Code Division Multiple Access
COMT 391 - Winter Quarter 2000
Michael Huge

Since its official introduction in 1989 as a commercial wireless technology, Code Division Multiple Access has gained international acceptance as a wireless technology that will improve system capacity and service quality. Many PCS operators in the United States have chosen CDMA to run their wireless networks [6].

CDMA has seized the reigns as the technology of third generation wireless applications. In North America and Asia, CDMA operates on the NA-95 standard. In Europe, the UTRA system is being used [5]. CDMA has a market presence in most parts of the world, except for Western Europe, where third generation is likely to bring in CDMA. In an article posted on the cdg.org webiste, it was reported that cdmaOne (the older version of CDMA) is in service in 27 countries, with 35 more countries in the deployment stage. This, in large part, is a result of the January 1998 European Telecommunications Standards Institute’s decision to deploy two flavors of CDMA (Wideband CDMA and Time Division CDMA) as its third generation standards.

In the U.S., 1999 was a big year for CDMA. It was developed and expanded to over 10% of the U.S. geographic regions (while GSM and TDMA PCS trailed with 7.3% and 6.2% respectively). The total number of CDMA handset subscribers increased by 440% in a one-year period concluding in March of 1999. However, CDMA still trails Global Systems Mobile communication (GSM) and analog service worldwide. In Western Europe, CDMA presently has almost no market presence in a GSM-dominated race. In North America, analog users are much more common (over 51 million users) than CDMA users (over 15 million users). CDMA is presently on par with TDMA in the U.S. with respect to total number of users [4].

The basis of CDMA is spread spectrum, which has been used in the United States military since the 1940’s. Spread spectrum uses bandwidths much greater than those needed for point-to-point communication at the same data rate. The military used spread spectrum to resist enemy jamming of communications (called anti-jamming or AJ) or to hide communications altogether (called low probability of intercept or LPI). It became a more practical option for civilian and commercial use with the advent of low-cost, high-density integrated circuits, which reduced the size, weight, and cost of mobile units. Also realized was the fact that "...optimal multiple access communications requires that all user stations (mobiles) regulate their transmitter powers to the lowest levels that will achieve adequate signal quality." [6]. Communication channels are separated by pseudo-random modulation coding that is applied and removed in the digital domain. The modulation is not based on frequency

The benefits of CDMA as applied to cellular and PCS systems are many. CDMA improves traffic capacity and voice quality. It also reduces dropped calls resulting from handoff failures because of better handoff techniques. CDMA has better potential for data movement, which is obviously a big plus as we move into an era of wireless e-mail, internet, and other data transfer needs. For example, QUALCOMM and Ericsson are working together on a wireless technology solution to support both the Bluetooth standard and the CDMA digital wireless standard [6]. Incorporating Bluetooth and CDMA could conceivably make almost any wireless device compatible with any other wireless device, while at the same time, linking them over a high-capacity, high-data rate link with CDMA [3].

CDMA’s method of spread spectrum is called direct sequence spread spectrum, or DSSS (the other spread spectrum method is frequency hopping). DSSS requires the transmitter and receiver to be synchronized with one another. The conventional wave is multiplied by a pseudonoise (PN) +/- 1 binary sequence in the transmitter. It is a chips over bits scheme. Because the chips have a much shorter period than the user's data symbol period, the available bandwidth is increased. The chip rate in spread spectrum is inversely proportional to the chip period. For example, the narrowband chip period (according to IS-95) is 800 nanoseconds. This results in a chip rate of 1.25 MHz. In third generation CDMA, the wideband chip period permits the chip rate to increase to levels between 4 and 16 MHz, which permits higher data rate services [5]. Since the transmitter and receiver are synchronized, the receiver can run the signal through a replica bit pattern (demultiplier, in effect) to obtain the original signal. This use of chips over bits also allows multiple users to access the same channel in a given cell. Narrowband filtering can reject most interference.

The actual spreading of the signal takes place at the baseband level. The spread signal is then modulated in the second stage. After transmission, the received signal is demodulated to recover the spread signal. Then, the spread signal is despread to recover the original signal information. Because each user has his or her own unique pseudorandom code, CDMA provides protection from casual detection without the need to encrypt the signal [3].

When compared to time division multiple access and frequency division multiple access, the reuse factor of CDMA is far superior, partially because it is applied in a different manner than TDMA or FDMA. For example, in FDMA, if there are seven users on a channel, the channel is divided into seven segments, with each user being allocated a channel. The reuse factor (L) in this situation is 7, meaning that each user has access to only 1/7 of the available bandwidth. The same is true in time division multiple access. In CDMA, the entire spectrum can be reused. Therefore, the reuse factor is: L = 1, meaning that all users have access to all available bandwidth. This allows CDMA to operate with 4 to 6 times more capacity than digital TDMA or FDMA. When analog FM/FDMA is considered, CDMA outperforms by a factor of almost 20. Also, with CDMA, there is no need for frequency management because one channel is used for everyone. This results in soft handoffs and soft blocking.

Upon first observation, it may seem that extra bandwidth is required for CDMA use because the channel needed for communication is wider than a channel needed for normal point-to-point transmission. This problem can be easily solved if the number of CDMA users in a cell is larger than the bandwidth expansion factor as illustrated below. Once a certain number of users are on the system, CDMA outperforms more standard methods, like TDMA.

Another problem with CDMA is the near-far effect. In order to maintain the best service for all users, power levels must be matched so that the base station receives all signals at an equal level. If power equality is not achieved, interference will result. The mobile transmitting the most powerful signal will cause interference for other users. For this reason, the base station must constantly monitor and control the power levels of all current users. It may seem that this problem could be easily solved by assigning each mobile unit to transmit at the same power for a given number of units in operation (if there are 5 users in the cell, transmit power should be x for everyone, if 10 users, then x/2). However, this won't work because of the distance factor. If two mobiles in an area are transmitting at the same power levels from differing distances, the one closest to the base station will create the stronger signal with respect to the base because the closer signal has less distance to attenuate over. To illustrate the effect of overpowering, a signal 10dB stronger than another signal will cause the weaker signal to perceive interference equivalent to ten users of the weaker signal user's level. In other words, the 10dB disparity causes the system's support ability to drop by nine users. The key is to have the same power level received from each mobile. This means that users at a greater distance are required to transmit a stronger signal than those close by. This requires constant feedback from the base station. It also requires the mobile to be able to dynamically alter its transmit level. The whole system can require and adjustment range of up to 80dB. The adjustments are obtained through algorithms and other advanced mathematics beyond the scope of this paper [3].

The limit of CDMA is said to be cell density. This dictates the speed and quality at which the network will operate. To ensure proper cell density, the outbuilding of the network (and its subsequent management) is key [6].

1. Bull, David. Insights into Managing Mobile Multimedia Communications. Academic Press, NY. 1999.

2. Glisic, Savo. Spread Spectrum CDMA Systems for Wireless Communications. Artech House, Inc. Boston. 1997.

3. Pahlavan, Kaveh. Wireless Information Networks. John Wiley and Sons, Inc. NY. 1995.

4. Schnayerson, Ben. "Get Wireless - Cellular Standards Worldwide". Wired Magazine March 2000

5. Swarts, Francis. CDMA Techniques for Third Generation Mobile Systems. Kluwer Academic Publishers. Boston. 1999.

6. www.cdg.org