Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
3~3
PHT 82.349 l 18.1l.1983
method of monitoring a radio link bett~en a base station and a mobile
station.
The invention relates to a method of monitoring a radio link
kett^~een a base station and a mobile station in a radio transmission
system.
In accordance with technological developments knot~n to those
involved, the radio transmission system consists of a hierarchically-
arranged series of networks. The lowest level is made up of radio
zones. The radius of these radio zones is, depending upon the height of
the antenna masts of the base stations BS and at a n~Yim~n transmission
power of 50 wa-tts, ketween 5 and 15 km. Each radio zone is served by
a base station BS, which can connect calls through to or from the public
telephone network via relay equipment. Several adjoining rcadio zones can
ye combined into what is known a5 a paging area. At the base stations
the locations of all the mobile stations MS are continuously registered
and stored in an address book. If a mobile station MS changes -to another
paging area, a corresponding change is made in the address book.
When a subscriker to a public telephone nett~rk wishes to n~ke
a call to a mobile station a selective call is transml-tted to all the
- radio zones of the paging area in which the mobile station is
registered at that moment.
The transmitting and receiving frequency band is between 860
and 960 ~Hz, for instance. A duplex distance of 45 MHz between the
transmittiny and -the receiving frequency band can be adopted, and the
-transmitting and receiving frequency band can be subdivided into a
channel spacing of 25 KHz. The channels are operated in the duplex
mode.
According to the level of traffic, a number of traffic
channels TCH for speech transmission and a-t least one control channel
CCH are assigned to each radio zone in the radio transmission systern,
different frequencies (control channels) being used in radio zones which
are in proximity to one another. The control channel CCH and the traffic
channel TCH have a special code to distinguish one from the other. In the
event of a failure or interference on a control channel CCH each traffic
channel TCH can take over the functions of the control channel CCH by
3~
Per 82.3~9 2 18.11.19~3
means of a change of code. It is hence possible to dispense with the
duplication of control channels Cal, which ~uld otherwise be required
for reasons of reliability.
A radio telephone system is known to us from DE-AS 25 50 26
in which nonitoring of the radio channels is carried out by means of
an evaluation of the received field strength of a radio receiver. A
central station is provided with a buffer stage and a gated buffer. The
buffer stage separates the transmitter voltage of dispersed fixed radio
receivers, which is conveyed via a junction circuit, from the demodulated
radio signal superimposed on i-t. In the gated buffer the fixed radio
receiver with the highest HF field strength is connected through in the
restitution and/or relay section of the central station for retrans-
mission or relaying. As well as the fixed central station, ~7hich in-
corporates a transmitter and at least two fixed radio receivers arranged
in different positions, the radio telephone system has at least one
mobile radio transmitter and receiver unit. The radio signal of the
mobile radio transmltter and receiver unit can be received hy at least
one of the Eixed radio receivers. Each fixed radio receiver incorporates
a trcmsmitting apparatus for proclucing a transmitter voltage which i.s
deI~endent upon the reception conditions of the fixed rad:io receiver -to
w}1ich it belongs. Each transmi-tting apparatus produces a d.c. vol-tage
proportionate to the received field strength of the fixed radio receiver
to which it kelongs, this voltage serving the fixed cen-tral station as
a basis for the monitoring of the radio channels in the manner descriked
akove, with the use of a buffer and a gated buffer.
A disadvantage of this kind of radio transmission system
(radio telephone system) is that in the evaluation of the received
field strength it is not possible to distinguish ketween wanted signals
(e.g. data and/or speech) and spurious signals (common channel inter-
ference, image frequency effects, interfering transmitters). The re-
ceived field strength is a compound signal made up of wanted and
spurious signals, the ratio ketween the level of wanted signals and
spurious signals not keing known. Wi-th an increase in the number of
transmission channels in a radio transmission system there is a dis-
proportionate increase in the spurious signals caused by the effectsof intermodulation. The thresholds for the release (clearing) of a
transmission channel must no-t go below a certain minimum value. On the
other hand, the minimum value which is adopted must not be too high, as
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PHT 82.349 3 18.11.1983
otherwise the release of a transmission channel occurs frequently.
The problem underlying the invention is to indicate a method
oE monitoring an existing radio link between a base station and a mobile
station in such a Jay that an evaluation of the received field strength
of the receiver concerned is not required.
This problem is solved by the features described in Claim 1.
The method on which the invention is based makes it possible
to distinguish between poor transmission quality caused by radio shadow
or by the large distance between the base station and the mobile station
and poor transmisstion quality caused by a failure of a transmitter. In
addition, the clearing of a traffic channel is made possible in a short
period of time even under unfavourable transmitting conditions. From the
monitoring of the number of messages a radio station (fixed or mobile
; station) knows the conditions indicating when a called station has
switched off its transmitter or left -the transmission channel.
Advantageous ways of cleveloping the invention are describxd
in the sub-cla:Lms.
The invention is explained and described in greater cletail
below on the basis of an example tried in practice.
A radio link (existing transmission channel, i.e. traffic
channel TCH or control channel CCH) exists between a mobile station MS
and a base station BS. The mobile station MS and the base station BS
register valid radio messages G and invalid radio messages U on the
transmission channel.
A radio message can bx structured in the form of a frame
synchronisation, an information block and a redundancy block. The frame
synchronisation displays, for instance, a block length of 16 bit and
precedes the information block. The beginning of a radio message can bx
recognised in the station (MS, BS) on the basis of the frame sync'nroni-
sation. The information block and the redundancy block can lx scrambled
for the purpose of recognising and/or correcting errors. Witll a view
to the simple processing of a radio message the block length of the
free synchronisation, the information block and the redw~dancy block
; is an integral multiple of 8 bit. The radio message can also bx preceded
by a preamble. The base station BS transmits a preamble with, for
instance, 16 bit on a control channel CCH. On a traffic channel TCH a
preamble is added only at the beginning of its occupation by base
station BS and mobile station MS. Speech and data (with, for instance,
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PHT 82.349 4 18.11.1983
150 bit/s in the upper band) can be transmitted on a traffic channel
TCH. Only data with, for instance, 2.4 Kbit/s is transmitted on a
control channel CCH. Radio messages can be transmitted in continuous
succession or only at certain times to a changeable frame.
s A radio signal is recognised as valid, for instance, when the
following conditions are met:
- There is not too much jitter
Jitter can be recognised over a given period of time by transitions
in level being detected in respect of time, averaged and evaluated.
During this period of time the jitter must not exceed a certain
amount (phase). This affords protection against residual errors.
- Frame synchronisation recognised
The received bits æe compared with a bit pattern specific to the
radio transmission system. If there is an approximate correspondence
lS between received and stored bit patterns, the frame synchronisation
is regarded as recognised.
- Radio message must be decodeable
after the frame synchronisation has been recognised, the received bits
of the information block and the redundancy block are stored. The
redundancy block can be used to recognise how many errors are contained
in a radio message. If the number of errors exceeds a certain value,
the radio message is regarded as not decodeable.
- Radio message must be in the frame
The radio stations (MS, BS) know the time of a new radio message. The
stations (MS, BS) check~whether the received radio message also meets
the time requirement (appears in the frame).
If at least one of these conditions is not met, this radio message is
classified as invalid. The radio stations (MS, BS) know that radio
messages are continually transmitted on the transmission channel (TCH,
CCH) and that the radio messages have a time pattern. A radio message,
therefore, is also classified as invalid if the radio station (MS, BS)
does not receive anything.
Case 1:
A radio link exists bet~7een mobile station MS and base station
BS on the control channel CCH.
- If the counter in the mobile station r~s has reached the position NCC~
(= 16), the mobile station MS does not being a search for a control
channel CCH.
PHT 82.349 5 18.11.1983
- If the mobile station receives a valid radio message G when the
counter reading is between NCCHW (= 16) and NCCHSC (= 48), the counter
is reset to zero and the mobile station MS can begin a search gain.
- When the counter reading NCCHSC (= 48) has been reached, the mobile
station switches to a spare channel CCHSC assigned to the control
channel CCH and checks whether a control channel message is being
received on it.
This method makes it possible to distinguish between a break
in communication caused by brief radio shadow and a break in
communication caused by the failure of a control channel CCH.
Case 2:
A radio link exists between mobile station MS and base
station BS on the traffic channel TCH.
- If the counter in the mobile station MS has reached the position
NTXOF (= 4), the mobile station MS switches off its transmitter and
continues to be assigned to this traffic channel TCH.
- If the mobile station receives NG (= 1) valid radio messages G when
the cc~mter reading is between NTXOF (= 4) and NTCHR (= 25), it
switches on its transmitter again and resets the ccNnter to zero.
- Len the counter reading NTCE-IR (= 25) is reached, the mobile sta-tion
MS leaves the traffic channel TCH.
This rrethod makes it possible to clear a traffic channel TCH
from the called station (BS) within a short period of tirre (4 radio
mlessages), as it is certain after this time that the rnobile station MS
has switched off its transmitter. After this period either the counter
in the mobile station US has reached a position of at least NTXOF (= 4)
or the mobile station MS has received one of NTXOF release messages.
This traffic channel TCH is thus available to other mobile stations MS.
If the m.obile station MS was not able to receive at least
NTXOF radio rnessages, its transmitter is indeed switched off but the
radio link is finally releæ ed only when the counter reading NTCHR
(- 25) has been reached. If as a result of radio shadow no radio
messages are received, there is a strong likelihood that the mobile
station MS has again left the radio shadow during this period and is
receiving NG valid radio messages G.
Case 3:
-
A radio lirk exists between a mobile station ITS and a base
station BSon the traffic channel TCH.
P~-~ 82.349 6 18.11.1983
- If the counter in the base station BS has reached the position NF3SP(= 4), the station increases the transmitter power of its transmitter
and feeds a command to the mobile station MS to increase the trans-
mitter power of its transmitter.
S - When the counter reading NCONV (= 25) has keen reached, the base
station BS transmits a maximum of NTXOF (= 4) release messages to the
mobile station MS. Irrespective of the reception quality of the radio
link in the mobile station MS, the traffic channel TCH is clear after
this period.
