Interference from nearby transmitters causes considerable problems for vehicle-mounted communications equipment, especially when the transmitter is co-located or cosited with a vehicle-mounted receiver. The interfering signals swamp the incoming signals making receipt of these signals difficult if not impossible through the interference.
The problem is especially severe when communications vans or vehicles have a number of transmitters with associated antennas on the vehicles, and where receivers within the vehicle are trying to listen to incoming communications signals at the same time the transmitters are transmitting. This is called cosite interference due to the co-location of antennas and communications equipment on the vehicle.
The problem exists in two situations, first for individuals seeking to transmit and receive at the same time using separate transceivers within the vehicle. Even if the signals are not on the same frequency spatter and unwanted harmonics oftentimes preclude receiving incoming signals.
Secondly, and perhaps more importantly, jamming signals are often employed to protect vehicles for instance from the detonating of improvised explosive devices (IEDs). Recently rather than utilizing detonator cords, cell phone activation of IEDs has been popular. This mode of detonation can be thwarted by providing jamming signals near the vehicle that swamps and/or defeats the transmissions from cell phones or other detonating transmitters. However in so doing, the jamming radiation also swamps the receiver in the vehicle and it is only with difficulty that this injected interference can be nulled.
As will be discussed, narrow band techniques involving numerous heavy coaxial delay lines have been employed. However, for narrow band systems in order to provide for instance a gigahertz of coverage the amount of space, weight and equipment is prohibitive.
By way of further background, oftentimes unwanted interference comes from signals which emanate from or exist near a receiver in which signals come from co-located or cosite transmitters located on a vehicle that has a number of transmitters that are used simultaneously. For instance, in a particular communications environment transceivers operating at different frequencies in the UHF and VHF bands are operated at the same time or with L band transceivers. Thus the transceivers are operated in different bands and with different modes of communication.
For example, when operating in the UHF region there may be a SATCOM transmitter nearby that is using a ground-based mode of communication in the same band as another SATCOM transceiver linked to a satellite.
As mentioned above, the other situation involving unwanted interference is when jamming signals are emitted from a vehicle for instance to jam transmitters that are trying to activate or detonate improvised explosive devices. This can occur through the use of cell phones or other transmitting devices. It will be noted that such devices operate over a wide range of frequencies and stepped jammers which frequency-hop their transmissions are in wide use.
Classical approaches to cosite mitigation involve passive frequency agile filters or active phase reversal techniques. In one active approach, power from each interfering transmit source is sampled using directional couplers, and then phase and amplitude matched for each interferer with the phase and amplitude matched signals injected out of phase into the receive path for phase-inversion cancellation, in which each frequency is a addressed in a separate channel.
Note that cancellation depth is highly dependant on how well the correlation (phase) and weighting (amplitude) hardware can match the interferer over frequency. As noted, cosite cancellation systems are exceedingly heavy and large in size driven largely by the length of delay lines required to match the interfering signal to the signal of interest. Moreover, lumped element components used to build the individual weight/correlators are heavy and large. Additionally, most suppliers of cancellation equipment avoid active electronics in the cancellation circuitry as they tend to generate self-noise which causes additional interference.
Put more simply, coaxial cable delay lines are utilized in the vehicles which snake around in the vehicle, with one delay line per frequency required. This makes a large tangle of delay lines within the vehicle simply to be able to select out and eliminate interfering signals, each at a different frequency. The reason for the requirement for so many delay lines is the fact that these systems are very narrow band and while they can be effective for the narrow banded signals, the weight of the delay lines, and the physical space of the snaking delay lines presents a problem.
Moreover, communications technology is moving away from low frequency, narrow band channels such as 25 kHz channels, to higher frequencies that can support wider waveforms capable of much greater data rates. These include wide band network waveforms and soldier radio waveforms. Inherently, lumped element components which are used to fabricate the amplitude the phase correction circuitry of the prior cancellers are narrow band. While filters and cancellers can be built to have wider bandwidths, the cost in terms of weight, size, insertion loss and fabrication complexity is prohibitive.
As a result an ideal canceller would desirably have a wide bandwidth, high dynamic range and be physically small.
In short, prior canceling systems involve narrow band cancellers. The narrow band cancellers operate to cancel the signals from an offending transmitter. This is accomplished by tapping some energy from the offending transmitter's antenna using a small directional coupler to bring the sampled signal to a place where one can make phase and amplitude adjustments so that one obtains the negative of the offending signal. The negative or phase reversed version of this signal is then coupled into the receive path to cancel the offending signal. Because of the narrow bandwidth of the cancellers, in order to do this one must adjust the phase and amplitude of the tapped signal and to do the adjustment every time one switches frequencies. Note that such multi-band cancellers typically only work over 1 MHz at most.
Such cancellers are thus inadequate to null the jamming signals now employed, which are frequency hopped and may cover a gigahertz in bandwidth. Since these systems are frequency agile, the offending signal can show up anywhere with in a very wide band spectrum. More importantly, the prior systems operate in the time domain as opposed to the frequency domain, the importance of which will be described below.