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Mobile Phone Patent Abstract
The object of the invention is a method to realize discontinuous
transmission (DTX) in a telecommunications network, e.g. in a GSM
(Global System for Mobile Communications) mobile phone network,
particularly on the downlink. The disadvantage of known methods
is that dummy bursts transmitted on the frequency of the broadcasting
channel cause disturbing responses in a mobile phone, when frequency
hopping is used. In the solution according to the invention the
bursts transmitted on the frequency of the broadcasting channel
are encoded so that after decoding the mobile phone prevents information
contained in a dummy burst to be interpreted as speech. In this
way the mobile phone responds with noise on the dummy bursts transmitted
on the broadcasting frequency.
Mobile Phone Patent Claims
We claim:
1. A method to realize discontinuous transmission (DTX) in a mobile
phone system, with a broadcasting channel (BCCH) and communication
channels transmitting speech information, in the transmission of
speech information from a base station to a mobile phone having
receiver circuits, comprising the steps of:
utilizing the broadcasting channel (BCCH) and the frequency of
the broadcasting channel to carry the speech information transmissions
of one or more communication channels by using frequency hopping,
transmitting dummy bursts, comprising frames containing one or
more blocks without speech information, on the frequency of the
broadcasting channel, during the DTX state when speech information
is not being transmitted, and
during the DTX state, encoding a dummy burst that is transmitted
on the frequency of the broadcasting channel in such a way that
the receiver circuits of the mobile phone are controlled to process
as bad frames the received frames which are read from the dummy
burst and comprise one or more blocks.
2. The method according to claim 1, characterized in that, during
discontinuous transmission, one or more blocks of the frame of a
dummy burst transmitted on the frequency of the broadcasting channel
are encoded as a signaling block.
3. The method according to claim 1, characterized in that, during
the DTX state, the second block in the frames of one or more bursts,
which frames have a first block being a Silence Information Description
(SID) block and which frames are transmitted on the frequency of
the broadcasting channel, is encoded as a signaling block.
4. The method according to claim 2, characterized in that said
encoding is made with the aid of a stealing bit in the transmitted
block.
5. The method according to claim 4, characterized in that the stealing
bit is given the value 1 in said encoding.
6. The method according to claim 1, characterized in that, during
the DTX state, the dummy bursts transmitted on the frequency of
the broadcasting channel use a training sequence comprising a bit
stream which is different from the training sequences transmitted
on the communication channels during the normal state.
7. The method according to claim 6, characterized in that the training
sequence of the dummy bursts is selected so that a received frame
is treated as a bad frame based on the result of a cross correlation
determination made in the mobile phone, between on one hand the
received training sequence and on the other hand one or more training
sequences used in the normal state.
8. The method according to claim 6, characterized in that the training
sequence of the dummy bursts is selected so that a received frame
is treated as a bad frame based on the result of a cross correlation
determination made in the mobile phone, between on one hand the
received training sequence and on the other hand a training sequence
made known to the mobile phone.
9. The method according to claim 6, characterized in that the bit
stream used as the training sequence of the dummy burst is selected
so that its cross correlation with one or more training sequences
in use is minimized.
10. The method according to claim 6, characterized in that the
bit sequence (0,1,1,1,0,0,0,1,0,1,1,1,0,0,0,1,0,1,1,1, 0,0,0,1,0,1)
is used as the bit stream of the training sequence of the dummy
burst transmitted on the broadcasting channel (BCCH).
11. The method according to claim 6, characterized in that the
dummy burst transmitted in the DTX state on the frequency of the
broadcasting channel utilizes a training sequence stored in the
receiver circuits of a mobile phone, which is different from the
training sequence used on a communication channel in the normal
state, and the identity of which is transmitted to the mobile phone.
12. The method according to claim 11, characterized in that an
encoding is performed on a burst by which the mobile phone is controlled
to generate comfort noise as a response to the burst when received.
13. The use of the method according to claim 12 in the GSM system.
14. The method according to claim 3, characterized in that said
encoding is made with the aid of a stealing bit in the transmitted
block.
15. The method according to claim 14, characterized in that the
stealing bit is given the value 1 in said encoding.
Mobile Phone Patent Description
The object of the invention is a method to realize discontinuous
transmission n a telecommnunications network. e.g. in a GSM (Global
System for Mobile Communications) mobile phone network. particularly
on the downlink.
The number of communication channels available in mobile phone
systems based on radio technology is limited, and therefore the
sane channels have to be used in several cells of the system. Transmission
activity on the same channel in nearby cells will then cause an
increased interference level.
Advanced mobile phone systems aim to reduce the co-channel interference
and its effect on the communication quality with several methods.
The most important of these are discontinuous transmission and frequency
hopping, which find applications in digital systems.
In discontinuous transmission the transmission activity is decreased
when there is no need to communicate information, or when the information
to be communicated is substantially noise. Because in a telephone
conversation the speech occurs alternately in both directions the
transmission time can be lowered to about the half, which correspondingly
reduces the interference level caused by the transmission activity
on the same channel. When the receiver during transmission does
not receive a signal of good quality it will internally produce
comfort noise. However, interrupted transmission can not be used
on a so called broadcasting channel (BCCH) frequency which is used
for measurements on the transmission path between the base station
and the mobile station, because the transmission power must be constant
on-this frequency. Among other things a mobile station monitors
the power level of signals received from different base stations
by receiving information transmitted on the broadcasting channel.
This information is used to select the base station. Dummy bursts
are transmitted on this frequency when there is no need for information
communication.
In frequency hopping the communication channel transmission frequency
is changed e.g. by pseudorandom sequence whereby the base stations
using the same frequencies have uncorrelated frequency hopping sequences.
Then the effect of serious disturbance sources will be averaged
on several connections.
The interference level can be very effectively reduced by using
both above mentioned methods simultaneously. However, technical
problems relate to the implementation of this, particularly when
the channels utilizing frequency hopping also utilize the frequency
of the broadcasting channel. Then also during discontinuous transmission
dummy bursts are transmitted on the transmission channel, when the
turn has come to use the frequency of the broadcasting channel.
When the mobile station receives the respective dummy bursts it
may interpret them to be normal bursts carrying speech information
and then the mobile phone's response is a faulty signal instead
of the comfort noise, which is heard as a disturbing sound. Due
to this problem it has not been common to have information communication
with discontinuous transmission on the downlink, i.e. from the base
station to the mobile phone.
The object of the invention is to create a method to realize discontinuous
transmission where the above disadvantages are solved. The method
according to the invention is characterized in what is presented
in the characterizing clause of claim 1. Preferred embodiments of
the invention are presented in the dependent claims.
The invention is described below with the aid of the enclosed drawings,
in which:
FIG. 1 shows the structure of the TDMA frame of the GSM system;
FIG. 2 shows bursts transmitted in the DTX mode, when frequency
hopping is not used;
FIG. 3 shows bursts transmitted in the DTX mode, when frequency
hopping is used;
FIG. 4 shows a method according to the invention in the form of
a flow diagram;
FIG. 5 shows a means to encode the burst according to the invention
in the form of a flow diagram; and
FIG. 6 shows an alternative embodiment of the means to encode the
burst according to the invention in the form of a flow diagram.
The following notations are used in the figures:
______________________________________ T tail D data block S stealing
bit TS training sequence G guard period I idle frame A SACCH signaling
channel VAD voice activity detection DTX-B burst encoding according
to the invention. ______________________________________
The invention is described in further detail by examples, in which
the invention is applied to the GSM system. Therefore we below first
consider the frame structure of the GSM system and the realization
of discontinuous transmission and frequency hopping in the realized
system. The GSM system is further described i.a. in the following
publications: M. Mouly, M-B. Pautet: The GSM System for Mobile Communications,
1992; and British Telecom Technology Journal, vol 8, no 1, January
1990, M. R. L. Hodges "The GSM Radio Interface", p. 31-43.
The GSM system is a Time Division Multiple Access System (TDMA)
which utilizes several transmission frequencies. FIG. 1 shows the
frame structure used in the system. One TDMA frame has eight time
slots, and one transmission channel burst can be transmitted in
each time slot. A so called normal burst comprises two blocks of
58 bits for the transmission of information. One bit of each block,
the so called "stealing bit" indicates whether the block
carries data or signaling information. In addition to the above
blocks there is in the middle of the burst a training sequence of
26 bits, and at both ends of the burst there are 3 tail bits. Besides
there is between two bursts a guard period which corresponds to
a period of 8.25 bits.
The speech signal is divided in speech frames comprising eight
blocks of 57 bits. These speech frames are transmitted in an interleaved
form, so that each burst contains one block from two consecutive
speech frames. Thus one speech frame is transmitted in eight consecutive
bursts. The first four of these bursts thus contain also data from
the prior speech frame, and the last four also contain data of the
next speech frame. It is to be noted that the bits of two data blocks
to be transmitted are interleaved on both sides of the training
sequence. Of course the speech frame can also contain other information
than speech. Below a "frame" and a "speech frame"
refer to this frame of transmitted information, and a frame comprising
time slots of different transmission channels is called a "TDMA-frame".
The training sequence is used to measure the response of the channel
and to adapt the receiver to channel burst by burst. The GSM system
defines eight different training sequences. They have good autocorrelation
characteristics, so that the channel response can be defined by
correlation measurement only. In addition the training sequences
have a low cross correlation, so that there should be low interference
of the same channel in a synchronized network. The training sequences
to be used are permanently programmed in the mobile phone, and the
base station informs the mobile phone of the identity of the respective
training sequence to be used.
Discontinuous transmission in the GSM system is defined as follows.
Voice Activity Detection VAD at the transmitter distinguishes the
speech and pause parts of a connection. No speech signal is transmitted
during a pause, and below this state is called the "DTX-state"
(Discontinuous Transmission). Correspondingly the state in which
the speech signal is transmitted will be called the "normal
state" in the following.
When the receiver detects that the transmitter has not transmitted
speech frames it marks the frame as "bad", which is called
the BFI function (Bad Frame Indication). The receiver generates
noise during this period, so that an interrupted input signal would
not be heard as a broken connection in the receiver. During the
DTX state the transmitter transmits instead of speech the parameters
describing the nature of the noise. However, the transmission of
these parameters requires only a negligible amount of data, and
thus a short transmission time compared to speech transmission.
The noise parameters are transmitted in so called SID frames (Silence
Information Description).
Below we consider the DTX state when the broadcasting channel frequency
is not used. When the transmitter enters the DTX state it transmits
a SID frame after the last speech frame, and thereafter regularly
one SID frame at intervals of 480 ms. FIG. 2 shows this multiframe
comprising 104 TDMA frames and having a duration of 480 ms. The
signaling channel SACCH (Slow Associated Control Channel) remains
active also in the DTX state. One SACCH frame is transmitted during
said period of 480 ms, and it uses four time slots A. The continuing
DTX state is thus periodic regarding the use of time slots, and
it contains always four active SACCH bursts and the SID frame transmitted
in eight bursts. No other bursts are transmitted. On the other hand
in the normal state one time slot is transmitted for each TDNA frame,
with the exception of the so called idle frames I.
The information of the speech frame is diagonally interleaved in
eight time slots, so a new speech frame begins at every fourth TDMA
frame, and each speech frame is transmitted in eight time slots.
Thus one burst will always carry data of two speech frames. The
receiver must distinguish the frames which were not transmitted
in the DTX state and mark them as bad frames (BFI; Bad Frame Indication).
Then the speech encoder generates background noise during the DTX
state. There are several means with which the receiver can identify
bad frames. The most common way is to use burst quality measurement
along with other means.
According to one embodiment the receiver separately measures the
quality of speech frame halves, the so called half-frames. If either
half-frame has at least one block which is read from a burst with
good quality, then the receiver usually interprets the speech frame
obtained in this way as a good frame and reproduces it.
In the situation shown in FIG. 2 the speech frames, all blocks
of which would be included in the TDMA frames 0 to 50 and 60 to
102, are not transmitted at all in the DTX state. Thus the receiver
interprets all these speech frames as bad frames and replaces them
with noise.
The speech frame before the SID frame comprises four time slots
during which no burst is transmitted, and four time slots which
transmit the information of the SID frame. The speech frame after
the SID frame corresponds to the speech frame before the SID frame.
Thus the receiver must also mark these incomplete frames as bad
frames.
Let us now consider the speech frame of FIG. 2, which would be
transmitted in the TDMA frames 47 to 55. SID information is transmitted
in the TDMA frames 52 to 55, so the respective bursts are transmitted.
Thus the receiver of the later half-frame of the speech frame could
falsely interpret a bad frame as a good frame. Bursts corresponding
to the first half of the speech frame will however not be transmitted
at all, so this half-frame is determined as a bad one. Because the
first half-frame of this speech frame is bad, and the second half-frame
is good, the receiver will interpret the whole speech frame as bad.
Correspondingly the speech frame which would be transmitted in the
TDMA frames 56 to 63 comprises a first half-frame, which is determined
as a good frame, and a second half-frame which is determined as
a bad one. The signaling frames A have no effect on the DTX function.
In the situation shown in FIG. 2 the receiver will thus process
all speech frames as bad frames.
FIG. 3 shows a situation where the discontinuous transmission is
used on the downlink, and frequency hopping is used on the communication
channel so that one of the used frequencies is the broadcasting
channel frequency. Three frequencies are used in the situation shown
in the figure, and the frequency is regularly changed, so that three
consecutive time slots always correspond to three different frequencies.
The time slots falling on the broadcasting channel are darkened.
When discontinuous transmission is used in combination with frequency
hopping the base station transmits on the BCCH carrier dummy bursts
in the DTX state. Let's again consider the speech frame, which in
the normal state would be transmitted in the TDMA frames 47 to 55.
The bursts are transmitted in the time slots of the TDMA frames
52 to 55, because they carry the SID information. Thus the second
half of the speech frame is determined as a good one in the mobile
phone. The first half-frame transmitted in the TDMA frames 47 to
50 contains the time slot transmitted in the TDMA frame 48, in which
a dummy burst is transmitted, because the time slot occurs at the
frequency of the broadcasting channel. The receiver determines this
burst as a good one, and because the first half-frame thus contains
one block determined as a good one the mobile phone could determine
also the first half-frame as a good one. Thus it is possible to
interpret the whole speech frame as a good one, even when the blocks
transmitted in the dummy bursts do not contain real information.
Correspondingly a speech frame which in the normal state would
be transmitted in the TDMA frames 56 to 63 would be interpreted
as a good one, because the two blocks of the second half-frame (the
TDMA frames 60 and 63) are included in the dummy bursts transmitted
on the frequency of the broadcasting channel.
Further a speech frame which in the normal state would be transmitted
in the TDMA frames 0 to 7 comprises a first half-frame, in which
the dummy bursts are transmitted in the time slots of the TDMA frames
0 and 3, and a second half-frame, in which a dummy burst is transmitted
in the time slots of the TDMA frame 6. Thus also this speech frame
could be falsely interpreted as a good frame in the mobile phone.
The half-frames of the speech frames contain in the above described
case always 1 to 4 bursts which do no contain information, but which
have a good signal quality. When a pseudo-random hopping sequence
is used even all blocks of a whole speech frame could be transmitted,
though they do not contain speech. In such cases the bad frame identification
based on the burst quality measurement will not work in a mobile
phone. This fault in the BFI function causes a response of bad quality
in the mobile phone because several speech frames will pass the
Cyclic Redundancy Check CRC even when they do not contain speech.
The bad frame indication base on the burst quality measurement could
be different from the above presented solution, but a corresponding
problem occurs also with other known alternative solutions.
The fundamental idea of the invention is that the dummy bursts
transmitted on the frequency of the broadcasting channel are encoded
so that the mobile phone is controlled to process the frames read
out from the dummy bursts as bad frames, whereby we can avoid to
reproduce faulty data as speech frames containing information.
FIG. 4 shows encoding of a TDMA frame when using the method according
to the invention. If the base station uses discontinuous transmission
on the downlink the voice activity detection VAD measures to know
whether the transmitted information is speech. The burst is encoded
as in the normal state and then transmitted if the information is
speech, or if no discontinuous transmission is used, or if the time
slot belongs to a BCCH carrier. A switch to the DTX state is made
if the VAD detects a speech pause. Then the encoding is made based
on the knowledge whether the transmitted time slot is on the frequency
of the broadcasting channel. If the time slot is not on the frequency
of the broadcasting channel then the burst is transmitted only if
it contains a SID block, otherwise the time slot is neglected. If
the time slot is on the frequency of the broadcasting channel the
burst is encoded according to the invention and transmitted. The
encoding according to the invention is called DTX-B.
FIG. 5 shows a burst encoding embodiment of the invention. Here
the dummy burst transmitted on the frequency of the broadcasting
channel use a training sequence which differs from the training
sequence used in the normal state. Then the mobile phone interprets
a received block as a bad one on the basis of the correlation measurements
made in the mobile phone. The new training sequence is advantageously
selected so that its cross correlation with the training sequence
used in normal situations is as low as possible. Then we obtain
the maximum probability for the mobile phone to interpret the received
block as a bad one. Of course a normal training sequence is used
in those bursts which transmit blocks of the SID frame.
It is particularly advantageous to select the training sequence
used in the dummy burst so that its cross correlation characteristics
are very good compared to all the training sequences used in the
system's normal state. Then one training sequence is sufficient
for this purpose, and it can be used in the dummy bursts of all
communication channels. When we select the bit sequence of the training
sequence so that its cross correlation characteristics are as good
with all training sequences as the cross correlation between all
training sequences used earlier, then we obtain on the same channel
an interference level which is as low as with other training sequences.
In the GSM system we can use as the training sequence for instance
the bit sequence: (BN61, BN62, . . . BN 86)=(0,1,1,1,0,0,0,1,0,1,1,1,0,0,0,1,0,1,1,1,0,0,0,1,0,1)
where BN (Bit Number) is the consecutive number of the bit in the
burst.
Alternatively, for the training sequence of the dummy burst we
can select another training sequence defamed in the system so that
this training sequence is not the same as is used on the respective
communication channel. Then the base station indicates to the mobile
phone the identity of the training sequence to be used in the normal
state, but uses another training sequence in the dummy bursts on
the frequency of the broadcasting channel. Because the mobile phone
performs a correlation measurement on the received signal related
to the training sequence indicated to it, also this case results
in a low correlation, and the received burst is interpreted to be
bad.
FIG. 6 shows another embodiment of the encoding according to the
invention. Here the parts of the speech frames transmitted during
the DTX state are marked as signaling with the aid of a stealing
bit. In the DTX state this stealing bit is set to the signaling
state (value is 1 in the GSM system) for the speech frame blocks
of bursts transmitted in connection with SID frames and for the
dummy bursts transmitted on the frequency of the broadcasting channel.
In the SID frame blocks the stealing bit is of course set to the
data communication state (value is 0).
When a stealing bit is used all bursts containing real signal power
during the DTX state are directed to the signal information receiving
branch of the telephone. Because the telephone uses a powerful error
check in the reception of signaling frames the frames directed to
the signaling channel will be rejected, and thus they will cause
no signaling errors. Interference is however avoided during discontinuous
transmission, because the frames directed to the signaling are treated
in the speech decoding like frames marked bad (BFI), and thus the
received frames are prevented from being reproduced as speech frames
containing information.
The method according to the invention provides considerable advantages
over the prior art. With the method we can avoid to reproduce faulty
data as speech frames in the mobile phone when discontinuous transmission
is used. Even when the frequency of the broadcasting channel is
used in the DTX state the mobile phone interprets the frames received
from the dumuny bursts as bad and produces noise as a response to
them.
This solution can be applied without any considerable modifications
in the existing mobile phone population. Thus the solution provides
a possibility to use discontinuous transmission on the downlink
in existing systems. Thus interference on the same channel can be
reduced in the system, and the communication channels can be utilized
more effectively.
Above we have presented some applications of the method according
to the invention. Naturally the inventive principle may be varied
within the scope of the claims, e.g. regarding the details of the
realization and the application area.
The invention is not limited to the GSM system, but it can also
find applications in other telecommunications systems. The method
is further well suited be used in the transmission of other information
in addition to speech. Correspondingly, here "data" has
been used to mean any kind of information transmitted in digital
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