Mobile Radio Propagation

Introduction

The mobile radio channel places fundamental limitations on the performance of wireless communication systems.

The wireless transmission path may be of two types:

1. Line of Sight (LOS)
2. Non Line of Sight (NLOS): Obstructed by buildings, cars, bridges etc.

• Radio Channels are random and often time varying.
• Modeling have been one of the difficult parts of the mobile radio design.

Propagation Basics

When electrons move, they create electromagnetic waves that can propagate through space. By attaching antenna of the appropriate size to an electrical circuit, the electromagnetic waves can be broadcast efficiently and received by a receiver some distance away. The radio, microwave, infrared, and visible light portions of the electromagnetic spectrum can all be used to transmit information. Information can be sent by modulating the amplitude, frequency or phase of the waves.

Properties of Radio Waves

• Are easy to generate
• Can travel long distances
• Can penetrate buildings
• May be used for both indoor and outdoor communication
• Are omni-directional – can travel in all directions
• Can be narrowly focused at high frequencies (greater than 100MHz) using parabolic antennas ( like satellite dishes)
• Frequency-dependence
  • Behave more like light at higher frequencies
    • Difficulty in passing obstacles
    • More direct paths (straight line paths
    • Absorbed by rain
  • Behave more like radio at lower frequencies
    • Can pass obstacles
    • Power falls off sharply with distance from source
• Subject to interference from other radio wave sources

Mobile Radio Propagation : Basics

At VLF, LF, and MF bands, radio waves follow the ground. AM radio broadcasting uses MF band.

At HF bands, the ground waves tend to be absorbed by the earth. The waves that reach ionosphere (100-500 km above earth surface), are refracted and sent back to earth.

VHF Transmission

• Directional antennas are used
• Waves follow more direct paths
• LOS: Line-of-Sight Communication
• Reflected wave interfere with the original signal

Modeling the radio channel is typically done in statistical manner. The statistical modeling is usually done based on measurement data made specifically for the intended communication system the intended spectrum. However, for higher frequencies (>10 GHz), deterministic modeling is used as statistical modeling starts failing.

By playing with the antenna (tilting and changing the height), coverage area can be controlled.

Propagation Models

We are interested in propagation characteristics and models for waves with frequency in range: few MHz to a few GHz

Modeling radio channel is important for:

• Determining the coverage area of a transmitter
  • Determine the transmitter power requirement
  • Determine the battery lifetime
• Finding modulation and coding schemes to improve the channel quality
  • Determine the maximum channel capacity

Radio Propagation Models

Transmission path between sender and receiver could be

• Line-of-Sight (LOS)
• Obstructed by buildings, mountains etc.

Even speed of motion affects the fading characteristics of the channel.

Radio Propagation Mechanisms

The physical mechanisms that govern radio propagation are complex and diverse, but generally attributed to the following three factors

1. Reflection
2. Diffraction
3. Scattering

Reflection

• Occurs when waves impinges upon an obstruction that is much larger in size compared to the wavelength of the signal
• Example: reflections from earth and buildings
• These reflections may interfere with the original signal constructively or destructively

Diffraction

• Occurs when the radio path between sender and receiver is obstructed by an impenetrable body and by a surface with sharp irregularities (edges)
• Explains how radio signals can travel urban and rural environments without a line-of-sight path

Scattering

• Occurs when the radio channel contains objects whose sizes are on the order of the wavelength or less of the propagating wave and also when the number of obstacles are quite large.
• They are produced by small objects, rough surfaces and other irregularities on the channel.
• Follows same principles with diffraction.
• Causes the transmitter energy to be radiated in many directions.
• Lamp posts and street signs may cause scattering.

As a mobile moves through a coverage area, these 3 mechanisms have an impact on the instantaneous received signal strength.

• If a mobile does have a clear line of sight path to the base-station, then diffraction and scattering will not dominate the propagation.
• If a mobile is at a street level without LOS, then diffraction and scattering will probably dominate the propagation.

Radio Propagation Models

As the mobile moves over small distances, the instantaneous received signal will fluctuate rapidly giving rise to small-scale fading.

• The reason is that the signal is the sum of many contributors coming from different directions and since the phases of these signals are random, the sum behave like a noise (Rayleigh fading).
• In small scale fading, the received signal power may change as much as 3 or 4 orders of magnitude (30dB or 40dB), when the receiver is only moved a fraction of the wavelength.

As the mobile moves away from the transmitter over larger distances, the local average received signal will gradually decrease. This is called large-scale path loss.

• Typically the local average received power is computed by averaging signal measurements over a measurement track of 5l to 40l. (For PCS, this means 1m-10m track

The models that predict the mean signal strength for an arbitrary-receiver transmitter (T-R) separation distance are called large-scale propagation models

• Useful for estimating the coverage area of transmitters

Small-Scale and Large-Scale Fading

What is Decibel (dB)

A logarithmic unit that is used to describe a ratio.

• Let say we have two values P1 and P2. The difference (ratio)between them can be expressed in dB and is computed as follows:
  • 10 log (P1/P2) dB
• Example: transmit power P1 = 100W,
                 received power P2 = 1 W
  • The difference is 10log(100/1) = 20dB.

dB

dB unit can describe very big ratios with numbers of modest size.

See some examples:

• Tx power = 100W, Received power = 1W
  • Tx power is 100 times of received power
  • Difference is 20dB
• Tx power = 100W, Received power = 1mW
  • Tx power is 100,000 times of received power
  • Difference is 50dB
• Tx power = 1000W, Received power = 1mW
  • Tx power is million times of received power
  • Difference is 60dB

dBm

For power differences, dBm is used to denote a power level with respect to 1mW as the reference power level.

Let say Tx power of a system is 100W. What is the Tx power in unit of dBm?

Solution:

Tx_power(dBm) = 10log(100W/1mW) = 10log(100W/0.001W) = 10log(100,0000) = 50dBm

dBW

For power differences, dBW is used to denote a power level with respect to 1W as the reference power level.

Let say Tx power of a system is 100W. What is the Tx power in unit of dBW?

Solution:

Tx_power(dBW) = 10log(100W/1W) = 10log(100) = 20dBW