8.15.4 What Is Code-Division Multiple Access (CDMA)?
- What Is Code-Division Multiple Access?
- Why Is It Called Code-Division Multiple Access?
- What Is Spread Spectrum?
- How Does CDMA Work?
- What Is a Chip?
- What Is a Spreading Code?
- What Types of Codes Are Used?
- What Is Processing Gain?
- Why Can Many Users Share the Same Frequency?
- Is There Any Interference?
- What Is the Near-Far Problem?
- How Is the Near-Far Problem Solved?
- Why Is Power Control So Important?
- What Is a RAKE Receiver?
- Why Is Multipath Helpful?
- Where Was CDMA Used?
- What Are the Advantages of CDMA?
- What Are the Disadvantages?
- Is CDMA Still Used Today?
- Why Was CDMA Such a Major Advance?
Description
Discover how CDMA allows many users to transmit simultaneously over the same frequency band using unique spreading codes. Learn about processing gain, spreading sequences, the near-far problem, power control, and why CDMA represented a major advance in mobile communications.
Introduction
The first generations of multiple-access systems solved the problem of sharing radio spectrum by separating users in either frequency or time. In Frequency-Division Multiple Access (FDMA), every user was assigned an exclusive frequency channel. In Time-Division Multiple Access (TDMA), users shared a common frequency but transmitted during different time slots. Both approaches proved highly successful and formed the basis of many early communication systems.
As the demand for mobile communications continued to increase, however, engineers searched for techniques that could make even more efficient use of the limited radio spectrum. Rather than separating users by frequency or time, they asked a more radical question: Could every user transmit at the same time, on the same frequency, and still be separated reliably by the receiver?
Surprisingly, the answer was yes.
The solution became known as Code-Division Multiple Access (CDMA). Instead of assigning each user a unique frequency or time slot, every user is assigned a unique digital code. Each transmitted signal is deliberately spread over a bandwidth far wider than is actually required. At the receiver, the correct code is used to recover the desired signal while treating all other users as background noise.
This concept represented one of the most important advances in wireless communications. CDMA dramatically increased network capacity, improved resistance to interference, and made much better use of available spectrum. It became the foundation of the IS-95 and CDMA2000 mobile-phone standards and influenced many later communication systems.
What Is Code-Division Multiple Access?
Code-Division Multiple Access (CDMA) is a multiple-access technique in which many users transmit simultaneously over the same frequency band using unique digital spreading codes.
Every user occupies the same:
- frequency;
- time interval; and
- communication channel.
The users are distinguished solely by their assigned codes.
Why Is It Called Code-Division Multiple Access?
Unlike FDMA and TDMA, CDMA does not divide the communication resource according to frequency or time. Instead, every user is assigned a unique code sequence. The receiver identifies the desired transmission by correlating the received signal with the appropriate code.
The communication resource is therefore divided by code.
What Is Spread Spectrum?
CDMA is based upon the principle of spread-spectrum transmission.
Instead of transmitting information within the minimum required bandwidth, the signal is intentionally spread across a much wider frequency range. At first this appears wasteful. However, spreading the signal provides several important advantages, including improved resistance to:
- interference;
- jamming;
- multipath propagation; and
- interception.
The original narrowband information is recovered only by a receiver possessing the correct spreading code.
How Does CDMA Work?
Before transmission, each data bit is combined with a much faster pseudo-random binary sequence called a spreading code.
This process replaces every information bit with many shorter binary elements known as chips. The transmitted signal therefore occupies a bandwidth much larger than the original information signal. At the receiver, the same spreading code is applied again. Only signals possessing the correct code collapse back into the original data stream.
Signals using different codes remain spread over the wider bandwidth and appear largely as low-level noise.
What Is a Chip?
A chip is one element of the spreading code.
It should not be confused with an information bit. For example, one data bit may be represented by:
- 32 chips;
- 64 chips;
- 128 chips; or
- even more.
The chip rate is therefore much higher than the original information bit rate.
What Is a Spreading Code?
A spreading code is a carefully designed binary sequence possessing excellent correlation properties. Good spreading codes:
- correlate strongly with themselves;
- correlate poorly with other codes;
- appear random; and
- provide reliable user separation.
Several families of spreading codes have been developed for practical CDMA systems.
What Types of Codes Are Used?
Common spreading sequences include:
- Walsh codes;
- Gold codes;
- Kasami sequences; and
- maximal-length pseudo-noise (PN) sequences.
Each possesses mathematical properties that assist in separating simultaneous users.
What Is Processing Gain?
One of the most important concepts in CDMA is processing gain.
Processing gain represents the improvement obtained by spreading the signal over a wider bandwidth. It is approximately equal to: spread bandwidth ÷ information bandwidth. A larger processing gain generally provides:
- greater interference rejection;
- improved receiver sensitivity;
- better resistance to jamming; and
- increased communication reliability.
Processing gain is one of the principal reasons CDMA performs so well in difficult radio environments.
Why Can Many Users Share the Same Frequency?
Every user's signal appears different because every user employs a different spreading code.
The receiver searches only for the desired code. Signals using other codes exhibit very low correlation and therefore contribute relatively little to the recovered information.
Although all users occupy the same spectrum simultaneously, the receiver can separate them mathematically.
Is There Any Interference?
Yes.
Unlike FDMA, CDMA users are not completely isolated.
Every additional user contributes a small amount of interference. As more users become active:
- background interference increases;
- receiver performance gradually decreases;
- capacity is eventually reached; and
- additional users cannot be supported reliably.
CDMA systems are therefore often described as interference limited rather than bandwidth limited.
What Is the Near-Far Problem?
One of CDMA's most significant challenges is the near-far problem.
Suppose one mobile phone is located very close to the base station while another is much farther away. The nearby transmitter arrives with much greater signal strength. Its transmission may partially mask the weaker distant signal, even though different spreading codes are used.
Without corrective action, nearby users could dominate the communication channel.
How Is the Near-Far Problem Solved?
The solution is power control.
Every mobile station continually adjusts its transmit power according to instructions from the base station. Nearby users transmit at lower power. More distant users transmit at higher power. The objective is for all received signals to arrive at approximately the same power level.
Power control is absolutely fundamental to successful CDMA operation.
Why Is Power Control So Important?
Without accurate power control:
- nearby transmitters would overwhelm distant users;
- interference would increase rapidly;
- network capacity would decrease; and
- communication reliability would deteriorate.
For this reason, CDMA systems employ extremely fast closed-loop power-control algorithms, often adjusting transmitter power hundreds of times each second.
What Is a RAKE Receiver?
Unlike many communication systems, CDMA can actually benefit from multipath propagation.
Different reflected signal components often arrive with sufficient time separation to be resolved individually. A RAKE receiver contains several parallel receiver branches, sometimes called fingers. Each finger processes one significant propagation path. The outputs are then combined coherently.
This allows energy from multiple reflected signals to contribute constructively to the received information.
Why Is Multipath Helpful?
Conventional communication systems often regard multipath as an impairment.
CDMA views it differently.
Provided reflected signals arrive with sufficient delay separation:
- each path contains useful information;
- the receiver processes them independently;
- signal energy is combined; and
- communication reliability improves.
This characteristic contributed greatly to CDMA's excellent mobile performance.
Where Was CDMA Used?
CDMA became the basis of several important communication systems.
Applications included:
- IS-95 cellular networks;
- cdmaOne;
- CDMA2000;
- military communications;
- satellite navigation systems; and
- numerous secure communication systems.
Although later cellular generations adopted OFDMA, many CDMA concepts remain influential.
What Are the Advantages of CDMA?
CDMA offers many important benefits.
These include:
- excellent spectrum utilization;
- resistance to interference;
- resistance to multipath propagation;
- graceful capacity degradation; and
- improved privacy compared with conventional narrowband systems.
These characteristics made CDMA one of the most advanced mobile communication technologies of its era.
What Are the Disadvantages?
CDMA also introduces significant engineering challenges. These include:
- complex receiver design;
- strict power-control requirements;
- the near-far problem;
- interference-limited capacity; and
- increased signal-processing complexity.
Fortunately, advances in digital signal processors made practical implementation possible.
Is CDMA Still Used Today?
Although most fourth- and fifth-generation mobile systems employ OFDMA rather than CDMA for multiple access, CDMA remains historically important.
Many of its concepts survive in modern systems. These include:
- spread-spectrum signalling;
- advanced power control;
- sophisticated receiver processing;
- interference management; and
- adaptive resource allocation.
Furthermore, spread-spectrum techniques continue to play an important role in military communications, satellite navigation, and secure wireless systems.
Why Was CDMA Such a Major Advance?
CDMA fundamentally changed the way engineers thought about sharing radio spectrum. Instead of avoiding interference by separating users in frequency or time, it demonstrated that carefully designed coding techniques could allow many users to occupy the same spectrum simultaneously. This innovation greatly increased communication capacity while providing excellent resistance to interference and multipath propagation.
The concepts developed for CDMA influenced not only third-generation mobile communications but also many later wireless technologies.
Summary
Code-Division Multiple Access allows multiple users to transmit simultaneously over the same frequency band by assigning each user a unique spreading code. Spread-spectrum transmission, processing gain, power control, and advanced receiver techniques enable the receiver to recover the desired signal while treating other users as low-level interference.
Although later mobile systems adopted newer multiple-access techniques, CDMA remains one of the most influential developments in wireless communications. Its principles continue to underpin many secure, satellite, and mobile communication systems and represent a major milestone in the evolution of modern telecommunications.
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