code division multiple access

-Offering a world of opportunities for multimedia applications
ABSTRACT

This paper provides an introduction to Code Division Multiple Access (CDMA) communications, which is a form of Direct Sequence Spread Spectrum communications. CDMA is a major protocol in wireless communications, which allows multiplex signals by assigning different codes to each subscriber to access the same bandwidth. It elucidates the important role played by this technology in providing better access to many subscribers at a time in wireless communications. In this presentation, the major difference between time division and frequency division multiple access (TDMA and FDMA) and CDMA techniques is emphasized graphically. The technical aspects involved in synchronous CDMA and asynchronous CDMA are discussed briefly. The advantages and disadvantages of CDMA techniques over GSM, which is another major technique used in mobile communications. The paper also gives a briefing about the history of the CDMA along with the techniques used by CDMA, like soft hand-off. Finally, it outlines the major applications and the emerging trends in CDMA technology such as wideband CDMA and CDMA2000.

INTRODUCTION

The world is demanding more from wireless communication technologies than ever before as more people around the world are subscribing to wireless. Adding to exciting Third-Generation (3G) wireless data services and applications - such as wireless email, web, digital picture taking/sending, assisted-GPS position location applications, video and audio streaming and TV broadcasting - and wireless networks are doing much more than just a few years ago. This is where CDMA technology fits in. Code Division Multiple Access (CDMA) is a technology developed by Qualcomm in the United States, and it is currently the dominant network standard in North America. CDMA consistently provides better capacity for voice and data communications than other commercial mobile technologies, allowing more subscribers to connect at any given time, and it is the common platform on which 3G technologies are built. CDMA is a "spread spectrum" technology, allowing many users to occupy the same time and frequency allocations in a given band/space. As its name implies, CDMA (Code Division Multiple Access) assigns unique codes to each communication to differentiate it from others in the same spectrum. In a world of finite spectrum resources, CDMA enables many more people to share the airwaves at the same time than do alternative technologies. The CDMA air interface is used in both 2G and 3G networks. 2G CDMA standards are branded cdmaOne and include IS-95A and IS-95B. CDMA is the foundation for 3G services: the two dominant IMT-2000 standards, CDMA2000 and WCDMA, are based on CDMA.

FREQUENCY SPECIFICATIONS AND WORKING PRINCIPLES IN BRIEF:
CDMA (Code-Division Multiple Access) refers to any of several protocols used in so-called second-generation (2G) and third-generation (3G) wireless communications. As the term implies, CDMA is a form multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. The technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.
CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum technology. Audio input is first digitized into binary elements. The frequency of the transmitted signal is then made to vary according to a defined pattern (code), so only a receiver whose frequency response is programmed with the same code, so it follows exactly along with the transmitter frequency can intercept it. There are trillions of possible frequency-sequencing codes, which enhance privacy and makes cloning difficult.
. The CDMA channel is nominally 1.23 MHz wide. CDMA networks use a scheme called soft handoff, which minimizes signal breakup as a handset passes from one cell to another. The combination of digital and spread-spectrum modes supports several times as many signals per unit bandwidth as analog modes. CDMA is compatible with other cellular technologies; this allows for nationwide roaming The original CDMA standard, also known as CDMA one and still common in cellular telephones in the U.S., offers a transmission speed of only up to 14.4 Kbps in its single channel form and up to 115 Kbps in an eight-channel form. CDMA2000 and wideband CDMA deliver data many times faster.

ACCESS SCHEMES- A GRAPHICAL COMPARISION:

For radio systems there are two resources, frequency and time. Division by frequency, so that each pair of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA). Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA). In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time. CDMA uses codes to identify connections.

Multiple Access Schemes

CODING TECHNIQUES USED IN CDMA

CDMA uses unique spreading codes to spread the base band data before transmission. The signal is transmitted in a channel, which is below noise level. The receiver then uses a device to correlate to spread the wanted signal, which is passed through a narrow bandpass filter. Unwanted signals will not be spread and will not pass through the filter. Codes take the form of a carefully designed one/zero sequence produced at a much higher rate than that of the baseband data. The rate of a spreading code is referred to as chip rate rather than bit rate.
CDMA codes are not required to provide call security, but create a uniqueness to enable call identification. Codes should not correlate to other codes or time shifted version of itself. Spreading codes are noise like pseudo-random codes, channel codes are designed for maximum separation from each other and cell identification codes are balanced not to correlate to other codes of itself.


CDMA spreading

Example OVSF codes, used in channel coding

SYNCHRONOUS AND ASYNCHRONOUS CDMA:
Synchronous CDMA exploits mathematical properties of orthogonality between vectors representing the data strings. For example, binary string "1011" is represented by the vector (1, 0, 1, 1). Vectors can be multiplied by taking their dot product, by summing the products of their respective components. If the dot product is zero, the two vectors are said to be orthogonal to each other. Since it is not mathematically possible to create signature sequences that are orthogonal for arbitrarily random starting points, unique "pseudo-random" or "pseudo-noise" (PN) sequences are used in Asynchronous CDMA systems. These PN sequences are statistically uncorrelated, and the sum of a large number of PN sequences results in Multiple Access Interference (MAI) that is approximated by a Gaussian noise process (following the "central limit theorem” in statistics). If all of the users are received with the same power level, then the variance (e.g., the noise power) of the MAI increases in direct proportion to the number of users.
HANDOVER

Handover occurs when a call has to be passed from one cell to another as the user moves between cells. In a traditional "hard" handover, the connection to the current cell is broken, and then the connection to the new cell is made. This is known as a "break-before-make" handover. Since all cells in CDMA use the same frequency, it is possible to make the connection to the new cell before leaving the current cell. This is known as a "make-before-break" or "soft" handover. Soft handovers require less power, which reduces interference and increases capacity. Mobile can be connected to more that two BTS the handover. "Softer" handover is a special case of soft handover where the radio links that are added and removed belong to the same Node B.

CDMA soft handover


ADVANTAGES

One of the main advantages of CDMA systems is the capability of using signals that arrive in the receivers with different time delays. This phenomenon is called multi path. FDMA and TDMA, which are narrow band systems, cannot discriminate between the multi path arrivals, and resort to equalization to mitigate the negative effects of multi path. Due to its wide bandwidth and rake receivers, CDMA uses the multi path signals and combines them to make an even stronger signal at the receivers. CDMA subscriber units use rake receivers. This is essentially a set of several receivers. One of the receivers (fingers) constantly searches for different multi paths and feeds the information to the other three fingers. Each finger then demodulates the signal corresponding to a strong multi path. The results are then combined together to make the signal stronger. Asynchronous CDMA is ideally suited to a mobile network where large numbers of transmitters each generate a relatively small amount of traffic at irregular intervals.
The best data transfer technology CDMA has to offer is the EVDO technology, allowing for a maximum download speed of about 2mb/s (about 700kbps in practice), which is similar to what a DSL line has to offer. EVDO is not available everywhere yet and requires a cell phone that is EVDO ready. GSM on the other hand offers EDGE, allowing for a maximum download speed of 384kbps (around 140kbps in practice). More technologies are being developed on top of EDGE such as HSDPA to boost the transfer rate to over 384kbps in practice. This technology requires an EDGE-ready cell phone.
CDMA offers faster data download. GSM is catching up fast, but its EDGE technology is subject to interferences. CDMA would therefore be the favored choice for data transfer.
APPLICATIONS

 Location-based services (LBS), which encompass a broad range of applications specific to a user’s position, represent an important source of differentiation and revenue for operators. CDMA2000 operators have introduced a wide variety of innovative services, such as traffic information, navigation and tracking. With these services, their consumer and enterprise users can obtain updates on traffic flow and directions to the nearest bank or restaurant, find their friends and manage their fleets.
 MMS enable graphics, pictures, video or music to be attached to text messages and sent to mobile devices or computers. These services are widely deployed by CDMA2000 operators in Korea and Japan and are gaining momentum elsewhere around the world.
 Push-to-Talk IPRS (IP Radio Service) always-on instantaneous communication technology enables one-to-one and one-to-many, two-way voice and data sessions that do not require dial-up – subscribers’ push a button and speak.
 CDMA2000 operators are leading the industry in the range of services they offer. Capitalizing on the high-speed data capabilities and flexibility of CDMA2000 technologies, operators have introduced a variety of applications which enable their consumers and enterprise customers to access information, surf the web, download music and video, send pictures and play games.


CONCLUSION: Hence, we can conclude that the CDMA technique is very advantageous and has revolutionized the mobile communication technology. CDMA subscribers enjoy one of the finest services available. And, it will only get better in the future.