Cellular Network Concepts: Handoff, Channel Assignment, and Interference
Methods for Handoff Procedures
Hard Handoff (Break-before-Make)
The connection to the old base station is terminated before a new connection is established with the next base station.
Used in FDMA and TDMA-based systems (e.g., GSM).
It can cause call drops if the new base station is not available immediately.
Example: GSM, AMPS (Advanced Mobile Phone System).
Soft Handoff (Make-before-Break)
The mobile device maintains connections with multiple base stations simultaneously before switching.
Used in CDMA-based systems (e.g., IS-95, W-CDMA).
Reduces call drops as the mobile unit gradually switches from one base station to another.
Example: CDMA, UMTS (3G).
Intersystem Handoff
Occurs when a mobile moves between two different cellular networks or service providers.
Requires coordination between different Mobile Switching Centers (MSCs).
Example: Moving from a GSM network to a CDMA network.
Mobile-Assisted Handoff (MAHO)
The mobile device actively measures signal strengths from neighboring base stations and reports them to the network.
The Mobile Switching Center (MSC) makes the handoff decision based on the received signal strength information.
Faster and more efficient, reducing dropped calls.
Example: GSM, 3G, 4G.
Channel Assignment Strategies
Channel assignment is a crucial aspect of cellular system design, as it determines how frequency channels are allocated to different cells to optimize network capacity, reduce interference, and improve call quality.
Fixed Channel Assignment (FCA)
Each cell is allocated a predetermined set of frequency channels.
Calls can only be served by the available channels in that particular cell.
If all channels are in use, new calls are blocked unless a borrowing strategy is implemented.
Example: Traditional GSM networks often use FCA for managing frequencies.
Dynamic Channel Assignment (DCA)
Channels are not permanently assigned to a particular cell.
When a call is made, the Mobile Switching Center (MSC) dynamically assigns a channel based on real-time traffic conditions.
The system continuously monitors channel usage and optimizes assignments to reduce interference.
Key Features of 4G Networks
High Data Rates
Supports up to 1 Gbps for stationary users and 100 Mbps for mobile users.
Enables HD video streaming, online gaming, and VoIP calls.
All-IP Network
Uses Internet Protocol (IP) for all communications, eliminating circuit-switched technology.
Enables seamless integration with Wi-Fi, broadband, and wired networks.
OFDM and MIMO Technologies
Uses Orthogonal Frequency Division Multiplexing (OFDM) for efficient data transmission.
Implements Multiple-Input Multiple-Output (MIMO) antennas to enhance signal quality and speed.
Low Latency
Reduced latency (~10 ms) enables real-time applications like video conferencing and remote healthcare.
Improved Spectrum Efficiency
Efficient spectrum usage allows more users to be served simultaneously.
Uses dynamic spectrum allocation for optimized performance.
Support for Multimedia Services
Enables high-definition voice and video calls (VoLTE).
Facilitates online gaming, mobile TV, and video-on-demand services.
Seamless Mobility & Handoff
Supports smooth handover between networks (e.g., LTE to Wi-Fi, LTE to 5G).
Ensures uninterrupted connections while moving between coverage areas.
Enhanced Security
Uses strong encryption and authentication methods to protect data.
Prevents unauthorized access with improved privacy mechanisms.
Cell Splitting and Sectoring
Grade of Service (GOS)
Definition:
Grade of Service (GOS) is a measure of the quality of service provided by a trunked communication system. It represents the probability of call blocking or delay due to limited system resources (e.g., frequency channels).
Interpretation:
A GOS of 0.02 means that only 2% of calls are blocked due to unavailable resources.
Lower GOS values indicate better service quality.
GOS is typically expressed in Erlangs using the Erlang B and Erlang C formulas.
Factors Affecting GOS:
Number of Available Channels → More channels reduce blocking.
Traffic Load (in Erlangs) → High demand increases blocking probability.
Trunking Efficiency → Better channel allocation improves GOS.
Trunking
Trunking is a method used in cellular and radio networks to allow multiple users to share a limited number of communication channels efficiently.
It optimizes resource utilization by dynamically allocating channels as needed.
How It Works:
Users do not have dedicated channels but access a common pool of frequencies.
When a user initiates a call, an available channel is assigned.
Once the call ends, the channel is returned to the pool for other users.
Benefits of Trunking:
Increases Network Capacity → More users can be served with fewer channels.
Reduces Call Blocking → Dynamic allocation improves efficiency.
Maximizes Spectrum Utilization → Calls are handled based on demand.
GOS=BLOCKED CALLS/TOTAL ATTEMPTS
Co-Channel and Adjacent Channel Interference
Interference reduces the capacity, quality, and efficiency of a cellular system. Two major types of interference in cellular networks are Co-Channel Interference (CCI) and Adjacent Channel Interference (ACI).
Co-Channel Interference (CCI)
Co-Channel Interference (CCI) occurs when multiple cells reuse the same frequency channels due to frequency reuse planning.
Interference is caused by signals from other cells using the same frequency band.
Impact on Cellular System Capacity:
Reduces Signal-to-Interference Ratio (SIR) → Degrades call quality and data rates.
Increases Dropped Calls → Higher call failure rates due to poor signal quality.
Limits Frequency Reuse Efficiency → More interference requires larger cluster sizes (N), reducing capacity.
Reduces Effective Coverage Area → Signals must be transmitted at lower power to minimize interference.
Solutions to Reduce CCI:
Increase Reuse Distance (D) → Use larger cluster sizes (N = 7, 12).
Use Sectoring → Directional antennas reduce interference.
Power Control → Optimize base station transmission power.
Adjacent Channel Interference (ACI)
Adjacent Channel Interference (ACI) occurs when signals from adjacent frequency bands leak into neighboring channels.
ACI is caused by improper filtering, overlapping frequency bands, or nonlinear amplification.
Impact on Cellular System Capacity:
Reduces Voice & Data Quality → Causes noise, cross-talk, and call distortion.
Leads to More Call Drops → Errors in signal decoding due to interference.
Increases Bit Error Rate (BER) → Degrades digital data transmission.
Reduces Effective Bandwidth → More frequencies need to be reserved as guard bands to prevent interference.
Solutions to Reduce ACI:
Use Guard Bands → Add frequency gaps between adjacent channels.
Improve Receiver Filtering → Use high-quality bandpass filters.
Optimize Frequency Assignment → Avoid placing adjacent frequencies in the same cell.
