Multiple Access Techniques in Wireless Networks: TDMA, FDMA, CDMA, OFDMA, and Random Access

Key Concepts and Techniques

1. Why Multiple Access?

Wireless networks must allow multiple users/devices to share the same communication channel without interference. This is critical because:

• Limited spectrum: Radio frequencies are scarce and expensive.
• Dynamic user demand: Users join/leave networks unpredictably (e.g., Pathao drivers, Ncell subscribers).
• Fairness and efficiency: All users should get fair access while maximizing throughput.

2. Channel Sharing Methods

Wireless multiple access techniques divide the shared medium (time, frequency, code, or a combination) among users. The three primary categories are:

A. Frequency Division Multiple Access (FDMA)

How it works:

• The total bandwidth is split into non-overlapping frequency channels.
• Each user is assigned a dedicated frequency band for transmission.
• Users transmit simultaneously but on different frequencies.

Real-World Example:

• Ncell’s 2G/3G networks use FDMA to assign unique frequencies to calls/texts.
• AM/FM radio: Each station broadcasts on a distinct frequency (e.g., 98.5 MHz for Radio Nepal).

Advantages:

• Simple to implement.
• No interference between users on different frequencies.
• Works well for circuit-switched services (e.g., voice calls).

Disadvantages:

• Inefficient spectrum use: Guard bands (unused frequencies between channels) reduce capacity.
• Scalability issues: Adding more users requires more bandwidth.
• Hardware complexity: Users need tunable radios to switch frequencies.

Worked Example: Suppose a wireless system has a total bandwidth of 1 MHz. If FDMA divides it into 5 channels with 20 kHz guard bands between each:

• Usable bandwidth per user = (1 MHz – (4 × 20 kHz)) / 5 = 160 kHz.
• If a user needs 100 kHz for data, only 5 users can share the channel.

B. Time Division Multiple Access (TDMA)

How it works:

• Users share the same frequency but take turns in time slots.
• Each user gets a time slot in a repeating frame.
• Guard times (short gaps) prevent collisions between slots.

Real-World Example:

• 2G GSM networks (used by Ncell and NTC) use TDMA to allow 8 users per frequency in a time frame.
• Digital TV broadcasting: Multiple channels share the same frequency but transmit at different times.

Advantages:

• Efficient spectrum use: No guard bands between frequencies (unlike FDMA).
• Scalable: More users can be added by increasing time slots.
• Supports bursty traffic: Users only transmit when they have data (e.g., WhatsApp messages).

Disadvantages:

• Latency: Users must wait for their time slot (e.g., delay in Pathao’s real-time ride updates).
• Synchronization required: All devices must be precisely timed.
• Inefficient for low-duty-cycle users: If a user has little data, their slot is wasted.

Worked Example: In a TDMA system with 4 users and a frame length of 4 ms:

• Each user gets 1 ms slot.
• If User 1 sends a 100-byte packet at 1 Mbps, it takes 0.8 ms to transmit.
• The remaining 0.2 ms is guard time or idle.
• Throughput per user = 100 bytes / 4 ms = 25 kbps.

C. Code Division Multiple Access (CDMA)

How it works:

• All users transmit simultaneously on the same frequency and time.
• Each user is assigned a unique code (pseudo-noise sequence).
• Signals are spread over a wide bandwidth (spread spectrum).
• Correlation at the receiver extracts the desired signal.

Real-World Example:

• 3G/4G LTE networks (used by Ncell and Smart Cell) use CDMA-like techniques (e.g., WCDMA in 3G).
• Military communications: CDMA is used for secure, jam-resistant links.
• GPS: Uses CDMA to allow multiple satellites to transmit simultaneously.

Advantages:

• High capacity: More users can share the same channel (e.g., 64+ in CDMA2000).
• Resistant to interference/jamming: Spread signals are harder to detect.
• Soft handoff: Users can switch base stations without dropping calls (smooth handover in Pathao rides).

Disadvantages:

• Complex hardware: Requires sophisticated spreaders/despreaders.
• Near-far problem: Strong signals from nearby users can drown out weak ones (solved by power control).
• Higher interference: More users = more noise.

Worked Example: In a CDMA system with processing gain of 100:

• Original signal bandwidth = 1 kHz.
• Spread signal bandwidth = 100 kHz.
• If 10 users share the channel, each gets 10 kbps (assuming 100 kbps total capacity).
• Interference rejection: The receiver can filter out 99% of unwanted signals.

D. Orthogonal Frequency Division Multiple Access (OFDMA)

How it works:

• The total bandwidth is divided into many narrow subcarriers (orthogonal frequencies).
• Each subcarrier is assigned to a different user.
• Fast Fourier Transform (FFT) modulates/demodulates signals efficiently.
• Used in 4G/5G (LTE-Advanced, 5G NR) and Wi-Fi (802.11ac/ax).

Real-World Example:

• 5G networks (Ncell, Smart Cell): Use OFDMA for ultra-high-speed downloads (e.g., 4K streaming on Pathao’s in-app browser).
• Wi-Fi 6/6E: Uses OFDMA to allow multiple devices to transmit simultaneously (e.g., 10+ devices on a single router).
• YouTube/Netflix: OFDMA enables adaptive bitrate streaming by assigning more subcarriers to high-demand users.

Advantages:

• High spectral efficiency: Subcarriers are packed tightly without guard bands.
• Resistant to multipath fading: Uses cyclic prefix to combat delays.
• Flexible allocation: Subcarriers can be dynamically assigned (e.g., Daraz’s peak-hour traffic).

Disadvantages:

• Complexity: Requires FFT processing at transmitter/receiver.
• Sensitive to carrier frequency offset: Small frequency mismatches cause interference.
• Higher latency: FFT computation adds overhead.

Worked Example: In a 5G OFDMA system with 100 MHz bandwidth:

• Divided into 1024 subcarriers (each 97.65625 kHz wide).
• If User A needs 10 Mbps, they get ~100 subcarriers.
• User B (streaming video) gets 500 subcarriers for 50 Mbps.
• Total capacity = 100 MHz × log₂(1 + SNR) ≈ 1 Gbps (theoretical max).

3. Random Access Methods

When users cannot be pre-assigned channels/slots (e.g., IoT devices, Wi-Fi), random access is used. Two key types:

A. ALOHA (Pure and Slotted)

How it works:

• Users transmit whenever they have data (no coordination).
• If collisions occur, users back off and retry randomly.

sequenceDiagram
    participant User1
    participant User2
    participant BaseStation
    User1->>BaseStation: Transmit (t=0)
    User2->>BaseStation: Transmit (t=0.5)  # Collision!
    BaseStation-->>User1: ACK (if no collision)
    BaseStation-->>User2: NACK (collision)
    User2->>User2: Backoff (random delay)
    User2->>BaseStation: Retry (t=2)

Real-World Example:

• Early packet radio networks (1970s).
• IoT devices (e.g., smart meters) may use ALOHA for low-power transmissions.

Advantages:

• Simple: No need for synchronization.
• Decentralized: Works in ad-hoc networks (e.g., emergency Wi-Fi hotspots).

Disadvantages:

• Low efficiency: Up to 18% (pure ALOHA) or 36% (slotted ALOHA) channel use.
• High collisions: More users = more retries.

Worked Example: In pure ALOHA:

• If N users transmit at rate λ packets/sec, collision probability ≈ λ²T²/2 (where T = packet time).
• For λ = 1 packet/sec, T = 1 sec, collision probability = 50%!

B. Carrier Sense Multiple Access (CSMA)

How it works:

• Users listen to the channel before transmitting.
• If the channel is idle, they transmit.
• If busy, they wait or back off.

Variants:

1. CSMA (non-persistent): If busy, wait random time before retrying.
2. CSMA/CD (Ethernet): Collision detection (used in wired networks).
3. CSMA/CA (Wi-Fi): Collision avoidance (used in 802.11).

stateDiagram-v2
    [*] --> Idle
    Idle --> Sense: Channel idle?
    Sense --> Transmit: Yes
    Sense --> Backoff: No
    Transmit --> Success: ACK received
    Transmit --> Collision: No ACK (retry)
    Collision --> Backoff
    Backoff --> Idle

Real-World Example:

• Wi-Fi (802.11): Uses CSMA/CA to avoid collisions (e.g., multiple laptops on a single router).
• Bluetooth: Uses a form of CSMA for ad-hoc connections.

Advantages:

• Higher efficiency than ALOHA (~80% channel use).
• Reduces collisions by sensing the channel.

Disadvantages:

• Hidden terminal problem: Two users out of range of each other may collide.
• Overhead: Sensing and backoff add latency.

Worked Example: In CSMA/CA (Wi-Fi):

• DIFS (Distributed Interframe Space): 50 µs (wait time before sensing).
• Slot time: 9 µs (minimum time between transmissions).
• If User A senses the channel busy for 20 µs, it waits DIFS + random backoff before retrying.

Comparison of Multiple Access Techniques

In the Real World

1. Ncell’s 4G LTE (OFDMA/CDMA Hybrid)

   • Ncell uses OFDMA for downlink (user → base station) and SC-FDMA (a CDMA variant) for uplink (user ← base station).
   • Why? OFDMA allows multiple users to download data simultaneously (e.g., streaming on Pathao), while SC-FDMA reduces power consumption for uploads (e.g., sending location data).

2. WhatsApp Voice Calls (TDMA in 2G/3G)

   • When you call someone on 2G (TDMA), your voice is split into time slots shared with 7 other users on the same frequency.
   • Example: If 8 users share a 200 kHz channel, each gets 25 kHz for their call (total 200 kHz). If one user hangs up, the slot becomes available for others.

3. Daraz’s Order Processing (CSMA/CA in Wi-Fi)

   • Daraz’s warehouse management systems use Wi-Fi (CSMA/CA) for inventory updates from multiple devices.
   • Problem: If 100 devices try to update stock simultaneously, collisions occur.
   • Solution: Devices sense the channel and wait if busy, reducing errors in order fulfillment.

4. Khalti’s Payment Gateway (FDMA in Microwave Links)

   • Khalti’s data centers use microwave links (FDMA) for secure, high-speed communication between branches.
   • Why FDMA? Microwave frequencies are licensed and isolated, preventing interference from other services (e.g., TV broadcasts).

5. Traffic Light Control (TDMA in V2X Communications)

   • Vehicular networks (V2X) use TDMA for cars to communicate with traffic lights (e.g., "green light ahead").
   • Example: A car approaching a junction gets a time slot to send its speed to the traffic light controller, which adjusts timings dynamically.

Exam Tip

This unit is heavily tested in TU/PU exams with:

1. Definitions and comparisons (e.g., "Differentiate FDMA and TDMA").
2. Calculations:
   • Bandwidth allocation in FDMA/TDMA.
   • Processing gain in CDMA.
   • Throughput in OFDMA (subcarrier count).
3. Scenario-based questions:
   • "Why does Ncell use OFDMA for 4G but TDMA for 2G?"
   • "How would you reduce collisions in a Wi-Fi network (CSMA/CA)?"
4. Diagrams:
   • Draw FDMA/TDMA/CDMA/OFDMA spectra.
   • Sketch CSMA/CA state diagrams.
5. Real-world applications:
   • Relate techniques to Nepali telecom (Ncell, NTC) or global tech (5G, Wi-Fi 6).

Common Mistakes to Avoid:

• Confusing FDMA (frequency) with TDMA (time).
• Forgetting guard bands in FDMA or guard times in TDMA.
• Ignoring near-far problem in CDMA.
• Mixing CSMA/CD (wired) with CSMA/CA (wireless).

High-Score Strategy:

• Memorize the comparison table (FDMA vs. TDMA vs. CDMA vs. OFDMA).
• Practice numericals (e.g., "If a 1 MHz channel is divided into 5 FDMA users with 20 kHz guard bands, what’s the usable bandwidth per user?").
• Link theory to real systems (e.g., "Why does YouTube use OFDMA?" → "For adaptive bitrate streaming").