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Claims  |
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We claim:
1. A digital cellular system, comprising at least one base station for
communication with subscriber equipment, and in which between a base
station that includes a transmitter and a receiver and a subscriber
equipment that also includes a transmitter and a receiver a radio channel
can be established for the transmission of a transmission burst by at
least one of said base station and said subscriber equipment, in which
radio channel a training sequence having a recognizable symbol sequence is
included in a transmission burst transmitted by said at least one of said
base station and said subscriber equipment, said recognizable symbol
sequence comprising a plurality of symbols for expressing a reference part
and at least one additional part, whereby a receiver of at least one of
said base station and said subscriber equipment can adapt itself to the
radio channel according to an impulse response measured in the radio
channel in accordance with the received training sequence, characterized
in that the at least one of said reference part and said other part of
said recognizable symbol sequence are not comprised, for all transmission
bursts, of a constant number of said plurality of symbols.
2. A digital cellular system according to claim 1, characterized in that in
the system a total number of symbols that comprise the training sequence
is constant.
3. A digital cellular system according to claim 2, characterized in that
when it is determined that an impulse response of the radio channel is
long, the number of symbols that comprise the other part is increased and
the number of symbols that comprise the reference part is decreased.
4. A digital cellular system according to claim 2, characterized in that
when it is determined that at least one of an impulse response of the
radio channel is short and a signal/noise ratio of the radio channel has
decreased, the number of symbols that comprise the reference part is
increased and the number of symbols that comprise the other part is
decreased.
5. A digital cellular system according to claim 1, characterized in that a
total number of symbols that comprise the training sequence is not
constant.
6. A digital cellular system according to claim 5, characterized in that a
total number of symbols that comprise the training sequence is increased
by increasing a number of symbols that comprise said other part when it is
determined that an impulse response of the radio channel has increased.
7. A digital cellular system according to claim 5, characterized in that a
total number of symbols that comprise the training sequence is increased
by increasing a number of symbols that comprise said reference part when
it is determined that a signal/noise ratio of the radio channel has
decreased.
8. A digital cellular system according to claim 5, characterized in that a
total number of bits that comprise said training sequence is decreased by
decreasing a number of bits that comprise said reference part when a
signal/noise ratio of the radio channel is determined to have increased,
whereby in a connection between the base station and the subscriber
equipment a total number of information bits of a transmission burst may
be increased by an amount equal to the number of bits by which said
reference part is decreased.
9. A digital cellular system according to claim 5, characterized in that a
total number of bits that comprise said training sequence is decreased by
decreasing a number of bits that comprise said other part when an impulse
response of the radio channel is determined to have decreased, whereby in
a connection between the base station and the subscriber equipment a total
number of information bits may be increased by an amount equal to the
number of bits by which said other part is decreased.
10. A digital cellular system according to claim 9, characterized in that
if a number of bits of said training sequence are decreased to zero, said
training sequence is not transmitted.
11. A digital cellular system according to claim 1, characterized in that
in a cell that is associated with a base station additional parts of all
training sequences are mutually of the same length in each radio channel
between the base station and subscriber equipments, and respectively,
reference parts are also of equal length.
12. A digital cellular system according to claim 1, characterized in that
both a number of bits that comprise said other part and a number of bits
that comprise said reference part are determined on a connection basis.
13. A digital cellular system according to claim 1, characterized in that
the training sequence is transmitted less frequently than in each
transmission burst.
14. A method for operating a digital cellular system of a type that
includes a base station in bidirectional wireless communications with a
user terminal through a communications channel, comprising the steps of:
transmitting a burst of data by at least one of the base station and the
user terminal between the base station and the user terminal, the burst
including an information data portion and a training data portion, the
training data portion being used to compensate a receiver of the burst for
an impairment in the communications channel;
wherein the training data portion includes at least a first sequence of
bits and at least one second sequence of bits; and wherein
the step of transmitting includes a step of adaptively varying at least one
of a number of bits of the first bit sequence and a number of bits of the
at least one second bit sequence in accordance with a degree of impairment
of the communications channel.
15. A method as set forth in claim 14 wherein the first sequence of bits
represents a reference bit sequence and wherein the at least one second
bit sequence represents a guard bit sequence.
16. A method as set forth in claim 14 wherein the step of adaptively
varying maintains a total number of bits of the training data portion
equal to a predetermined, constant value.
17. A method as set forth in claim 14 wherein the step of adaptively
varying operates so as to vary a total number of bits of the training data
portion.
18. A method for operating a digital cellular system of a type that
includes a base station in bidirectional wireless communications with a
user terminal through a communications channel, comprising the steps of:
transmitting a burst of data by at least one of the base station and the
user terminal between the base station and the user terminal, the burst
including an information data portion and a training data portion, the
training data portion being used to compensate a receiver of the burst for
an impairment in the communications channel;
wherein the training data portion includes at least a reference sequence of
symbols and at least one guard sequence of symbols; and wherein
the step of transmitting includes a step of adaptively varying at least one
of a number of symbols of the reference symbol sequence and a number of
symbols of the at least one guard symbol sequence in accordance with a
degree of impairment of the communications channel.
19. A method as set forth in claim 18 wherein the step of adaptively
varying operates so as to maintain a total number of symbols of the
training data portion equal to a predetermined, constant value.
20. A method as set forth in claim 18 wherein the step of adaptively
varying operates so as to increase or decrease a total number of symbols
of the training data portion, and further comprising a step of
proportionately decreasing or increasing a number of symbols of the
information data portion. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention relates to a digital cellular radio telephone system
provided with at least one base station and a plurality of subscriber
apparatus, and in which in a radio channel between the base station and a
subscriber apparatus a training sequence is transmitted in a transmission
burst, according to which the receiver fits the channel equalizer into the
radio channel.
BACKGROUND OF THE INVENTION
Many information transmission problems in the radio phone system involve
time-variant or statistical sources of signal degradation in a radio
channel. An advantage of the digital radio telephone system compared with
the analog one is that it can be designed to monitor the channel and be
adapted to the changes thereof. Any transmission channel, be it a
transmission line or a radio channel, affects the amplitude of the
waveform, frequency or phase of the signal, thus producing intersymbol
inteference among the bit pulses. In a mobile station, like a radio phone
in a car, the characteristics of the channel change constantly over time.
A general solution in digital cellular systems known in the art is to use
adaptive channel correction. This means that certain distortion
characteristics of a channel are measured periodically or continuously,
and the predicted distortions in the transmitted pulses are subtracted
from the received waveform. The system is capable of monitoring the
quality of the channel by measuring the bit error ratio and/or other
parameters, such as signal strength and delays.
The subscriber device of the cellular system may mean, depending on the
system, a so-called mobile station, i.e. mobile communicating equipment,
between the antenna and the base station whereof being provided a radio
channel, or it may refer to a phone which is by means of a wire connection
connected with a transmitter/receiver at a remote distance, between the
antenna whereof and the antenna of the base station being provided a radio
channel. Reference is made below primarily to a mobile communicating
equipment, though it is useful to note that the same features are
applicable to the subscriber device of the latter definition as well. The
signal strength and the delay are bound to the signal propagation distance
between the base station and the mobile station. As is well known in the
art, the transmission rate is high because of the TDMA-transmission used
in digital systems so that the multiple-path propagation characteristic of
the radio path is visible in the reception not only in the form of rapid
so-called Rayleigh fading of the envelope of the RF signal but also as an
intersymbol interference between the detected bits. In view of the
intersymbol interference, the signal propagation model has been so
expanded in the digital systems that a received signal is no longer an
individual Rayleigh-faded signal but a sum of independently
Rayleigh-fading signals and signals including a different delay.
The impulse response of a radio channel can be illustrated in the time
domain by means of tap presentation as shown in FIG. 1. Therein, the
height of an individual tap illustrates the average strength of a
Rayleigh-faded signal and the location of the tap illustrates the
transmission delay. The distribution of the taps is dependent on the power
levels used and the environment conditions, and the fading frequency of
the taps is dependent on the speed of the mobile station, e.g. of a car.
In various systems, some of such propagation models have been defined in
order to illustrate various environments and vehicle velocities.
On the basis of what is said in the foregoing it is obvious that since the
radio channel changes rapidly, the intersymbol interference of the
detected bits caused by signal transmission across the radio channel must
be corrected by measuring the impulse response of the channel and by
adapting the receiver to the tap configuration of the channel. This is
usually carried out in the systems so that the base station or the mobile
communicating equipment transmits a known bit configuration in the
transmission burst thereof, i.e. a constant-length sequence of consecutive
bits. The sequence is called a training sequence. The receiver has earlier
received an encoded piece of information about what kind of bit pattern,
that is training sequence, will be transmitted. The receiver correlates
with the prior art training sequence corresponding to the training
sequence it received and equivalent to the encoded data accessed from the
memory. As a result of the correlation, an estimate on the radio path
(i.e. delay) is received and the receiver sets the channel equalizer so
that the delay distributions are corrected on a given length. For
instance, in the GSM system the delay distributions are corrected up to 16
.mu.s.
One TDMA frame, for instance in the GSM system, comprises eight time
intervals. The signal is transmitted in the form of bursts, of which a
so-called standard burst is shown in FIG. 2. It consists of first three
tail bits, whereafter 58 data bits follow, said bits containing data or
speech. They are followed by a training sequence of length of 26 bits,
then again followed by 58 data bits, and finally, by three tail bits.
Between the time intervals of the frame a 8.25 sec a guard period is
provided. As shown in the figure, the training sequence is located in the
middle of a burst as a uniform sequence, its constant length being 26
bits. Eight training sequences differing in bit configuration are
provided, and pre-information has been transmitted to the phone about the
type of training sequence to be transmitted by the base station.
The training sequence need not be located in the middle of a burst.
Therefore, in a digital radio phone system used in the U.S.A. a frame
consists of six time intervals, each containing 162 symbols. One symbol
may comprise 2 bits, as in the QPSK modulation used in said system, or
even more bits, depending on the modulation system. In a burst to be
transmitted from a base station to a mobile station, a transmission time
interval always contains first a 14 symbol synchronization burst used as a
training sequence. Let it be noted that the length of a training sequence
is constant. In said system six different sequences of training sequences
are provided.
Let it be noted that the training sequence is transmitted both from the
subscriber device to a base station (Up Link) and from the base station to
the subscriber device (Down Link). The symbol sequences of the training
sequences need not necessarily be the same in both directions. Whatever
the system, endearours are made to provide such sequences of training
sequences that they are provided with as good autocorrelation properties
as possible, i.e. on both sides of a peak in the middle of an
autocorrelation function a sufficient amount of zeroes are provided. A
given training sequence is appropriate for a given environment. For
instance in city areas the multiple path propagation of a signal is
dominating, and the training sequence can be therefore different from that
in the countryside where few obstacles causing signal reflections exist.
In systems currently used the length of a training sequence is the
constant length typical of the system and it has been selected according
to the so-called worst case, whereby it is necessary to be prepared to
correct the delay distortion time-wise on a long distance and it is
assumed that the impulse response of the channel is multiple-tap type.
FIG. 3 shows the design of a typical training sequence. The example is
selected from the GSM system. The training sequence comprises a reference
part, on both sides whereof being an additional part. The length of the
reference part is 16 bits, and the length of each guard part is 5 bits.
The shape of a training sequence ie thus 5+16+5. FIG. 4 presents the bit
sequences included in the training sequences used. As mentioned above, the
sequences have been so selected that they are provided with good
autocorrelation properties. The length of the guard part determines how
long impulse response in said training sequence is estimatable. In the
present training sequence a six-tap impulse response can be estimated. The
length of the additional part in GSM has been selected to conform to the
worst instance, i.e. training sequences similar in configuration are used
all over the network, although not all six taps need to be estimated: if
the delay distribution is small, as it is in the countryside, estimation
of only a few taps would be enough.
The guard part need not be located on both sides of the reference part,
such as in the GSM system, instead, it can be only one guard part which is
located before or after the reference part. In practice, the guard part is
so produced that the first and/or last symbols are selected for the
symbols thereof.
The length of the additional parts of the training sequence and the
reference part has an essential significance: the longer the reference
part (the more bits or symbols), the better channel estimate is obtained
because when using a long reference part the noise becomes averaged, thus
not distorting the result. On the other hand, the longer is the guard part
(as symbols or bits), the longer bit distributions can be measured. Now,
reservations have been made e.g. in the GSM system against the most
difficult instance by setting 5 bits for the length of the additional
part, whereby a six-tap impulse response can be estimated.
Setting the reference part and the guard part fixed in length involves
certain drawbacks. If the multipath propagation is insignificant, i.e. the
impulse response of the channel is short in duration, it is of no use to
utilize a long additional part, and instead, a long reference part would
be preferred, whereby a better estimate of the radio channel than today
could be obtained. Thus, in areas where no obstacles exist to a disturbing
degree, a good quality of the connection could be provided also in long
distances. On the other hand, in areas where the multiple-path propagation
is dominant, it would be better to use as long as additional part as
possible, whereby a multiple-tap impulse response would be provided and a
channel equalizer can be disposed to correct the delay distribution
time-wise on a great distance. In favourable conditions the length of the
entire training sequence need not be very long. Now, capacity would be
released in the burst to transfer more speech and data information.
SUMMARY OF THE INVENTION
As taught by the invention, the drawbacks mentioned above can be avoided by
utilizing in the cellular system the training sequence which is defined in
the accompanying claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of this invention are made more apparent in the ensuing
Detailed Description of the Invention when read in conjunction with the
attached drawings, wherein:
FIG. 1 illustrates the impulse response of a received radio channel in the
time domain, in a so-called conventional tap-type presentation;
FIG. 2 depicts a conventional standard burst in the GSM system;
FIG. 3 illustrates one typical and conventional structure of a training
sequence;
FIG. 4. illustrates bit sequences of a training sequence in the
conventional GSM system; and
FIG. 5 illustrates several examples of adaptive sequences in accordance
with the teaching of this invention.
DETAILED DESCRIPTION OF THE INVENTION
The invention is based on the aspect that since the delay distribution of a
channel varies significantly from location to location, it is useful to
make the training sequence adaptive. It can be embodied at least in two
ways.
According to a first embodiment, the training sequence can be of a fixed
length while the lengths of the guard part and the reference part vary
from situation to situation. Hereby, when the impulse response of a
channel is short, so that the guard part required is also short, or, along
with the deteriorating signal--noise ratio of the channel, the reference
part can be lengthened at the cost of the additional part, whereby a
better estimate about the radio channel can be made. Respectively, if the
impulse response of the channel in the area of a cell is long or if it
becomes longer, the guard part is lengthened at the cost of the reference
part. An advantage of a training sequence of a fixed length is that the
burst remains constant in length and shape. Let us assume that the length
of a training sequence is the constant 30 symbols. A sequence could be of
form 7+16+7 (i.e. guard part+reference part+additional part) when the
impulse response of the channel is long. With such configuration, not more
than eight taps can be estimated. When the impulse response of the channel
is shorter, the training sequence could be 5+20+5 or 3+24+3, or even
1+28+1 in form, depending on the number of bits required. In the table of
FIG. 5 more examples are presented on adaptive sequences. The "exemplary
sequence" on the left in the table is used e.g. when the impulse response
is long, and the "adaptive" sequence can be transmitted when the impulse
response is short, so that the length of the reference part can be
increased at the cost of the additional part, and a better estimate on the
channel can be provided. The binary sequences presented here are merely
exemplary in character. According to the second embodiment, the total
length of a training sequence is not constant but varies. Now, in
situations in which the impulse response ie short or it becomes shorter,
and a good channel estimate with a short guard part can be produced, the
guard part can be reduced while the reference part remains the same in
length. The length of a training sequence becomes shorter, thus, the
symbols thereof can be transferred into other use, for instance for
speech/data transfer of the user, or for transferring of signalling data.
Similarly, in adverse situations in which the impulse response of the
channel is very long or it becomes longer, the training sequence can be
lengthened by lengthening the additional part, whereby by means of the
long additional part, the long delay distribution can be taken into
consideration. If no essential changes occur in the impulse response of
the channel, the signal/noise ratio of the channel gets worse, the length
of the reference part ie increased so that a better estimate on the
channel can be made and the signal/noise ratio is enhanced. Respectively,
together with the improved signal/noise ratio, the reference part can be
shortened. In said two last-mentioned instances, the length of the guard
part will not be changed, but along with the changing length of the
reference part the total length of the training sequence changes.
The variation of the total length of the training sequence according to
said second embodiment can be implemented in the systems known in the art,
e.g. by varying the length of the time interval in which the training
sequence ie transmitted, whereby an increase in the length of the training
sequence is not carried out at the cost of the rest of symbols of the
transmission burst. A second embodiment is such that the training sequence
can be extended to the range reserved for the speech/data symbols of the
data field, provided there is room for them at that moment. Also the
modulation system utilized has an influence on how many bits can be
transmitted in a given number of symbols; therefore, when the modulation
method is changed, also the number of the bits available for a training
sequence is also affected.
Both of the embodiments also include such a possibility that the training
sequence can be transmitted only every now and then, not in each frame.
Hereby, instead of transmitting the symbols of a training sequence, user
information (speech/data) or system data can be transmitted. The second
embodiment makes the possibility feasible that the training sequence can
be reduced to zero, whereby it will not be transmitted.
Various ways for transmitting an adaptive training sequence can be adopted.
First, the operator can measure the environment of the cell and define
what kind of training sequence is most appropriate and use it. When a
connection is produced between a base station and a mobile station, the
base station transmits information, by using e.g. some method known in the
art, about which type of training sequence is in use for said training
sequence. The training sequence information may also not be transmitted,
whereby the mobile station tests what kind of training sequence is most
appropriate for the received training sequence, and makes a decision of
using it. The same training sequence is in use in the cell area. Secondly,
a connection-specific training sequence can be used. It can be implemented
so that right at the beginning of a connection a long training sequence is
used (according to the worst instance). Thereafter, the base station
changes the training sequence by changing the length of the reference
part, the length of the additional part, the total length of the training
sequence, or a combination thereof. Information about a change of the
training sequence is transmitted to the mobile station. Thus, the base
station selects the best training sequence available for said connection,
and from that moment onwards, the most appropriate training sequence is
used. In practice, such method may, for instance, be adopted that an
appropriate length of the guard part is concluded e.g. on the basis of the
impulse response of the channel measured by the subscriber device. When
the training sequence is transmitted from a base station to a subscriber
device, the subscriber device measures the impulse response of the
channel. If it finds out that the guard part is unneccessarily long or too
short, it informs the base station thereof, whereby the training sequence
is equally changed.
The aim of the adaptive training sequence described above is to provide as
good channel estimate as possible for the radio link to be used, but the
invention may equally be used for achieving a good synchronization,
because if the mobile station is adapted as well as possible into the
channel, also a good synchronization is achieved, in addition to a
correction of the delay distribution and the correct channel estimation.
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Description |
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