Can receiver performance predict communication range?

Introduction

In the previous article, “Understanding receiver specifications“, we looked at the various parameters that you would typically find in the datasheet for a radio receiver. While we can measure the influence a parameter has on the received signal, then the next step is – how well will our communication channel perform? If the user cannot confidently predict whether the radio module can receive the desired data, the user will not bother investigating the module further.

Why the focus on the reciever?

If we predict the communication performance just in terms of the radio circuitry and components (not counting external factors such as the antenna gain, cable loss, protocol etc.), then the receiver is a good place to start.

The transmitter’s job is to generate the radio frequency signal and deliver it to the transmit antenna. In comparison to a receiver, this process is quite straight forward.

Since the transmitter parameters are fixed by the regulations, we can assume this part of the communication link to be constant and focus on the receiver aspect. The design of the receiver is something we have more control over.

How much can we predict our communication based on the receiver specifications?

First what qualifies as “successful communication”? And how does it differ between transmission of continuous signals and a data stream?

For continuous signals such as audio transmission, the signal gets progressively weaker until it is overwhelmed by noise. There is no “threshold” , only a rough level where the signal becomes “unintelligible”. However there does exist a standardised measurement on such receivers for determining the input level that is able to produce “intelligible” signals.

For more details, see this article, “What is 12 dB SINAD?”

For a data stream, a perfect communication link would always produce exactly the same output as what was input. However no radio can produce a completely error free link – there always exists some probability of an errored packet being received which only worsens with distance. Also are various outside influences such as interfering signals from other devices. For industrial applications where safety can be critical, it is necessary that the receiver output exactly what was input at the transmitter. So usually a good protocol and a system to detect/correct errors is necessary in addition to a good receiver.

For more details, see this article, “Requirements for transparent interface”

Understanding the user requirements in terms of the receiver type needed.

It can be tricky to link the communication performance the user is envisioning to the exact receiver that needs to be used. The user, who may or may not have an extensive RF background can describe the communication link, but only in general terms. There is a third step needed to intepret these requirements and determine which aspects of the receiver need the most attention.

Communication range

The range is the distance between the transmitter and receiver where the requirements for successful communication are satisfied.

To make a preliminary estimation, engineers construct a link budget. Since path loss depends on distance, this is included in the link budget. The sensitivity as one of the receive parameters, is the lowest signal level presented to the receiver where the signal can still be demodulated by the receiver. If the path loss is too great, the signal level will fall to a level too weak for demodulation.

See these articles, “Link budget” and “Propagation“for more details.

 

 

We can see that a link margin exists to account for fluctuations to create stable reception. By having a link margin and knowing the sensitivity, we can visualise a useable range.

Noise

However, the link margin assumes this signal is the only one we’re receiving. It does not account for any noise in the environment which may reduce the sensitivity. In the link budget above, this will reduce the link margin and communication stability, so the link budget would need to be re-calculated.

Multiple transmitters and the near far problem

Unfortunately, a higher sensitivity can also make a receiver sensitive to signals you do not want to receive. This will happen if there are transmitters using nearby channels. So while a higher sensitivity would improve your range, it would also mean having to space these other transmitters out to reduce their impact. To avoid this, it is necessary to also increase channel selectivity along with sensitivity.

Number of communication channels

The user may want more than one communication link, for example, a communication network. This would require radio modules with the ability to communicate on its own channel. The assumption is we can create multiple communication links, each one having the same communication performance as a single link.

As long as the receiver allows only the desired channel through, while keeping all the other signals out – there is no problem. Unfortunately, filters in the receiver are not ideal filters and strong signals from other channels and bands can also leak through. These will interfere with the signal you want if the receiver has poor adjacent channel selectivity and blocking. 

Multiple channels in the same area

A receiver antenna will pick up many signals in the same area and present these to the receiver.

Typically a receiver contains a low noise amplifier as one of the initial stages. Normally an amplifier is a linear product but any strong signals in the same area can push it into its non-linear region. The result is the creation of intermodulation products that limits the channels available to use.

For more details, see “Third order intermodulation / Channel planning”

Data rate

For data communication, the user may desire a certain data rate.

Generally, data rate will determine the bandwidth of the RF signal that is to be transmitted. This signal needs to “fit” within the window of the receiver and is referred to as the channel bandwidth. For a superheterodyne model, the channel bandwidth will be the width of the receiver’s IF filter.

The closer this channel bandwidth matches to the transmitted signal, the better. If this bandwidth is too narrow, our signal will be difficult to resolve. On the other hand, a channel bandwidth that is too wide has a higher chance of receiving unwanted noise. Also internal noise that is bandwidth dependent (thermal noise) will further lower the signal to noise ratio.

See the article and chapter, “Isolating the intermediate frequency” for more information.

Conclusion

Since the receiver contains many more stages than a transmitter, it is has more complexity and therefore the most influence.

In this article, we tried basing our predictions on the receiver parameters alone. However, in order to properly evaluate the communication link, it has to be tested in that environment using the actual module. Due to environmental factors such as obstructions, antenna placement, Fresnel zone and so on, you may find the range to be less than the optimum.

For example, Circuit Design has an evaluation program for testing the MU series modems here. It is designed for using on a laptop with an interface board to communicate with the radio modem allowing you to perform real life communication tests in the field.