What is CDMA (Code-Division Multiple Access)?

CDMA stands for Code-Division Multiple Access. It is a method that allows many users to share the same frequency channel. This process uses spread-spectrum technology.

Unlike Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA), CDMA assigns unique codes to each user’s signal. As a result, multiple transmissions can occur at the same time without interference.

This feature improves network capacity. Additionally, it enhances resilience against noise and signal fading. CDMA technology is standard in various mobile communication systems. For example, it is key in 3G cellular networks. Overall, it helps deliver voice and data services effectively.

What is the CDMA Mobile Telephone Network?

Spread-spectrum technology is crucial in wireless communication. It is also used in fiber optic and cable systems.

One key challenge is how to share a single channel among multiple users. This allows several communications to occur at once.

Without proper organization, signals can interfere with each other. This is where multiplexing comes in. It manages the distribution of the communication channel.

A hub at the end of a fiber optic cable is a classic example. Multiplexing works seamlessly for end-user devices.

In contrast, media access control organizes communication between user terminals. For instance, it’s used when many mobile phones connect to a network. A specific scheme is needed to prevent interference among users.

To tackle this, CDMA employs spread-spectrum technology. Each transmitter receives a unique code, ensuring that signals do not overlap. The receiver captures all signals at once, thanks to this coding method.

Additionally, other multiplexing methods exist. These include frequency division (FDMA), time division (TDMA), and space division (SDMA). Each serves to separate communications and reduce interference.

In real systems, multiple strategies enhance communication quality. However, issues arise when many people want to speak simultaneously.

To manage this, one person can talk loudly while another speaks softly. They may also position themselves differently or use different languages.

This is similar to how CDMA works. Only those who understand the code can communicate effectively. Code division is widely used in mobile phones, WiFi, and satellite navigation (GPS) systems.

Additionally, CDMA refers to a wireless data communication standard developed by Qualcomm and adopted as IS-95 by North America’s TIA.

Features

In CDMA, signals use a broader bandwidth than needed. This technique is known as spread spectrum multiple access.

Each user’s data is unique. It is XORed with a transmission code that has a higher bandwidth.

The data signal lasts a duration of Tb. Meanwhile, the transmission code has a jittered time, Tc. Since Tc is much smaller than Tb, the emitted signal has a larger bandwidth. This is why we call it spread spectrum.

Every CDMA user has a different code to modulate their signals. Choosing these codes is vital for system performance. We select the relevant signal by cross-correlating it with the user’s code.

The best communication happens with clear separation between signals. When we search for a specific signal, the correlation results are very high. This allows the system to recover the desired signal.

If the signal is not needed, correlation should be near zero. Each user’s code is different, which helps achieve this.

Additionally, any non-zero time delay in correlation should also trend towards zero. This concept, called autocorrelation, helps reject multipath interference.

What are the Types of CDMA?

Overall, CDMA codes can be divided into two types: synchronous and asynchronous.

1. Simultaneous CDMA

Simultaneous CDMA uses unique codes to transmit data. It relies on the mathematical properties of orthogonality. In this context, two vectors represent the codes for different users.

When their dot product equals zero, the vectors are orthogonal. This characteristic ensures that the codes do not interfere with each other.

When the receiver gets a mix of signals, it can still isolate its data. The intended user knows its unique code.

Using the scalar product, the desired signal can be extracted from the mix. As a result, only the user’s data remains, while all other signals are canceled out.

This method can work for any number of users. However, there must be enough orthogonal codes available. To accommodate more users, the length of the code can be increased.

In CDMA, all users transmit at the same time, without frequency filtering. This efficiency simplifies the process.

For example, IS-95 uses 64-bit orthogonal Walsh codes. These codes help to encode signals and separate users effectively. Thus, CDMA allows multiple users to share the same channel seamlessly.

2. Asynchronous CDMA

Synchronous CDMA systems function well without long signal delays. The radio links between cell phones and base stations are carefully coordinated. However, terminals can move and face obstructions. This movement leads to varying arrival delays for signals.

Asynchronous CDMA systems use pseudo-random sequences. In theory, creating perfectly orthogonal codes at all times is impossible. The PN code looks random, but can be predicted. This sequence encodes and decodes the signals for asynchronous CDMA users.

In contrast, synchronous systems rely on orthogonal codes. PN sequences lack statistical correlation. Therefore, combining multiple PN sequences resembles a Gaussian noise process. This follows the central limit theorem.

If all users’ signals have equal strength, the interference, or MAI, increases. More users lead to more interference. Consequently, unwanted signals appear as noise. Thus, higher user numbers result in greater disruption.

The randomness of these sequences has a benefit. They spread power over a wide bandwidth. Therefore, it becomes harder to detect them. Background noise can confuse attempts to intercept the signals.

Military communication used this feature in the 20th century. All CDMA types benefit from processing gain. This property helps filter out unwanted signals.

When the receiver gets signals with a PN code, it treats others as noise. This noise can be reduced or eliminated. However, users generate MAIs, making power control essential. Synchronous CDMA, TDMA, and FDMA can eliminate unwanted signals due to their orthogonality.

On the other hand, asynchronous CDMA cannot completely reject unwanted signals. If stronger unwanted signals arrive, they mix with the desired signal.

To address this challenge, system design requires controlled emitter power. This control helps synchronize energy for incoming signals.

In cellular telephone systems, base stations use closed-loop power control. This method precisely manages each phone’s emission power.

Conclusion

In conclusion, CDMA has changed wireless communications significantly. It allows efficient sharing of frequency channels among many users.

By using unique coding techniques, CDMA enhances network capacity. Furthermore, it reduces interference, ensuring strong connectivity for voice and data services.

Its adaptability in synchronous and asynchronous systems shows its effectiveness. This adaptability helps meet the dynamic needs of mobile communication.

As technology advances, CDMA’s core principles remain essential. They promise even greater improvements in user connectivity and network resilience in the future.