Cellular Communications: Cells, Channels & Mobility

TAKEAWAYS:

• Cellular networks divide coverage into hexagonal cells to reuse frequencies efficiently, balancing capacity and interference.
• Frequency reuse and channel allocation (FDMA, TDMA, CDMA) determine how users share limited spectrum without collisions.
• Handoffs (soft vs. hard) enable seamless mobility between cells, critical for calls/data continuity in moving devices.
• System architectures (1G analog → 5G mmWave) evolve to support voice, SMS, and ultra-low-latency IoT.
• Interference management (co-channel, adjacent-channel) is solved via power control, sectorization, and dynamic allocation.
• Real-world tie: Ncell’s 4G/LTE uses OFDMA (a CDMA variant) for data, while NTC’s 5G trials employ beamforming for high-speed connections.

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Core Concepts: Cells and Frequency Reuse

1. Cell Structure and Coverage

Cellular networks partition geographic areas into hexagonal cells (theoretical model) to minimize overlap and maximize coverage. Each cell has:

• A Base Transceiver Station (BTS) (the tower).
• A sectorized antenna (typically 3 sectors per cell for directional beams).
• A frequency band assigned by regulators (e.g., NTC in Nepal allocates 800 MHz, 1800 MHz, 2600 MHz).

Why hexagons?

• Minimize overlap: Hexagons tile the plane with the least gap/overlap (vs. circles/squares).
• Reuse distance (D): The distance between co-channel cells to avoid interference. Formula: $D=R\sqrt{3N}$, where $R$ = cell radius, $N=$ reuse factor (typically 3, 4, or 7).

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2. Frequency Reuse and Channel Allocation

Limited spectrum is shared via frequency reuse: the same frequencies are reused in non-adjacent cells. Key terms:

• Cluster: A group of cells using the same set of frequencies.
• Reuse factor (N): Number of clusters (e.g., $N=7$ means 7 unique frequency sets).
• Co-channel interference (CCI): Signals from co-channel cells interfering at the edge.

Worked Example: Ncell’s 4G LTE in Kathmandu

• Band: 1800 MHz (Band 3).
• Channel bandwidth: 20 MHz.
• Reuse factor: 3 (typical for urban areas).
• Calculation:
  • Cell radius $R=1$ km.
  • Reuse distance $D=1\times \sqrt{3\times 3}=3$ km.
  • Result: Co-channel cells are 3 km apart, reducing CCI.

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Channel Access Techniques

1. FDMA (Frequency Division Multiple Access)

• How it works: Divides the frequency band into fixed channels (e.g., 30 kHz per channel in 1G).
• Example: 1G analog networks (e.g., old NTC’s AMPS).
• Limitation: Inefficient for data (fixed slots waste bandwidth).

2. TDMA (Time Division Multiple Access)

• How it works: Users share the same frequency by taking turns in time slots (e.g., 8 slots per frame in GSM).
• Example: 2G GSM (used by Ncell until 2020).
• Advantage: Supports multiple users dynamically.

3. CDMA (Code Division Multiple Access)

• How it works: All users transmit simultaneously on the same frequency using unique codes (e.g., Walsh codes in IS-95).
• Example: 3G (UMTS/WCDMA), 4G LTE (OFDMA is a CDMA variant).
• Advantage: Higher capacity, softer handoffs (no call drops during mobility).

Comparison Table:

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Mobility Management: Handoffs and Roaming

1. Handoff (Handover) Mechanisms

When a mobile device moves between cells, a handoff ensures seamless connectivity. Two types:

• Hard handoff: Breaks connection before establishing a new one (used in GSM/TDMA).
• Soft handoff: Maintains connections to both old and new cells briefly (used in CDMA/WCDMA).

Mermaid Sequence: Soft Handoff in 3G

Real-World Tie: Pathao Driver’s Handoff

• A Pathao driver moving from Thapathali to Bhatbhateni (Kathmandu) experiences:
  1. Soft handoff between Ncell’s 4G sectors (seamless data).
  2. Inter-RAT handoff (if switching from 4G to 5G in a trial zone).

2. Roaming and Inter-System Handoffs

• Intra-system roaming: Moving between cells of the same operator (e.g., Ncell’s 4G to 5G).
• Inter-system roaming: Switching between operators (e.g., Ncell → NTC) or technologies (4G → Wi-Fi).
• Example: NEPSE’s stock traders use dual-SIM (Ncell + NTC) for failover.

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Cellular Generations: 1G to 5G

Key 5G Innovations:

• mmWave: Uses 24 GHz+ bands for ultra-high speeds (but short range).
• MIMO: Multiple antennas for beamforming (focused signals).
• Network slicing: Dedicated virtual networks (e.g., one slice for NTC’s IoT, another for Ncell’s gaming).

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Interference Management

1. Types of Interference

2. Mitigation Techniques

• Power control: Adjust transmit power to avoid overwhelming nearby cells.
• Sectorization: Divide cells into 3 sectors (120° each) to reduce CCI.
• Dynamic channel allocation (DCA): Assign channels on-demand (used in 4G/5G).

Worked Example: NTC’s 5G Trial in Lalitpur

• Problem: High CCI in dense areas (e.g., Pulchowk campus).
• Solution:
  • Beamforming: Direct signals to users (reduces wasted energy).
  • Reuse factor N=1: Uses mmWave’s high bandwidth to avoid reuse entirely.

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In the Real World

1. eSewa (Nepal):

   • Uses 3G/4G CDMA for secure transactions.
   • Handoffs ensure payments work while moving (e.g., bus to market).
   • Interference: eSewa’s servers use load balancers (like dynamic channel allocation) to distribute traffic.

2. Pathao (Ride-Hailing):

   • Real-time handoffs: Driver’s phone switches between Ncell/NTC towers without dropping the app.
   • 5G potential: Future AR navigation will require ultra-low latency (<1 ms).

3. NEPSE (Stock Exchange):

   • Dedicated microwave links: Use frequency-hopping spread spectrum (FHSS) to avoid interference from NTC/Ncell towers.
   • Backup: If primary 4G fails, switches to satellite (VSAT).

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Exam Tip

1. Diagrams are worth 20% of marks:
   • Draw hexagonal cells with reuse factor N=7 and label co-channel cells.
   • Sketch a soft handoff sequence diagram (as above).
2. Formulas to memorize:
   • Reuse distance: $D=R\sqrt{3N}$.
   • Capacity: $C=\frac{B}{N}$, where $B$ = total bandwidth.
3. Compare technologies:
   • FDMA vs. TDMA vs. CDMA (table above).
   • 4G LTE vs. 5G (latency, speed, use cases).
4. Real-world applications:
   • Explain how Ncell’s 4G uses OFDMA (a CDMA variant) for data.
   • Describe Pathao’s handoff during a ride in Kathmandu traffic.
5. Common pitfalls:
   • Don’t confuse hard handoff (GSM) with soft handoff (CDMA).
   • Remember: 5G uses mmWave for speed, not coverage (short range).