Method of covering shadow areas in a cellular mobile radio system and radio booster for implementing this method

A mobile radio network comprises a base transceiver station for transmitting and receiving radio signals to and from mobiles on different basic frequencies and a radio booster for receiving, amplifying and retransmitting radio signals to and from at least one shadow area. To cover the shadow areas a radio signal received by a radio booster from the base transceiver station on a basic frequency is retransmitted to a mobile station on a translated frequency different from the basic frequency and associated with the latter by a translation law. This law is such that, firstly, each of the basic frequencies is either associated with at least one translated frequency or is not associated with any translated frequency and, secondly, the difference between the value of a translated frequency and the value of the associated basic frequency is not the same for all the translated frequencies. A radio signal received by a radio booster from a mobile on a translated frequency is retransmitted to the base transceiver station on the associated basic frequency.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns a method of covering shadow areas in a 
cellular mobile radio system and a radio booster for implementing this 
method. 
2. Description of the Prior Art 
In cellular mobile radio systems the environment of fixed and mobile 
transceiver stations includes obstacles due to the terrain or to 
buildings, for example. 
Because radio propagation is more or less directional, radio shadow areas 
exist in the vicinity of these obstacles, i.e. areas in which radio 
transmissions can be partially blocked or absorbed by some feature of the 
environment, with the result that they can be strongly attenuated. 
Shadow areas can also exist at the extreme range of the transmitters of the 
fixed stations of the network. 
Accordingly, the signal transmitted can deteriorate significantly. 
Various solutions to this problem have been put forward. 
A first solution is to use a radio booster comprising a transceiver and an 
associated antenna provided with an amplifier located in the shadow area. 
The radio receiver is a broadband receiver, i.e. it can receive and 
retransmit a major part of the frequency spectrum used in the radio 
network to which it belongs. 
Its function is to receive radio transmissions in the shadow area, to 
amplify them and then to retransmit them on the same frequency, at a 
different angle. It is therefore virtually "transparent", i.e. it modifies 
only the amplitude of the retransmitted signal. The advantage of using a 
booster is that it is autonomous: no cables have to be connected to the 
booster. This solution is therefore of low cost. 
This solution is unsatisfactory, however, because of a phenomenon similar 
to the Larsen effect that occurs, especially if the booster is not far 
away from the fixed transceiver station it relays. The Larsen effect is a 
spurious oscillation which occurs when the output of an electro-acoustic 
system, such as the sound from a loudspeaker, for example, impinges on its 
input, usually the microphone, and the effect produced is a whistling. In 
the case of radio waves this phenomenon produces interference in the 
signal transmitted. 
One solution to this problem is to use a booster which converts or 
translates the frequency of the signals received before retransmitting 
them. A solution of this kind is described in patent application GB-A-2 
253 324, for example. 
It has been found that this solution is unsatisfactory in the case of 
cellular mobile radio systems because the base transceiver station cannot 
determine if frequency translation has been applied unless devices to 
carry out the converse frequency translation are used at the boundaries 
between cells, which is costly in hardware. Another solution for cellular 
mobile radio systems avoids these problems. This solution increases the 
number of cells, and therefore the number of base transceiver stations, by 
creating additional cells to cover the shadow areas. 
This solution is effective and has the advantage of avoiding the problems 
that arise with the previous solution. 
It has drawbacks of its own, however. 
First of all, it is very costly in installation terms because, unlike a 
radio booster, a cellular network base transceiver station must be 
connected by cables to a base station controller and to a network 
operation and maintenance center. 
Also, given its complexity, a cellular network base transceiver station is 
inherently a high cost item. 
An object of the present invention is therefore to provide a method of 
covering shadow areas in a cellular mobile radio system that is not costly 
to implement but which guarantees good quality radio signals so that the 
base transceiver stations can determine if the signals received are at a 
frequency resulting from frequency translation in a booster without 
needing to use devices to carry out the converse frequency translation. 
SUMMARY OF THE INVENTION 
To this end the present invention proposes a method of covering shadow 
areas in a cellular mobile radio system comprising a plurality of cells 
each covered by a base transceiver station adapted to transmit and to 
receive radio signals to and from mobile stations in the cell on different 
basic frequencies, each shadow area being in one of the cells, a radio 
booster being associated with each of the shadow areas to receive, amplify 
and retransmit the radio signals to and from mobile stations in the shadow 
area, the booster adapted to cover a given shadow area being subordinate 
to the base transceiver station of the cell in which the shadow area is 
located. 
According to the present invention a radio signal received by one of the 
boosters from the base transceiver station on a basic frequency is 
retransmitted to one of the mobile stations by the booster on a translated 
frequency different from the basic frequency and associated therewith by a 
translation law known to the control means of the base transceiver 
station. 
Conversely, a radio signal received by one of said boosters from one of the 
mobile stations on a translated frequency is retransmitted to the base 
transceiver station by the booster on the basic frequency associated with 
the translated frequency, the frequency of the broadcast control channel 
of each cell being always associated with a translated broadcast control 
channel of each of the boosters. 
When a mobile station in a shadow area of a cell transmits an access 
request radio signal on a specific access channel conveyed by a translated 
broadcast control channel in order to establish a radio connection with 
the base transceiver station of the cell, so that a traffic channel is 
assigned to it, the booster adapted to cover the shadow area, after 
receiving the access request, retransmits it to the base transceiver 
station on broadcast control channel after modifying it. A control unit, 
to which the modified access request is retransmitted, is able to 
determine that the mobile station is in a shadow area, and to deduce the 
translation law for obtaining the translated frequencies associated with 
that shadow area, so that the signal transmitted by the control unit and 
adapted to advise the mobile station which traffic channel and which 
frequencies it must use to transmit and receive contains the translated 
frequencies instead of the associated basic frequencies. 
Because the frequencies retransmitted undergo frequency translation, the 
Larsen effect type phenomenon usually encountered with radio boosters does 
not occur. 
The boosters of the invention have several functions. The first of these 
functions is to translate the received frequencies and a second is to 
modify the received access requests. These functions are much simpler than 
those implemented by a conventional base transceiver station, however, 
with the result that the hardware cost of a booster used in the method of 
the invention, although greater than that of a conventional booster, is 
much less than that of a base transceiver station. 
The booster which receives an access request from a mobile station, 
transmitted by the latter as if it were adjacent the base transceiver 
station, i.e. with a null timing advance relative to the base transceiver 
station, retransmits the corresponding modified access request with a 
so-called partial timing advance corresponding to the distance between it 
and the mobile station. Therefore the timing advance indicated thereafter 
by the base transceiver station to the mobile station and to be used by 
the latter for any subsequent transmission to the base transceiver station 
is equal to the sum of the partial timing advance and the timing advance 
corresponding to the distance between the booster and the base transceiver 
station. 
A third function of a booster for implementing the method of the invention 
is therefore to determine the primary timing advance. This function also 
enables access requests to be decoded correctly, which is necessary for 
modifying them in the specific case mentioned above. 
It is highly advantageous to enable the booster to retransmit the modified 
access request to the associated base transceiver station with a non-null 
timing advance. In this way the base transceiver station can determine 
directly the total timing advance that the mobile station must use. 
Moreover, if frequency hopping is used in a cell in which there is a shadow 
area and if the booster covering this shadow area uses fewer frequencies 
than the base transceiver station of the cell, the traffic channel 
assigned to a mobile station in the shadow area is conveyed either by 
basic frequencies each having an associated translated frequency or by a 
channel with no frequency hopping. 
A fourth function of boosters in the method of the invention is therefore 
to apply frequency hopping. 
All the translated broadcast control channels of a given broadcast control 
channel are preferably held in memory in control means. 
To prevent interference, the translated frequencies are chosen so that 
adjoining shadow areas use separate sets of translated frequencies, so 
that classes of shadow area are defined, a shadow area class comprising 
all the shadow areas using the same set of translated frequencies and 
being represented by a translation law different from those representing 
the other shadow area classes. 
If frequency hopping is applied in a cell by means of a so-called 
repetition law, frequency hopping can then also be applied in the shadow 
areas in this cell, the repetition laws used by the boosters covering 
these shadow areas being either identical to or different from the 
repetition law used by the base transceiver station of the cell, the 
signalling channel conveyed by the broadcast control channel being always 
a channel with no frequency hopping, however. 
The control unit can then deduce the shadow area class from the modified 
access request. 
If the cellular mobile radio system uses time-division multiple access, the 
access request can be modified by changing channel, for example: the 
booster retransmits the access request on a so-called modified access 
channel whose position in time is different from that of the access 
channel, the modified access channel being known to the control means as 
specific to retransmission of access requests from mobile stations in a 
shadow area of a predetermined class. 
This solution is particularly simple to implement. 
In this case, the downward channels, i.e. the channels in the direction 
from the base transceiver station to the mobile stations, associated with 
the modified access channels are highly advantageously either not used for 
traffic or used for booster control and monitoring. 
If a mobile station in a shadow area accessing a base transceiver station 
through the booster covering that shadow area must access directly the 
base transceiver station in whose coverage area that shadow area is 
located, a handover procedure is carried out. 
If a mobile station accessing a base transceiver station, either directly 
or through a booster covering a first shadow area, must access the booster 
of a second shadow area, the base transceiver station communicates to the 
control unit the translated broadcast control channel associated with this 
second shadow area and the control unit deduce therefrom the translation 
law required to obtain the translated frequencies associated with the 
second shadow area, so that the signal transmitted by the control unit to 
tell the mobile station which traffic channel and which frequencies it 
must use to transmit and receive contains the translated frequencies used 
by the booster of the second shadow area rather than the associated basic 
frequencies. 
In an advantageous embodiment of the invention the base transceiver station 
holds in memory all the translated broadcast control channels related to 
its broadcast control channel by the translation law. 
The translation law is preferably an increasing monotonous function if the 
radio system uses frequency hopping. 
Finally, the translation law to be used by each booster is supplied to it 
by the network operation and maintenance unit, this law being modifiable. 
The present invention also concerns a booster for implementing the method 
as explained above. 
This booster can include: 
at least one transceiver, 
at least one antenna associated with said transceiver, 
means for translating the frequency of signals received from said mobile 
stations or said base transceiver stations, 
means for applying frequency hopping, 
means for determining the timing advance corresponding to the distance 
between it and a mobile station, 
means for modifying access requests from said mobile stations, 
means for effecting its operation and maintenance by radio from network 
operation and maintenance means. 
Other features and advantages of the present invention emerge in the 
following description of a method in accordance with the invention and one 
embodiment of the latter given by way of non-limiting and purely 
illustrative example only.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Items common to more than one figure are always identified by the same 
reference number. 
Throughout the remainder of this description, for reasons of clarity, the 
method of the invention is described by means of a specific example which 
is, of course, given purely by way of illustration. 
This example concerns application of the method of the invention to a cell 
of a GSM type cellular mobile radio system. FIG. 1 shows an "umbrella" 
cell CP whose general features are conventional in the GSM system: 
it covers a broadly circular geographical area, 
it has at its center a base transceiver station BTSP which includes a radio 
transceiver with a transmit and receive antenna A, 
the base transceiver station BTSP can transmit radio signals on several 
separate frequencies, referred to as basic frequencies, which are assigned 
to it when the network is configured in such a way that there is no 
interference between the cell CP and the adjoining cells (not shown), each 
frequency reserved for transmission (in the downward direction) being 
associated with one only separate frequency reserved for reception (in the 
upward direction); from the protocol point of view, the cell CP is 
associated with numbers, each number representing a pair of frequencies, 
i.e. an upward frequency and a downward frequency. 
There are various obstacles (not shown) to radio waves in the cell CP and 
these create shadow areas Z1, Z2, Z3. A respective radio booster R1, R2, 
R3 is installed at the center of the each shadow area Z1, Z2, Z3, for 
example. The radio coverage of each booster is shown by a circle. 
FIG. 2 is a simplified representation of the shadow area phenomenon: it 
shows the base transceiver station BTSP whose antenna A is mounted on a 
mast M, an obstacle O on top of which is located the booster R1, 
comprising an antenna A1 disposed to transmit and receive signals to and 
from the base transceiver station BTSP, and an antenna for transmitting 
and receiving radio signals respectively to and from mobile stations 
provided with transceivers, and in particular one such station MS located 
in the shadow area Z1 (shaded in FIG. 2) caused by the obstacle O. 
It is assumed that four transmit frequencies f.sub.P1, f.sub.P2, f.sub.P3 
and f.sub.P4 are associated with the base transceiver station BTSP. The 
corresponding receive frequencies are identified by the same alphanumeric 
symbol but "primed". The frequency of the broadcast control channel of the 
cell CP is the frequency f.sub.P1. 
There follows a description of the exchange of radio signals between the 
base transceiver station BTSP and the mobile station MS in the shadow area 
Z1 in the case of radio signals other than access requests transmitted by 
the mobile station during the access procedure. 
If the base transceiver station BTSP transmits a radio signal to the mobile 
station MS in the shadow area Z1 on the frequency f.sub.P2, for example, 
it is received by the antenna and then by a receiver RR12 (see FIG. 3) of 
the booster R1. Before retransmitting this signal to the mobile station MS 
the booster R1 translates its frequency. This operation is schematically 
represented by the block T in FIG. 3. 
The signal is then retransmitted to the mobile station MS by the 
transmitter ER12 and the antenna of the booster R1 on the translated 
frequency f.sub.R12 associated with the frequency f.sub.P2. 
To prevent cochannel interference the frequency f.sub.R12 is separate from 
the frequency f.sub.P2 and all other frequencies associated with the base 
transceiver station BTSP and all translated frequencies associated with 
the latter. For the same reason, during configuration of a network using 
the method of the invention the translated frequencies chosen for each 
shadow area are made sufficiently far away from those associated with 
other shadow areas and those of cells adjoining the cell CP. 
The translation can be schematically represented by a so-called translation 
law, such that each of the basic frequencies of the base transceiver 
station BTSP is either associated with at least one translated frequency 
(the specific case in which a basic frequency is associated with more than 
one translated frequency is discussed later) or is not associated with any 
translated frequency. Also, the difference between the value of a 
translated frequency and the value of the associated basic frequency must 
not be the same for all the translated frequencies. 
When a transmit frequency is associated with a translated frequency the 
corresponding received frequency is also associated with a translated 
frequency. 
Also, the frequency of the broadcast control channel must naturally always 
be associated with a translated frequency, as it conveys the signalling 
channels. In this example this translated frequency is the frequency 
f.sub.R11. 
The following description concerns the situation in which a basic frequency 
is associated with at most one translated frequency. 
When a signal from the mobile station MS on the frequency f'.sub.R12 is 
received by a receiver R'.sub.R12 via the antenna , the booster R1 applies 
the converse translation law, this operation being symbolically 
represented by the block T.sup.-1 in FIG. 3. It then retransmits this 
signal on the frequency f'.sub.P2 to the base transceiver station BTSP by 
means of a transmitter E'.sub.R12 and the antenna A1. 
To avoid the problems encountered in the prior art the mobile station must 
use only translated frequencies to transmit and to receive. 
The description so far applies to transmission and reception of all signals 
other than access requests either on the broadcast channel used for 
traffic or on the other frequencies. 
The translation law(s) used (it is not necessary for all the boosters to 
use the same translation law, and in some cases this is even undesirable, 
for reasons to be explained later) can be determined once and for all when 
the network is installed. However, for obvious reasons of flexibility and 
to allow adaptation to changing call traffic densities, it is preferable 
to make provision for subsequent modification of these laws. 
For example, the translation law(s) can be communicated to the various 
boosters by a network operation and maintenance center, preferably by 
radio. 
In the GSM system, each base transceiver station is controlled by a base 
station controller. The latter must hold the translation laws in memory. 
The access procedure is described next with reference to FIGS. 3 and 4. In 
this context, the access procedure is the procedure carried out when a 
mobile station attempts to access the network the first time after it is 
powered up, i.e. when it is attempting to connect by radio to a base 
transceiver station of the network, primarily to send and receive calls. 
As a general rule, in a GSM type network each base transceiver station 
transmits a signalling channel on the broadcast control channel BCCH. As 
this type of network uses time-division multiple access, this signalling 
channel is always transmitted by the base transceiver station in the first 
time slot TS0 of each frame. 
When a mobile station attempts to access a base transceiver station it 
transmits in TS0 an access request (Random Access--RA) signal on the 
upward broadcast control channel (known as the Random Access 
CHannel--RACH). 
On receiving the access request RA the base station controller BSC, through 
the intermediary of the base transceiver station, assigns the mobile 
station a traffic channel, i.e. a pair of frequencies (or several pairs if 
frequency hopping is used) and a specific time slot (steps 20, 21 and 22, 
FIG. 6). It also determines the timing advance with which the mobile 
station must transmit signals to allow for the propagation time of 
electromagnetic waves between the mobile station and the base transceiver 
station. 
For the mobile station MS in the shadow area Z1 the signalling channel 
(BCCH in FIG. 4) from the base transceiver station BTSP received via the 
antenna A1 by the receiver RR11 of the booster R1 is retransmitted by the 
transmitter ER11 and the antenna of the booster R1 on the translated 
frequency f.sub.R11 of the broadcast control channel f.sub.P1 to all 
mobile stations in the shadow area Z1. The mobile station MS receives the 
frequency f.sub.R11 and regards this as the downward broadcast control 
channel; it deduces from it the translated frequency f'.sub.R11 for the 
upward broadcast control channel f'.sub.P1. 
To access the base transceiver station BTSP the mobile station then sends 
an access request in time slot TS0 at the frequency f'.sub.R11 (see FIG. 
4). 
In accordance with the invention, instead of retransmitting this access 
request on the upward broadcast control channel f'.sub.P1 the booster R1 
modifies it and then retransmits a modified access request to the base 
transceiver station BTSP. 
Modification of the access request tells the base transceiver station BTSP 
and thus the base station controller BSC of the latter that the access 
request is from a mobile station in a shadow area. It also indicates, 
where appropriate, the class to which the shadow area in which the mobile 
station MS is located belongs. 
A class of shadow area comprises all shadow areas using the same set of 
translated frequencies within the same cell, where more than one shadow 
area exists within the same cell. As already mentioned, the translated 
frequencies must be distributed in such a way that shadow areas close 
together do not use the same frequencies. However, to make best possible 
use of the frequency spectrum there may be more than one shadow area using 
the same set of frequencies provided that they are sufficiently far apart. 
In this example the shadow areas Z1 and Z3 are in the same class and the 
shadow area Z2 is in a different class. 
To use separate sets of translated frequencies each shadow area class can 
be represented by a different translation law. 
If the modified access request (step 21, FIG. 6) tells the base station 
controller BSC that the mobile station MS is in a shadow area and the 
class appropriate to this shadow area and thus the associated translation 
law, the base station controller BSC can assign to the mobile station MS, 
by way of the Immediate Assignment (IA) message transmitted by the base 
transceiver station BTSP, a channel, i.e. a frequency (or several 
frequencies if frequency hopping is used) and a specific time slot, the 
frequency or frequencies thus communicated (f.sub.R11, f.sub.R12) being 
the translated frequencies for the frequencies (f.sub.P1, f.sub.P2) that 
would have been communicated to it had it been outside the shadow area Z1 
(steps 21, 23, 24 and 25, FIG. 6). The mobile station MS can deduce the 
upward frequencies once it knows these downward frequencies. 
The remainder of the access procedure is entirely conventional. 
As already mentioned, in accordance with the invention the mobile station 
MS must use only translated frequencies. If the conventional access 
procedure were used the message IA would contain not the translated 
frequencies but the associated basic frequencies (f.sub.P1, f.sub.P2) of 
the base transceiver station BTSP, which would therefore be unsuitable. 
In a specific application mentioned here purely by way of illustrative 
example, the modification of the access request carried out by the 
booster R1 entails, rather than retransmitting the access request in time 
slot TS0, retransmitting it on another channel, called the modified access 
channel, at the frequency f'.sub.P1 of the broadcast control channel. 
Naturally, the modified access channel must be known not only to the base 
transceiver station BTSP but also to the base station controller BSC so 
that the latter expects to receive on this modified access channel an 
access request from a mobile station in a shadow area. The modified access 
channel can be used only to transmit modified access requests. 
There follows a description with reference to FIG. 5 of one possible 
configuration of the traffic channels and access channels in a frame with 
eight SDDCH (Standalone Dedicated Control CHannels). 
A conventional frame with eight SDCCH is described in "The GSM System for 
Mobile Communications", by M. MOULY and M. B. PAUTET, published by the 
authors, on pages 204 and 205. 
In this configuration time is represented in the form of a helix and each 
box represents a time slot according to the time-division multiple access 
principle. The time slots are grouped into cycles of eight and the frame 
comprises 102 such cycles. 
In the simplest form of this frame a channel corresponds to the repetition 
of a time slot every eight time slots. On the other hand, in the frame 
with eight SDCCH a given channel includes eight sub-channels, each 
sub-channel being called SDCCH and corresponding to a set of four time, 
slots, one per cycle of eight over four consecutive cycles, in the same 
time position relative to the cycle origin, and repeating every 51 cycles. 
In FIG. 5, which shows the downward frame only, the SDCCH are marked SDCCH0 
through SDCCH7. To clarify the description there is no discussion 
hereinafter of the use of sub-channels other than the SDCCH and the seven 
time slots separating one time slot of an SDCCH from the next time slot 
are not shown. 
According to the invention, four of the eight SDCCH are reserved for 
traffic, for example: SDCCH0, SDCCH2, SDCCH4 and SDCCH6; the other four 
SDCCH, known as dedicated SDCCH, are reserved for modified access 
channels: SDCCH1, SDCCH3, SDCCH5 and SDCCH7. 
If there are four shadow areas classes, for example, each of the four time 
slots constituting each dedicated SDCCH can be reserved to a separate 
shadow area class. If there are fewer or more than four shadow area 
classes the time slots of the dedicated SDCCH, in time order, are reserved 
in turn for each class (in FIG. 5, there is shown in the first half of the 
frame an example with two shadow area classes C1 and C2 and in the second 
half of the frame an example with five shadow area classes C'1 through 
C'5). 
The duration of the access request messages is less than the duration of a 
time slot. The access requests are decoded, demodulated, modulated and 
coded again for retransmission by the booster R1 in the time slot of a 
dedicated SDCCH corresponding to the class of the shadow area Z1 in which 
the mobile station MS from which they originate is located. This 
retransmission is effected with the same timing advance, known as the 
partial timing advance, representing the distance between the mobile 
station MS and the booster R1, as that with which the booster R1 received 
the message RA. 
It is therefore necessary for the booster R1 to be capable of measuring the 
timing advance, of course. It is sufficient to provide it with the 
appropriate means, which can be similar to those used by a conventional 
base transceiver station which has to implement this function. 
The booster R1 also needs to know the partial timing advance in order to 
decode access requests received from the mobile station MS. 
The fact that the boosters retransmit the modified access requests with the 
partial timing advance enables the base transceiver station BTSP to deduce 
the total timing advance directly. This is the sum of the partial timing 
advance and the timing advance representing the distance between the 
booster R1 and the base transceiver station BTSP. It is this total timing 
advance which must be communicated to the mobile station MS for delaying 
its subsequent transmissions, as all signals transmitted by the mobile 
station MS are relayed by the booster R1. 
The base transceiver station BTSP decodes the modified access requests 
and, thanks to its internal clock, knows the timing reference of the time 
slot in which they were transmitted. It transmits this time reference to 
the base station controller BSC. As the base station controller BSC knows 
the modified frame configuration, it can deduce from it the shadow area 
class and therefore assign the mobile station MS an appropriate traffic 
channel. 
If frequency hopping is applied in the cell CP, i.e. if the carrier 
frequencies used on a traffic channel change according to a predetermined 
repetition law, and if the number of frequencies used by the base 
transceiver station BTSP is greater than the number of translated 
frequencies used by the booster R1 (i.e. if some basic frequencies have no 
associated translated frequency), the base station controller BSC must 
assign to the mobile station MS a traffic channel using only translated 
frequencies associated with translated frequencies of the booster R1. 
A possible alternative solution is to allocate the mobile station a channel 
with no frequency hopping, the frequency of this channel evidently being a 
translated frequency at the booster R1. A channel with no frequency 
hopping is typically the signalling channel (BCCH). However, it is 
possible to provide other channels with no frequency hopping when 
configuring the network. 
In an alternative implementation the booster R1 can apply, in addition to 
the translation law T, a repetition law which is specific to it and which 
can be different from that applied by the base transceiver station BTSP to 
effect frequency hopping. 
In this case, however, the signalling channel carried by the broadcast 
control channel is always a channel with no frequency hopping, both at the 
base transceiver station BTSP and at the booster R1. 
This implementation does not cause any problems if the base transceiver 
station BTSP and the booster R1 use the same number of frequencies. 
If the booster R1 uses fewer frequencies than the base transceiver station 
BTSP (in particular if few frequencies are available for the shadow 
areas), it is feasible not to apply frequency hopping in area Z1 although 
it is applied in CP. In this case, the repetition law for the area Z1 is 
significantly different from that for the cell CP. 
If the booster R1 uses more frequencies than the base transceiver station 
BTSP (for example in the event of fading in the shadow area Z1), i.e. if 
some basic frequencies are associated with more than one translated 
frequency, it is feasible to apply frequency hopping in area Z1 but not in 
cell CP. Once again, the repetition law specific to area Z1 is 
significantly different from that associated with cell CP. 
In the latter case, an appropriate law for selecting the translated 
frequency in the downward direction must be applied by the booster R1 to 
retransmit the received signals, since it can retransmit on more than one 
translated frequency a signal conveyed by a single basic frequency. 
If the repetition laws used by the boosters are different from those used 
by the base transceiver stations it is possible to define shadow area 
classes not only by the associated translation law but also by the 
repetition law. This makes it possible to increase the diversity of the 
booster classes and therefore to reduce further the risk of interference. 
When frequency hopping is used in a GSM type network, the translation law 
must be an increasing monotonous function. In GSM type networks a 
repetition law is defined by a list of frequencies to be used and by a law 
for choosing from this list, this law being used to determine that a given 
frequency, for example the ith frequency, must be used at a given time. 
If the translation law is not an increasing monotonous function, for 
example if it interchanges the order of the first two frequencies, when 
the base transceiver station BTSP transmits on the first of these two 
frequencies the booster R1 retransmits on the second translated frequency 
instead of the first. 
There follows a description of the specific features of the handover 
procedure associated with the method of the invention. 
In the conventional way, when a mobile station is accessing a base 
transceiver station it knows the broadcast control channel frequencies of 
the adjoining cells. It therefore "monitors" these broadcast control 
channels on the signalling channel and measures the level at which it 
receives them as well as the level at which it receives the broadcast 
control channel of the cell which it is currently accessing, referred to 
as the "old cell". The measured values are transmitted to the base 
transceiver station of the old cell, which compares the various receive 
levels. 
As soon as the receive level of the broadcast control channel of the old 
cell is below the receive level of one of the other broadcast control 
channels, the base transceiver station advises the base station controller 
of this to tell it that a change of base transceiver station (handover) is 
desirable. The mobile station then receives a handover instruction which 
assigns it a traffic channel from the base station controller, via the 
base transceiver station of the old cell. The mobile station then 
transmits access messages specific to the handover procedure to the base 
transceiver station of the new cell which it is to access, in order for 
the latter to determine the timing advance to be used by the mobile 
station. 
In accordance with the invention, a mobile station in cell CP or in area 
Z1, Z2 or Z3 knows, in addition to the frequencies of the broadcast 
control channels of the adjoining cells, the translated frequencies 
associated with the frequency f.sub.P1 in areas Z1, Z2 and Z3, i.e. the 
frequencies f.sub.R11 and f.sub.R21 (remember that areas Z1 and Z3 are of 
the same class and therefore use the same translated frequencies); it 
measures their receive level in the same way. 
The mobile station MS in the shadow area Z1 transmits its measured values 
on the translated frequency of the upward broadcast control channel 
f'.sub.R11 to the base transceiver station BTSP; these measured values are 
transmitted to the base transceiver station BTSP on the upward broadcast 
control channel f'.sub.P1 via the booster R1. When the measured values 
indicate that the mobile station MS should access CP rather than the 
shadow area Z1, for example if the receive level for f.sub.P1 is greater 
than that for f.sub.R11, a conventional handover procedure ensues, as if 
the shadow area Z1 were a normal cell and the booster R1 a conventional 
base transceiver station. 
If the measured values of a mobile station accessing the base transceiver 
station BTSP and outside any shadow area or accessing the base transceiver 
station BTSP through a booster covering a first shadow area indicate that 
the receive level for f.sub.R11 is greater than that for f.sub.P1 or 
f.sub.R22, then the base transceiver station BTSP communicates to the base 
station controller BSC the broadcast control channel received best. The 
base station controller BSC knows that this broadcast control channel 
cannot be the broadcast control channel of an adjoining cell and that it 
is not that of the old cell. The base station controller BSC then has only 
to apply to this frequency the converse translation law to determine the 
class of the associated shadow area and thus, by means of an appropriate 
message, to assign a correct traffic channel to the mobile station, i.e. a 
traffic channel using translated frequencies. 
The remainder of the procedure is the same as conventional handover. 
The base station controller BSC knows all the translated frequencies used 
in the various shadow areas. The base transceiver station BTSP necessarily 
holds in memory, and therefore knows, the translated frequencies 
associated with its broadcast control channel, in order to be able to 
avoid instigating handover to itself. 
In accordance with a final aspect of the present invention the boosters can 
be controlled and monitored by radio from the network operation and 
maintenance centre using the downward SDCCH corresponding to the dedicated 
upward SDCCH, for example. 
The control function consists in sending to the boosters information as to 
changes to the radio configuration, for example (changing of modified 
access channels, modification of the associated translation law, etc). 
As for monitoring, this can consist in sending a status request to the 
booster periodically, for example, to which the booster responds by radio. 
Of course, the invention is not limited to the embodiment just described. 
Firstly, the method of the invention can be applied in the case of a GSM or 
other type cellular system. 
The method of modifying the access requests can be different from that 
described, providing it enables the control means to tell that a mobile is 
in a shadow area and the class of that shadow area, if necessary. 
A complete channel of a simpler frame could be used rather than half the 
SDCCH in a frame of eight SDCCH, for example. 
Also, to modify the received access request it is not necessary to decode 
it and then demodulate it first. However, in this case, even if the 
implementation is less complex at the booster, it involves a loss of time 
slots, in particular because a complete time slot is needed for each class 
if there is more than one class of shadow area. However, it may be 
feasible to use fewer time slots than there are shadow classes, for 
example, by retransmitting only access requests whose level is above a 
predetermined threshold, which would reduce the number of traffic channels 
translated into modified access channels. 
Also, instead of executing a handover procedure on moving from one shadow 
area booster to another or to a base transceiver station, or on moving 
from a base transceiver station to a shadow area booster, the mobile 
station could be sent a message of the "Frequency Redefinition" type, to 
use the GSM terminology. This message is a conventional message for 
reconfiguring frequencies in the network and will include in these various 
cases the new frequencies to be used by the mobile station, for example 
the translated frequencies of the shadow area which it must access. 
Another possible application of the method of the invention is to provide 
boosters at the extreme range of the base transceiver station 
transmitters, especially in two-way transmission media (roads, railroads) 
for very large radius cells (35 km). This increases the cell range without 
increasing the number of transmission sites requiring cable connections, 
which is particularly advantageous. 
Finally, any means as described herein can be replaced by equivalent means 
without departing from the scope of the invention.