edVirtus Reference

What Is Multiple-Access Interference?

What Is MAI?

Preview: Learn more about multiple-access interference (MAI) and why it limits the capacity of spread-spectrum communication systems.

Multiple-access interference (MAI) is the unwanted interference that occurs when several users simultaneously share the same communication channel. It is most commonly associated with Code Division Multiple Access (CDMA) and other spread-spectrum communication systems, in which many users transmit at the same time over the same frequency band. Although each user is assigned a unique spreading code, practical codes are not perfectly independent, and each transmission therefore contributes a small amount of interference to every other user.

One of the principal advantages of spread-spectrum communication is that multiple users can occupy the same spectrum simultaneously. Rather than separating users by frequency or time, as in Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA), CDMA distinguishes users by assigning each one a different pseudorandom (PR) spreading sequence. At the receiver, the desired signal is recovered by correlating the received waveform with the appropriate code, while signals using different codes ideally appear as low-level background noise.

In practice, however, no family of spreading codes possesses perfect orthogonality under all operating conditions. Although the cross-correlation between different codes is usually very small, it is rarely zero. As a result, a receiver cannot remove the transmissions of other users completely. After despreading, each unwanted signal leaves a small residual contribution that adds to the receiver noise. This accumulated interference from all the other active users is known as multiple-access interference.

The amount of MAI depends strongly on the number of simultaneous users sharing the communication channel. Each additional user introduces another source of interference, causing the total interference power to increase. As more users become active, the signal-to-interference ratio gradually decreases, making reliable reception progressively more difficult. Eventually, the communication system reaches a point where additional users cause an unacceptable increase in the bit error rate. For this reason, MAI is one of the principal factors limiting the capacity of CDMA systems.

The near-far effect can significantly worsen multiple-access interference. If one transmitter is received much more strongly than another, its residual interference after despreading may become comparable to—or even greater than—the desired signal from a more distant user. Consequently, effective power control is essential in practical CDMA systems. By ensuring that all users arrive at the receiver with approximately equal signal strength, the network minimizes the dominance of any individual transmission and substantially reduces the impact of MAI.

Another important factor influencing MAI is the propagation environment. In ideal laboratory conditions, carefully chosen spreading codes may exhibit excellent orthogonality. In real wireless channels, however, reflections from buildings, terrain, and other objects create multipath propagation, causing delayed copies of the transmitted signal to arrive at the receiver. These delays can reduce the effective orthogonality of the spreading codes, increasing the level of multiple-access interference. Modern receivers therefore employ techniques such as RAKE receivers, equalization, and adaptive signal processing to recover energy from the various propagation paths while minimizing interference.

Several engineering techniques have been developed to reduce MAI. Careful selection of spreading codes minimizes cross-correlation between users, while fast closed-loop power control maintains approximately equal received signal strengths. Advanced receiver architectures, including multi-user detection and interference cancellation, estimate the signals transmitted by other users and subtract them from the received waveform before decoding the desired signal. These techniques can significantly increase system capacity, although they require additional computational complexity.

Multiple-access interference is not unique to cellular communication. Similar effects occur in satellite communication systems employing spread-spectrum multiple access, military communication systems, wireless sensor networks, and satellite navigation systems such as the Global Positioning System (GPS). In GPS receivers, for example, signals from multiple satellites occupy the same frequency band and are distinguished by unique PR codes. Although the satellite signals are intentionally designed to minimize mutual interference, some level of MAI is unavoidable and must be considered during receiver design.

It is important to distinguish MAI from other forms of interference. Thermal noise arises from random physical processes within electronic components, while co-channel interference is produced by transmitters using the same radio frequency without code separation. MAI, by contrast, results specifically from the imperfect separation of multiple users that are intentionally sharing the same communication resources through different spreading codes. It is therefore an inherent characteristic of spread-spectrum multiple-access systems rather than an external impairment.

Today, multiple-access interference remains one of the principal factors governing the capacity and performance of spread-spectrum communication systems. The success of CDMA-based cellular networks, satellite navigation systems, and numerous military communication systems depends upon careful management of MAI through code design, power control, receiver processing, and interference cancellation. Understanding multiple-access interference is therefore essential to understanding how many users can reliably share the same radio spectrum simultaneously.