Z Technology 's RF Newsletter - Wireless Edition

Wireless News Edition 1

Rayleigh Fading vs. Communication Effectiveness

Rayleigh Fading, or reflection multipath, affects almost every transmission to some degree. Although math and physics can explain it, we don't need a lot of math to see it can cause real-world problems when dealing with RF signal coverage. In practice, the effects of Rayleigh Fading can be easily characterized.

Rayleigh Fading occurs when there is more than one signal path between the transmitting and receiving antennas, i.e., a direct path and a reflected path. The fading is noticed as the relative path lengths change due to motion of the transmitter, receiver, or reflecting object. This is the effect you see on your television signal when an airplane makes a low pass over your home…. or when you are trying to receive a wireless transmission and your vehicle is moving among tall buildings.

When a signal is being "distorted" due to Rayleigh Fading its received signal level will vary from weak-to-strong-and-back-to-weak in a repeating pattern. The rate of the fade depends on the frequency of the signal, the angle between the two signals as they arrive at the receiving antenna, and the speed and direction of travel of the receiving antenna. For our purpose, we will consider a fastest fade rate, which would occur when a receiving antenna is travelling directly towards or away from a primary transmitter and a reflection of that signal is arriving at an angle close to 90 degrees. At some locations, the two signal paths would add; at some locations the resulting signal would be weaker. Rayleigh Fading is a concern when mobile communications must be reliable, and a challenge when taking coverage measurements.

This Diagram shows a signal being transmitted from a fixed antenna, with a strong secondary reflected signal. Weak signal level locations are marked along the driven route.

If we were to stop at every location, we could measure a signal level that would accurately illustrate the strong and weak signal locations. While the depth of signal fading is dependant on the relative signal strength between the primary signal source and the reflection, the repetition rate for this "weak-to-strong-to-weak" pattern is dependant (other factors being held almost constant) on the frequency of the signal being received and the rate of change of distance between the transmitter and the receiver.

Rayleigh Fading in your world

One advantage to wireless technology is the ability to communicate while in motion. However, a moving receiver is susceptible to Rayleigh Fading. Many communications networks are impacted by this phenomenon.

Modern day wireless networks use several communications bands within the VHF and UHF frequency spectrums. Some of the common frequencies in use are listed below with the closest repetition distances which may occur due to Rayleigh Fading:

Operating Frequency

Rayleigh Fading
Repetition Distance

150 MHz

78 inches

400 MHz

29 inches

850 MHz

14 inches

Let's assume we are receiving a signal at 150 MHz. For an operator of a receiver traveling along a road, directly towards or away from the transmitter, with a reflection coming in at a 90 degree angle from the left or right, the Rayleigh Fading phenomenon will cause weak signal levels to occur every 78 inches along the route of travel. If the reflected component of the signal is strong enough, which could be the case if the primary signal path is blocked by some obstacle, this may well affect the ability to communicate as the operator drives along or stops in a signal null. In practice, the operator will often keep moving and the effect of Raleigh Fading will appear as an interference in a voice contact, or a temporary loss of data in a data transmission.

Characterizing Rayleigh Fading

If we want to measure and record signals under conditions such as those experienced under Rayleigh Fading, we must be able to make measurements in such a way as to see minimum points of signal strength as we move along the path to be measured. In order to capture each and every minimum along a path with a Rayleigh Fading pattern, we must make several measurements during each repetition cycle. In fact we need to make at least two measurements per cycle.

Theoretically, by making at least two measurements per cycle, we can reproduce the signal being received, (in all its detail) as we drive along. This is a wonderful idea for an ideal situation and for a theoretically perfect repetition pattern. In the real world we do not have perfect repetition patterns and we do not necessarily need to reproduce the complete Rayleigh Fading pattern in all of its detail. What we really would like to do is be reasonably sure, as we drive along, we capture one of the signal's low-points fairly often. Using this approach, we can reasonably decide to require our measurement method be such that we make a least one measurement per each Rayleigh Fading repetition cycle. Using this method, one is reasonably sure over time, and over a relatively short distance, to have a measurement point fall at one of the notches located in the repetitive fade cycle. In this way, the user can be comfortable that over time, a signal strength reading at a minimum will be captured and recorded.

To better understand this, let's return to the 150 MHz example above. Assume an operator is driving at 60 miles per hour (88 feet per second). If this operator wants to measure a typical amount of Rayleigh Fading along a road, the test equipment must be able to repeatedly sample signal levels at least 13.5 measurements per second, a sample rate of at least 74 mS. If it is important to log every minimum of an "ideal" fading pattern signal, a sample rate of 27 samples per second (two samples per repetition distance) would be required for worst-case measurements at 60 mph.

Using the one sample-per-Rayleigh-Fading-cycle, the required measurement rate is summarized as below:

Sample rate required to take one measurement per Rayleigh Fading cycle

Operating
Frequency

Minimum Rayleigh Fading Repetition Distance

@ 60 MPH

@30 MPH

150 MHz

78 inches

74 mS

148 mS

400 MHz

29 inches

28 mS

56 mS

850 MHz

14 inches

13 mS

26 mS

By taking test samples more often than every 37 mS, the operator will have a better chance of recording a sufficient number of readings along a route and will capture the weakest and the strongest 150 MHz signals to be found at locations over the road being driven at 60 mph. Although a sample rate of 37 mS or faster is required to catch worst-case events 150 MHz when traveling 60 mph, a sample rate as slow as 74 mS is adequate to identify most problem locations. Traveling more slowly will result in more samples taken along the route. This is called over-sampling and also produces proper measurement results.

To summarize, a least three (3) parameters effect the ability to accurately measure the Rayleigh Fading phenomenon:

  1. The Frequency being measured
  2. The Vehicle Speed, and therefore how fast the receiver is moving
  3. The Measurement Speed of the test equipment

Making Rayleigh Fading measurements

Z Technology provides DriveTest Systems with sample speeds capable of measuring Rayleigh Fading for signals in these communications bands while the measurement vehicle is in motion.

Our standard DriveTest Systems makes repeated measurements on one frequency at up to a 60 mS rate. Under typical reflection conditions, standard systems accurately depict Rayleigh fading in the low and high VHF bands at speeds up to 60 mph (see table above). In practice, the standard system will discover the high and low signal levels along a route with sufficient accuracy to determine where signal coverage needs to be improved. Standard systems can also measure up to 50 different frequencies during a DriveTest, at a time at a reduced measurement rate, but detailed effects of Rayleigh fading should not be expected.

The Z Technology S5010STR Streaming-Mode DriveTest System enhances this capability with measurement speeds of more than 200 measurement per second (this is a rate of 5 mS) for any one frequency. This is a lot of data to be recorded; therefore we have come up with a unique method of recording the critical information. The information is summarized and recorded to a data file every time a new GPS location is available, once per second. The 200 samples per second are characterized over this time period. During this time, the maximum, average and minimum signals measured by the S5010STR system are captured and stored in the data file. A new record is added to this file every second; that is for each new GPS location.

Using the streaming system, the number of measurements are more than sufficient (more than two samples per cycle as shown below) to effectively characterize a signal impacted by Rayleigh Fading.

Number of measurements per Rayleigh Fading cycle using the S5010STR

Operating
Frequency

Rayleigh Fading
Repetition Distance

@ 60 MPH

@30 MPH

150 MHz

78 inches

14.8

29.6

400 MHz

29 inches

5.6

11.2

850 MHz

14 inches

2.6

5.2

With the S5010STR Streaming-Mode DriveTest System, mobile measurements may be made for frequencies up to 1000 MHz, at full driving speeds, effectively determining the Rayleigh maximum and minimum values to determine the likelihood of successful communications.

For more information on our DriveTest systems, ask your Z Technology Representative or contact Z Technology directly.