Primary requirements for any wireless mode of communication include high quality of service and data secrecy. Realizing that these two factors are to be fulfilled in the most optimized ways without the costs going too high, CDMA, a spread spectrum based technology came into existence. Initially restricted to the armed forces, this technology was commercially launched in 1995 by Qualcomm Telecommunications and now, as per Q4 of the year 2011, CDMA has over 8 billion voice and data customers in the 122 countries that it operates.
What CDMA Means?
Code: It refers to the string of binary sequence that the transmitter and the receiver share. This code encodes the information into a low frequency signal before it is transmitted over a channel. This same code is used by the receiver to decode the information. The receiver attains the code with the help of the nearest base station.
Division: In CDMA a single channel is divided into numerous slots which can be used by multiple users. This is possible because of the use of unique code.
Multiple Accesses: Due to code based communication, multiple users can communicate and access the same channel simultaneously without any undesirable interference and loses.
Why we need CDMA?
CDMA is regarded as an improvement over GSM technology whose need can be easily understood by taking a simple example. Consider 5 couples that have their respective partners in different rooms. The partners are permitted to communicate only to each other and each is provided with a communication instrument for the purpose. The instruments are aided with a medium that facilitates the communication. The medium can be wired or wireless and is termed as “channel” in communication terminology. Since there is only one channel, the users are allotted some amount of time for which they can utilize the channel. In this case, let it be 5 seconds. So, every user communicates for 5 seconds and then the channel is used by other users.
The channel has a certain limit of allotting time slots and cannot accommodate more users after that. Let’s assume that in this case maximum number of users that the channel can accommodate is 6 i.e. 3 couples can use the channel. Hence, if all the couples want to communicate, 2 couples might have to wait till the channel has an empty slot. Also, there are chances that one couple might interfere in communication of the other due to sharing of same channel. This can be termed as “cross correlation” and is a serious problem in GSM operations. This is how a usual GSM system works. In normal GSM, the channel utilization time or “time burst” is determined by dynamic scheduling where number of users determine the time for which channel is used by a user.
When we go the CDMA way, every user’s voice is converted to a unique code which only the intended recipient instruments can understand. The code here is a “spreading sequence” of digital bits and is common between both the transmitter and the receiver. Since, code is digital, the information to be sent is also required to be converted to a digital format. With this method, there are no time boundations and even if all users are using the channel at the same time, there will be no interference and secrecy will be maintained. In CDMA, since the code is unique for every transmitter, it is determined by the receiver in two steps: Acquisition and tracking.
Under acquisition, the receiver acquires the sent signal and generates the decoding sequence which it receives from the base station. In tracking, it keeps synchronization between the received signal and decoding sequence so that the output is exactly same as the input.
History of CDMA
The idea of using CDMA as commercialized or licensed technology came in the year 1988. In 1993, the first protocol under CDMA IS-95 was introduced and in 1995, it was commercialized. Since that time, lots of changes in CDMA technology have occurred in terms of resource allocation, data usage, bit rate and several such factors.
Interim Standard-95 or TIA-EIA-95 is the first 2G-CDMA based cellular standard by Qualcomm and has been branded as cdmaOne. IS-95 defines forward (Base station to mobile) and reverse (mobile to Base station) link specifications. CDMA2000 is a 3G standard, backward compatible with IS-95. IS-2000 or CDMA2000 1X is the core wireless air interface standard. CDMA 2000 differs from IS-95 is that it includes beam forming; this increases the gain at the mobile and allows better SNR and a larger number of users. Also, CDMA2000 has double the capacity of IS-95.
WCDMA was developed in order to support high bandwidth applications like gaming, multimedia services, etc. The later version of CDMA is WCDMA systems which combine the CDMA air interface with GSM based networks. In contrast to cdmaOne and CDMA2000 (which uses 1.25 MHz wide radio signal), WCDMA uses a 5 MHz wide radio signal and a chip rate of 3.84 Mbps, which is about three times higher than the chip rate of CDMA2000 (1.22 Mbps). Thus WCDMA offers higher capacity and QoS.