Have you heard of the telecom wars recently? Recently some telecom service providers say ’’Our service is based on CDMA technology, the efficient one’’ and some others say ’’GSM is the true mobile’’ etc. What are these CDMA and GSM and their features?
Since the radio spectrum is a limited resource and has to be shared by all the users a number of techniques have been devised to allow multiple access of the same frequency band by different users. These techniques are TDMA (Time division multiple access), FDMA (Frequency division multiple access) and CDMA (Code division multiple access).
Coming to the two mobile systems CDMA and GSM:
CDMA (Code Division Multiple Access)is a form of multiplexing (access to the same resource will be given to more than one user) , which allows the use of a particular frequency for a number of signals, optimizing the use of available bandwidth. It is a cellular technology that uses spread-spectrum techniques. In CDMA technology every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence.
CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum technology. Audio input is first digitized (ADC) into binary elements. The frequency of the transmitted signal is then made to vary according to a defined pattern (code), so it can be intercepted only by a receiver whose frequency response is programmed with the same code, so it follows exactly along with the transmitter frequency. There are trillions of possible frequency-sequencing codes; this enhances privacy and makes cloning difficult. The technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.
CDMA was first used during World War II by the English allies to foil German attempts at jamming transmissions. The allies decided to transmit over several frequencies, instead of one, making it difficult for the Germans to pick up the complete signal.
GSM (Global System for Mobile Communications) is a form of multiplexing, which divides the available bandwidth among the different channels.
GSM is a combination of Time and Frequency-Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart. Each of these carrier frequencies is then divided in time, using a TDMA scheme. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are grouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), which forms the basic unit for the definition of logical channels. One physical channel is one burst period per TDMA frame. Thus GSM allows eight simultaneous calls on the same radio frequency.
GSM was first introduced in 1991 and until recently before the establishment of CDMA networks, GSM was the only mobile communication system present in the market.
Since bandwidth is the major problem in the modern times the CDMA has a very clear advantage over the GSM in these terms. The number of channels(users) that can be allocated in a given bandwidth is comparatively higher for CDMA than for GSM. The cost of setting up a CDMA network is also comparatively less than the GSM network. Due to these advantages there is high probability that CDMA technology will dominate the future of mobile communications.