UNIT-I: CELLULAR CONCEPT AND SYSTEM DESIGN FUNDAMENTALS

UNIT-I: CELLULAR CONCEPT AND SYSTEM DESIGN FUNDAMENTALS

What is Communication?

- Communication is the process of transmitting information or data from one point to another through a physical medium (e.g., air or wireless) using electronic systems and protocols.

Frequency Reuse in Cellular Concept:

- Cellular systems use many low-power transmitters instead of a single high-power transmitter.

- The service area is divided into hexagon-shaped cells, each with a base station and a group of radio channels.

- Frequency Reuse Planning involves assigning different frequency groups to cells in a pattern.

- A Cluster contains N uniquely assigned cells (S = K × N); System capacity: C = M × S

- Frequency Reuse Factor = 1/N (N = i² + ij + j², where i and j are integers)

Channel Assignment Strategies:

1. Fixed Channel Assignment:

   - Pre-assigned voice channels to each cell.

   - New calls are blocked if all channels are in use.

2. Dynamic Channel Assignment:

   - Channels are assigned on request.

   - Reduces blocking, increases capacity and flexibility.

Handoff Strategies:

- Handoff is the handover of an ongoing call/data session as users move between base stations.

Types of Handoff:

1. Guard Channel:

   - Reserves some channels for handoff requests only.

2. Handoff Queuing:

   - If no channel is available, the handoff request is queued (avoids dropped calls).

Practical Handoff Considerations:

- Received Signal Strength (RSSI), Signal-to-Noise Ratio (SNR)

- Speed and mobility of user

- Network load and available resources

- Call type priority (e.g., emergency)

- Use of hysteresis margin, delay timers, predictive handoff algorithms

- Inter-system compatibility

Interference and System Capacity:

Types:

- Co-Channel Interference (CCI): Caused by identical frequencies in different cells.

- Adjacent Channel Interference (ACI): Caused by nearby frequencies due to filtering issues.

Co-Channel Interference:

- Increases with reduced reuse distance.

- Mitigation: Use directional antennas, increase distance, power control.

System Capacity:

- Depends on bandwidth, modulation, SINR, interference.

- Trade-off: Lower reuse factor increases capacity, but also interference.

- Capacity ∝ SINR↑ → Data rate↑, QoS↑

Channel Planning:

1. Understand network requirements

2. Divide area into hexagonal cells

3. Choose reuse pattern (e.g., N=3, 4, 7)

4. Assign frequencies per reuse pattern

5. Optimize using simulations and field data

Adjacent Channel Interference:

- Occurs due to imperfect filtering or power mismatch.

- Common in Wi-Fi (e.g., Channel 1, 6, 11 avoid overlap).

- Effects: Reduced throughput, higher BER.

Power Control:

- Adjusts transmission power to maintain quality and reduce interference.

Techniques:

- Open/Closed loop control, Adaptive algorithms

Pros:

- Improved signal quality

- Energy efficiency

- Battery saving

- Network capacity increase

Cons:

- Delay in dynamic response

- May struggle in congested networks

Trunking and Grade of Service (GOS):

Trunking:

- Allows multiple users to share a limited number of channels.

- Efficient resource utilization using statistical multiplexing.

Grade of Service:

- Defines probability of call being blocked or delayed.

- Measured using Erlang B (no queuing) or Erlang C (with queuing).

Improving Coverage & Capacity:

1. Cell Splitting:

   - Sub-divides large cells into smaller ones (microcells).

   - Increases capacity but needs more base stations and handoffs.

2. Cell Sectoring:

   - Divides cell into 3 or more sectors (typically 120°).

   - Each sector has its antenna; improves frequency reuse and lowers interference.

Pros:

- Higher SIR

- Channel reuse improves

- Efficient for high-density areas

Cons:

- Increases cost and antenna complexity

- May reduce overall efficiency in some cases

-- END OF UNIT-I NOTES --