Since the beginning of PMR radio, there has been a constant juggling act between available spectrum and channel size. As filter and modulation technology has advanced the channel size has progressively reduced, 100 kHz, then 50, followed by 25 and then the 12.5 kHz.
Achieving 6.25 kHz channeling was impossible to do using analog technology, so it became necessary to develop a new digital protocol. Availability of high quality low bit rate voice codecs meant that 6.25 kHz was a practical plan if a suitable modulation scheme could be identified. Several methods were considered, including ACSB and the proposed APCO Project 25 Phase II CQPSK. However, both required a more expensive linear amplifier in the transmitter and neither is compatible with existing analog FM hardware.
Instead, 4-Level FSK (4FSK) modulation was selected using FDMA for the access method.
This method has a number of advantages:
- better communication range
- simpler design
- easy to maintain and service
- lower cost for business and industry customers
- compatible with existing FM radio hardware
The first 6.25 kHz capable radios were introduced to the market in 2006. There are now 13 manufacturers that have 6.25 kHz FDMA products (dPMRTM, NXDNTM and Japan ARIB standards based products) with more manufacturers still to develop products.
These products operate in both digital conventional and digital trunking modes. To enable backwards compatibility, they also operate in 25 kHz and 12.5 kHz channel bandwidths.
Categorised in: News Feature
This post was written by aa radio