Understanding on Motley–Keenan Model for WCDMA

Site-specific models of indoor propagation, as the name implies, take the specific characteristics of a particular building as their input. Often these models count the number of walls, floors or other obstacles between the transmitter and the receiver to produce a value of attenuation for propagation along that path. If we repeat this procedure for many hypothetical receiver (EU) positions, we can trace the contours of the received signal strength equivalent.

These propagation predictions are ideal for computer automation. At the time of this writing, several propagation forecasting tools covered in the market use variations of this technique. One of these models, often quoted, has come to be known as the Motley-Keenan model after the researchers who published the first time. This model takes the form of a free space propagation model with attenuation added for each wall, floor, or obstruction encountered along the route.

Mathematically, we can write the LO path loss as:

LO = PO + 20 log(d) + p × WAF + k × FAF

where:

Po= 20Log[4 πf/C]

d = distance between transmitter and receiver

p = number of walls between transmitter and receiver

WAF = Wall Attenuation Factor

k = number of floors between transmitter and receiver

FAF = Floor Attenuation Factor

The free space propagation model should be evident from the first two terms in the equation. The additional terms represent the loss through any obstacle. This site specific model is general enough to accommodate a wide range of frequencies with the appropriate choice of attenuation factors.

The literature is full of attenuation values measured for a variety of materials and at different frequencies, and more are published regularly. In general, the frequencies used for personal communication (800-2100 MHz), the inner walls of a modern office building have aWAF of about 3 dB.

Brick interior walls and structural supports have a WAF of about 10 dB. Floors vary depending on the building, but they usually have a FAF between 13 and 18 dB.

Understanding the Motley–Keenan Model for WCDMA

The Motley–Keenan model is a widely used empirical indoor path loss model that helps predict signal attenuation in wireless communication systems, particularly in environments such as buildings and urban interiors. It is highly relevant in planning and designing indoor WCDMA (Wideband Code Division Multiple Access) networks, which require accurate radio propagation models due to the complex and variable nature of indoor environments.

This model provides a relatively simple yet effective method for estimating signal strength loss by accounting for the number and types of obstacles (like walls and floors) between the transmitter and receiver. It has become particularly important in 3G systems such as WCDMA, where indoor coverage is essential for consistent quality of service (QoS), capacity planning, and interference management.

Structure and Formula of the Motley–Keenan Model

The Motley–Keenan path loss model extends the basic free space path loss model by introducing fixed losses for each wall, floor, and other obstacles in the propagation path. The formula is expressed as:

L = L₀ + 10 * n * log₁₀(d) + ΣWAF + ΣFAF

Where:

L – Total path loss in dB
L₀ – Path loss at reference distance (typically 1 meter)
n – Path loss exponent (usually ranges from 2 to 6 depending on environment)
d – Distance between transmitter and receiver (in meters)
ΣWAF – Wall Attenuation Factors (sum of losses due to all walls the signal passes through)
ΣFAF – Floor Attenuation Factors (sum of losses due to floors in multi-level buildings)

Each wall or floor introduces an additional fixed attenuation. For example, a concrete wall may introduce 12–15 dB of loss, while a glass wall may cause 4–6 dB. These factors are summed up and added to the free-space loss to compute the total expected path loss.

Typical Attenuation Factors

Obstacle Type	Attenuation (dB)
Light partition (drywall)	3–5 dB
Brick wall	8–12 dB
Concrete wall	12–15 dB
Glass partition	4–6 dB
Floor (concrete)	15–20 dB per floor

These values are not fixed and can vary based on building materials, layout, and measurement frequency. In WCDMA systems (operating around 2.1 GHz), higher frequencies tend to experience more attenuation than lower frequencies, which should be considered in system planning.

Application in WCDMA Network Planning

WCDMA uses spread-spectrum technology where signal strength and quality are crucial for maintaining connections, especially indoors. The Motley–Keenan model is useful in the following scenarios:

Indoor coverage prediction: It helps radio engineers predict how signal strength will degrade within buildings and determine base station placement.
Small cell deployment: It supports femtocell and picocell design to ensure localized, strong indoor signals.
Capacity optimization: Helps manage user distribution and avoid oversaturation of indoor nodes.
Interference estimation: Understanding signal degradation assists in reducing inter-cell interference in dense environments.

By using the Motley–Keenan model in simulations, network engineers can adjust transmission power, antenna placement, and frequency reuse plans to maximize indoor performance and meet WCDMA’s strict QoS requirements.

Advantages and Limitations

The Motley–Keenan model provides several benefits, but it also has limitations due to its empirical nature.

Advantages	Limitations
– Simple to use and implement – Works well in office buildings – Easy integration into planning tools	– Not suitable for outdoor or large open spaces – Inaccurate with complex 3D propagation paths – Assumes uniform wall losses

Despite its limitations, it remains one of the most practical and frequently applied models for indoor network design, especially in WCDMA and LTE pre-deployment assessments.

Related Concepts for Broader Understanding

Free-Space Path Loss (FSPL): The baseline model for ideal, unobstructed environments.
ITU Indoor Model: Another standardized indoor propagation model used for comparison and regulatory reporting.
Ray Tracing: A more advanced method used in 3D indoor modeling for accurate but computationally intensive planning.
WCDMA Cell Breathing: An effect where cell size shrinks under load; accurate propagation modeling helps minimize its impact.
Indoor Site Surveys: Empirical measurements used to validate the predictions of models like Motley–Keenan.

In summary, the Motley–Keenan model is a foundational tool in indoor RF planning. For WCDMA networks, it offers a practical balance between complexity and accuracy, aiding engineers in providing seamless indoor coverage and capacity management in both enterprise and residential deployments.