Cdma Background

CDMA, in contrast to FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access), poses no restrictions to the time interval and frequency band to be used for the transmission of different users. All users can transmit simultaneously while occupying the whole available bandwidth (Figure 2). They are separated by uniquely (per user) assigned codes with proper low cross-interference properties. Thus, while interuser interference is strictly avoided in TDMA and FDMA systems by assigning different portions oftime (time slots [TSs]) or bandwidth to different users, respectively, interuser interference, referred to as multiple-access interference (MAI), is inherent in CDMA techniques and is the limiting capacity factor (interference-limited systems).

Figure 2. FDMA, TDMA, and CDMA principles Figure 3. DS/CDMA principle

Figure 2. FDMA, TDMA, and CDMA principles Figure 3. DS/CDMA principle

Figure Fdma Tdma Cdma

Although CDMA has been known for several decades, only in the last two decades has interest peaked regarding its use for mobile communications because of its enhanced performance compared to standard TDMA and FDMA techniques. Greater capacity, exploitation of multipath fading through RAKE combining, soft handover, and soft capacity are some of CDMA's advantages (Viterbi, 1995). The first commercial CDMA mobile application was IS-95 (1993). The real boost ofCDMA applications, though, was the adoption of the WCDMA air interface for UMTS.

CDMA is applied using spread-spectrum techniques, such as frequency hopping (FH), direct sequence (DS), or hybrid methods. The DS technique, which is used in UMTS, is applied by multiplying the information symbols with faster pseudorandom codes of low cross-correlation between each other, which spreads the information bandwidth (Figure 3). The number of code pulses (chips) used for spreading an information symbol is called the spreading factor (SF). The higher the SF, the greater the tolerance to MAI is. A simplified block diagram of a CDMA transmitter and receiver is given in Figure 4. The receiver despreads the signal with the specific user's unique code followed by an integrator or digital summing device. Coexistent users' signals act as additive wideband noise (MAI).

With properly selected codes (of low autocorrelation), multipath propagation can turn into diversity gain for CDMA systems as soon as multiple paths' delays are spaced more than the chip duration (these paths are called resolved). In such a case, a RAKE receiver is employed (Figure 5), which performs a full reception procedure for each one of the resolved paths and properly combines the received signal replicas. In any case, discrimination between CDMA users is feasible with conventional receivers (no multiuser receivers) only when an advanced power-control method is engaged. Otherwise the near-far effect will destroy multiple-access capability.

There is no universally accepted definition for what is called WCDMA. From a theoretical point of view, a CDMA system is defined as wideband when the overall spread bandwidth exceeds the coherence bandwidth of the channel (Milstein, 2000). In such a case, the channel appears to be frequency selective, and multipath resolvability is possible. Compared to narrowband CDMA, beyond multipath exploitation, WCDMA presents enhanced performance through certain advantages, such as a decrease ofthe required transmitted power to achieve a given performance, greater tolerance to power-control errors, fading-effects reduction, the capability to transmit higher data rates and multimedia traffic, and so forth.

Figure 4. Transmitter and receiver models of DS/ CDMA

Figure 5. RAKE receiver realization example

Figure 4. Transmitter and receiver models of DS/ CDMA

Figure 5. RAKE receiver realization example

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