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This is not a new concept but is one has been used for many years on analogue medium wave networks especially in the UK and Spain. The opposite concept to a single frequency network (SFN) is a multi-frequency network (MFN) and a good example of this is RDS controlled analogue FM/VHF networks. In the OFDM world it may seem counter intuitive that an SFN can work better than an analogue equivalent. An SFN is created when all the transmitters in the network operate on the same nominal carrier frequency. The transmitters are designated as co-channel. In theory sub-carriers arriving from a transmitter outside the local service area can positively reinforce reception and replace a carrier lost through local anomalous propagation effects. With OFDM the transmitters have to radiate not just the same but an identical on air signal meaning that:
- The frequencies and phases of the sub-carriers have to be radiated to a very high tolerance.
- The individual sub-carrier frequencies have to appear at the same time.
- The individual sub-carriers have to carry the same data.
Frequency Domain
The OFDM frequencies have to be generated to a very high frequency tolerance, which means that each sub-carrier must be in its assigned slot. For a DRM signal using Mode A in a 9 kHz bandwidth this means that each of the 204 carriers must be exactly on frequency relative to the carrier. For example positive carrier number 99 must be spaced 4125.033 Hz from the nominal centre frequency of the channel.
A frequency standard signal divided down to 1 pulse per second is now usually generated from an off-air GPS receiver rather than from an atomic based system.
Time Domain
Synchronizing the OFDM symbols in the time domain is a new challenge for the transmitter engineers who have up to now only had to worry about frequency accuracy. Each transmitter has to radiate the same OFDM symbol (this is a QAM encoded sub-carrier frequency that has a certain phase and amplitude) at the same time so that the data is synchronized in the time domain. The key factor here is the guard time interval specified for the mode of operation. The guard time or interval governs the maximum delays or echoes that an OFDM-based system can tolerate. It also influences the maximum distance between transmitters. An OFDM receiver samples the received signal for a pre-determined period of time at regular intervals. In between these sampling times (during the guard interval) the receiver ignores any received frequencies.
Creating an SFN
To get each transmitter synchronized in both the time and frequency domains some extra data is added to the serial data streams (these contain the encoded audio and data inputs) the sent to the transmitters. This additional data stream is essentially a time reference signal inserted into the network. At each transmitter the OFDM modulator uses this time stamp to calculate the local delay so that a common on-air time is achieved.
SFN Networks on DRM
Unlike line of sight networks that occur at Band III and Band IV/V implementing an SFN on short wave needs a little more thought. However Deutsche Welle in Germany has tested this system from two transmitters where one was located in Germany and the other in Portugal. DW decided on a control point in Germany (a designated geographical location) and then calculated the signal transit time from Sines and Wertachtal via the ionosphere. Sines is the longer path so the signal from the Wertachtal transmitter was delayed by about 4 milliseconds to ensure that both signals arrived at the control point at the same time. The concept worked well as the signal decoded both before and after the SFN was in use.
RTL also tried to use the SFN concept using a transmitter in Julich, Germany and in Junglinster in Luxembourg. The control point was in France and even though reception was very good in the UK the signal did not decode at all. I am not sure just how often the broadcasters will use this technique in the future.
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