Code Division Multiple Access

Comt 391

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.

CDMA has seized the reigns as the technology of third generation wireless applications. 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 it’s 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.

History

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." (www.cdg.org) 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.

Effects on cellular/PCS

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 also has better potential for data movement, and this 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. Incorporating Bluetooth and CDMA could conceivably make almost any wireless device compatible with any other wireless device.

CDMA’s method of spread spectrum is called direct sequence spread spectrum, or DSSS (the other 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. 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. Narrowband filtering can reject most interference.

(diagram here)

Comparison to TDMA/FDMA

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 for 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.

Disadvantages

Some disadvantages in CDMA are self-jamming and the near-far effect. In the latter problem, distance becomes an issue. Because distance from transmitters dictates power levels in CDMA, differing power levels can result, and thus cause interference. To correct this problem, the base sends signals to the mobiles, instructing them to adjust their power levels accordingly.

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.

Resources

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

5. www.cdg.org