Multiple Access Techniques for Mobile Communications

Introduction to Multiple Access Techniques

What is Multiple Access?

Multiple Access is a technique that allows multiple users to share a common communication channel simultaneously. In mobile communications, this is essential because spectrum is limited and expensive, while the number of users is large and growing.

The Multiple Access Problem

In wireless communications, we face a fundamental challenge: how do we allow multiple users to communicate over the same medium without interfering with each other?

Key Principle: Multiple access techniques separate signals in different domains to allow simultaneous communication:

Frequency Domain - Different frequencies (FDMA)
Time Domain - Different time slots (TDMA)
Code Domain - Different codes (CDMA)
Space Domain - Different spatial locations (SDMA)

Historical Evolution

1G (1980s) - FDMA

Analog systems using Frequency Division Multiple Access. Each user assigned a unique frequency channel.

2G (1990s) - TDMA & CDMA

Digital systems. GSM used TDMA, IS-95 used CDMA. Improved capacity and quality.

3G (2000s) - CDMA Evolution

W-CDMA and CDMA2000 provided higher data rates using spread spectrum techniques.

4G LTE (2010s) - OFDMA

Orthogonal Frequency Division Multiple Access enabled broadband mobile internet.

5G NR (2020s) - NOMA & Advanced OFDMA

Non-Orthogonal Multiple Access and massive MIMO for ultra-reliable low-latency communications.

System Capacity Considerations

System Capacity C = B × log₂(1 + SNR)

Where B = Bandwidth, SNR = Signal-to-Noise Ratio

Trade-off: Increasing the number of users typically reduces the bandwidth or time available per user, affecting data rates and quality of service.

Frequency Division Multiple Access (FDMA)

Definition

FDMA divides the total available bandwidth into non-overlapping frequency channels. Each user is allocated a unique frequency band for the entire duration of the communication.

Principle of Operation

Total bandwidth B divided into N channels
Each channel has bandwidth B_c = B/N
Guard bands between channels prevent interference
Continuous transmission - users transmit simultaneously but on different frequencies

Channel Bandwidth: B_c = (B - (N-1)×B_guard)/N

Spectral Efficiency: η = N×B_c/B

Interactive FDMA Visualization

Number of Users: 4

Total Bandwidth (MHz): 20

Guard Band (MHz): 1

Mathematical Representation

The transmitted signal for user i:

s_i(t) = A_i(t) cos(2πf_it + φ_i)

where f_i is the carrier frequency for user i

Advantages & Disadvantages

✅ Advantages

Simple implementation
No synchronization needed between users
Continuous transmission (good for voice)
Low delay
Robust against fading (narrowband channels)

❌ Disadvantages

Inefficient spectrum usage (guard bands)
Fixed channel allocation
Requires high-quality filters
Frequency planning complexity
Not suitable for bursty data traffic

Applications

AMPS (1G): 30 kHz channels in 800 MHz band
GSM (partial): 200 kHz carriers with TDMA inside
Satellite Communications: Transponder channels
Broadcast Radio/TV: Different stations on different frequencies

Time Division Multiple Access (TDMA)

Definition

TDMA divides the time axis into frames, and each frame is further divided into time slots. Users transmit in rapid succession, one after another, each using the entire bandwidth but only during their assigned time slot.

Principle of Operation

Time divided into frames of duration T_f
Each frame contains N time slots
Each user gets one slot per frame
Requires precise synchronization
Guard times between slots to prevent overlap

Slot Duration: T_s = (T_f - T_guard)/N

Data Rate per User: R_u = R_total/N

Frame Efficiency: η = (N×T_s)/T_f × 100%

Interactive TDMA Frame Structure

Users per Frame: 4

Frame Duration (ms): 4

Guard Time (%): 10

GSM Frame Structure Example

GSM TDMA Specifications

Carrier bandwidth: 200 kHz
Frame duration: 4.615 ms
Slots per frame: 8
Slot duration: 0.577 ms
Bit rate: 270.833 kbps
Bits per slot: 156.25

TDMA Frame Hierarchy

1 Frame = 8 time slots
1 Multiframe = 26 frames (120 ms)
1 Superframe = 51 multiframes (6.12 s)
1 Hyperframe = 2048 superframes (3h 28m)

Advantages & Disadvantages

✅ Advantages

Higher capacity than FDMA
Digital technology benefits (encryption, error correction)
Flexible bit allocation
Handoff procedures simpler
Supports multiple services

❌ Disadvantages

Strict synchronization required
Guard time overhead
Complex equalization needed
High peak power requirement
Slot allocation delays

Applications

GSM (2G): Most widely used cellular standard
IS-136 (D-AMPS): North American TDMA standard
PHS: Personal Handy-phone System
TETRA: Professional mobile radio

Code Division Multiple Access (CDMA)

Definition

CDMA allows all users to transmit simultaneously over the entire bandwidth. Users are separated by unique codes (spreading codes) that are orthogonal or nearly orthogonal to each other. This is a spread spectrum technique.

Spread Spectrum Principle

CDMA uses Direct Sequence Spread Spectrum (DSSS):

Narrowband signal multiplied by high-rate spreading code (chip sequence)
Spreading factor (SF) = Chip rate / Symbol rate
Processing gain provides interference resistance
All users share same frequency and time

Processing Gain: G_p = 10 log₁₀(SF) dB

Chip Rate: R_c = SF × R_s

Capacity (approximate): N ≈ 1 + (G_p/E_b/N₀)

Interactive CDMA Code Visualization

Number of Users: 4

Spreading Factor: 8

Show Correlation:

Walsh Codes and Orthogonality

CDMA systems use orthogonal codes (Walsh codes) for channel separation:

Orthogonality Condition: The cross-correlation between any two different codes is zero:
∫₀^T c_i(t) × c_j(t) dt = 0 for i ≠ j

Near-Far Problem and Power Control

Critical Issue: The Near-Far Problem occurs when a transmitter close to the receiver overwhelms distant transmitters. CDMA requires precise power control to ensure all signals arrive at the base station with equal power.

Power control accuracy: typically within ±1 dB

Advantages & Disadvantages

✅ Advantages

Higher capacity than FDMA/TDMA
Soft capacity (graceful degradation)
Soft handoff possible
Multipath resistance (Rake receiver)
Privacy (spread spectrum)
No frequency planning needed

❌ Disadvantages

Complex power control required
Near-far problem
Self-interference (MAI)
High computational complexity
Requires precise timing

Applications

IS-95 (cdmaOne): 2G CDMA standard
CDMA2000: 3G evolution
W-CDMA (UMTS): 3G standard in Europe
GPS: Uses CDMA for satellite signals

Orthogonal Frequency Division Multiple Access (OFDMA)

Definition

OFDMA combines OFDM (Orthogonal Frequency Division Multiplexing) with multiple access. It divides the bandwidth into multiple orthogonal subcarriers and allocates subsets of these subcarriers to different users.

OFDM Principles

High-speed data stream split into many lower-rate streams
Each stream modulates one subcarrier
Subcarriers are orthogonal: Δf = 1/T_s
Cyclic prefix (CP) eliminates ISI
Implemented efficiently using IFFT/FFT

Subcarrier Spacing: Δf = 1/T_s

OFDM Symbol Duration: T_s = 1/Δf

Total Bandwidth: B ≈ N × Δf

where N = number of subcarriers

Interactive OFDMA Resource Grid

Number of Subcarriers: 12

OFDM Symbols: 7

Number of Users: 3

LTE Resource Block Structure

LTE Parameters

Subcarrier spacing: 15 kHz
Subcarriers per RB: 12
RB bandwidth: 180 kHz
Slot duration: 0.5 ms (7 symbols)
Subframe: 1 ms (2 slots)
Frame: 10 ms (10 subframes)

Resource Allocation

Minimum allocation: 1 RB (12 subcarriers × 1 slot)
Localized allocation: contiguous RBs
Distributed allocation: scattered RBs
Supports frequency-selective scheduling

Advantages & Disadvantages

✅ Advantages

High spectral efficiency
Robust against multipath fading
Flexible resource allocation
Supports MIMO easily
Low complexity equalization
Scalable bandwidth (1.4-20 MHz)

❌ Disadvantages

High PAPR (Peak-to-Average Power Ratio)
Sensitive to frequency offset
Requires accurate synchronization
Out-of-band emissions

Applications

LTE (4G): Downlink uses OFDMA
IEEE 802.11 (WiFi): OFDM/OFDMA in WiFi 6
WiMAX: 802.16e mobile broadband
5G NR: Enhanced OFDMA with flexible numerology

Comparative Analysis

Technical Comparison

Feature	FDMA	TDMA	CDMA	OFDMA
Separation Domain	Frequency	Time	Code	Frequency + Time
Spectrum Efficiency	Low	Medium	High	Very High
System Complexity	Low	Medium	High	High
Synchronization	Not required	Strict	Precise	Strict
Handoff Type	Hard	Hard	Soft	Hard
Capacity	Fixed	Fixed	Soft (interference limited)	Flexible
Power Control	Not critical	Not critical	Critical	Important
Multipath Resistance	Poor	Poor	Good (Rake)	Excellent (CP)
Frequency Planning	Required	Required	Not required	Required

Capacity Comparison

FDMA: N = B/(B_ch + B_guard)

TDMA: N = T_f/(T_s + T_guard)

CDMA: N ≈ G_p/(E_b/N₀)_req × (1 + β)

OFDMA: N = N_RB × N_symbols/(Resource per user)

Evolution and Trade-offs

Spectrum Efficiency Evolution

FDMA → TDMA → CDMA → OFDMA represents progression toward higher spectral efficiency and better handling of multipath environments.

Implementation Complexity

As efficiency increases, so does implementation complexity. Modern DSPs and ASICs make complex techniques like OFDMA practical.

Latency Considerations

FDMA offers lowest latency (continuous), while TDMA/OFDMA introduce frame-based delays. Critical for URLLC in 5G.

Future Trends

NOMA (Non-Orthogonal MA), SCMA (Sparse Code MA), and RSMA (Rate-Splitting MA) for 5G/6G.

Engineering Calculators

FDMA Channel Capacity Calculator

Total Bandwidth (MHz):

Channel Bandwidth (kHz):

Guard Band (kHz):

TDMA Efficiency Calculator

Frame Duration (ms):

Slots per Frame:

Guard Time (μs):

CDMA Processing Gain Calculator

Chip Rate (Mcps):

Symbol Rate (ksps):

Required Eb/N0 (dB):

OFDMA Resource Calculator

System Bandwidth (MHz):

Subcarrier Spacing (kHz):

Users to Schedule:

Practical Applications & Case Studies

Cellular System Evolution

Launched: 1983 (AT&T)
Frequency: 824-849 MHz (uplink), 869-894 MHz (downlink)
Channel bandwidth: 30 kHz
Duplex: FDD (Frequency Division Duplex)
Total channels: 832 (2×416)
Modulation: FM

Launched: 1991
Multiple Access: FDMA + TDMA hybrid
200 kHz carriers with 8 TDMA slots
GMSK modulation (0.3 BT)
Peak data rate: 9.6 kbps (voice), 14.4 kbps (data)
GPRS/EDGE enhanced data rates up to 384 kbps

Launched: 2001
Technology: W-CDMA (FDD)
Chip rate: 3.84 Mcps
Channel bandwidth: 5 MHz
Spreading factors: 4-512
Peak data rate: 2 Mbps (HSPA: 14 Mbps)

Launched: 2009
Downlink: OFDMA, Uplink: SC-FDMA
Subcarrier spacing: 15 kHz
Bandwidth options: 1.4, 3, 5, 10, 15, 20 MHz
MIMO support: up to 8×8
Peak rate: 300 Mbps (4×4 MIMO, 20 MHz)

Launched: 2019
Flexible numerology: 15, 30, 60, 120 kHz subcarrier spacing
Bandwidth: up to 400 MHz (single carrier), 800 MHz (CA)
Massive MIMO: up to 256 antennas
New multiple access: NOMA, SCMA for mMTC
Peak rate: 20 Gbps (theoretical)

System Design Considerations

Voice vs. Data Optimization

FDMA/TDMA optimized for constant-rate voice. OFDMA better suited for bursty, high-rate data traffic.

Spectrum Availability

CDMA works well in fragmented spectrum. OFDMA requires contiguous bands for optimal performance.

Mobility Support

CDMA provides seamless soft handoff. OFDMA requires careful handoff procedures but offers better throughput.

Device Complexity

Modern smartphones support all techniques through software-defined radio (SDR) and advanced DSPs.

Future Directions

6G Vision (2030+):

AI-native air interface
Holographic communications
Terahertz frequencies (100-300 GHz)
Intelligent reflecting surfaces (IRS)
Cell-free massive MIMO