RF Performance Advantage

What is it?

rf-perforrmance-advantageThe RF performance of all Base Station, Mobile or Portable radios is quantified in a set of specifications given in deciBels (dB). One set of RF performance specifications describes the Transmitter performance, another describes the Receiver performance. For some RF performance specifications, a larger number is better; and for others, a smaller number is best. There are minimum acceptable RF performance specifications prescribed by regulatory authorities such as ETSI (Europe) and FCC (USA). Meeting these is mandatory.

Some RF performance specifications are more important than others to the real life operation of the radio system. This post will explain:

  • What the most important RF specifications are and whether a larger or smaller number is best.
  • Why these give ‘real life’ benefits such lower system cost and improved reliability.

Key RF Specifications for Receivers

Sensitivity

  • This is the minimum RF received signal level that results in an understandable audio output quality.
  • Usually quoted in dBm, and a smaller number is best, eg: -120dBm is better than -115dBm.
  • There are two types of sensitivity measurement:
    1. Static Sensitivity is a test measurement to prove compliance against ETSI or FCC regulations.
    2. Dynamic Sensitivity is the minimum received signal level required in a real system.
  • Better sensitivity performance reduces system cost because it means greater coverage and a smaller number of sites.

Adjacent Channel Selectivity

  • The Adjacent Channel Selectivity determines how much stronger a signal in the neighboring (known as ‘adjacent’) channel can be than a weak signal in the desired channel before it starts to degrade communication performance.
  • Usually quoted in dB, and a larger number is best, eg: 90dB is better than 75dB.
  • A receiver with better adjacent channel selectivity will allow a radio operating on an adjacent channel to get closer to a site before it causes interference.
  • In short, receivers with better adjacent channel selectivity contribute to a more reliable, robust system.

Intermodulation

  • When many strong interfering signals are received, they mix in the receiver to produce ‘intermodulation products’ that can swamp weak wanted signals.
  • So, the intermodulation performance of a receiver indicates how much stronger the unwanted signals can be before it starts to degrade communications performance.
  • Usually quoted in dB, and a larger number is best, eg: 85dB is better than 65dB.
  • At fixed sites, strong signals from co-located transmitters can cause intermodulation products at levels that deafen site receivers.
  • This means distant terminals would be unable to reach the site, preventing communication.
  • Large, costly cavity filters can protect against this, so base station receivers with better intermodulation performance reduce the quantity, size and cost of the external filters required.

Key RF Specifications for Transmitters

Adjacent Channel Power (ACP, ACPR)

  • Transmitters produce small amounts of RF power outside the channel they are operating on.
  • The amount of RF power produced in the neighboring channel is called the adjacent channel power.
  • Usually quoted in dBc, which means dB below the power generated in the wanted channel.
  • A larger number is best, eg: 65dBc is better than 60dBc.
  • Transmitter Adjacent Channel Power has the same impact already discussed for Receiver Adjacent Channel Selectivity.
  • A transmitter with lower adjacent channel power will allow a radio operating on an adjacent channel to get closer to a site before it causes interference.
  • In short, transmitters that emit lower power in the adjacent channel contribute to a more reliable, robust system.
  • Transmitter adjacent channel power is often the limiting factor for the overall adjacent channel performance of the system.
  • If the transmitter adjacent channel power is poor, then further improvements in receiver adjacent channel selectivity will not improve the overall system performance.

Sideband (Wideband) Noise

  • Transmitters continue to produce small amounts of RF power well beyond the neighboring channel.
  • Transmitter sideband noise power is the amount of energy the transmitter produces at large offsets from operating frequency.
  • Usually quoted in dBc/Hz for specific frequency offsets.
  • A lower number is better, eg: -160dBc/Hz is better than -150dBc/Hz.
  • Sideband noise from co-located transmitters can deafen receivers operating nearby even though they are widely separated in frequency.
  • This means distant terminals would be unable to reach the site, preventing communication.
  • Site transmitters with better sideband noise performance require less expensive, large cavity filters to attenuate the noise below the level where it will deafen neighbouring receivers. This reduces size and cost.
  • Also, a transmitter with better sideband noise will require less filtering and achieve a greater radiated power. This will result is a better coverage range and less sites.
  • Terminals with better sideband noise can be parked closer together at an incident before one vehicle’s receiver is deafened when the other vehicle transmits.

Important Takeaways

Be sure to determine if the specifications are given as guaranteed, or typical. Be very careful when comparing the two. “Guaranteed” is the minimum performance. “Typical” performance is not guaranteed. Typical specifications are often 2 – 5dB better than guaranteed.

Better RF performance means lower system cost:

  1. Better base station and terminal RF performance means less sites are required.
  2. Better base station RF performance means less complex third party combining equipment is needed and so further reduces costs.
  3. Better Terminal (Mobile & Portable) RF performance means terminals will carry on working further away from the site.

Better RF performance mean improved reliability:

  1. When deploying base stations on already busy sites, superior base station RF performance means less interference with existing systems, whereas deploying equipment with poorer specifications will likely suffer blocking & intermodulation problems.
  2. When deploying terminals onto busy systems, or into areas where many LMR or cellular radio systems co-exist, superior terminal RF performance means less chance of being interfered with.

Questions to think about

  • Do you have an existing analogue FM LMR radio system you are looking to replace? If so, what type of system is it (conventional, trunked eg: MPT1327)?
  • How many sites does your system use? How far apart are they? Do you have good coverage now, or are there dead spots you need to fill?
  • What frequencies / channels do you have licenced? Do you want to re-use these?
  • Are you deploying new equipment on to already busy sites and towers?
  • Is system reliability and flexibility important to you?
  • Do you maintain your own radio system? Would you consider it being managed?

If you are considering purchasing a radio system and are concerned about RF Performance and need help with working through the above questions or with a deeper dive into your system design, please feel free to contact us. The RF performance of Tait Base Stations, Mobiles and Portables often exceeds recommended specifications giving important benefits in terms of system cost and reliability.

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Comments

  1. Dean says:

    Thanks for taking the time to write this intriguing post on RF performance. This is important and often confusing, ““Guaranteed” is the minimum performance. “Typical” performance is not guaranteed.” Appreciate the share!

  2. What about Passive Intermod products from aging feeders and oxidized connectors etc ? How do they affect RF performance relative to the above discussion ?

    • Hi David,

      In this blog post we were trying to deal with the basic parameters to identify which ones are the most important and why. The subject of Passive Intermodulation is deeper than we were trying to cover in this blog post.

      To answer your question, at a site when you have several transmitters combined on to a single antenna, and damage to the antenna or feeder cable will cause harmonics of the transmit frequencies to be generated which then mix and generate sum and difference (intermodulation) products. Unless the site transmit frequencies are chosen wisely, some of these generated passive intermodulation products land right on top of the site receive frequencies. If the antenna is duplexed, any passive intermodulation produced at a receive frequency will pass straight through the antenna to receive path of the duplexer and deafen one or more receivers. A portion of the predicted coverage is then lost. For this reason, we recommend using separate transmit and receive antennas for multi channel systems, and paying careful attention to which transmit channels are combined together.

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