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January 30, 20182 min read

What is the difference between C4FM and LSM Modulation for P25?


More than 1/3 of US Public Safety Agencies are choosing Simulcast networks. There are many benefits to Simulcast networks, including:

  • Easier Operation – the radios just work everywhere
  • Better coverage and capacity with existing frequencies
  • The ability to reach your entire organization with a single call
  • Expertly-designed Simulcast coverage can eliminate dead spots

How do simulcast networks operate? Essentially, simulcast (short for simultaneous broadcasting) means multiple base stations transmitting the same signal on the same frequency at the same time. Check out this video from the Tait Radio Academy to learn more and see how it works:

Should I choose C4FM or LSM?

The two most common types of simulcast modulation are Continuous Four Frequency Modulation (C4FM) and Linear Simulcast Modulation (LSM). LSM is a type of Phase Shift Keying and C4FM is a type of Frequency Shift Keying.

The P25 standard allows for both C4FM and LSM. If you’re migrating from an analog simulcast network to a P25 simulcast network, it’s absolutely vital that you maintain the same level of coverage after the migration. You don’t want to invest significant funds into a P25 network, only to find your coverage is worse and you need to invest in more sites.

Delay Spread


The primary drawback of C4FM is that the maximum delay spread tolerance before the received audio becomes distorted is less than half that of analog simulcast. LSM, however, can withstand delay spreads similar to the delay spread tolerance of analog FM. In other words, if we compare two simulcast networks, one using C4FM and one using LSM, the LSM network would require fewer sites, as the site separation could be much greater. Your total network cost is related to site separation, and therefore the LSM simulcast network will costs less, even though LSM may require more expensive linear base station transmitters.

Benefits of LSM

To recap, here are the benefits of choosing an LSM P25 network over a C4FM one:

  • Require fewer sites compared to non-LSM (the usual P25 C4FM).
  • You will have more flexibility in site selection. You can choose to use sites you already own, or it may be cheaper to lease a site further away from where you would have to have one for C4FM.
  • Better audio quality in crossover areas using existing sites compared to analog sites and C4FM on the same sites.
  • Easier to do coverage planning/launch time calculations as there is more margin for error.

Tait P25 Conventional solutions are available in LSM. To learn more about how we can ensure you maintain great coverage when you migrate from analog to P25, contact us today. If you’d like to dig into the technical reasons that LSM requires fewer sites, then check out our new white paper from Principal Engineer, Ian Graham: P25 Simulcast Coverage Explained.


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