# Compact Bistatic Iterative Passive Radar Based on Terrestrial Digital Video Broadcasting Signals
## 3 Proposal Description
### SFN Considerations  
A single-frequency network (SFN) is a transmission scenario where multiple transmitters broadcast the same signal on the same frequency. This is commonly used in DVB-T and DVB-T2 systems. The key advantage of SFN is that it allows for the coverage of large areas with a reduced spectrum usage. In an SFN, all transmitters are synchronized to transmit the same signal at the same time, effectively minimizing interference between them and providing robust reception over a wide geographical region.  
However, implementing SFN in DVB-T or DVB-T2 systems involves addressing several technical challenges. One of the most significant hurdles is ensuring that the transmission signals from different transmitters remain time-aligned. Any misalignment can cause inter-symbol interference (ISI), which degrades the quality of the reception. The synchronization process relies on precise timing, which can be influenced by factors such as the distance between transmitters, network topology, and the stability of the clocks used in each transmitter.  
To mitigate these challenges, DVB-T and DVB-T2 systems utilize time-frequency synchronization techniques. These techniques ensure that the signals transmitted from different transmitters are synchronized, maintaining signal coherence and reducing potential interference. Furthermore, advanced error correction mechanisms, such as forward error correction (FEC), are often employed to enhance signal robustness in SFN environments. Additionally, there are some limitations on the maximum distance those multiple transmitters can be. In order to avoid multipath problems, a guard interval (GI) is left to absorb the echoes and avoid ISI. This GI is flexible and will depend on the scenario. The longest GI in DVB-T is 224 (1/16 of the OFDM symbol period) \(\upmu\)s and in DVB-T2 is 532 \(\upmu\)s (19/128). If the signal from other transmitters arrive with a delay longer than this, it will not be canceled by GI and will appear as interference and the synchronization process will fail too. Thus, the maximum distance for the other transmitters when SFN configuration is used is around 67 km and 160 km for DVB-T and DVB-T2, respectively.  
In terms of performance, SFN provides several benefits. It enables seamless coverage across large areas with minimal gaps or overlaps between transmission zones, improving signal availability, especially in urban environments or rural areas with challenging terrains. Moreover, SFN allows broadcasters to economize on spectrum usage, making more efficient use of available frequencies.  
On the flip side, managing SFN in DVB-T and DVB-T2 systems requires the careful planning of transmitter locations, transmission power levels, and the timing synchronization of the entire network. Additionally, the multipath effects caused by the reflection of signals off buildings or natural obstacles can lead to destructive interference, particularly when the signals from multiple transmitters arrive at the receiver at slightly different times.  
Regarding our system in SFN configuration, since the initialization phase estimates the channel without targets, it will also characterize the possible echoes from other transmitters and thus, this configuration does not affect the performance.