Multiple Access Techniques

When multiple users share a common communication medium, a strategy is needed to prevent their signals from interfering with each other. Multiple access techniques define how the available radio resource — frequency, time, or code space — is divided and assigned among users. The three classical techniques are FDMA, TDMA, and CDMA, and they form the foundation of every cellular generation from 1G through 3G.

Core Concept Explanation

In Frequency Division Multiple Access (FDMA), the available bandwidth is divided into non-overlapping sub-bands, and each user is permanently assigned one sub-band for the duration of the call. This is the oldest and simplest scheme. Guard bands are inserted between sub-bands to prevent adjacent channel interference. FDMA was used in 1G analog systems like AMPS and still appears in satellite communication.

In Time Division Multiple Access (TDMA), all users share the same frequency band but are assigned different time slots within a repeating frame. Each user transmits in short bursts during its allocated slot and is silent at all other times. Guard times are inserted between slots to handle propagation delay variations. GSM uses TDMA with 8 slots per frame on a 200 kHz carrier.

In Code Division Multiple Access (CDMA), all users transmit simultaneously over the entire available bandwidth using unique spreading codes. The receiver uses the known code to despread the desired signal while all other users appear as wideband noise. This requires precise power control because a strong nearby user can mask a distant user — the near-far problem.

Mathematical Expression

For FDMA, if total bandwidth B is divided among N users with guard band Bg between each pair, the bandwidth per user is:

Bu = (B - (N-1) * Bg) / N

For TDMA with frame duration Tf and N users, the slot duration per user is Ts = Tf / N (ignoring guard time). The bit rate per user equals the channel bit rate divided by N, so TDMA requires burst transmission at N times the information rate.

For CDMA, system capacity is interference-limited. The number of users K that can be supported given processing gain G = W/R and required Eb/N0 is:

K = 1 + G / (Eb/N0)

Practical Understanding

FDMA wastes spectrum when a channel is allocated but the user is silent (like pauses in speech). TDMA is more efficient because idle slots can be reassigned dynamically. CDMA is the most spectrally efficient in a fully loaded system but requires careful power control and sophisticated receivers. Modern LTE uses OFDMA for downlink, which combines FDMA and OFDM principles.

Handoff behavior also differs: FDMA and TDMA use hard handoff (break before make), while CDMA enables soft handoff (the mobile communicates with multiple base stations simultaneously), which improves coverage at cell edges.

Given:
Total bandwidth B = 12.5 MHz
Number of users N = 10
Guard band per user Bg = 25 kHz

Why this formula applies:
FDMA splits the band into equal sub-channels with guard bands removed.

Formula:
Bu = (B - (N-1) * Bg) / N

Substitution:
Bu = (12,500,000 - 9 * 25,000) / 10
Bu = (12,500,000 - 225,000) / 10

Calculation:
Bu = 12,275,000 / 10

Final Answer: Bu = 1.2275 MHz per user

Exam Tip: In GATE problems, TDMA burst rate = N times the individual user data rate. CDMA capacity formula K = 1 + G/(Eb/N0) is commonly tested. Also note: FDMA uses hard handoff, CDMA uses soft handoff.

• FDMA: fixed frequency sub-band per user, guard bands required, used in 1G and satellite systems.
• TDMA: shared frequency, time slots per user, burst transmission at N times info rate, used in GSM 2G.
• CDMA: all users share time and frequency, separated by orthogonal PN codes, capacity is interference-limited.
• CDMA capacity: K = 1 + G/(Eb/N0) where G = W/R is the processing gain.
• Soft handoff is unique to CDMA and improves cell-edge coverage and reliability.

Quick Revision

• FDMA: separate frequency bands, guard bands, 1G, hard handoff.
• TDMA: time slots on shared frequency, burst at N x rate, GSM 2G, hard handoff.
• CDMA: unique spreading codes, all share BW and time, interference-limited, soft handoff.
• FDMA bandwidth per user: Bu = (B - (N-1)*Bg) / N.
• CDMA capacity: K = 1 + G/(Eb/N0), G = W/R (processing gain).
• GATE trap: TDMA burst rate = N times user info rate; do not confuse with channel bit rate.
• Soft handoff uses make-before-break; hard handoff uses break-before-make.