UMTS (WCDMA) Radio Wave Propagation Models

📖 Course Overview

Learning Objectives: By the end of this study guide, you will be able to:

Understand WCDMA-specific propagation characteristics and system features
Apply propagation models (Okumura-Hata, COST-231) for 3G frequency bands (2.1 GHz)
Perform WCDMA link budget calculations with processing gain and Eb/N0 requirements
Analyze cell breathing effect and load-dependent coverage in CDMA systems
Evaluate uplink noise rise and its impact on network capacity
Understand soft handover gains and overhead in propagation analysis

What is UMTS (WCDMA)?

UMTS (Universal Mobile Telecommunications System) is a third-generation (3G) cellular technology based on Wideband CDMA (WCDMA). Key characteristics:

Carrier Bandwidth: 5 MHz (vs 200 kHz in GSM)
Chip Rate: 3.84 Mcps (mega-chips per second)
Frequency Bands: 2100 MHz (Europe/Asia), 850/1900 MHz (Americas)
Multiple Access: CDMA with spreading codes
Frame Length: 10 ms radio frames

Key Difference from GSM: WCDMA is interference-limited (not hard-blocked). Coverage depends on cell loading - this is called "cell breathing".

System Architecture

UMTS consists of:

UE (User Equipment): Mobile terminal
NodeB: Base station (equivalent to BTS in GSM)
RNC (Radio Network Controller): Controls multiple NodeBs
Core Network (CN): Connection to external networks

Multi-Service Environment

UMTS supports multiple services simultaneously with different QoS requirements:

Service Class	Typical Application	Bit Rate	Processing Gain	Required Eb/N0
Conversational	Voice, Video call	12.2 kbps	25 dB	5-7 dB
Streaming	Video streaming	64-384 kbps	18-12 dB	3-5 dB
Interactive	Web browsing	64-128 kbps	18-15 dB	4-6 dB
Background	Email, FTP	16-64 kbps	23-18 dB	5-8 dB

Progress: 7 sections to complete

🎓 WCDMA Propagation Fundamentals

Key Propagation Mechanisms

The propagation mechanisms are similar to GSM but with different frequency characteristics:

📡 Free Space Propagation

Ideal LOS conditions at 2.1 GHz

L_fs = 20log₁₀(d) + 20log₁₀(f) + 32.44 dB
d = km, f = MHz (2100 MHz for UMTS)

🏢 Reflection & Scattering

More pronounced at 2 GHz due to smaller wavelength (λ ≈ 14 cm)

Critical for multipath diversity

📐 Diffraction

Loss increases with frequency - affects edge coverage

Knife-edge diffraction model used

🌿 Penetration Loss

Higher at 2.1 GHz vs 900 MHz

Building penetration: 10-20 dB additional loss

WCDMA-Specific Propagation Effects

Processing Gain and Link Budget

In WCDMA, the spreading process increases the signal bandwidth, providing processing gain:

Processing Gain (PG):
PG = 10·log₁₀(W/R) = 10·log₁₀(3.84×10⁶ / R) dB
W = chip rate (3.84 Mcps), R = bit rate (bps)

Why it matters: Higher processing gain (for lower bit rates) means better immunity to interference, allowing lower transmit power for the same quality.

Required Eb/N0 vs C/I

WCDMA uses Eb/N0 (Energy per bit to Noise ratio) instead of C/I:

Required Received Power:
P_rx = N₀ + PG + Eb/N0_required dBm
N₀ = thermal noise (typically -108 dBm)

Uplink Noise Rise

As more users connect, the uplink noise floor rises, reducing cell coverage:

Noise Rise:
NR = 10·log₁₀(1/(1-η)) dB
η = cell load factor (0 to 1)

Critical Concept: At 50% load (η=0.5), noise rise = 3 dB. At 75% load, noise rise = 6 dB. At 90% load, noise rise = 10 dB, significantly reducing cell range.

Cell Breathing

Unlike GSM, WCDMA cell size shrinks as load increases due to interference:

Cell Range Reduction:
ΔR = 10^(ΔL/(10·n)) where ΔL = noise rise + interference margin
n = path loss exponent (3-4)

Soft Handover Gain

UE can connect to multiple cells simultaneously, providing macro-diversity gain:

Combining Gain: 2-4 dB reduction in required Eb/N0
Shadowing Diversity: 3-5 dB gain against slow fading
Total Soft Handover Gain: Typically 5-7 dB

Impact: Soft handover reduces transmit power from both UE and NodeB, decreasing interference and increasing capacity. Overhead is 20-40% of connections.

📊 Propagation Models

1. COST-231 Hata Model (UMTS Standard)

Extended Okumura-Hata for 2 GHz PCS/UMTS bands (1.5-2.0 GHz). Based on measurements and adapted for European cities.

Urban Area (1.5-2.0 GHz):
L_U = 46.3 + 33.9·log₁₀(f) - 13.82·log₁₀(h_b) - a(h_m) + [44.9 - 6.55·log₁₀(h_b)]·log₁₀(d) + C_M dB

a(h_m) = (1.1·log₁₀(f) - 0.7)·h_m - (1.56·log₁₀(f) - 0.8) dB
C_M = 0 dB (suburban/rural), 3 dB (dense urban)
f = frequency (MHz), h_b = NodeB height (30-200m), h_m = UE height (1-10m), d = distance (1-20 km)

Parameter	Range	Typical UMTS Values
Frequency (f)	1.5 - 2.0 GHz	2100 MHz (Europe)
Distance (d)	1 - 20 km	0.5 - 5 km typical
NodeB Height (h_b)	30 - 200 m	30 m (urban), 50 m (suburban)
UE Height (h_m)	1 - 10 m	1.5 m (handheld)

2. Walfisch-Ikegami Model (Microcells)

Site-specific model for urban microcells where NodeB antenna is below rooftop.

When to Use:

Urban microcells (NodeB below rooftop)
Distance < 1 km
Frequency: 800-2000 MHz
For NLOS street canyon propagation

3. Ray-Tracing Models (Advanced)

Deterministic approach for accurate microcellular prediction using ray-optical theory.

Key Features:

Computes actual ray paths (reflections, diffractions)
More accurate than empirical models
Higher computational complexity
Requires detailed building database

4. COST207/259 Channel Models

Statistical multipath models for WCDMA system simulation:

Profile	Environment	Delay Spread	Typical Use
RA (Rural Area)	Rural macrocell	0.1 μs	Outdoor coverage
TU (Typical Urban)	Urban macrocell	1.0 μs	Standard urban simulation
BU (Bad Urban)	Dense urban	2.5 μs	City centers
HT (Hilly Terrain)	Mountainous	20 μs	Complex terrain

Model Selection Guidelines

📍 Macrocells (NodeB > Rooftop)

Model: COST-231 Hata

Radius: 1-10 km

Planning: Coverage prediction

🏢 Microcells (NodeB < Rooftop)

Model: Walfisch-Ikegami or Ray-tracing

Radius: 0.1-1 km

Planning: Urban hotspots

💻 System Simulation

Model: COST207/TU3/RA6

Purpose: Link-level simulation

Used for: Eb/N0 requirements

📱 UMTS-Specific Considerations

UMTS Frequency Bands

Band	Uplink (MHz)	Downlink (MHz)	Region	Typical Cell Radius
Band I (2100)	1920-1980	2110-2170	Europe/Asia	1-3 km (urban)
Band II (1900)	1850-1910	1930-1990	Americas	1-4 km (urban)
Band V (850)	824-849	869-894	Americas	2-5 km (urban)

WCDMA Link Budget

Unlike GSM, WCDMA link budgets are service-specific and asymmetric.

Uplink Budget Components

Parameter	Voice (12.2 kbps)	Data (64 kbps)	Notes
UE TX Power	21 dBm (125 mW)	24 dBm (250 mW)	Max power
UE Antenna Gain	0 dBi	0 dBi	Integrated
Body Loss	3 dB	0 dB	Voice only
EIRP	18 dBm	24 dBm
Path Loss	140 dB	135 dB	Typical urban
NodeB Antenna Gain	17 dBi	17 dBi	3 sectors
NodeB Cable Loss	2 dB	2 dB
Receiver Noise Floor	-105 dBm	-105 dBm	Thermal + NF
Required Eb/N0	5 dB	2 dB	With diversity
Processing Gain	25 dB	18 dB	3.84M/R
Interference Margin	3 dB	3 dB	50% load
SHO Gain	4 dB	2 dB	Macro-diversity
Fade Margin	2 dB	2 dB	Fast fading
Sensitivity	-124 dBm	-122 dBm	Calculated
Link Margin	0 dB	2 dB	Feasible?

Load Factor and Noise Rise

Uplink Load Factor:
η = Σ (1 / (1 + PG_i / (Eb/N0_i × ν_i)))
ν = voice activity factor (0.5-0.67 for speech), i = each user/service

The noise rise limit determines maximum cell capacity:

Practical Limit: 3-6 dB noise rise (50-75% load)
Pole Capacity: Theoretical maximum when η → 1
Coverage-Limited: Low load, coverage determined by link budget
Capacity-Limited: High load, coverage shrinks due to noise rise

Service Coverage Thresholds

🎤 Voice (12.2 kbps)

Sensitivity: -124 dBm

Max Path Loss: 145 dB

Coverage: Best (highest PG)

📱 Data (64 kbps)

Sensitivity: -122 dBm

Max Path Loss: 140 dB

Coverage: Good

📶 Data (384 kbps)

Sensitivity: -115 dBm

Max Path Loss: 130 dB

Coverage: Limited (low PG)

Key Principle: The tightest service requirement determines site density. Typically, high-speed data services require more sites than voice coverage.

🧮 Interactive Calculators

1. COST-231 Path Loss Calculator (UMTS 2100 MHz)

Frequency (MHz):

Distance (km):

NodeB Height (m):

UE Height (m):

Environment:

2. WCDMA Uplink Budget Calculator

Service:

UE TX Power (dBm):

Processing Gain (dB):

Required Eb/N0 (dB):

Load Factor (%):

NodeB Antenna Gain (dBi):

Interference Margin (dB):

3. Cell Breathing Calculator

Cell Load (%):

Reference Range @ 0% Load (km):

Path Loss Exponent (n):

Noise Rise: 3.0 dB
Range Reduction: 0%
Effective Cell Radius: 3.00 km

4. Load Factor & Capacity Calculator

Number of Voice Users:

Number of Data Users (64 kbps):

Voice Activity Factor (ν):

✍️ Practical Exercises

Exercise 1: WCDMA Urban Cell Planning

Scenario: Design a UMTS 2100 network for urban area with 15,000 subscribers.

Given:

Subscriber density: 100 subscribers/km²
Voice usage: 25 mErlang/subscriber
Data usage: 10% users, 64 kbps average
NodeB height: 30 m
Target loading: 50% (3 dB noise rise)
Environment: Urban (n=4.0)

Questions:

1. Calculate the required number of NodeBs for voice coverage (max PL=140 dB):

2. Estimate the uplink capacity per cell (pole capacity for 12.2 kbps voice):

3. Determine if coverage or capacity is limiting:

Exercise 2: Cell Breathing Analysis

Scenario: Analyze cell breathing for a UMTS cell with reference radius 2.5 km.

Given:

Reference range @ 0% load: 2.5 km
Path loss exponent n = 3.5
Acceptable noise rise: 6 dB max

Questions:

1. Calculate the range at 50% load (η=0.5):

2. Calculate the range at 75% load (η=0.75):

3. Explain the impact on network planning:

Exercise 3: Propagation Model Selection

Scenario: Match the UMTS deployment scenario to the appropriate propagation model.

Match each scenario:

A. UMTS 2100 macrocell covering suburbs (radius=5 km, hb=50 m)

B. UMTS 2100 microcell in city center (radius=0.8 km, hb=15 m)

C. WCDMA system simulation for performance verification

📝 Knowledge Check Quiz

Question 1: What is the primary factor that makes WCDMA coverage load-dependent?

Higher frequency

Uplink noise rise

Lower transmit power

Smaller bandwidth

Question 2: Which model is most appropriate for a UMTS microcell with NodeB below rooftop?

COST-231 Hata

Walfisch-Ikegami

Free Space

Okumura-Hata

Question 3: What happens to cell radius as cell load increases from 0% to 90%?

Increases by 50%

Remains constant

Decreases significantly

Doubles

Question 4: Which service typically determines site density in UMTS networks?

Voice (12.2 kbps)

High-speed data (384 kbps)

Control channels

SMS

Question 5: What is the typical soft handover gain in WCDMA?

5-7 dB

1-2 dB

10-12 dB

0 dB

Question 6: At 50% cell load, what is the approximate noise rise?

3 dB

1 dB

6 dB

10 dB

Question 7: Which parameter is most critical for calculating receiver sensitivity in WCDMA?

Processing gain + Required Eb/N0

Frequency band

Antenna height

Chip rate only

Question 8: What is the main advantage of CDMA technology in propagation?

Multipath diversity and soft handover

Lower path loss

Higher transmit power

Less interference

📖 Course Overview

What is UMTS (WCDMA)?

System Architecture

Multi-Service Environment

🎓 WCDMA Propagation Fundamentals

Key Propagation Mechanisms

📡 Free Space Propagation

🏢 Reflection & Scattering

📐 Diffraction

🌿 Penetration Loss

WCDMA-Specific Propagation Effects

Processing Gain and Link Budget

Required Eb/N0 vs C/I

Uplink Noise Rise

Cell Breathing

Soft Handover Gain

📊 Propagation Models

1. COST-231 Hata Model (UMTS Standard)

2. Walfisch-Ikegami Model (Microcells)

3. Ray-Tracing Models (Advanced)

4. COST207/259 Channel Models

Model Selection Guidelines

📍 Macrocells (NodeB > Rooftop)

🏢 Microcells (NodeB < Rooftop)

💻 System Simulation

📱 UMTS-Specific Considerations

UMTS Frequency Bands

WCDMA Link Budget

Uplink Budget Components

Load Factor and Noise Rise

Service Coverage Thresholds

🎤 Voice (12.2 kbps)

📱 Data (64 kbps)

📶 Data (384 kbps)

🧮 Interactive Calculators

1. COST-231 Path Loss Calculator (UMTS 2100 MHz)

2. WCDMA Uplink Budget Calculator

3. Cell Breathing Calculator

4. Load Factor & Capacity Calculator

✍️ Practical Exercises

Exercise 1: WCDMA Urban Cell Planning

Questions:

Exercise 2: Cell Breathing Analysis

Questions:

Exercise 3: Propagation Model Selection

Match each scenario:

📝 Knowledge Check Quiz

Quiz Complete!