edVirtus Reference

What Is Multiple Access?

Preview: Learn more about multiple-access techniques.

Multiple access is the process of allowing many independent users to share a common communications channel without causing unacceptable interference with one another. Rather than providing every user with a dedicated communication path, multiple-access techniques coordinate transmissions so that the available bandwidth, time, or coding resources can be shared efficiently. These techniques form the foundation of virtually every modern wireless communications system, including mobile telephone networks, satellite communications, Wi-Fi, and many Internet of Things (IoT) applications.

The need for multiple access became increasingly important as communications networks expanded during the twentieth century. Early radio systems often assigned a dedicated frequency to each user or service. While this approach was simple, it quickly became impractical as the number of users increased because the available radio spectrum is a limited natural resource. Engineers therefore sought methods that would allow many users to communicate simultaneously while making more efficient use of the available spectrum.

Several different multiple-access techniques were developed to address this challenge. One of the earliest was Frequency Division Multiple Access (FDMA), in which each user is allocated a separate portion of the available frequency spectrum. As long as the frequency assignments do not overlap, many users can communicate simultaneously without interfering with one another. FDMA was widely used in early cellular telephone systems and remains important in many satellite and radio communication networks.

Another important technique is Time Division Multiple Access (TDMA). Instead of assigning each user a separate frequency, TDMA allows users to share the same frequency by transmitting during different time slots. Each user is allocated a small portion of a repeating transmission cycle, enabling many independent communications to share a single radio channel. TDMA formed the basis of several second-generation digital cellular systems, including the widely adopted GSM mobile telephone standard.

A different approach is Code Division Multiple Access (CDMA). Rather than separating users by frequency or time, CDMA assigns each user a unique mathematical code. All users transmit simultaneously over the same frequency band, and the receiver separates the desired signal by recognizing its assigned code. This spread-spectrum technique provides excellent resistance to interference and multipath propagation and became widely used in third-generation (3G) mobile communications and satellite navigation systems such as GPS.

Modern wireless networks increasingly employ Orthogonal Frequency Division Multiple Access (OFDMA). This technique divides the available bandwidth into many closely spaced subcarriers, which can be allocated dynamically to different users according to their communication requirements and channel conditions. OFDMA provides excellent spectral efficiency and flexibility and is used extensively in 4G LTE, 5G mobile networks, Wi-Fi 6, and many broadband wireless systems.

It is important to distinguish multiple access from multiplexing, although the two concepts are closely related. Multiplexing generally combines several information streams at a common location before transmission over a single communication channel. Multiple access, by contrast, coordinates transmissions from many independent users, often located at different geographical locations, who all wish to communicate using the same shared infrastructure. In practice, many communications systems employ both multiplexing and multiple-access techniques simultaneously.

Efficient multiple access is particularly important in wireless communications because radio spectrum is both finite and highly valuable. Every mobile phone, wireless sensor, satellite terminal, and Wi-Fi device competes for access to limited frequency resources. Modern communication systems therefore employ sophisticated scheduling, resource allocation, and interference-management techniques to maximize network capacity while maintaining acceptable quality of service for every user.

Today, multiple-access techniques are used throughout communications engineering. Cellular networks support millions of subscribers by dynamically sharing radio channels among active users. Satellites simultaneously serve thousands of Earth stations using carefully planned frequency, time, and beam allocations. Wi-Fi networks coordinate transmissions between numerous devices connected to the same access point, while Internet of Things networks enable enormous numbers of sensors to share limited communication resources efficiently.

Multiple access therefore represents far more than a method of sharing radio spectrum. It is one of the enabling technologies of modern communications, allowing billions of users and devices to communicate simultaneously using a finite amount of infrastructure and spectrum. As demand for wireless connectivity continues to grow, increasingly sophisticated multiple-access techniques will remain essential for supporting the next generation of communications systems.