Undergraduate Electrical Engineering

Mobile Hand-off
in GSM Networks

A comprehensive study guide covering the principles, procedures, and algorithms of handover mechanisms in Global System for Mobile Communications (GSM).

What is Hand-off?

Handover (or handoff) is the process by which a mobile station (MS) transfers an ongoing call or data session from one base station (BTS) to another as the user moves through the coverage area of the cellular system [^1^].

The primary objective is to preserve ongoing calls when the mobile station moves from one cell to another, ensuring seamless mobility and preventing call drops [^5^].

Key Concept: MAHO

GSM uses Mobile Assisted HandOver (MAHO), where the mobile measures signal strength from neighboring cells and reports to the network, but the network (BSC) makes the final handover decision [^1^][^4^].

Network Architecture Active
MS
Mobile Station
Measures & Reports
BTS
Base Transceiver Station
Radio Interface
BSC
Base Station Controller
Handover Decision Point
MSC
Mobile Switching Center
Inter-BSC Coordination

Types of GSM Handover

GSM implements four primary types of handover, classified by the network elements involved [^1^][^11^].

1

Intra-BTS

Within same Base Transceiver Station

  • • Changes frequency/time slot
  • • Same BTS, different channel
  • • Due to interference
2

Inter-BTS Intra-BSC

Different BTS, same BSC

  • • BSC controls handover
  • • Assigns new channel
  • • Releases old BTS
3

Inter-BSC

Different BSCs, same MSC

  • • MSC controls handover
  • • Involves channel change
  • • More complex signaling
4

Inter-MSC

Different Mobile Switching Centers

  • • MSCs negotiate control
  • • Most complex type
  • • Inter-network handover

Hard vs. Soft Handover

Hard Handover (GSM)

"Break before Make" - The connection to the old base station is terminated before the new connection is established [^2^][^4^].

Connection Type: Single (TDMA/FDMA)
Switching: Discontinuous
Implementation: GSM, LTE

Soft Handover (CDMA)

"Make before Break" - The mobile connects to the new base station while maintaining the old connection temporarily [^2^][^7^].

Connection Type: Multiple (CDMA)
Switching: Continuous
Implementation: WCDMA, CDMA2000

Handover Procedure

1. Measurement Phase

Mobile Station continuously measures signal strength (RSSI) of serving cell and up to 6 neighboring cells during idle slots in TDMA frame [^1^][^5^].

1
SACCH Period: 480ms
Reports sent: 2/second
BSC Evaluation:
RSS + Quality + Distance
2

2. Decision Phase

BSC analyzes Measurement Reports (MR). If RSS drops below threshold or better cell available, handover decision is initiated [^5^][^9^].

3. Allocation Phase

BSC/MSC allocates new channel resources. New BTS activates channel and waits for MS. Timing advance value calculated [^3^][^6^].

3
Handover Command:
New Freq + TS + TA + Power
Access Bursts:
4 (sync) or 64 (non-sync)
4

4. Execution Phase

MS breaks old connection, retunes to new channel, sends access bursts to synchronize. Upon success, old channel released [^1^][^6^].

Handover Initiation Algorithms

The decision to initiate handover is based on various criteria involving received signal strength (RSS), quality, and network conditions [^4^][^5^].

Relative Signal Strength

Handover occurs when received signal from new BS exceeds current BS. Simple but causes "ping-pong" effect [^4^].

Handover if: RSS_new > RSS_current

Relative Signal with Threshold

Handover only occurs if current signal drops below a threshold (T) AND new signal is stronger [^4^].

Handover if: RSS_current < T AND RSS_new > RSS_current

Relative Signal with Hysteresis

Prevents ping-pong by requiring new signal to be stronger by margin (H). Most common approach [^4^].

Handover if: RSS_new > RSS_current + H

Power Budget (PBGT)

Minimizes transmit power while maintaining quality. Handover preferred over power increase [^3^].

Handover if: RXLEV_n - BSTXPWR_n > RXLEV_s - BSTXPWR_s + H

Timing Advance (TA)

Critical for handover success. GSM uses TDMA where timing is crucial. The Timing Advance ensures the MS signal arrives at the BTS in the correct time slot, compensating for propagation delay [^1^].

  • • Range: 0-63 bits (0-233 μs)
  • • Each bit ≈ 3.69 μs ≈ 1.1 km distance
  • • Maximum cell radius: ~35 km
MS Distance Timing Advance
0 km
0
10 km
9
20 km
18
35 km
63

Interactive Handover Simulation

Visualize the handover process as a mobile moves between cells. Adjust parameters to see different behaviors.

Simulation Parameters

Slow Fast
0 dB 3 dB
-100 dBm -85 dBm
Status: IDLE
Serving: Cell A
Handovers: 0
Cell A
Cell B

Key Takeaways

Mobile Assisted

GSM uses MAHO where MS measures neighbors but BSC decides, balancing load between mobile and network [^1^][^4^].

Hard Handover Only

Due to TDMA/FDMA, GSM cannot do soft handover. "Break before make" is used, requiring fast execution [^2^][^11^].

Hierarchical Control

Intra-BSC handled by BSC; Inter-BSC/MSC requires MSC involvement, increasing complexity [^1^][^8^].

Timing Critical

Timing Advance is crucial for synchronization. Access bursts (4 or 64) used depending on sync status [^1^][^3^].