COMMON
CHANNEL COMMUNICATIONS
(stolen from an IRE convention proceedings of many years ago )
Presented
by S.V. Judd
Plessey
Electronic Systems Research Limited, UK
INTRODUCTION
Voice
communications to remote and isolated areas and also for mobile use depend upon
a radio link to provide the service.
The demand for such services is growing at a tremendous rate but the band
of frequencies suitable for such applications is relatively limited.
.There is considerable pressure, therefore to optimise the use of the
available spectrum which for all practical purposes ties below 1000MHz.
In particular a number of investigations of narrow-band modulation
schemes are in progress to maximise the number of available voice channels (1). Ideally one would also wish to be able to carry
on a two-way conversation using a single radio channel.
Until recently this has not proved to be feasible without recourse either
to time and/or spatial multiplexing or some form of cancellation technique (2).
(The simplex mode of operation of a radio link may be considered as an
extreme form of time multiplexing). However,
research at the Electronic Systems Research Laboratory of Plessey has
demonstrated that it is possible to design equipments which can cope with
transmit to receive power ratios in excess of 130dB using omnidirectional
antennas. Currently this
standard of performance is restricted to angle modulated signals and amplitude
modulation results to date have been limited to approximately lOOdB of dynamic
range. This paper describes the techniques used and results which have been
achieved in a VHF manpack FM transmitter/receiver operating in the frequency
band 30-76MHz and considers future possibilities.
COMMON
CHANNEL FUNDAMENTALS
Typical
conventional single channel simplex mobile radio equipments exhibit a path loss
capability of approximately 150dB. Ideally a common channel duplex design
"should approach this figure if system range performance is to be
maintained.
Essentially
the problem reduces to that of maintaining receiver sensitivity independent of
the transmitter power for signals at the same frequency (FIGURE 1).
The problem is familiar to designers of mobile radio systems as an
extension of the interference performance requirements they have to meet in a
crowded spectrum. For a conventional superhet receiver the situation is
shown schematically in FIGUPE 2. The
transmitter output is shown as a large carrier exactly on the receive frequency.
At the left the equivalent ideal translation to an intermediate frequency
is shown. Since, in the limit
this transmitter signal is some 150dB greater than the received signal, it is
not surprising that the receiver is blocked or desensitised.
In
general four possible approaches to solve this problem have been reported.
They are:-
•
Aerial Isolation
•
RF/IF Cancellation
•
Time Division Duplexing
•
Tracking Notch Fitter
Aerial Isolation
In
this scheme spatial separation of the transmit and receive polar diagrams in the
vicinity of the transmitter/receiver is used to reduce the transmit signal level
at the receiver. The method
is not practicable for mobile equipments and also suffers from potential
"gaps' where the transmit and receive polar diagrams do not overlap.
RF/IF
Cancellation
Considerable
work has been reported using this technique.
A single channel system is shown in FIGURE 3. The signals from the transmitter are coupled to the
receiver through a number of different paths, including leakage through the
antenna coupler, reflections from mismatch at the antenna and reflections from
the surrounding terrain. A
complex (amplitude and phase) weight is used to duplicate as far as possible the
amplitude and phase of the composite coupling path in order to cancel the
transmitter signal at the receiver. Because
the coupling paths have frequency - dependent transfer functions and because
this is essentially a negative feedback, considerations of loop gain and
bandwidth place severe limitations on achievable performance.
In practice some 40-50dB of cancellation is achieved.
Time
Division Duplexing
In
this system the interference problem is solved by ensuring that the transmitter
is muted while the receiver is receiving and vice versa.
This method in its basic form is a cheat however, in that either the
information rate is halved or the bandwidth has to be doubled for the same
information rate and hence cannot legitimately be called common channel.
However, for a speech signal in which, typically, 40% of the time is
"silence* it should be possible to arrange for quasi-common channel
operation.
Tracking
Notch Filter
This
technique can only be applied to angle modulated signals.
In this case the receiver local oscillator is modulated in phase with the
transmitter signal via the transmission path. Thus in the receiver the
modulation on the unwanted transmitter signal is reduced to a very low bandwidth
(<?OHz) which can, in principle be filtered out. FIGURE 4.
In
practice it has not been found practicable to achieve a notch of sufficient
depth within the IF filter of a conventional superheterodyne receiver. However,
by utilising a direct conversion or zero -IF receiver such a notch is entirely
practicable. Since the FM spectrum becomes 'folded' about zero frequency a
phasing type of receiver has to be used (3).
FIGURES 5, 6. Currently
some 75dB of cancellation is realisable using this technique.
COMMON
CHANNEL PRACTICAL LIMITATIONS
Common
channel radio equipments may be used to convey duplex transmissions on a single
channel or to re-broadcast a weak signal at the same frequency by applying the
demodulator output to the modulator input. In practice, in order to achieve an
acceptable standard of performance, a combination of the techniques mentioned
above is necessary. Since it
is difficult to generalise the contributions required by each technique because
system design constraints are usually unique, an example of a practical design
will be used to demonstrate the principles involved.
COMMON
CHANNEL REPEATER FOR COMBAT NET RADIO
In
the military land tactical radio field the frequency band 30-76MHz is employed
for relatively short range communications, 2-20 kilometres approximately, using
frequency modulation. Several
versions of compact, lightweight manpack radio sets for operation in this band
have been developed across the world and these usually provide an analogue
speech link at an output power of 0.5 watt to a few watts.
In the role for which they are designed and at the frequency at which
they operate the nature of the terrain has a considerable influence on the
performance of these sets. Mountainous,
or even undulating wooded country restricts ranges and causes gaps to appear in
the area-coverage capability. Hitherto, in order to overcome this problem
repeaters have been sited in suitable positions which use two frequencies for
the reasons discussed earlier. This results in frequency planning problems and
also operational limitations. A single frequency repeater, albeit with reduced gain
compared to the two frequency version, offers operational advantages.
From
an extensive research study of the attractions and practical limitations of
common channel duplex and repeater techniques the following broad performance
specification seemed attainable using a tracking notch direct conversion
approach:
Narrow
band F.M.(5kHz deviation)
Frequency
range 30 - 76MHz
Modulation
Channelling
25kHz (or 50kHz
Repeater
Gain lOOdB
In
order to provide adequate performance margins for production the following
contributions for each technique were postulated:-
Aerial
isolation^ 25dB (2 whip aerials)
Tracking
notch ^ ^ 70dB
Cancellation
15dB
Total
~ 110 dB
At
this point it is necessary to look at the causes of the dynamic range limitation
because a problem arises concerning the choice of transmitter power output.
If
the leakage signal present at the mixer of the two aerial transmitter/receiver
is a perfect replica of the local oscillator waveform, then it is converted to
zero frequency and notched out. In
practice the leakage signal suffers distortion in a number of ways which results
in a residue at the mixer output. This
residue occupies the same bandwidth as the wanted signal, limits the sensitivity
of the receiver (a wanted signal at audio which is weaker than the residue is
lost in the demodulation process) and hence defines the dynamic range.
There is a maximum power output consistent with this limiting receiver
sensitivity. The transmitter power output may be increased above
this level but only at the expense of reduced receiver sensitivity.
The causes of leakage signal distortions
are:-
•
A.M. noise on the transmitter output.
•
P.M. to A.M. conversion.
•
Time delay on the leakage path.
•
Reflections from mobile objects.
The
first two can be minimised by good engineering design and in practice have been
reduced to the extent that they are not limiting performance.
Time
delayed leakage signal The latter two effects are related but wilt be discussed
separately.
If
it takes longer for the leakage signal to reach the mixer than the local
oscillator signal, the phase difference at the mixer will vary as a function of
frequency. Hence frequency
modulation of the local oscillator produces a residue which is proportional to
the F.M. deviation and the differential delay.
Provided that the F.M. deviation is small compared with the reciprocal of
the timo delay the conversion characteristic can be considered to be linear.
However, the situation which exists in a two aerial system is not
straightforward and many leakage paths exist. These result in primary residues
and secondary effects due to nearby objects.
Thus a compromise is needed to try and minimise the effect over the
operating band. A suitable
fixed delay, comprising a length of coaxial cable, has been found as a result of
considerable experimentation."
3
Reflections
from Mobile Objects
A
reflection from a stationary local object (buildings, etc) causes a secondary
residue as mentioned above, due to time delay. Reflections from moving objects modify the residue in
two ways. Firstly, if the relative velocity is high enough, the reflections
contain doppler components which lie outside the tracking notch bandwidth.
Secondly, the residue(s) which is present as a result of other effects mentioned
above is amplitude modulated at the doppler frequency.
In addition a further type of residue is produced when a signal is
amplitude or phase modulated on reflection at an audio rate.
These
effects are site dependent and are the ultimate limitation to performance.
Early
versions of the receiver design showed that residue levels of about -95dBm could
be achieved with a dynamic range of approximately 110dB.
A maximum power output setting of 1 watt was chosen therefore with two
lower settings 100 milliwatts and 10 milliwatts .
A
compact manpack repeater incorporating simplex, common channel duplex and common
channel repeater modes has been designed which not only meets the original
specification but in terms of dynamic range now provides 130dB with adequate
margin for production.
OPERATIONAL
ASPECTS
Within
the limitations imposed by realisable dynamic range, common channel systems
offer unique performance capability. However,
like all new concepts significant misconceptions can arise unless steps are
taken to prevent this happening. The inclusion of a common channel repeater, having a
path toss capability some 20dB less than conventional simplex radios, in a
mobile radio system requires careful consideration due to the asymmetry not
normally present.
Multipath
effects are a fundamental problem in communications systems and unless
precautions are taken a similar effect can be introduced by common channel
repeaters if a receiver can receive a direct and repeated signal simultaneously.
FUTURE
SYSTEMS
The
basic tracking notch filter appears to have some 80dB of rejection for angle
modulated signals. To this
figure can be added aerial transmit/receive isolation, which may take the form
of physical separation or of duplexing on a single aerial, together with residue
cancellation at base-band. Further work is required on the residue effects of
broad-band signals before system specifications can be formulated.
CONCLUSIONS
The
radio frequency spectrum is becoming increasingly congested and the demand for
mobile radio channels is rising. Common
channel communication techniques have been investigated and shown to have some
potential for spectrum conservation. A practical realisation in the form of a V.H.F. manpack
repeater has provided a detailed insight into some of the problems which have to
be addressed and at the same time given confidence for further embodiments.
REFERENCES
1.
Wells. R., 1981, "SSB Modulation for VHP Mobile and Hand-Portable
Transceivers". Electronics and Power 27.
2.
Abrams, B.S. et at, 1976, "Adaptive Same
Frequency
Repeater Study", Final Technical Report RADC-TR-76-78.
3.
Vance, I.A.W., 1980, "An integrated circuit V.H.F. Radio
Receiver", The Radio and Electronic
, 50. pp 158-164.
Look at this URL it is a direct conversion FM receiver ( for analogue) on 900 MHZ http://www.numatechnologies.com/pdf/NT2904%20Datasheet.pdf