Simulcast when digitizing the FM band
When introducing digital radio "gracefully" in the FM band, one strives to do this gradually in stages. This is in
contrast to "hard" introduction where the transition from analog to digital is made at a certain date. In order to
make such gradual introduction, the simple method of "simulcast" can be used, where the operator/radio
station is broadcasting analog and digital content simultaneously.
However, this requires that there is sufficient spectrum available in the band that is to be digitized. The FM
band covers 87.5 MHz to 108 MHz, a total available bandwidth of 20.5 MHz. The Swedish Post and Telecom
Authority allocate frequency permits on a grid roughly rounded to 400 kHz increments. This would give a total
number of presumptive FM transmitters to approximately 50. The situation is however a bit more complicated
than that. Different transmitter output power levels have to be taken into account as well as their geographic
location in order not to interfere with each other in the receivers (crosstalk). Therefore the permits are
allocated on a wider frequency grid at higher output powers and a denser grid at lower output powers.
Furthermore, a weak transmitter should not be placed near a strong transmitter, in frequency or vice versa,
as the stronger transmitter may saturate the receiver that is tuned to the weaker one. Altogether, this makes
frequency planning rather complicated since it involves a multidimensional optimization problem with many
variables to be taken into account.
Simulcast is visualized below with a simple example involving two FM transmitters and a DRM+ transmitter.
FM1 and DRM+ are broadcasting in simulcast and FM2 is autonomous. FM1 and DRM+ in this example has a
center/center distance of 200 kHz.
In order not to interfere with the listener's FM receiver, the DRM+ transmit power must be reduced (backed-
off) at least 20dB in the case where the DRM+ transmitter is situated 150 kHz from the FM transmitter.
Reducing the distance to less than 150 kHz is not recommended. This implies a power reduction of 100 times
for DRM+. If the FM transmitter transmits at 1 kilowatt, the DRM+ transmitter is allowed to transmit at a
maximum of 10 watts. It should be noted that the DRM+ transmitter in this first case is about 20 dB weaker
than the FM transmitter (1/100th). However, the DRM+ modulation mode-E at 4-QAM, exhibits a significantly
higher efficiency than analog FM modulation, thus the transmit coverage area should be close to identical to
the FM coverage.
If the DRM+ transmitter FM1 and DRM+ transmitter has a center/center frequency distance of 200 kHz as
depicted above, one may reduce the power difference with about 10 dB. This means that in the case of the 1
kilowatt FM transmitter, the DRM+ transmitter should broadcast on max 100 watts in order not to interfere
with the FM receiver. It should be noted that the DRM+ transmitter in this second case is about 10 dB weaker
than the FM transmitter (1/10th). However, the DRM+ modulation mode-E at 16-QAM, exhibits a higher
efficiency than analog FM modulation (but less than 4-QAM), thus the transmit coverage area should be close
to identical to FM coverage.
Hence there is a given ratio between the frequency spacing between the FM transmitter and the DRM+
transmitter (delta-f), which gives a maximum allowed output power of the DRM+ transmitter in watts (delta-P)
related to delta-f. The closer the DRM+ and the FM signals are to each other, the lower the allowed output
power of the DRM+ transmitter, in order not to interfere with the FM receiver. These so-called protection ratio
or protection levels have been established both by theoretical calculations and practical measurements using
different brands of radio receivers.
A delta-f of 150 kHz gives a minimum delta-P, of about 20dB (1 % output of DRM+ relative to FM).
A delta-f of 200 kHz gives a minimum delta-P, of about 10dB (10 % output of DRM+ relative to FM).
Note that this applies to all frequency planning on for example the FM band regardless of simulcast or not. A
digital broadcaster could broadcast with DRM+ on the FM band and as long as there are analog FM
transmitters in the band, the same recommendations as above apply.
It should be noted that the least problems with crosstalk and interference occur when the FM receiver is of
high quality, meaning that is has a steep input filter and high selectivity. Such characteristics are usually not
found in the low-cost receivers sold cheaply in tech stores.
Crosstalk in the other direction - the problem of FM broadcasts leaking into the DRM+ receiver - is virtually
nonexistent as the radio modem is digital with high selectivity at the receiver and which do not pass through
any FM signal. However, a strong FM transmitter may jam a DRM+ receiver if it is in close proximity to the FM