One of the most important concepts to any cellular telephone system is that of “multiple access”, meaning that multiple, simultaneous users can be supported. In other words, a large number of users share a common pool of radio channels and any user can gain access to any channel (each user is not always assigned to the same channel). A channel can be thought of as merely a portion of thelimited radio resource, which is temporarily allocated for a specific purpose, such as someone’s phone call. A multiple access method is a definition of how the radio spectrum is divided into channels and how channels are allocated to the many users of the system.
Now a days, there is an ever-increasing demand for mobile communications. Existing mobile systems such as AMPS (Advanced Mobile Phone Services), GSM (Global System for Mobile communication), can only provide a limited capacity, because in these systems, traditional multiple access techniques (FDMA, TDMA, or combination of them) are used. Severe congestion of mobile communication spectrum in urban business areas dramatically illustrates the need to seek a new cellular system which uses the spectrum more efficiently. This is the drive behind CDMA initiative. CDMA is a modulation and multiple access technique that is based on spread spectrum technology. It is designed to combat the problem of capacity shortage in mobile telecommunication world. With CDMA, multiple user canshare the same frequency band without interfering each other if the transmit power of mobile station (MSs) is carefully controlled. The same frequency can be used in all neighboring cells (almost 1/1 frequency reuse). On the other hand there is no hard limit on the number of active users that can be serviced by a base station (BS). When the number of active users exceeds the design value, more traffic channels can be provided by degrading quality of service. This is usually referred to as soft capacity. Therefore, the capacity of CDMA systems is potentially unlimited.
IS-95CDMA is designed to co exist with AMPS system in America. That is, CDMA supports dual-mode operation. Mobile stations can switch between AMPS and CDMA when necessary. CDMA has many attractive advantages, due to these advantages CDMA has also been proposed as a candidate for the third generation mobile and personal telecommunication systems such as global FPLMTS (Future Public Land Mobile Telecommunication Systems), the Europeans UMTS (Universal Mobile Telecommunication Systems) and PCN (Personal Communication Network).
What is CDMA?
CDMA it stands for Code Division Multiple Access. In this technique we can say that alot many subscribers can talk at a particular time that is as many as 95million subscribers worldwide can talk. The great attraction of CDMA technology from the beginning has been the promise of extra ordinary capacity increase over narrow band multiple access wireless technologies. CDMA is aspread-spectrum technology that allows multiple frequencies to be used simultaneously. CDMA codes every digital packet it sends with a unique key(code). A CDMA receiver responds only to that key and can pick out and demodulate the associated signal. Simple model suggests that the capacity improvement may be more than 20 times that of the existing narrowband cellular standard, such as AMPS in North America, NTM in Scandinavia, TACS in United Kingdom. Historically the capacity was calculated using simple arguments. Reality, of course, is much more complicated than the idealized model. Realcell coverage area is highly irregular, not the neat hexagon. Offered load isnot spatially uniform, changes dramatically with time of day, and is often subject to other uncontrollable influences.
Adigital wireless technology that uses a "spread spectrum" technique to scatter a radio signal across a wide range of frequencies. CDMA provides greater total capacity than other technologies, along with outstanding voice quality and fewer dropped calls. CDMA is the technology of choice for new third-generation systems.
CDMA works by converting speech into digital information, which is then transmitted as a radio signal over a wireless network. Using a unique code to distinguish each different call, CDMA enables many more people to share the air waves at the same time - without static, cross-talk or interference.
Advantages of CDMA Technology
CDMA is a new technology, which is developing at agreat speed these days. Compared with other cellular systems based on FDMA or TDMA, like analog and digital cellular systems, CDMA cellular system has advantages in several aspects: capacity, tolerance to noise, and frequency planning. They can be summarized as below:
- Soft Capacity - This means high capacity gain i.e. thecapacity of the cdma channel is much more than analog and digital channel. Wecan say that much data can be transmitted at a high speed.
- Soft hand off - When a subscriber moves from one cell toanother a connection is established between the subscriber and the new BTSautomatically this technique is called make before break and so it is called soft handoff. When thereis a break, when a subscriber moves from one cell to another and then aconnection is established it is called break before make.
- High capacity- CDMA system provides anincrease in capacity in order of 4 to 6 fold over digital TDMA (e.g., GSM,IS-54) and 10 fold over analog FM/FDMA (AMPS). A number of CDMA featurescontribute to the increased capacity. High capacity is due to the singlefrequency channel, with one to one frequency reuse. The total bandwidth inwhich the CDMA signal propagates is 800MHz-1900MHz which is of very high bandwidth so thecapacity is also high. as many as 95 million subscribers worldwide can use thistechnology at the same time.
- Multiple forms of diversity- The CDMA systememploys three types of diversity—time diversity, frequency diversity, space(path) diversity, and thus effectively reduces the impact of multipath fading.
- Noise suppression- Based on spectrumspreading , the CDMA receiver can operate in very low SNR (Signal-to-NoiseRatio) environment (say less than 0db), due to its high processing gain (21db).
- Low RF transmit power- The CDMA transmitters(MS/BS) always transmit at the lowest possible power level. A combination ofopen-loop and closed-loop power control mechanism is implemented to control thetransmit power of mobile stations. In fact, the average mobile transmit poweris less than 7 mW. This helps minimize interface and save mobile battery life.
- No hard handoff- Since every cell uses thesame RF frequency, the only difference is the code sequence. Therefore, nohandoff from one frequency to another frequency while from moving from one cellto another cell. Furthermore, handoff is performed in a make before breakmanner, which means the mobile will maintain connection with both old and newcell sites before completely switched to the new cell. This is so called soft-handoff.
- Support variable bit rate- The vocoder(voice encoder/decoder) in the CDMA system converts speech into a binary streamwhose bit rate is dependent on the speech activity. Four rates are supposed:9600 kbps, 4800 kbps, 2400 kbps, 1200 kbps. The result of implementing variablebit rate is suppression of background noise and good voice transmission even ina noisy environment.
- Minimal frequency planning- In FDMA andTDMA systems, frequency planning must be carried out with great care so as toreduce co-channel interface. In the CDMA system, frequency planning is almostunnecessary because only one common frequency channel is used in all cellsites.
- Dual mode operation- IS-95 CDMA cellular systemsupports both AMPS and CDMA services. The compatibility ensures a gradualevolution from analog to digital system.
Many subscribers can make a call at the same timethis means that as many as 95 million subscribers worldwide can use thistechnology at the same time.
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