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Claims  |
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What is claimed is:
1. A method for deciding when to handing-over a radio connection when a
mobile radio station (MS) moves from one radio cell into another radio
cell in a digital radio transmission system with frequency-division
multiplexing of adjacent radio cells and wherein co-channel radio cells
are separated by different spread codes, comprising the steps of the
reception quality criteria measurements to be effected in the mobile
station (MS) for assigning itself one of the radio cells and consequently
for the hand-over decision are effected such that additionally during the
existence of a radio connection to a near base station (BS) the reception
quality of the co-channel message transmission channels are measured in
the mobile radio station (MS).
2. A method as claimed in claim 1, characterized in that the spread codes
used by remote base stations (BS) for co-channel message transmission
channels are transmitted via the message transmission channel by the near
base station (BS) maintaining the connection, to the mobile station (MS).
3. A method as claimed in claim 1, characterized in that the results of the
measurements of the reception quality of the co-channel message
transmission channels effected in the mobile station (MS) are transmitted
by the mobile station (MS) via the message transmission channel to the
near base station (BS) maintaining the connection.
4. A method as claimed in claim 3, characterized in that the near base
station (BS) maintaining the connection stores the received results and on
the basis of the results stored in the store takes the decision if and to
which of the adjacent base station (BS) hand-over of the radio connection
is to be effected.
5. A method as claimed in claim 1, characterized in that the spread codes
used for the co-channel message transmission channels of the remote base
stations (BS) for the purposes of synchronization and multi-path
resolution differ from each other and are also used in the mobile radio
station (MS) for measuring the reception quality.
6. A method as claimed in claim 5, characterized in that a signal
characterizing the multi-path profile at the output of a correlator (K)
which is included in the mobile station (MS) and is adjusted to the spread
code to be evaluated by means of a control arrangement (ST) and on the
basis of a spread code stored in a store (SPC), is used as the reception
criterion.
7. A circuit arrangement for performing the method as claimed in claim 1,
characterized in that the mobile radio station (MS) includes a control
arrangement (ST) connected to a correlator (K), that the output of the
correlator (K) is connected to an evaluation circuit (AW) for evaluating
the measured reception quality of the co-channel message transmission
channels of remote base stations (BS) and that the evaluated measuring
values are stored in a first store (SPM) connected to the evaluation
circuit (AW).
8. A circuit arrangement as claimed in claim 7, characterized in that the
mobile station (MS) incorporates a second store (ST) which is connected to
the first store (SPM) and to the control arrangement (ST) and in which
predetermined threshold values transmitted by the remote station (BS) via
the message transmission channel to the mobile station (MS) for the
reception quality of other base stations (BS) and/or in which measuring
values evaluated in previous measuring cycles by the evaluation circuit
(AW) of the mobile station (MS) are stored and that on the basis of the
evaluated measuring values stored in the second store (SPT) the control
arrangement (ST) decides on hand-over of the radio connection and conveys
this decision via a message transmission channel to the near base station
(BS) maintaining the connection.
9. In a radio transmission system comprising multiple radio cells, adjacent
cells being frequency multiplexed and at least some non-adjacent radio
cells being co-channel, said co-channel radio cells being separated by
different spread codes, the method for handing-over a radio connection via
a first message transmission channel between a mobile radio station and a
first base station in a first cell to a radio connection via a second
message transmission channel between said mobile radio station and a
second base station in an adjacent radio cell comprising the steps of:
during the existence of a radio connection between said mobile radio
station and said first base station, receiving in said mobile radio
station at least one co-channel message transmission channel from at least
one base station in at least one non-adjacent radio cell;
measuring in said mobile radio station the reception quality of said at
least one received co-channel message transmission channel;
using the measured reception quality of said at least one-channel message
transmission channel to decide whether to hand-over said radio connection
via said first message transmission channel between said mobile radio
station and said first base station to a radio connection via a second
message transmission channel between said mobile radio station and a
second base station in an adjacent radio cell.
10. The method of claim 9 wherein said last recited step is carried out in
said mobile radio station.
11. The method of claim 9 further comprising the step of transmitting from
said first base station to said mobile radio station via said first
message transmission channel the spread codes used for co-channel message
transmission channels by base stations in non-adjacent radio cells.
12. The method of claim 9 further comprising the step of transmitting from
said mobile radio station to said first base station via said first
message transmission channel the measured reception quality of said at
least one co-channel message transmission channel.
13. The method of claim 12 further comprising the step of receiving and
storing in said first base station the measured reception quality of at
least one co-channel message transmission channel measured in said mobile
radio station, the decision whether to hand-over said radio connection via
said first message transmission channel between said mobile radio station
and said first base station to a radio connection via a second message
transmission channel between said mobile radio station and a second base
station in an adjacent radio cell being made in said first base station.
14. The method of claim 9 wherein said base stations use spread codes for
the purposes of synchronization and multi-path resolution and wherein said
spread codes used for co-channel message transmission channels by base
stations in non-adjacent radio cells for the purposes of synchronization
and multi-path resolution differ from each other and are used in said
mobile radio station to identify the base station transmitting the
co-channel message transmission channel while measuring the reception
quality of the same.
15. Mobile radio station apparatus for use in a radio transmission system
comprising multiple radio cells, adjacent cells being frequency
multiplexed and at least some non-adjacent radio cells being co-channel,
said co-channel radio cells being separated by different spread codes,
a receiver for receiving message transmission channels from said first and
second base stations and from base stations in radio cells non-adjacent to
said first cell, said receiver without changing frequencies receiving a
message transmission channel from said first base station and also
receiving co-channel message transmission channels from base stations in
radio cells non-adjacent to said first cell,
measuring means for measuring the reception quality of at least one
received co-channel message transmission channel,
evaluation means for evaluating the measured reception quality of said at
least one co-channel message transmission channel and deciding whether to
hand-over a radio connection established via a first message transmission
channel between said mobile radio station and a first base station to a
radio connection via a second message transmission channel between said
mobile radio station and a second base station. |
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Claims |
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Description |
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The invention relates to a method of handing-over a radio connection when a
mobile radio station moves from one radio cell to another radio cell in a
digital radio transmission system.
In the non-prepublished German Patent Application, file No. P 35 27 331.3 a
digital radio transmission system is proposed in which in the forward and
return directions of the message transmission channel a different
modulation method is used in each direction. In the direction from the
base radio station to the mobile radio stations associated therewith each
message transmission channel is spread by bandspread modulation. The
spread message transmission channels thus formed are superimposed on each
other in the base radio station and the broadband sum signal obtained
thereby is transmitted in a common frequency band. In the direction from
the mobile radio stations to the stationary radio station the message
transmission is effected in separate, narrow-band frequency channels.
For speech transmission, from the base station to the mobile stations, the
bandspread modulation used is selected by the base station and
communicated to the mobile station when building-up the communication
link. For the transmission of synchronizing symbols to the mobile radio
stations assigned to the base station, a bandspread modulation which is
common to all the mobile radio stations is used in the direction from the
base radio station to the mobile radio stations.
In digital radio transmission systems the base stations are spatially
arranged in a cellular system. When frequencies are allocated in digital
radio transmission systems, a plurality of radio cells is combined into
one cell cluster, different sets of channels being used in the several
cells of a cluster. In this situation it is alternatively possible to
allocate within a radio cell several sets of channels to the base station.
Spatially the distribution of channel sets in a cell cluster is
periodically repeated. The size of the cell cluster determines a
co-channel reuse distance, it being possible to choose for the network
design (frequency allocation) the co-channel reuse distance and,
consequently, the size of the cell cluster such that the requirements as
regards the degree of freedom of interferences in the digital radio
transmission system are satisfied.
A set of channels of a base station can, for example, consist of 32 message
transmission channels. The individual message transmission channels for
the several mobile radio stations are separated from each other in a set
of channels by, for example, different spreadcode modulation words (CDMA)
and/or different time slots (TDMA). The structure of a set of channels is
obtained in that, for example, a plurality of time slots, each one
containing the information for every one subscriber, are combined into a
time-division multiplex frame, the time-division multiplex frame having,
for example, four time slots. Immediately thereafter the information
contained in such a time-division multiplex frame is spread with suitably
chosen code words, which allow the simultaneous transmission of a
plurality of time-division multiplex frames on the same carrier frequency.
Spreading of these time-division multiplex frames is effected by means of
one code word which, in this set of channels, is only assigned to this
specific time-division multiplex frame. If, for example, eight different
code words are used for each set of channels, then the set of channels
comprises eight different time-division multiplex frames, each having four
time channels, yielding a total of 32 message transmission channels per
channel set.
Each set of channels has at least one control channel assigned to it, which
is accessed by the mobile stations for establishing a link and via which
the connection is set-up and some special services are performed. The
mobile stations know the frequency position of the possible sets of
channels, the corresponding time channel and the code words for the
control channels provided within the digital radio transmission system.
Having this knowledge, the mobile station can search for an appropriate
control channel and can receive there all the information necessary for
accessing (for example, the frequency of the narrow-band (return)
direction from the mobile station to the base station of the control
channel) and for setting-up the connection (for example time channel and
code word for message transmission in the direction from the base station
to the mobile station, and also the frequency for the transmission in
narrow-band frequency channels in the direction from the mobile station to
the base station).
Separating the message channels from adjacent base stations is effected
either by using the frequency-division multiplex method (different
RF-carriers for the set of channels used in these base stations) or by
using the code-division multiplex method (different sets of code words for
the sets of channels) or by using a combination of these multiplexing
methods. The same channel set (RF-carrier and/or code word set) can be
repeated in a further radio cell when spaced sufficiently apart
(determined by co-channel interferences produced). If adjacent radio cells
are separated from each other by different carrier frequencies and
co-channel radio cells are separated from each other by different code
words, then cell clusters having, for example, 3 to 4 radio cells in each
cluster can be formed for the broadband transmission in the direction from
the base station to the mobile stations. For a cell cluster having three
radio cells of identical carrier frequencies it is obtained that when
three different code sets are used, a frequency and code set repetition
occurs in the cell structure every ninth radio cell. If four different
code sets are used then the same frequency-code combination occurs only
after every twelfth radio cell.
For a total overall band width of 25 MHz, available in, for example, the
digital radio transmission system, 20 sets of channels (each having 32
message transmission channels) can be formed with a transmission-bandwidth
of approximately 1.25 mHz. For the transmission direction from the mobile
radio stations to the base station, the message transmission is effected
in frequency channels the carrier frequencies of which are spaced 25 KHz
apart. The frequency allocation in the radio cells themselves is not
fixed, the frequencies are rather freely allocated by the base radio
station. Because it is possible for a mobile radio station to travel
during the conversation from one radio cell to another, it is necessary
that the then existing radio connection is switched through from the
previous radio cell to another one, a process that is commonly known as
"hand-over". Also the occurrence of unexpectedly high co-channel
interferences may render it necessary that within the radio cell a change
to a message transmission channel with a higher transmission quality is to
be made. Extensive research has proved that for the direction of
transmission from the mobile radio station to the base station
(narrow-band transmission) there are hand-over criteria for the radio
connection on the basis of the field strength or jitter or the beat
frequency. For the direction from the base station to the mobile stations
(broadband transmission) criteria can be determined on the basis of the
field strength and the multi-path profile properties.
DE AS No. 26 34 080 discloses a method of hand-over of a radio connection,
in which additional receivers and radio receivers are provided in the base
stations. The receive voltages of the radio receivers and the receive
voltages of the additional receivers provided in adjacent base stations
are compared to each other in a comparator in the base station of the
radio traffic area. When in the radio receiver the receive voltage falls
short of a preset threshold value, then a switch is effected with the aid
of the control arrangement of the radio cell to that additional receiver
for which the comparator has found the highest receiving voltage.
In the new radio base station an additional transmitter is switched to the
speech channel used in the prior radio cell and a channel command for a
speech channel which in the new base station is identified as being free
is transmitted to the mobile radio station via said speech channel. After
evaluation of the channel command in the mobile radio station it is
switched to the new speech channel and the radio connection for the new
radio cell is established again and switched through in the switching
center connected thereto, which is effected by transmitting the
identification of the mobile radio station to the base station.
The invention has for its object to provide a method of radio connection
hand-over by the mobile radio station when the mobile station passes from
one radio cell into another, that the hand-over is effected automatically
and without interrupting the conversation and that the base station does
not require a field strength measuring receiver for obtaining a hand-over
criteria.
This object is accomplished using a method in accordance with the
characterizing features of the Patent Claim 1.
By measuring the reception quality criteria in the mobile radio station it
is possible for areas of varying building densities to prevent, when the
mobile station moves only once from a radio cell into another, a
conversation from being repeatedly hand-over, which occurs when only the
field strength criterium is used. In addition, arrangements only present
in the mobile station, more specifically for measuring the reception
quality criteria, can be used advantageously. Having the switching center
participate in the decision whether a conversation must be
switched-through or not, is not required in the method according to the
invention, so that the dynamic load on the switching center is reduced.
Because of the fact that the mobile station measures the reception quality
of the co-channel message transmission channels of remote base stations
during the existence of a radio connection to a base station, no
synthesizer change-over is required in the mobile station. The co-channel
message transmission channels of the remote base stations differ from the
near base station only in their code words. The receiver provided in the
mobile radio station can consequently be additionally used to monitor the
existing radio connection during the conversation. In the digital radio
transmission system the base radio stations transmits in time-division
multiplex frames synchronizing signals which are inserted between the data
symbols at regular intervals, such as is proposed in, for example, the
non-prepublished German Patent Application bearing the official file
number P 35 11 430.4. The base radio station transmits the synchronizing
symbols in all the message transmission channels in time-parallel and in
synchronism. The identical synchronizing symbols transmitted
simultaneously in all the message transmission channels are received in
the mobile station with a significantly higher energy, compared to user
informations. If the transmission field strength of the remote base
stations are used as a basis for the hand-over criterion, the evaluation
can be effected in a simple manner.
If in accordance with Patent Claim 2 the mobile radio station is informed
of the spread-code words of the remote base radio stations by the near
base radio station maintaining the connection, then the code-words can be
altered during system gyration, if necessary, without necessitating
modifications of the mobile stations' software.
In accordance with the method claimed in Patent Claim 4, the base radio
station includes a store in which the measuring values transmitted by the
mobile radio station are stored. These measuring values do not relate to
message transmission channels of the base radio stations adjacent to the
base radio station, as in the method described in De-AS No. 26 34 080, but
to co-channel message transmission channels of remote base radio stations.
By weighting the stored measuring values it is possible in the digital
radio transmission system to effect an adaptation to the terrain also
during operation. In a channel-assigning method described in DE-OS No. 29
37 108 it is determined, before the network is made operative, by means of
measurements where co-channel interferences may occur. The propagation
conditions in a radio transmission system constantly change during
operation (for example by weather conditions), so that by combining the
measuring values with the weighting factors an adaptation to the channel
properties can be effected and the transmission reliability can be
increased. As already described in the foregoing, the synchronizing
symbols are transmitted simultaneously and consequently with a higher
average power in all the message transmission channels. As a result
thereof the synchronizing symbols are received substantially without
interferences in the mobile radio station and can be used, in accordance
with Patent Claim 5, for measuring the reception quality.
As already described in the foregoing, inserting the synchronizing symbols
in the continuous data flow of user information is effected with
predetermined time intervals, which are determined such that the
multi-path reception can also be used at high vehicle speeds. By a
phase-locked (not necessarily equal-phase) addition of the synchronizing
symbols an (uncontrolled) relative cancelling of inherently identical
synchronizing symbols in the individual message transmission channels is
prevented. If in accordance with Patent Claim 5 the multi-path profile is
used as the reception quality criterion, then the message transmission
channels can be adapted in the receiving arrangements to the channel
properties on the basis of the measured multi-path profile, and the
transmission reliability can be further increased. The mobile radio
station only requires, as an additional device, an additional store for
storing the measuring values, and arrangements already present in the
mobile radio station can also be used advantageously.
The method according to the invention will now be described and illustrated
by way of example with reference to the embodiments shown in the
accompanying drawings. Therein:
FIG. 1 shows the cell structure of the digital radio transmission system,
FIG. 2 shows the block circuit diagram of a first embodiment and
FIG. 3 shows the block circuit diagram of a second embodiment of the
circuit arrangement used in the mobile radio station in the method
according to the invention.
FIG. 1 shows an embodiment for a cell structure of the digital radio
transmission system. Base radio stations BS are arranged in the centre of
the various radio cells. The direction in which a mobile radio station MS
moves is indicated in FIG. 1 by the arrow indicating the direction of
travel.
In digital radio transmission systems sets of channels are formed, for
example by a sequence of code-division multiplex and frequency-division
multiplex operations performed on the message signals to be transmitted.
Separating the message transmission channels of adjacent base radio
stations BS is effected using the frequency-division multiplex method,
that is to say different RF-carriers for the sets of channels used in
these base stations, which is illustrated in FIG. 1 by the first digit.
The radio cells can be combined into cell groups (clusters), different
channel cells being used in the several radio sets of a cluster and the
spatial distribution of the channel sets in a cluster being periodically
repeated. During designing of the network, the co-channel reuse distance
and consequently the size of the cell clusters can be chosen such that
predetemined requirements for the freedom of interference in the network
are satisfied. Co-channel radio cells are separated in the digital radio
transmission system by means of different code words, which is illustrated
in FIG. 1 by the capital letters A to G. For locating itself within the
radio cell assignment and consequently the decision for hand-over,
reception quality criteria measurements are effected in the mobile
station. To that end, during the existence of a radio connection to the
near base station 1/A (centrally in the network structure shown in FIG. 1)
the reception quality of the co-channel message transmission channels of
remote base stations BS are additionally measured in the mobile station
MS. In the practical example of the network structure shown in FIG. 1 the
mobile station MS measure the transmission field strength of the base
stations BS denoted by 1/B, 1/C, 1/D, 1/E, 1/F and 1/G. The decision
whether the radio connection must be handed-over is made in the near base
station BS which maintains the connection. For that purpose the measuring
results are transmitted to the near base station BS, which maintains the
connection, in a low-rate signalling channel (ACCH). This is done in such
a manner that it is inaudible for the subscriber, in parallel with the
transmission of the digitized speech. A further possibility is to transmit
the measuring results instead of the digitized speech during the speech
pauses and to extract these measuring results in the base station before
the reconversion of the speech.
The base station BS incorporates a store for storing the measuring values
transmitted by the mobile station MS via the message transmission channel.
This store contains predetermined threshold values for the reception
quality of the co-channel message transmission channels from remote base
stations and by comparing these values to the measuring values determined
by the mobile station MS for these base stations, a decision can be taken
in the base station BS 1/A about handing-over of the radio connection.
This comparison is consequently based on a radio bearing, the radio
bearing effected being reflected in the increase and decrease of the
measuring results. It is possible, without modifying the method of the
invention, to effect a plurality of measurements for each base station BS
throughout a measuring period and to increase the decision reliability by
taking the average value. To adapt the factors which influence the
propagation condition, for example terrains, weather etc., the measuring
values can be combined with weighting factors prior to storage.
FIG. 2 shows the block circuit diagram of a first embodiment of the circuit
arrangement provided in the mobile radio station MS when performing the
method according to the invention. The signals received by the aerial are
applied to a receiver E whose output is connected to an
analogue-to-digital converter ADW. The receiver E has an RF-input stage,
an oscillator as well as an IF-portion containing a preset amplifier. The
signals converted to the basehand thereby are applied for further
processing via the analogue-to-digital converter ADW to a correlator K
connected thereto. The correlator K is connected to both a control device
ST and to an evaluation circuit AW. With the aid of the evaluation circuit
AW the measured reception quality of the co-channel massage transmission
channels from the remote base stations BS are evaluated. The evaluated
measuring values are stored in a first store SPM connected to the
evaluation circuits AW and these measuring values are transmitted to the
base station BS by means of a transmitter S via the message transmission
channel. The control arrangement ST is connected to both the transmitter S
and to a third store SPC in which the code words are stored. A store
suitable for this purpose is, for example a read-only memory.
When the cost and design effort of the store in the mobile radio stations
MS are to be reduced, then it is possible to omit also the first store SPM
without altering the method of the invention, which creates the situation
that then on the request of the base station BS the first measurement of
the reception quality criteria is efected in the mobile station MS, this
measuring value is transmitted to the base station BS via the message
transmission channel, whereafter the second measurement is effected etc.
FIG. 3 shows the block circuit diagram of a second embodiment of a circuit
arrangement used during the performance of the method according to the
invention. A second store SPT which is connected to the first store SPM
and to the control arrangement ST is arranged in the mobile radio station
MS. The measuring values evaluated in the preceding measuring cycles by
the evaluation circuit AW of the mobile radio station MS are stored in the
second store SPT. It is also possible to store in this store SPT the
receiving quality threshold values of the remote base stations BS which
are transmitted by the base station BS to the mobile radio stations MS via
the message transmission channel. On the basis of the evaluated measuring
values stored in the second store SPT the control arrangement ST decides
on handing-over the radio connection.
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