Code Division Multiple Access

What is CDMA?

Code Division Multiple Access (CDMA) is a method to implement multiple access among different communication systems. Here, Multiple Access means multiple users/subscribers and transmitters share a common communication channel at the same time to send/receive information from different locations. Radio spectrum is divided in to channels that are a portion of radio spectrum originated for the completion of a subscriber call. Multiple Access defines how this spectrum has to divided and how channels has to be allocated to different users. As multiple users use the same channel for transmission there are possibilities of interrupt. To overcome this CDMA or code division multiple access was proposed and implemented in the early nineties which allowed the users to use full frequency band for the user and giving a unique pseudo-code for each signal that is being transmitted. CDMA converts information in speech format to digital form and sends it through wireless network as radio signal.

Different standards of CDMA include cdmaOne,CDMA2000, WCDMA.

cdmaOne

This is the digital cellular standard based on CDMA or code division multiple access introduced by Qualcomm. cdmaOne is the brand name for Interim Standard 95 (IS-95). It is a standard meant for mobile telecommunications in 2G network that use CDMA for the transmission of voice and data between mobiles and cell stations. IS-95 describes a set of protocols for the connection between mobile telephones and network as an air interface. It is 3-layered protocol with L1 as the physical layer (PHY), L2 layer consists of Media Access Control (MAC) and Link-Access Control (LAC) sub-layers and L3 refers to call-processing state machine. Physical Layer has transmission of signals from network-to-mobile (forward direction) and mobile-to-network (reverse direction). In forward direction, that is, from network-to-mobile, the base transceiver stations (BTS’s) transmits radio signals. Transmissions are strictly time controlled with every BTS being synchronized with GPS receiver. Signals are spread with Walsh code and pseudo-random code and transmissions in forward direction are QPSK modulated. Mobiles transmit radio signals in case of reverse direction. Walsh code and pseudo-random code are used to spread the signals with the reverse transmissions being OQPSK modulated. The usage of traffic channel is restricted to the mobile once the call is established. The traffic channel in the MAC which allows the voice/data bits, that is being multiplexed with message formats for signaling gets a defined frame format. The LAC combines together the fragments of signaling messages. These messages are then passed to the L3 layer.

CDMA2000

It is a family with 3G mobile telecommunication standards which uses CDMA code division multiple access methodology to send voice and data between mobile phones and cell sites. CDMA2000 consists of a number of family of standards including CDMA2000 1xRTT, CDMA2000 1xEV-DO(with revisions), CDMA2000 1xEVDV. CDMA2000 1x (1x) is the standard in the case of CDMA2000 wireless air interface. The 1X supports a data speed of about 153kbits/s. CDMA 1xEV-DO( Evolution-Data only) is a standard used in telecommunications basically in broadband internet areas for sending radio signals. CDMA and TDMA included as multiplexing techniques are used to increase the throughput of the system as well as the individual user.

WCDMA

WCDMA along with other standards is found in 3G mobile communications as one of the air interface standard. The standard supports voice, MMS and text messages, also carries data at higher speeds which allows streaming and broadband internet usage by delivering higher bandwidth for mobile operators. The Core Network that is being used is same as 2G GSM network which allows mobile operation in dual mode along the GSM/EDGE. This is a feature that is being shared with UMTS family members.

CDMA Modulation

The multiple access involved in CDMA code division multiple access is a spread spectrum one. For the same power being transmitted, the data bandwidth is spread uniformly. The pseudo-random code used as the spreading code has an ambiguity function, kind of narrow. Data to be transmitted runs at a lower rate than a local code in a CDMA. The combining of data to be transmitted with the faster code is done via bitwise XOR. The data to be transmitted with pulse duration T s and the faster code with duration T f is XORed. 1/T s is the bandwidth of the data and 1/T f is the bandwidth of code signal. Since the code signal or spread spectrum signal is much smaller than the data signal, its bandwidth will be much larger. T s /T f is the spreading factor and describes the total users upper limit that is supported by the base station simultaneously.

Code used by each user in CDMA code division multiple access to modulate their corresponding signal is different. Codes selected affect the performance of the CDMA systems. The signal of the desired user and that of the local user should have a good separation to get the best performance. The received signal is correlated with that of the desired user’s local code for creating a separation between the signals. The system is able to extract the signal when the correlation function is high as the signal is matched with the desired user’s signal. The correlation will be close to zero when the codes do not match each other and that is called cross-correlation. In CDMA code division multiple access, user groups share a code, many of the codes are being occupied by the same channel but user’s part of a particular code can only communicate. There are two categories for CDMA or code division multiple access mainly: synchronous Code division multiple access and asynchronous CDMA.

Advantages of CDMA

Capacity increases and simplifies site selection

Reliable mode of data communication

Voice quality improvement and substantial reduction fading in multipath.

Average transmitted power is reduced.

Allocation of resources are flexible

Nationwide roaming is available as CDMA ,Code division multiple access is compatible with other mobile technologies.

Disadvantages of CDMA