What Is Multiaccess Communication?

Multiaccess communication is the set of techniques and protocols that allow multiple transmitters to share a common physical medium or radio channel without prohibitive levels of mutual interference. Rather than dedicating a full channel to each user at all times, multiaccess schemes partition channel resources, whether by frequency, time slot, spreading code, spatial beam, or a combination of these, and assign those partitions to users on demand or by fixed schedule. The discipline encompasses both the physical-layer signal design that makes such sharing possible and the link-layer protocols that govern how access is coordinated among potentially many concurrent users.

The field draws on information theory, digital communications, signal processing, and queuing theory. Shannon's multiple-access channel (MAC) capacity region, which defines the jointly achievable data rates for a collection of users sharing a channel, provides the theoretical foundation, while practical standards such as 2G GSM, 3G CDMA, and 4G LTE represent successive engineering approximations to that theoretical limit. Delay estimation plays a crucial role in protocol design because the end-to-end delay experienced by a user depends on both the transmission time assigned by the access scheme and the queuing delay incurred while waiting for a channel resource.

Multiple Access Schemes

The classical access schemes divide the channel by three orthogonal dimensions. In frequency division multiple access (FDMA), the available spectrum is divided into non-overlapping sub-bands, each assigned to one user, as in early analog cellular systems and satellite telephony. Time division multiple access (TDMA) assigns each user a repeating time slot on a single carrier, allowing multiple users to share one frequency channel with interleaved bursts; GSM uses an 8-slot TDMA frame on each 200 kHz carrier. Code division multiple access (CDMA) spreads each user's signal across the full available bandwidth using a unique pseudorandom spreading code. All users transmit simultaneously in the same frequency band, and receivers apply matched filters keyed to each user's code to separate signals. As described in fundamentals of communications access technologies, CDMA's interference-averaging property provides a gradual capacity degradation as users are added, in contrast to the hard capacity limits of TDMA and FDMA systems.

3G Mobile Communication and CDMA

The third generation (3G) of mobile cellular standards standardized CDMA as the dominant access technology through two related families: WCDMA (used in UMTS) and cdma2000. Both employ wideband CDMA with direct sequence spreading over 5 MHz or 1.25 MHz carriers respectively. The 3G CDMA system architecture provides data rates sufficient for mobile internet access, with downlink rates of up to 14.4 Mbps under HSPA enhancement. The fundamental capacity challenge in CDMA systems is managing the near-far problem: a nearby user's stronger signal can overwhelm a distant user's weaker signal at the base station, requiring fast closed-loop power control to equalize received power levels.

Channel Capacity and Delay

Information-theoretic analysis of the multiple-access channel defines the capacity region as the set of all rate tuples simultaneously achievable by the users sharing the channel. For a Gaussian MAC with K users, successive interference cancellation (SIC) at the receiver achieves any point on the boundary of the capacity region. Stanford's analysis of multiuser capacity and opportunistic communication demonstrates that scheduling users when their channel conditions are favorable, opportunistic scheduling, can approach the capacity region boundary with simple receivers. Delay constraints complicate this picture because waiting for good channel conditions introduces queuing latency, creating a fundamental throughput-delay trade-off that protocol designers must balance against application requirements.

Applications

Multiaccess communication has applications in a wide range of disciplines, including:

• Cellular mobile telephone and broadband data networks
• Satellite communication systems serving multiple ground stations simultaneously
• Wi-Fi local area networks using OFDMA for multi-user downlink scheduling
• Industrial wireless sensor networks requiring low-latency machine-type access
• Cable television return channel systems for upstream data from subscribers
• Tactical military communication networks with frequency hopping and spread spectrum