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Claims  |
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I claim:
1. Digital radio transmission system for a network of cells using a
phase-coded spread spectrum technique, in which spectrum spreading of a
transmitted information carrying signal takes place by multiplication of
the information carrying signal with an auxiliary function and despreading
of a received information carrying signal takes place by using this
auxiliary function, comprising several user stations within a cell, each
having a transmitter, a receiver for multipath reception and a control
unit and further comprising a base station with a plurality of
transmitter-receivers and a base control unit, characterized in that
(a) exactly one set of several sets of spreading sequences used as the
auxiliary function is allocated to each cell and an inverse of the one
spreading sequence set is used as the despreading sequence;
(b) paired sequences of different sets have absolute values for normalized
periodic cross-correlation functions which are less than one for all
values;
(c) in sequences with a set, values for paired cross-correlation functions
are minimized around a zero point of the paired cross-correlation
functions;
(d) data synchronous generation of the transmitted information carrying
signals take place in the base stations for all simultaneous connections;
and
(e) a data clock of a received information carrying signal is sued as a
data clock of a transmitted information carrying signal within a given
user station.
2. Radio transmission system according to claim 1, characterized in that
the base station has a service channel which is provided for establishment
of connections and is operated with a sequence selected from the set of
spreading sequences, the sequence selected having an auto-correlation
function which facilitates synchronization.
3. Radio transmission system according to claim 2, characterized in that
the spreading sequences used are from the family of gold codes, and are
grouped into 20 sets of 17 sequences each.
4. Radio transmission system according to claim 3, characterized in that
generation of gold codes in code generators takes place by means of two
linear feedback shift registers with programmable feedback taps,
individual spreading sequences differing only in programmable initial
values of the two shift registers in the case of fixed feedback taps.
5. Radio transmission system according to claim 4, characterized i that
each code generator has a synchronization input by means of which the
generation of the gold code can be controlled with respect to time.
6. Radio transmission system according to claim 5, characterized in that
the receivers of the base station and of the user station are designed as
multichannel correlation receivers and have an HF reception part, a
N-channel despreader a N-channel reference code generator, a N-channel
envelope or square law detector, a synchronization circuit, a L-channel
demodulator and a combiner, with N corresponding to the number of
reception channels and L corresponding to the number of demodulation
channels.
7. Radio transmission system according to claim 6, characterized in that
conversion of the received signal to an intermediate frequency signal
takes place in the HF reception part.
8. Radio transmission system according to claim 7, characterized in that
the despreader has a correlator which includes a mixer for each of said N
channels for multiplying the intermediate frequency signal with a
reference signal, and a band pass filter with a selectable bandwidth a
reception code produced by the reference code generator being used for
despreading the received signal.
9. Radio transmission system according to claim 8, characterized in that N
versions of the same sequence code, which versions are shifted from one
another in time, are produced in the reference code generator and in that
these N versions are transmitted to the mixers of the despreaders.
10. Radio transmission system according to claim 9, characterized in that
delay times of the N versions can be programmed individually for the N
channels.
11. Radio transmission system according to claim 10, characterized in that
output signals of the despreaders are transmitted to the demodulators and
to the envelope or square law detectors with which signal intensity at
outputs of the correlators is measured independently of received data, and
control signals for the synchronization circuit are obtained from these
measurements.
12. Radio transmission system according to claim 11, characterized in that
integration of N envelope detector signals over an adjustable time takes
place in the synchronization circuit and in that the N envelope detector
signals which have thus been integrated undergo analog - digital
conversion and are transmitted to a digital processor which controls the
integration and calculates control signals for controlling the reference
code generator.
13. Radio transmission system according to claim 12, characterized in that
DPSK (=Differential Phase Shift Keying) is chosen as primary modulation
and the demodulator is operated using DPSK and in that for obtaining data
recovery which is insensitive to frequency offset and Doppler shift,
correlator signals in each of the L channels are transmitted to
attenuators and subsequently to a stage for differential demodulation.
14. Radio transmission system according to claim 13, characterized in that
the stage for differential demodulation further includes a CCD delay line
which delays an input signal to the differential demodulation stage by one
bit duration Tb, a 90.degree. phase shifter which generates a 90.degree.
phase shifted version of the input signal and two mixers which multiply
the input signal and the phase sifted input signal, respectively, with the
delayed input signal from the CCD delay line to provide output signals in
two channels for transmission to an integration stage which integrates the
the two channel output signals over one bit duration Tb.
15. Radio transmission system according to claim 14, characterized in that
the integrated output signals of the two channels of all the L channels
are summed an entered in a rectangular coordinate system, and a data value
of each transmitted information bit is determined by a decision logic.
16. Radio transmission system according to claim 15, characterized in that
an I/Q plane for the two channel output signals representing a signal pair
is sub-divided into eight sectors and in that comparators are provided at
an input of the decision logic, which comparators determine to which of
the said sectors the incoming signal pair belongs.
17. Radio transmission system according to claim 16, characterized in that
in the synchronization circuit, coarse synchronization takes place by
measurement of an impulse response in the receiver.
18. Radio transmission system according to claim 17, characterized in that
the reference code generator slides over the code generator of the
transmitter and energy falling into a reception window of the receiver is
calculated and an energy maximum is detected, and in that the reception
window which contains several reception paths is centered around a point
of the energy maximum.
19. Radio transmission system according to claim 18, characterized in that
only a portion of the N reception paths of the receiver is used for
centering the reception window and in that for fine synchronization, the
remaining paths are used for continuously scanning the background impulse
response of the channel by means of a scanning window which is wider than
the duration of the impulse response and another position of an energy
maximum is sought for a new centering of the reception window for fine
synchronization.
20. Radio transmission system according to claim 18, characterized in that
after each coarse synchronization, a verification based on a signal/noise
ratio is carried out and optionally a new coarse synchronization.
21. Radio transmission system according to claim 20, characterized in that
a criterion for loss of synchronism in the receiver is determined by a
measurement based on the signal/noise ratio together with an error rate of
the demodulator, a signal in the reception window and noise in the
scanning window being determined.
22. Radio transmission system according to claim 1, characterized in that
only a portion of N reception paths of the receiver is used for centering
a reception window of the receiver around a point of maximum energy and in
that for fine synchronization, the remaining paths are used for
continuously scanning a background impulse response of the channel using a
scanning window which is wider than the duration of an impulse response in
the receiver and a position of an energy maximum is sought for a new
centering of the reception window for fine synchronization.
23. Radio transmission system according to claim 1, characterized in that a
criterion for loss of synchronism in the receiver is determined by a
measurement based on a signal/noise ratio and an error rate of a receiver
demodulator using a signal in a given reception window and noise in a
background scanning window. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The spread spectrum technique is based on the interchangeability of
signal/noise ratio and bandwidth. Spread spectrum systems (SSS) employ an
auxiliary function for spectrum spreading. The waveform of this function
is known to the receiver. This results in the highly advantageous property
that these systems can operate even under very difficult signal/noise
ratios. The auxiliary functions employed may be signals which themselves
have a large bandwidth and transmit this to the transmission signal when
linked with the information signal. Due to the large product of time x
bandwidth an advantageous autocorrelation function (AKF=) may be produced,
with a sharp peak at the origin and low side lobe values, and the
necessary synchronization of the code sequences forming the auxiliary
functions may be produced in the receiver by means of this property.
Another property of SSS is that several spread spectrum signals can be
transmitted simultaneously in one and the same channel of a given
bandwidth under the condition that the auxiliary functions of different
users differ distinctly in their cross-correlation properties. It is
thereby possible to realise networks with multiple access (Code
Division Multiple Access=CDMA). The nature of the spectrum of SSS to a
certain extent allows operation on frequency bands which are already in
use by narrow band services without significant interference between these
two systems. Moreover, by using pseudorandom code sequences with short so
called chip duration Tc (=smallest rectangular impulse duration of the
auxiliary function), it is possible to resolve individual radio signals
that are propagated over several natural paths (so called multipath
connections), and utilize them effectively as diversity components.
This invention relates to a digital radio transmission system for a network
built up of cells, using the spread spectrum technique, in which spreading
of the spectrum is achieved at the transmission end by multiplication of
the information carrying signal with an auxiliary function while
despreading is brought about at the receiving end by utilizing the same
auxiliary function, the system comprising several user stations within a
cell, each station equipped with a transmitter, a receiver for multipath
reception and a control unit, and further comprising a base station with a
plurality of transmitter-receivers and a base control unit.
Systems of this type, in which spectrum spreading is brought about by
multiplication of the information carrying signal with the auxiliary
function, are known as Direct Sequence-Spread-Spectrum Systems (DS-SSS)
(see, for example, R. C. Dixon: "Spread Spectrum Systems", John Wiley
Interscience, 1984). The methods known in the literature for the
realization of DS-SSS receivers (A. Baier: "A Low-Cost Digital Matched
Filter for Arbitrary Constant-Envelope Spread-Spectrum Waveforms" IEEE
Trans. Comm., Vol. COM-32, April 1984, page 354; M. Kowatsch:
"Synchronisation in a Spread-Spectrum Communication Modem Based on SAW
Convolvers", IEEE Milcom '84, Los Angeles, October 1984, page 9.5. 1; U.S.
Pat. No. 4 672 658) may be roughly divided into two categories, the
matched filter type and the correlation type receiver. Both processes aim
at highly accurate synchronization auxiliary sequence in the receiver,
with the transmission auxiliary sequence. The main parameter for the first
type is the time x bandwidth product of the matched filter Nowadays,
sufficiently high values can only be achieved with CCD (Charged Coupled
Devices) and SAW (Surface Acoustic Wave) technologies. The advantage of
the matched filter receiver is the rapid synchronization but the great
disadvantage is the limitation of process gain of this technology, i.e.
the limited correlation time and hence the period duration of the
auxiliary function. The SAW technology has the disadvantage of a small
dynamic range due to its high intrinsic losses while CCD are limited in
the clock frequency and digital matched filters are not optimal due to the
amplitude quantization and the chip surface required.
The correlation type receiver, on the other hand, has the disadvantage of a
longer synchronization time, although this is not found to be a
disadvantage in most applications. During the process of synchronization,
the receiver code is shifted continuously or stepwise according to the
time delay to the transmitter and correlated with the received signal
until the maximum of the correlation function has been found, i.e. the
residual shift is smaller than the chip duration. This search process is
dependent upon the length M of the auxiliary function but there are no
restrictions to the period duration of the auxiliary function.
A rough principle of operation is already known from the above literature
by R.C. Dixon. It is also known that the receiver structure can be
extended so that several correlations with displaced auxiliary functions
can take place simultaneously. This leads to somewhat more rapid
acquisition (finding of the synchronous moment in time) and when there are
several propagation paths, these may each be correlated synchronously
independently of one another so that the natural diversity of the
different paths can be utilized. H. Ochsner describes such an architecture
in "An Antimultipath Spread-Spectrum Receiver and its Application to
Portable Radio Telephone", IEEE Globecom '86, Houston, December 1986, page
31.7.1, but without giving any practical details for realizing such a
system. Ochsner also fails to disclose any strategy for initial
synchronization and its maintenance.
SUMMARY OF THE INVENTION
The present invention serves to provide a DS-SS system which can be
realized in practice, having the basic structure of a receiver of the
correlation type for several paths, which system is suitable in particular
for digital data transmission in building and similar local areas for
building up a network with cordless connections (so called cordless LAN).
To solve this problem according to the invention,
(a) exactly one set of several sequence sets of the spreading sequences
used as auxiliary function is associated with each cell and the inverse of
the spreading sequence is used as despreading sequence;
(b) sequences from different sets in pairs have the property that the
absolute values of the normalized periodic cross-correlation functions are
much less than one for all values;
(c) for sequences within a set, the cross-correlation function in pairs is
optimally small around the origin;
(d) a data synchronous generating of the transmission signals takes place
in the base station for all simultaneous connections; and
(e) the data clock of the received signal is used as data clock of the
transmitted signal of the given user station.
The following advantages are obtained from the system parameters and the
properties of the code sequences according to the invention:
Non-synchronous DS interference signals (DS=direct sequence) are
sufficiently suppressed.
Interference signals of DS transmissions within the same cell which arrive
datasynchronously in a receiver are almost completely suppressed.
In the sum of the signals received by the user stations, the signal
components of the various connections of a cell are data synchronous.
At the central station, the offset in the data clock between two DS signals
from the same cell only depends on the difference in propagation time of
the two signals.
Data clock regeneration is reliably carried out by means of special
acquisition and tracking algorithms. The chosen receiver blocks allow for
a wide margin for the choice of system parameters by systematic use of
digital signal processing. Due to this choice of parameters, the SS
technique may be adapted to digital speech transmission as well as to data
transmission with practically any data rate. Compared with conventional
radio transmission systems, the multipath spread is optimally utilized in
buildings or in hilly terrain and there is a limited possibility of
simultaneous operation with existing radio systems in the same frequency
band.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is described below with the aid of an exemplary embodiment
illustrated in the drawings, in which
FIG. 1 is a block circuit diagram of a direct sequence spread spectrum
system (DS-SSS),
FIG. 2 is a diagram to illustrate the functions of the system,
FIG. 3 is a schematic representation of the structure of a microcellular
system,
FIG. 4 is a block circuit diagram of the structure of a cell with possible
interference influences,
FIG. 5 shows a linear feedback shift register for the production of gold
codes,
FIGS. 6a and 6b are each a table of the initial values for optimised code
sets with two particular polynomials,
FIG. 7 is a block circuit diagram of a DS-SS trans mitter,
FIG. 8 is a block circuit diagram of a multipath receiver,
FIG. 9 is a block circuit diagram of the demodulator of the receiver of
FIG. 8 and
FIGS. 10, 11a, 11b, 12, 13, 14 and 15 are graphs to illustrate the
operations.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates the principle of a direct sequence spread spectrum
system (DS-SSS). In the transmitter, the data signal d(t) is modulated in
a modulator 1. The signal s(t) obtainable at the output of the modulator 1
is coded with an auxiliary function or spreading sequence PNs(t) produced
in a code generator 2, i.e. it is multiplied with this auxiliary function
or spreading sequence, and is transmitted to a receiver as data signal
x(t). The receiver decodes the received signal y(t) by means of the same
auxiliary function PNe(t) produced in a local generator 2'. The signal
z(t) obtained from this decoding passes through a band-pass filter 3 and a
demodulator 4 at whose output the data signal d'(t) is obtainable.
The various signals are illustrated schematically in FIG. 2, where the
signal d(t) or d'(t) is shown in line a, the auxiliary function PN(t) in
line b, the signal s(t) or z(t) in line c and the signal x(t) in line d.
The auxiliary function PN(t) are pseudo-random binary code sequences which
can easily be produced, for example by linear feedback shift registers.
Their power spectrum approximately corresponds to a (sin x/x).sup.2 shaped
envelope with a first zero point at the frequency 1/Tc where Tc is the
smallest rectangular impulse duration of the auxiliary function, the so
called chip duration (line b). Tb (line a) is the symbol duration of the
information carrying signal d(t), d'(t), which is chosen to be equal to an
integer multiple of the chip duration Tc. In systems used in practice,
functions of length M chip with a period duration of M.Tc are used
repetitively for PN(t) functions. Typical values for 1/Tc are several 10
MHz and a typical value for M is, for example, 1023, and Tb is frequently
chosen to be equal to M.Tc.
The principle of the cellular structure is know from the mobile radio in
which several mobile users within the network can simultaneously maintain
data connections or digital speech connections with other mobile or fixed
stations. By reducing the size of the cell, such a cellular structure may
be used as a microcellular system, for example in a building or a factory
area.
FIG. 3 shows such a sub-division of the coverage area into microcells,
where on microcell MZ may cover, for example, one story of a building or a
single factory workshop. The system described below is suitable for
operating a cordless LAN within a microcell MZ. The operation of
independent microcells side by side is also provided for by the code
selection. Radio communications beyond the microcells MZ and between them
is not a subject matter of the present patent application. FIG. 4 shows
the structure of a microcell with possible interference influences.
According to FIGS. 3 and 4, each cell MZ contains a number k of mobile
stations MS1 to MSk and a central base station BS. Each mobile station
(the two stations MS1 and MSk are shown schematically in FIG. 4) contains
a transmitter S, a receiver E and a control unit SE. The central base
station BS consists of n transmitter-receivers SE, a switching matrix SM
with a connection, indicated by a double arrow, to external users
connected by lines, a control stage STS for control and synchronization
and an optional service channel transmitter DK. The number k of possible
LAN users in one cell MZ is normally much greater than the number n of
possible simultaneous connections corresponding to the n channels of the
base station BS. The ratio of k to n may be, for example, 200 to 20. The
connections within the cell proceed according to a simple plan, which will
be described hereinafter.
All the connections between the users MS of a cell MZ and the base station
BS use two broad frequency bands separated according to direction. This
means that all transmitters SE of the users MS have a centre frequency of,
for example, 914 MHz and all transmitters of the base station BS have a
frequency of, for example, 959 MHz. The access procedure used is CDMA
(multiple access) so that several radio connections can be effected
simultaneously between the mobile stations and the base station, and these
connections may be simplex, semiduplex or fully duplex.
As indicated in FIG. 4, the connection between a particular mobile station
MS and the base station BS may be impaired by various interferences. Thus
for any particular user, all the other users in the same cell may
constitute interferences, which are determined by the cross-correlation
properties of the codes user. Another disturbing factor is that the
multi-path spread caused by reflection (paths P1, Px in FIG. 4) in
buildings and their surroundings produces frequency selective fading of
the received signal, which may give rise to considerable transmission
faults in narrow band transmission systems. Electrical apparatus AP and
narrow band radio services FD operating in the same frequency band are
also sources of interference.
Successful operation of such a DS-SSS requires careful choice of the
spreading code used and efficient realization of the receiver. The most
important units for the latter are the digital signal processors and a
flexible cell structure can be built up by optimum combination of the
programmable transmitter and receiver blocks. The receiver is a
correlation type receiver, the advantages of which, in particular the
multipath diversity and the free choice of code length and data rate, come
into full effect.
As already mentioned, the data signal in DS modulation is coded with a
spreading sequence and the received signal is decoded by means of the
inverse of the spreading sequence. In a binary sequence with the permitted
states of "+1" and "-1", the despreading sequence and its inverse are
identical. If the despreading sequence is produced in time in the
demodulator such that the effects of coding and decoding virtually cancel
each other at all times, then synchronism is said to exist between the
spreading sequence in the received signal and the despreading sequence
produced in the demodulator. For synchronous demodulation, therefore, the
despread sequence must be the inverse of the spreading sequence and the
time shaft between the sequence in the received signal and the despreading
sequence produced in the receiver must be zero.
If the spreading sequence is repeated periodically, then the above
mentioned time shift may be a multiple of the period duration. Systems in
which the period duration of the spreading sequence is exactly equal to
the data bit duration Tb (FIG. 2) have special properties. Assuming that
the code sequence consists of M chips and under ideal conditions each chip
is represented by a rectangular impulse of duration Tc=Tb/M, the following
conclusions may be made:
(a) For DS signals which arrive data synchronously at the receiver, i.e. in
which the aforesaid time shift delta s of the individual signals at all
times differs only by a multiple of the data bit duration Tb=M.Tc, a
special situation arises if the demodulator code is synchronized to the
code of an individual reception signal. In such a case, the delta s values
of all signals are a multiple of M.Tc and only even and odd correlation
functions with the argument zero appear mathematically. For the wanted
signal, odd and even autocorrelations assume the value M at the zero
point, while for interference signals the odd and even cross-correlation
functions are equal. The code sequences may be optimize in such a manner
that these interference terms are reduced to the minimum.
(b) For incoming DS signals which are not synchronous with the despreading
sequence, i.e. when delta s may assume any value, the odd and even
correlation functions may also assume any values. This means that for
interference signals, the values for cross-correlation function should be
as small as possible while for non-synchronous wanted signals the values
for autocorrelation function should be as small as possible.
On the basis of these two conclusions, the following parameters and
measures for determining the code sequences are proposed for microcellular
systems:
Exactly one set from several sequence sets is allocated to each cell.
Sequences from different sets must in pairs have the property that the
absolute value of the normalized periodic cross-correlation functions is
much less than one for all values. Non-synchronous DS interference signals
are thereby sufficiently suppressed. It follows from this parameter that a
sequence may only belong to one set and that versions of this sequence
with a cyclic shift must not occur in any other set. PN sequences are
types of sequences which satisfy these requirements, e.g. gold sequences
and kasami sequences.
In the sequences within a set, the cross-correlation functions must in
pairs be optimally small about the zero point. Interference signals of DS
connections within the same cell which arrive data synchronously in a
receiver are thereby almost completely suppressed.
At least one sequence with advantageous properties for synchronization is
allocated per set.
In the central base station, the transmission signals are processed data
synchronously for all simultaneous connections so that the signal
components of the various connections of a cell are data synchronized in
the summation signal received by the mobile stations.
The mobile stations employ the data clock of the reception signal as data
clock of their transmission signal, with the result that in the base
station, the offset in the data clock of two DS signals of the same cell
depends only on he propagation time of the signals.
In addition, the base station may be arranged to transmit control signals
for synchronizing the signals of the mobile stations and regulating their
power.
Each cell has one set of n+1 spreading sequences, one of which has a
particularly advantageous shape of its autocorrelation function for
synchronization. This special sequence is used for operating the service
channel DK which forms a simplex connection from the base station BS to
the mobile stations MS and is reserved for the establishment of the
connection. The other n sequences are used exclusively for data
transmission, including signalling between the mobile stations MS and the
base station BS.
A computer search for optimum sequences of a family of god codes for
fulfilling these criteria resulted in a total of 20 sets each with 17
sequences which may be used in a microcellular system. These sequences may
be generated with two linear feedback shift registers SR1 and SR2 of the
type illustrated in FIG. 5. The individual code generators 2, 2' (FIG. 1)
for the 340 sequences differ only in the initial values a1, a2 of the two
shift registers SR1 and SR2. These initial values are given in FIGS. 6a
and 6b for the 340 optimized code sets with the polynomials h1=3025 (octal
8) and h2=2527 (octal 8), for cells 1 to 10 in FIG. 6a and cells 11 to 20
in FIG. 6b. The first sequence of each cell denotes the service channel
sequence. If the 20 possible sequence sets are used again in a cell
situated at some distance, then a cellular system with any number of cells
may be built up by means of these optimized sequences.
To make full use of the advantages of the optimized codes, the code
generators are synchronized in the base station so that the signals of all
n transmitters of the base station arrive data bit synchronously at the
mobile stations and hence the optimized correlation values are obtained.
The mobile station in turn synchronizes its code sequence bit
synchronously to the signal received from the base station. Since the
arrival of the code sequence at the base station is then at the most
delayed by an amount equal to twice the propagation time between base
station and mobile station, the uncertainty of arrival time of the signal
is minimal. At typical distances of up to 50 meters, this corresponds to
only a few code chips. If, the spreading sequence is sent out exactly once
per data bit, and the transmissions of all the channels in operation in
the base station are synchronized, then a channel change (=sequence
change) may take place without synchronization loss and the disadvantage
of the longer synchronization time of the correlation receiver compared
with that of a matched filter receiver is eliminated.
Each mobile station MS in which the transmitter and receiver unit SE (FIG.
4) has a programmable code generator knows all the sequences used for data
transmission and the service channel code in its cell and may produce them
itself. Each mobile station is characterized by a subscriber address which
occurs only once in the system (identification), and the base station
knows all the subscriber addresses of its cell. So long as the subscriber
addresses can be allocated uniquely, any number of mobile stations may be
allotted to a cell. It is only the number of mobile stations in operation
at the same time which is limited, namely to the number n of the channels
of the base station.
All mobile stations which are not actively in a connection are tuned in to
the service channel, which plays an essential role for the establishment
of connections If the base station has not occupied all n data
transmission channels, it communicates the free channels to all mobile
stations on the service channel. When a mobile station is required to be
called from the base station, this also takes place on the service
channel. Since all the transmitters can be programmed for any sequences, a
free transmitter or, optionally, a transmitter specially reserved for this
purpose, may be used for the service channel, as indicated by the service
channel transmitter DK in FIG. 4.
Connections within the cell take place according to the following scheme:
(a) Base station BS calls mobile station MS: BS calls MS with a particular
identification by way of the service channel and informs MS on which
channel (sequence) it expects the reply. The mobile station then has the
possibility of switching over to the required sequence and to start
communication. BS at the same time begins a coarse synchronization in the
corresponding channel (sequence),which can be carried out very rapidly
owing to the small time uncertainty. Data transmission can then begin.
Subsequent information such as signalling, power control, etc. are
transmitted within the stream of data.
(b) Mobile station calls base station: MS learns from the information on
the service channel on which channel (sequence) BS waits for the
establishment of a new connection. It adjusts its code generator to this
sequence and starts the call and at the same time transmits the address of
the desired mobile station. BS then takes up communication with the
desired mobile station and switches the two MS together by means of the
switching matrix. During data transmission, BS monitors the signals
between the two MS in order to separate the connection when the call is
finished. In the meantime, it may itself allow further control data to
flow in, e.g. for power control or synchronization of the MS.
FIG. 7 is a schematic circuit diagram of a spread spectrum transmitter S
which is used both in the base station BS and in the mobile stations MS.
It consists, as shown in the drawing, of the following blocks: coder 5 for
error control, differential coder 6 (both optional),programmable code
generator 7, control unit ST, chip coder 8, modulator 9, HF converter 10
(optional) and antenna 11.
The programmable code generator 7 which is programmed by the control unit
ST produces a certain spreading code from the code set defined above. For
establishment of the connection, this is the service code (service channel
sequence) and the code determined by the base station is subsequently
selected in the base station as well as in the mobile station. The code
sequence is normally produced once per data bit. In the practical
embodiment, the code generator 7 is composed of two feedback shift
registers SR1 and SR2 (FIG. 5) with programmable feedback and initial
value registers. The codes produced are exactly controlled in time by a
synchronization input 12 of the code generator 7 so that the base station
produces all transmission codes in a well defined mutual phase position
and can thus utilize the good cross-correlation properties of the
specially selected codes.
It should be understood at this point that the receiver E (FIG. 4) also has
a code generator. This code generator of the receiver may also be used by
the transmitter S in the mobile stations but if the transmitter has its
own code generator then the transmitter code is synchronized to the
receiver code. This synchronization reduces the time uncertainty for the
arrival of the signal of the mobile station at the base station to a few
code bits (chips), with the result that the search time required for
acquisition is drastically reduced.
The coder 5 for error corrections serves to provide the data which are to
be transmitted with a redundancy for error recognition and/or error
correction, for example with linear block codes or with convolutional
codes. The differential coding by the coder 6 serves to simplify
demodulation. The uncoded or suitably coded data are linked to the
spreading code in the chip coder 8. In the simplest case, this is achieved
by a modulo-2-addition of data and code.
In the modulator 9, the coded data are modulated on a carrier, preferably
by binary phase shift-keying (BPSK), and filtered. The modulated signal is
transferred to the antenna 11 for transmission, either directly or after
conversion into a different frequency band by the HF converter 10. The
modulator 9 of the mobile station may in addition contain a device for
power control which receives control signals through an input 13 from the
control unit ST which converts the control sequences received by the base
station.
FIG. 8 is a block circuit diagram of a correlation receiver according to
the invention, comprising the following units for the base station and for
each mobile station: HF front-end 14, N-channel despreader 15, N-channel
reference code generator 16, N-channel envelope or square law detector 17,
acquisition and tracking circuit 18, L-channel demodulator 19 and an
optional error control decoder 20.
The broad band reception signal reaching the antenna of the receiver by way
of the paths P1 to Px is filtered in a band-pass filter 141 in the HF part
14 to suppress signals outside the frequency band used. The signal is then
amplified and converted to an intermediate frequency IF (mixer 142, filter
143, local oscillator 144, for example with 889 MHz). An automatic gain
control amplifier AGC controls the gain level so that narrow band
interference signals are prevented from saturating the amplifier or over
driving the correlators. The receiving signal lies in two broad frequency
bands with a central frequency of, for example, 914 MHz and 959 MHz. The
converted IF signal at the output of the HF front-end may have a central
frequency of, for example, 70 MHz.
At the despreader stage 15, the IF signal is multiplied with a reference
signal in a mixer 151 and is then band-pass filtered (filter 152). When
synchronism exists between the spreading code in the IF signal and the
code in the reference signal, despreading produces a narrow band signal of
bandwidth about 2/Tb from the broadband input signal of bandwidth about
2/Tc (FIG. 2). The band-pass filter 152 has a programmable bandwidth and
suppresses the interference signals spreaded by the despreading process.
This enables the bandwidth to be kept relatively large during acquisition
to allow for a higher search rate.
The reference code generator 16 can be programmed flexibly and contains a
stage 161 for simultaneously generating the received code for each of the
N channels with any code phase. N versions of the same code shifted in
time in relation to one another are thus produced. Each of the N channels
contains a filter 162, a mixer 163 and an amplifier 164. The delay times
between the individual code versions can be programmed individually for
all the N channels. The resolution with which the shifts in relation to
one another can be adjusted amounts to a fraction, preferably one half, of
a chip. Stage 161 is clocked by a code clock oscillator 166 and the mixer
163 is fed by a local oscillator 165, for example of 80.7 MHz.
The despreaded signals are transmitted both to the envelope or square law
detectors 17 and to the demodulators 19. The signal amplitude at the
outputs of the N correlators (despreaders 15) is measured by the N
detectors independently of the received data and the results are used for
obtaining the control signals required for the acquisition (=coarse
synchronization) and tracking (=fine synchronization) in the circuit 18.
In the acquisition and tracking circuit 18, the N detector signals are
integrated over a time Ti (variable parameter) and converted by
analog-digital conversion and transferred to a digital processor (in
particular a digital signal processor, DSP). The integration time is
controlled by the DSP. This enables the behaviour of the synchronization
circuit to be continuously adapted to the instantaneous signal/ noise
ratio. The control signals for controlling the N-channel reference code
generator 16 are calculated by means of special algorithms and strategies.
The processor at all times knows the delay times of the N channels of the
generator 16 in relation to one another. It can therefore dynamically
adapt the N channels optimally to the multipath profile according to the
nature of this profile. The allocation of the channels may be chosen for
optimum demodulation and code tracking. Additional functions may also be
obtained, e.g. measurement of the signal/noise ratio. This built in
intelligence allows for rapid acquisition of the received code and robust
tracking in the case of multipath spread. In addition, the weighting of
the individual N channels for the selection in the demodulator is
calculated in the processor.
The advantages of the processor (DSP) lie mainly in the optimum utilization
of the information concerning actual propagation conditions since the
transmission channel is constantly measured in real time. In contrast to
analog acquisition and tracking circuits, in which in most cases only
weighted and usually also averaged) sums are used for the control process,
the processor can obtain additional information from the N individual
channels. For example, the delay times between the N channels of the
generator 16 can be programmed in such a way that most of the channels
obtain a very powerful reception signal. In an analog system, only limited
flexibility is obtainable taking into account the circuit complexity
involved. Individual channels may therefore sometime onl | | |
