📡 GSM Virtual Laboratory

ECE 525E - Global System for Mobile Communications - Architecture, Frequency Planning, Channels & Handover

Learning Objectives

Upon completion of this laboratory, students will be able to:

Pre-Lab Questions

Before starting the laboratory, answer the following questions:

  1. What is the frequency separation between uplink and downlink in GSM 900?
  2. How many time slots constitute one TDMA frame in GSM?
  3. What is the difference between FDMA and TDMA?
  4. Why is frequency reuse planning necessary in cellular systems?
  5. What triggers a handover in GSM networks?

Fundamental Theory

1. GSM Overview

GSM (Global System for Mobile Communications) is a second-generation (2G) digital cellular standard developed by ETSI. It uses a combination of FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access) to provide voice and data services.

Key Specifications:
  • Frequency Bands: 890-915 MHz (Uplink), 935-960 MHz (Downlink) - GSM 900
  • Channel Spacing: 200 kHz
  • Duplex Spacing: 45 MHz
  • Modulation: GMSK (Gaussian Minimum Shift Keying)
  • Data Rate: 270.833 kbps per carrier
  • TDMA Frames: 8 time slots per frame, 4.615 ms frame duration

2. Multiple Access Techniques

FDMA (Frequency Division Multiple Access)

The available spectrum is divided into 200 kHz channels. Each carrier frequency supports 8 time slots.

ARFCN (Absolute Radio Frequency Channel Number): 0-124 for GSM 900, representing specific frequency pairs.

TDMA (Time Division Multiple Access)

Each carrier is divided into 8 time slots. Each user gets one time slot for transmission and one for reception (3 slots apart due to duplex timing).

Frame Structure: 4.615 ms frames containing 8 slots of 576.9 μs each.

3. Frequency Planning Concepts

Frequency planning involves allocating channels to cells to minimize interference while maximizing capacity. Common reuse patterns include:

The 4/12 pattern provides greater reuse distance, reducing co-channel interference but offering lower capacity per cell compared to 3/9.

4. Handover Fundamentals

Handover is the process of transferring an ongoing call from one cell to another as the mobile station moves. GSM uses hard handover (break-before-make) because the mobile must retune to different frequencies.

Handover Types:

  • Intra-BTS: Between sectors of same BTS (BSC not involved)
  • Intra-BSC: Between BTSs connected to same BSC
  • Inter-MSC: Between different MSCs (most complex)

5. Channel Organization

GSM logical channels are divided into:

Experimental Procedure

📋 Pre-Experiment Setup: Ensure you have reviewed the Theory section and understand basic GSM concepts before proceeding with experiments.

🔬 Experiment 1: GSM Architecture Identification

Required Tools:

  • GSM Virtual Laboratory Interface
  • Architecture Diagram Worksheet
  • Network Component Database

Step 1: Network Element Identification

Navigate to the Architecture tab. Click on each network component (MS, BTS, BSC, MSC, HLR, VLR, AuC, EIR) to reveal detailed descriptions.

Record the primary function of each component in Table 1.1 below.

Step 2: Interface Mapping

Study the interface connections between components. Identify the Um, Abis, A, B, C, D, and E interfaces.

Complete Table 1.2 by matching interfaces with their connecting elements.

Step 3: Subsystem Classification

Classify each component into either Radio Subsystem (RSS) or Network and Switching Subsystem (NSS).

Verify your classification with the provided solution.

Table 1.1: Network Component Functions

Component Full Name Primary Function Subsystem
MS
BTS
BSC
MSC
HLR
VLR

Table 1.2: Interface Identification

Interface Between Elements Physical Medium Protocol
Um
Abis
A
0%

🔬 Experiment 2: ARFCN and Frequency Planning

Required Tools:

  • ARFCN Calculator
  • Frequency Reuse Pattern Simulator
  • Spectrum Analyzer (Virtual)

Step 1: ARFCN Calculation (GSM 900)

Navigate to the Frequency Plan tab. Select GSM 900 band.

  1. Enter ARFCN = 1 in the calculator. Record uplink and downlink frequencies.
  2. Calculate frequencies for ARFCN 50, 100, and 124.
  3. Verify that duplex spacing is consistently 45 MHz.
Record results in Table 2.1. Verify calculations using the formula:
Fup = 890 + 0.2 × n (MHz), Fdown = Fup + 45 (MHz)

Step 2: GSM 1800 Frequency Calculation

Switch to GSM 1800 band. Calculate frequencies for ARFCN 512, 600, and 885.

Record results in Table 2.2. Note the different duplex spacing (95 MHz).

Step 3: Frequency Reuse Pattern Analysis

Using the Reuse Pattern Calculator:

  1. Set total available ARFCNs = 36
  2. Calculate for 3/9 pattern (3 sectors, 9 cells)
  3. Calculate for 4/12 pattern (3 sectors, 12 cells)
  4. Compare carriers per cell and reuse distances
Complete Table 2.3 comparing reuse patterns.

Table 2.1: GSM 900 ARFCN Calculations

ARFCN Uplink (MHz) Downlink (MHz) Duplex Spacing (MHz)
1
50
100
124

Table 2.3: Reuse Pattern Comparison

Parameter 3/9 Pattern 4/12 Pattern Advantage
Cluster Size
Carriers/Cell
Reuse Distance

🔬 Experiment 3: Logical Channel Analysis

Required Tools:

  • TDMA Frame Visualizer
  • Channel Type Analyzer
  • Multiframe Structure Viewer

Step 1: TDMA Frame Structure

Navigate to the Channels tab.

  1. Examine the TDMA frame structure (8 time slots)
  2. Click on each time slot (TS0-TS7) to identify channel types
  3. Record which slots carry control vs. traffic channels
Complete Table 3.1 identifying channel assignments.

Step 2: Multiframe Analysis

Study the 26-frame and 51-frame multiframes:

  1. 26-frame multiframe: Identify TCH, SACCH, and IDLE frame positions
  2. 51-frame multiframe: Observe control channel organization
  3. Calculate multiframe durations (26 × 4.615 ms and 51 × 4.615 ms)
Record frame compositions in Table 3.2.

Step 3: Logical Channel Function Mapping

Use the Channel Analyzer to study:

  1. BCCH: System information broadcast
  2. SDCCH: Call setup and signaling
  3. SACCH: Measurement reports and power control
  4. FACCH: Fast signaling (handover)
Document channel functions in Table 3.3.

Table 3.1: TDMA Frame Structure

Time Slot Channel Type Direction Primary Use
TS0
TS1
TS2-TS7

Table 3.2: Multiframe Structures

Multiframe Type Frames Duration (ms) Channel Usage
26-frame
51-frame

🔬 Experiment 4: Handover Mechanisms

Required Tools:

  • Handover Simulator
  • Signal Strength Monitor
  • Network Topology Viewer

Step 1: Handover Types Identification

Navigate to the Handover tab.

  1. Study the three handover types: Intra-BTS, Intra-BSC, Inter-MSC
  2. Identify which network elements are involved in each type
  3. Note the complexity level and signaling requirements
Complete Table 4.1 comparing handover types.

Step 2: Handover Decision Parameters

Analyze the handover decision criteria:

  1. RXLEV (Received Signal Level): Thresholds for triggering
  2. RXQUAL (Received Signal Quality): Bit error rate considerations
  3. Timing Advance (TA): Distance-based handover
  4. Power Budget: Path loss calculations
Document parameter thresholds in Table 4.2.

Step 3: Handover Procedure Simulation

Run the Handover Animation:

  1. Observe mobile station movement between cells
  2. Monitor signal strength variations (serving vs. neighbor)
  3. Identify the handover decision point
  4. Note the "break-before-make" characteristic
Record handover latency and signal levels at handover point.

Table 4.1: Handover Types Comparison

Handover Type Between Decision Maker Elements Involved Complexity
Intra-BTS
Intra-BSC
Inter-MSC

Table 4.2: Handover Decision Parameters

Parameter Typical Threshold Measurement Handover Trigger
RXLEV
RXQUAL
Timing Advance

🔬 Experiment 5: Link Budget and Coverage Planning

Required Tools:

  • Path Loss Calculator
  • Cell Radius Estimator
  • Link Budget Analyzer

Step 1: Free Space Path Loss Calculation

Navigate to the Calculators tab.

  1. Set frequency = 900 MHz, distance = 5 km
  2. Calculate path loss using FSPL formula
  3. Vary distance (1, 2, 5, 10 km) and observe loss increase
  4. Repeat for 1800 MHz and compare
Record results in Table 5.1. Note: Higher frequencies experience greater path loss.

Step 2: Cell Radius Estimation

Using the Cell Radius Calculator:

  1. Input transmitter power = 40 dBm (BTS)
  2. Set receiver sensitivity = -90 dBm
  3. Apply fade margin = 10 dB
  4. Calculate maximum cell radius
Record calculated radius in Table 5.2.

Step 3: Complete Link Budget Analysis

Perform uplink and downlink budget calculations:

  1. Downlink: BTS (40 dBm, 15 dBi gain) → MS (0 dBi gain)
  2. Uplink: MS (30 dBm, 0 dBi gain) → BTS (15 dBi gain)
  3. Calculate received power at both ends
  4. Verify link balance (difference should be minimal)
Complete Table 5.3 with link budget details.

Table 5.1: Path Loss vs. Distance

Distance (km) 900 MHz Loss (dB) 1800 MHz Loss (dB) Difference (dB)
1
2
5
10

Table 5.3: Link Budget Analysis

Parameter Downlink (BTS→MS) Uplink (MS→BTS)
Tx Power (dBm)
Tx Antenna Gain (dBi)
EIRP (dBm)
Path Loss @ 5km (dB)
Rx Antenna Gain (dBi)
Received Power (dBm)

⚠️ Safety and Best Practices

  • Record all observations immediately after each experiment
  • Double-check calculations using alternative methods
  • Save screenshots of simulation results for your report
  • Verify frequency calculations against standard tables
  • Ensure handover thresholds align with GSM specifications (3GPP TS 05.08)

Experimental Workflow

Pre-Lab
→
Theory Review
→
Exp 1: Architecture
→
Exp 2: Frequency
→
Exp 3: Channels
→
Exp 4: Handover
→
Exp 5: Link Budget
→
Report

GSM Network Architecture

GSM System Architecture

Mobile Station (MS)
ME + SIM
Base Transceiver Station (BTS)
Radio transmission/reception
Base Station Controller (BSC)
Radio resource management
Mobile Switching Center (MSC)
Call switching & mobility
HLR
VLR
AuC
EIR
PSTN / ISDN / Other Networks

Subsystem Division

Radio Subsystem (RSS)

  • MS (Mobile Station)
  • BTS (Base Transceiver Station)
  • BSC (Base Station Controller)
  • TRAU (Transcoder Rate Adaptation Unit)

Handles all radio-specific aspects including air interface, modulation, and radio resource management.

Network and Switching Subsystem (NSS)

  • MSC (Mobile services Switching Center)
  • HLR, VLR, AuC, EIR
  • GMSC (Gateway MSC)

Manages switching, mobility management, subscriber databases, and interconnection to external networks.

Interfaces

Interface Between Description
Um MS ↔ BTS Air interface (radio)
Abis BTS ↔ BSC Typically 2 Mbps link
A BSC ↔ MSC Standardized interface
B MSC ↔ VLR Internal interface
C MSC ↔ HLR For routing queries
D HLR ↔ VLR Location updates
E MSC ↔ MSC Inter-MSC handover

Frequency Planning & ARFCN

GSM Frequency Bands

Parameter Uplink (MS→BTS) Downlink (BTS→MS)
Frequency Range 890 - 915 MHz 935 - 960 MHz
ARFCN Range 1 - 124 1 - 124
Channel Spacing 200 kHz
Duplex Spacing 45 MHz

ARFCN Calculator

Results:

Frequency Reuse Patterns

3/9 Reuse Pattern

Cluster Size: 9 cells (3 sectors × 3 sites)

Carriers per Cell: 4 (if 36 carriers available)

Advantage: Higher capacity per cell

Disadvantage: Lower reuse distance, higher interference

Reuse Distance: D = R × √(3 × 9) = R × 5.2

4/12 Reuse Pattern

Cluster Size: 12 cells (3 sectors × 4 sites)

Carriers per Cell: 3 (if 36 carriers available)

Advantage: Greater reuse distance, less interference

Disadvantage: Lower capacity per cell

Reuse Distance: D = R × √(3 × 12) = R × 6.0

ARFCN Visualization (GSM 900)

Uplink (890-915 MHz)
Downlink (935-960 MHz)

Cell Planning Exercise

Scenario: You have 36 ARFCNs available for a GSM 900 network.

GSM Logical Channels

TDMA Frame Structure

One TDMA frame consists of 8 time slots (0-7), each lasting 576.9 μs. The frame duration is 4.615 ms.

Control Channel (TS0)
Traffic Channel (TS1-7)

Channel Categories

Traffic Channels (TCH)

  • TCH/F (Full Rate): 13 kbps voice, 9.6 kbps data
  • TCH/H (Half Rate): 6.5 kbps voice, 4.8 kbps data
  • TCH/EFR (Enhanced): Better quality at 13 kbps

Uses 26-frame multiframe structure (120 ms duration).

Control Channels

Broadcast Channels (BCH):

  • FCCH: Frequency correction (pure sine wave)
  • SCH: Synchronization (frame number, BSIC)
  • BCCH: System information, neighbor cells

Common Control Channels (CCCH): PCH, RACH, AGCH

Dedicated Control Channels (DCCH): SDCCH, SACCH, FACCH

Multiframe Structures

26-Frame Multiframe (Traffic Channels)

Duration: 120 ms (26 × 4.615 ms)

● TCH Traffic Channel ● SACCH Slow Associated Control Channel ● IDLE Idle Frame

51-Frame Multiframe (Control Channels)

Duration: 235.37 ms (51 × 4.615 ms) - Used for signaling and control

Channel Mapping

Logical Channel Type Direction Function
FCCH BCH Downlink Frequency correction for MS
SCH BCH Downlink Synchronization, TDMA frame number, BSIC
BCCH BCH Downlink Broadcast system info, neighbor cells, frequencies
PCH CCCH Downlink Page mobile for incoming calls
RACH CCCH Uplink Random access for channel request
AGCH CCCH Downlink Access grant, assigns SDCCH
SDCCH DCCH Both Call setup, authentication, location update
SACCH DCCH Both Measurement reports, power control, timing advance
FACCH DCCH Both Fast signaling (handover), steals TCH bursts

Interactive Channel Analyzer

Select a channel type to see detailed analysis.

Handover Mechanisms

Handover Types

Intra-BTS Handover

Between: Sectors of same BTS

Decision: BTS

Complexity: Low

Occurs when moving between sectors of the same cell. MSC is not involved.

Intra-BSC Handover

Between: Different BTSs, same BSC

Decision: BSC

Complexity: Medium

BSC manages the handover without MSC involvement. New channel assignment via BSC.

Inter-MSC Handover

Between: Different MSCs

Decision: MSC/BSC

Complexity: High

Requires anchor MSC, call routing changes. Includes Handover Forward and Handover Back types.

Handover Decision Criteria

Handover Procedure

BTS 1
-65dBm
BTS 2
-85dBm
Serving: -65 dBm
Neighbor: -85 dBm
State: Connected to BTS 1

Handover Signaling Sequence

  1. Measurement Reports: MS sends SACCH reports every 480ms containing RXLEV and RXQUAL of serving and neighbor cells.
  2. Decision: BSC evaluates measurements against thresholds (RXLEV_MIN, HO_MARGIN).
  3. Handover Command: BSC sends HANDOVER COMMAND via FACCH on old channel.
  4. Access: MS sends HANDOVER ACCESS bursts on new channel with handover reference.
  5. Physical Info: BTS sends timing advance and synchronization info.
  6. Handover Complete: MS confirms successful channel change.
  7. Release: Old channel resources are released.

Handover Types Deep Dive

Type Trigger Network Elements Duration
Emergency (Quality) RXQUAL > 4 (bad quality) BSC decides immediately < 1 second
Level Handover RXLEV < threshold BSC evaluates neighbors ~2-3 seconds
Distance (TA) Timing Advance > 63 BSC forces handover Immediate
Power Budget Better cell available Prevent ping-pong Configurable delay
Inter-MSC Target in different MSC Anchor MSC involved 100-200 ms additional

Engineering Calculators

Path Loss Calculator (Free Space)

Cell Radius Estimator

Frequency Reuse Distance

Timing Advance & Distance

Note: 1 TA unit = 550 m (3.69 μs round-trip propagation time)

Link Budget Calculator

Downlink (BTS → MS)

Uplink (MS → BTS)

Interactive Simulations

1. TDMA Frame Visualizer

Click on time slots to see their functions:

Hover over or click a time slot to see details

2. Frequency Hopping Simulator

Simulate Slow Frequency Hopping (SFH) used in GSM to improve interference diversity:

3. Call Setup Procedure

Trace the channel usage during a Mobile Terminated Call:

Click "Start Animation" to begin

4. Knowledge Check

Q1: What is the duplex spacing in GSM 900?

Q2: Which channel carries the Base Station Identity Code (BSIC)?

Q3: In handover, which entity makes the decision for Intra-BSC handover?

Laboratory Report Guidelines

1. Title Page

  • Experiment Title: GSM Architecture, Frequency Planning and Handover
  • Student Name, ID, Date
  • Course Name and Code

2. Objectives

3. Theory Summary

Summarize the key theoretical concepts learned:

4. Experimental Procedures

Document the steps you followed:

  1. Explored GSM architecture components and their functions
  2. Calculated ARFCN frequencies for different bands
  3. Analyzed frequency reuse patterns (3/9 vs 4/12)
  4. Examined logical channel structures and multiframe organization
  5. Simulated handover procedures and decision criteria
  6. Performed link budget and path loss calculations

5. Results and Calculations

Include your calculated values:

6. Analysis and Discussion

Answer these questions in your report:

  1. Why is the duplex spacing different for GSM 900 (45 MHz) vs GSM 1800 (95 MHz)?
  2. Compare the advantages of 3/9 vs 4/12 reuse patterns. When would you use each?
  3. Explain why GSM uses slow frequency hopping rather than fast frequency hopping.
  4. What is the significance of the 3-time-slot offset between uplink and downlink?
  5. Describe the trade-offs involved in handover decision thresholds.

7. Conclusion

8. Post-Lab Questions

Provide detailed answers to:

  1. Calculate the uplink and downlink frequencies for ARFCN 75 in GSM 900.
  2. A GSM network has 42 ARFCNs available. Compare the capacity per cell for 3/9 and 4/12 reuse patterns.
  3. Explain the difference between hard handover (GSM) and soft handover (CDMA).
  4. Why is the BCCH carrier usually not frequency hopped?
  5. Calculate the maximum cell radius limited by timing advance (TA=63).