Receiver

Details Receiver Receiver

1999-12-28

2004-09-21

Chin, Stephen (Department: 2634)

Pulse or digital communications

Spread spectrum

Direct sequence

C375S148000, C375S150000, C375S140000, C375S134000, C375S350000, C370S342000

Reexamination Certificate

active

06795487

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a receiver for receiving radio-frequency pseudo-random encoded signals from satellites of a universal ranging system.
BACKGROUND OF THE INVENTION
A positioning or timing receiver in a universal ranging system must receive and process signals from several satellites, such as those of the GPS or Glonass constellations, to obtain a measurement of its position or to establish a timing reference. The signals from a given satellite are processed in a dedicated channel, a conventional example of which is shown schematically in FIG.
1
.
Referring to
FIG. 1
, a receiver channel
10
of a positioning receiver comprises generally an analogue section
11
, a digital section
12
and a digital/software interface
13
. The interface
13
is connected to a micro-processor (not shown) which processes interface output signals with software. The primary signal path of the analogue section comprises, from an antenna
14
, a radio frequency (RF) amplifier
15
, a first mixer
16
, a first intermediate frequency (IF) amplifier
17
, a second mixer
18
, a second IF amplifier
19
and an analogue to digital converter (ADC)
20
. A 20 MHz reference frequency oscillator
21
provides a 20 MHz reference signal to a phase lock loop (PLL)
22
, which provides local oscillator signals on outputs
23
and
24
to the mixers
16
and
18
respectively.
The ADC
20
provides a three-level output signal (i.e. occupying two bit lines) sampled at 15.42 MHz to signal inputs of each of two digital mixers
25
and
26
, which define an input of the digital section
12
. The oscillator
21
is connected to feed the 20 MHz reference
30
frequency signal also to a digital frequency divider
27
, which provide orthogonal phase digital cos and sin signals at 5 MHz to local oscillator inputs of the mixers
25
and
26
respectively. In-phase (I) and quadrature (Q) digital signals are therefore provided on mixer outputs
28
and
29
respectively. A carrier numerically controlled oscillator (NCO)
32
is connected to receive the 20 MHz signal from the oscillator
21
. The carrier NCO
32
is controlled to provide oscillator signals on an output
33
at such a frequency as to cause the mixers
30
and
31
to provide baseband I and Q signals on their outputs
34
and
35
respectively. These baseband signals are modulated only (in the case of signals from GPS satellites) by the C/A code at 1.023 MHz and by the data which is carried on the signals at 50 bits per second.
A code NCO
36
is connected to receive the 20 MHz signal from the oscillator
21
and is controlled to provide code clock signals at an output
37
. The code clock signals are referenced to the frequency of the oscillator
21
but are equal in frequency to the code, (1.023 MHz for GPS L
1
signal codes). During code tracking, a code replica generator
38
receives the code clock signals and is controlled to provide prompt code replica signals on an output
39
and early-minus-late code replica signals on an output
40
. The early-minus-late code replica signals are generated by subtracting code replica signals which are phase-delayed with respect to the prompt code replica signals from code replica signals which are phase-advanced with respect to the prompt signals. First and second prompt code mixers
41
and
42
are connected to mix the baseband I and Q signals on the outputs
34
and
35
with the prompt code replica signals to provide prompt I and prompt Q signals on respective outputs
43
and
44
. First and second early-minus-late mixers
45
and
46
similarly provide early-minus-late I and early-minus-late Q signals on respective outputs
47
and
48
by mixing the baseband I and Q signals with the early-minus-late code replica signals. Each of the signals provided by the mixers
41
,
42
,
45
and
46
is accumulated in a respective accumulator
49
-
52
. The accumulators
49
-
52
are clocked by the oscillator
21
, and subsequently buffered into data form by respective buffers
53
-
56
. The buffers
53
-
56
are clocked by a signal obtained from the oscillator
21
by a frequency divider
57
. The output signal of the frequency divider
57
thereby defines accumulation intervals. The outputs of the buffers
53
-
56
are collated into an output
58
for subsequent software processing. A feedback path (not shown) allows the frequency and phase of both the carrier NCO
22
and the code NCO
36
to be dynamically controlled to maintain alignment with the 30 received signal.
In another known positioning receiver, code replicas may be switched to vary the delay of the early-minus-late code signals so that the receiver channel can function as a conventional correlator during signal acquisition and as a narrow correlator during signal tracking. In either case, a prompt code replica is aligned with the C/A code modulated onto the received signal when an output signal of an early-minus-late correlator is zero.
In a multipath environment, signals which are reflected before arrival at the receiver are delayed in time with respect to the arrival of the direct signals. Although reflected signals are of lower amplitude than the direct signals (except when the line of sight is obstructed), they cause problems with alignment of the prompt correlator with the early-minus-late correlator, when the reflected signals are within around one period or chip of the code of the direct signals. This is a recognised problem which is addressed at least in part by the narrow correlator operation mentioned above. Narrow correlators have the effect of reducing the effect that a reflected signal has on the alignment of the correlators by reducing the effect of the reflected signals. Two other approaches have been taken in an attempt to improve signal resolution in the presence of multipath signals.
Firstly, it is known to use in a receiver a plurality, typically 4 or 6, of parallel correlators each using a different delay of the early-minus-late code signals. This approach, in effect, involves sampling the discrimination pattern at a number of points, equal to the number of correlators, to provide data which can be resolved by software as a series of simultaneous equations. The solutions to the simultaneous equations identify the reflected signals, which can thus be isolated from the direct signal. Obviously, a greater number of correlators results in a greater resolution of the signals and, therefore, more accurate position measurements. However, such an approach requires a considerable increase in hardware (the extra correlators) and in processing power to resolve the signals. Although a similar effect can be achieved by assigning the use of plural channels for the resolution of one signal, each channel having a differently phased early-minus late code signal, the same disadvantages are present.
A second approach has been to generate code signals comprising a set of recurring non-zero three-level gating signals having equal positive and negative areas and a polarity at a centre point which depends on whether a corresponding edge of the code modulated on the received signals is a rising or a falling edge. This approach provides an error signal or discrimination pattern which allows steering around the alignment point but which has zero response to multipath signals falling more than a short distance from the alignment point.
SUMMARY OF THE INVENTION
According to the present invention, a receiver, for receiving and processing radio-frequency pseudo-random encoded signals, comprises in a channel thereof:
a code replica generator arranged to provide code replica signals on an output thereof;
a mask generator device controllable to provide any one of at least two predetermined time varying mask signals at an output thereof;
a frequency translator arranged to receive and to frequency translate the encoded signals and to provide translated signals in response thereto;
a signal path of the channel comprising between the frequency translator and an accumulator:
a code mixer arranged to mix the translated signals with the

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