450-470 MHz band alignment
Annex B: Constraints of the Band Plan
It is possible to construct a phased transition process to arrive at a band plan for the aligned spectrum. This is based on moving spectrum blocks to achieve the desired configuration; however, it is subject to a number of constraints which dictate where spectrum blocks can be moved to. This band plan has met with almost unanimous agreement within the Industry Working Group (IWG) as a way in which alignment could be taken forward.
Emergency Service migration
Migration of the Emergency Services to their new system will leave two 3 MHz holes in the spectrum at 14 MHz duplex. Effecting a single move of base and mobile frequencies within the band is considered to be the least disruptive way of achieving alignment. It is possible to move both legs into vacant spectrum, but this limits the moves that are possible to achieve an aligned plan.
The transition between the base-receive and base-transmit frequencies occurs in the middle of the band at 460 MHz. This is the most sensitive part of the band, with the possibility of base receivers just below 460 MHz being adversely affected by transmitters placed just above 460 MHz. It will be necessary to consider very carefully how the spectrum in this area is allocated.
Television channel 21
Another consideration when planning the upper end of the band is the presence of television channel 21. The terrestrial television spectrum immediately above the band is divided into 8 MHz channels, the first of which (470-478 MHz) is denoted channel 21. This lowest channel only came into widespread use with the start of Channel 4. Studies conducted in the early 1970s showed that there was a compatibility problem between narrowband base stations (e.g. 12.5 kHz PMR bases) and domestic reception of television signals on this channel if the narrowband bases were close geographically to the television receiver and they operated above about 466 MHz.
When the current plan for 450-470 MHz was devised, it was not known where channel 21 television transmitters would be located, so 467-470 MHz was kept clear of base transmitters. In the new plan, base transmitters are to be placed above 460 MHz, so the constraint is to avoid interference to domestic television reception on channel 21.
The locations of analogue television transmitters and the first phase of digital transmitters on channel 21 are now known. It is possible, therefore, for newly planned base transmitters above 466 MHz to be co-ordinated with the television service. This will mean that users of bands above 466 MHz may have geographical limitations on use of their spectrum. Digital television receivers are known to be less sensitive to out-of-band interference, but new digital transmitters are likely to be introduced as analogue switchover approaches. The pattern of restrictions is therefore likely to be a moving target, and further research is required in this area.
Flexibility of the band plan
As implementation of the band alignment is scheduled to begin in 2005, the greatest difficulty is predicting the technological market share that will have arisen by then. To cope with this uncertainty, RA has attempted to design a technology-neutral plan that does not specify whether the technology will be analogue or digital, proprietary or open standard. Instead we have opted for a co-existence mask. This is a compromise between waiting to see what technology prevails in five years (an option which would not leave enough time for proper planning) and mandating current technology (which would run the risk of not being forward-thinking enough nor able to introduce the then current market-favourite technologies).
The use of CDMA has been discounted by the IWG, which felt that the technology would not be suited to the use currently existing in the band. Instead, RA favours creating a channel raster from the highest common factor. A raster of 1.25 kHz would allow channel widths appropriate to narrowband and wideband technologies.
A more detailed look at the proposed band plan can be found in Annex C.
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