Radio spectrum is very valuable. Although all waves from long wave and micro wave have been developed and used, the need of the society is not satisfied.
For this reason, frequency spectrum management authorities were set up all over the world. Users can only use the frequencies granted, and divide them into channels to avoid mutual interference.
As spread spectrum communication uses the correlation reception technology, the signal transmission power is extremely low (<1 W, usually 1 mW ~ 100 mW), and can work in channel noise and hot noise background.
Therefore, a frequency can be easily reused in the same area, and the frequency can also be shared with the now various narrow band communications.
- Anti multipath interference
In the wireless communication, anti multipath interference is a persisting problem that is difficult to solve.
With the correlation between spread spectrum codes, the most powerful useful signals can be extracted from multipath signals at the receiving end with a related technology.
Also, the same code sequence waveform from multiple paths can be added for reinforcement, to achieve effective anti multipath interference.
- Spread spectrum communication is a form of digital communication, particularly suitable for synchronous transmission of digital voice and data. Spread spectrum communication offers the encryption function for good confidentiality, making it easy to launch various communication services.
Using multiple new technologies including code division multiple access, and voice compression, spread spectrum communication is more suitable for transmission of computer network and digitized voices and images.
- Spread spectrum communication involves mostly digital circuitry. Its equipment is highly integrated, easy to install and maintain, compact, and reliable and easy to mount/expand, and has a long MTBF.