GB2265527A - Technique for combatting FMTV satellite interference with digital data carriers - Google Patents

Abstract

The technique is a symbol-synchronous frequency-hopping scheme for combatting FMTV (frequency modulated television) satellite interference with digital data carriers. Each digital data carrier is frequency hopped on a symbol-by-symbol basis within a specified frequency "hop-bandwidth", thereby causing FMTV interference to be shared between all digital carriers rather than concentrated on one digital carrier. This makes the interference appear "noise-like". The technique may be used to reduce the effect of other sources of interference on digital communication links. <IMAGE>

Description

Technique for Combatting FVTV Satellite Interference With Digital Data Carriers The EUTELSAT II satellites present a severe interference environment for 64 Kbps and other low-rate coded QPSK digital SCPC carriers. EUTELSAT specifies in ref 1 that acceptable degradation of a digital carrier is achieved by specifying a minimum frequency-separation of the digital carrier from each interfering Frequency-Modulated Television (FMTV) carrier. In a VSAT environment this will limit the available bandwidth in the transponder within which EUTELSAT II interference criteria can be met. It may also impact VSAT equipment cost by increasing the required terminal EIRP.

We describe here a low-cost approach to reducing the effects of fMFV interference, reducing VSAT costs and increasing service reliability. FMTV interference is so damaging because a worstcase analysis has to be taken of its effects: the whole of the interfering power of the TV carrier must be assumed to be concentrated on one digital carrier. However by frequency hopping each digital carrier on a symbol-by-symbol basis within a specified frequency "hop-bandwidth", it is possible to simultaneously share out the FMTV interference between all digital carriers. This makes the interference appear "noise-like" and so reduces its effect in proportion to the frequency hopbandwidth. The frequency-hopping can be implemented with little impact on modem complexity, and at negligible cost in Eb/No.

The two dominant sources of crosstalk into digital carriers, which interfere on both uplink and downlink, are: (i) "co-frequency" cross-polar FMN carriers (ii) "co-frequency" co-polar adjacent satellite FVTV carriers For wide-aperture ground-terminals interference tends to be dominated by cross-polar interference; for VSAT applications adjacent satellite interference is also important. For 36 MHz EUTELSAT II transponders, the cross-polar FMTV carriers are equistaggered with respect to co-polar FMTV carriers, being located in the frequency gap between co-polar transponders. However the adjacent satellite co-polar FMTV carriers are located in the centre of the co-polar transponders.Thus, while the requirement to minimise cross-polar interference tends to drive low-rate digital carriers towards the centre of the 36 MHz transponder, the requirement to minimise adjacent satellite co-polar interference conversely tends to drive them to the edge of the transponder. Even with large aperture terminals only a few hundred KHz may typically be available for low-rate digital carriers in the transponder centre. But for small aperture VSAT terminals, which are also strongly affected by adjacent satellite interference, it may be the case that no bandwidth is available at all for low-rate digital carriers providing "acceptable" degradation performance.

The levels of interference that can be tolerated with digital carriers in the EUTELSAT II environment are such as to cause a degradation to the digital carriers that may be up to several dB.

This implies that digital uplink EIRP needs to be several dB greater than otherwise appropriate in an interference-free environment. This means a larger aperture and/or a larger High Power Amplifier (HPA) power-rating and increases VSAT equipment cost as well as destroying the predictability of reliable VSAT service.

The effect of viIv interference on low-rate digital carriers will be not only to introduce extra noise at the symbol-decisions, thus increasing Bit Error Rate (BER), but also to affect carrier and clock sync circuits. Sync circuit effects will vary with specific modem design, and will probably be the dominant source of degradation. Likely effects are regular cycle-slip, a regular tendency to lock to frequency components in the FMTV interference, and regular frequency-acquisition searches. FMTV interference will also strongly impact the performance of Viterbi decoders, producing regular long error bursts. Viterbi decoders work best with random error events, and error correction performance degrades when correcting the error bursts that N4IV interference produces.Viterbi decoders should at least be protected by interleaving, and it is also appropriate to consider the use of concatenation with a Reed-Solomon codec. The effects of FMTV interference will have a large potential impact on VSAT service reliability.

The route to reducing the impact of FMTV interference will be to employ some form of spread spectrum technique. The particular form of spread spectrum to be chosen should be such as not to impact unnecessarily on equipment cost or performance against thermal noise. Preferably the signal waveform should also appear similar to conventional Single Channel Per Carrier (SCPC). The conventional frequency hopping spread spectrum technique tends to fall short on thermal noise performance, while direct sequence spread spectrum (ie Code Division Multiple Access, COMA) modulation presents a significant cost impact. Also CDMA would need to be operated symbol-synchronously if there were not to be an internal interference constraint, or the need for greater bandwidth.

A cost-effective approach that does not sacrifice Eb/No margin is to frequency hop symbol-synchronously. A group of SCPC channels of identical bit-rate (such as 64 Kbps) is assigned along conventional lines, with an allocated channel spacing and rolloff factor. The channels need not necessarily be contiguous. The active bandwidth is the "hop-bandwidth". Each terminal transmits one symbol at a time in the normal way, except that the assigned frequency of the symbol hops from symbol to symbol. Different terminals transmit a symbol at different frequency: each terminal transmits the same hop-frequency pattern, but offset from all the others and wrapped around as necessary to fit within the operating frequency hop-bandwidth. Thus the satellite supports a full multiplex of carriers without any clashing.By this means the signal is uplinked with no more EIRP than for the same SCPC signal (thus incurring no extra terminal-EIRP cost), and may be sufficiently randomized with respect to FMTV interference to reduce this interference to "equivalent noise". The satellite transponder receives an apparently conventional SCPC multiplex.

The demodulator removes the imposed frequency-hop pattern from the received signal and recovers the bitstream in the conventional way. It is important to note that the signalling waveform of each transmitted symbol lasts for a specific number of (typically 5) symbols. The whole of a symbol signalling waveform is transmitted at one frequency, while the whole of the next symbol signalling waveform (which overlaps it in time) is transmitted at another frequency, and so on. Thus a given terminal is transmitting into several (typically 5) SCPC channels at the same time (along with typically 4 other terminals, each on different symbols of the symbol signalling waveform). There is no degradation of Eb/No or the splattering of spectral power that occurs with conventional frequency-hopping. Each symbol is transmitted and matched-filtered coherently without degradation, at the correct frequency.The modem required to implement this scheme is a slightly modified conventional single-channel modem.

The satellite operator sees a waveform that is effectively indistinguishable from that of the conventional SCPC multiplex.

In order to further clarify these concepts, we present in Figs 1 and 2 representative signalling waveforms for the conventional SCPC approach and the frequency-hopping scheme described here. In Fig 1 the conventional SCPC waveform transmitted by a given terminal consists of a train of symbol signalling waveforms, one for each symbol, each lasting several symbols in time, all at the same frequency. In Fig 2 the frequency-hopped waveform (as required to implement the scheme described here) transmitted by the given terminal has each different symbol-signalling waveform at a different frequency. At any time the modulator is transmitting simultaneously at as many different frequencies as there are symbols in the signalling waveform. The symbolsignalling waveforms from the other terminals are received with nominally identical clock-timing phase at the satellite.No two symbol-signalling waveforms from different transmitters are received at the satellite nominally simultaneously at the same frequency. In general (typically 5-1 = 4) other terminals' symbol signalling waveforms will be overlapping in time at the same frequency as the symbol signalling waveform of the given terminal, all nominally offset in time by a whole number of symbols.

Any hop-pattern of frequencies can be used, providing no two transmitters ever simultaneously transmit a signalling waveform centre at the same frequency. However synchronisation is simplified if: (i) the pattern is repetitive with the shortest acceptable period, and (ii) any modulator uses the same pattern as the other modulators, but shifted in time. The frequency repetition pattern should be chosen such that the interference itself has a sufficiently small probability of following the same frequency pattern. From this we can conclude that a 50 Hz sawtooth, the FMTV dispersal pattern itself, would not be an appropriate choice. A suitable choice would be a pseudorandomlychosen sequence of frequencies with the shortest possible period, that is sufficiently unlikely to be found in the FMTV signal.

In order to clarify further these concepts, an example modulator and demodulator are shown in Figs 3 and 5, for the frequencyhopped signal described here. Although these are drawn for the case where the symbol-signalling waveform has duration N = 5 symbols, the duration N could generally take any chosen value.

The symbol-signalling waveform generator and receiver shown in Figs 4 and 6 are fundamental to the operation of the frequencyhopping modulator and demodulator. These consist of a mixer driven by a Voltage-Controlled Oscillator (VOW) or Numerically Controlled Oscillator (NCO) and a shaping filter whose response has a duration of N symbols. Each of the symbol-signalling waveform generators or receivers handles one symbol, taken over an elapsed period of the selected number of N symbols. The N symbol-signalling waveform generators or receivers operate in sequence, each staggered one symbol later than the previous. The switches arrange for the transfer of a symbol to/from each unit and a newly-selected frequency to each unit every N symbols. Each unit is transmitting/receiving a modem signal all of the time. A synchronisation process arranges for the pattern of frequencies at the demodulator to be in the correct phase with respect to the received signal.

This architecture could be particularised to the case of the conventional single-frequency modem, by arranging for the frequencies not to change. In this case all of the symbolsignalling waveform generators or receivers become identical, though still displaced in time by one symbol.

Claims (1)

1. Claims

   - the symbol-synchronous frequency-hopping scheme described above for reducing the effect of FMN satellite interference on a digital data carrier and also for reducing the effect of any source of interference on any digital communication link - the symbol-synchronous frequency hopping scheme described here as a means of frequency-hopping in any communications application, which provides the benefit of being able to achieve arbitrarily small loss of signal-to-noise ratio and arbitrarily small spectral splatter - as a means of forming a waveform that appears to satellite transponder operators to be indistinguishable from a conventional single-channel-per-carrier (SCPC) signal multiplex.