Methods and apparatus for excluding communication channels in a radio telephone

A portable radio telephone (5) communicates with base stations (1, 2) in a digital radio telephone system, such as DECT, employing TDMA transmission. The portable radio telephone is programmed with an algorithm which is operative to select a communication channel by storing a blind slot mask (7) representative of the time slots in each frame which are excluded, either by virtue of being occupied by the base station or by the portable radio telephone itself. Hence, any channel attempted and failed in an occupied time slot is excluded and marked blind in the blind slot mask. Further, the algorithm is operative to check the time slots for availability in a predetermined sequence which corresponds to the temporal sequence of the slots (or the reverse thereof) to promote orderly filling of the time slots at the base stations.

CROSS-REFERNNCE TO A RELATED APPLICATION 
This patent application is related to commonly assigned U.S. patent 
application Ser. No. 08/584,054, filed on even date herewith, entitled 
"Portable Radio Telephones and Methods of Operation", to N. Tat et al. 
BACKGROUND OF THE INVENTION 
This invention relates to portable radio telephones and in particular to 
such telephones for communication with base stations in a digital cellular 
radio telephone system employing transmission by a plurality of carrier 
frequencies in frames, such as TDMA frames, each consisting of a 
predetermined number of time slots. The invention also relates to a method 
of operation of such radio telephones, frequently called handsets. The 
invention is concerned with the allocation of a channel (that is a 
combination of a carrier frequency and time slot) to a portable radio 
telephone when a connection is first required ("call set-up") or when a 
change in channel is required ("handover") during a call to maintain call 
quality. Handover can either be to a different channel at the same base 
station (intracell) or to a different channel at a different base station 
(intercell). 
Most of the currently manufactured DECT base station equipment only 
contains a single transceiver, and is therefore unable to open more than a 
single communication channel on different frequency carriers at the same 
instant. This restricts single transceiver base stations to only using a 
single time slot at any one time, effectively excluding the other carriers 
on this time slot and thereby turning them "blind". The DECT standards 
have foreseen this and include the blind slot information message to 
inform the handset of time slots it should avoid using. However, this 
information is not totally reliable, particularly on intercell handover 
when it is very difficult to obtain the blind slot information of any base 
station other than the one that the handset is connected to unless a 
separate `observation` channel is opened. In addition to slots "blinded" 
by the base station, a handset will be unable to switch communication 
channels to a slot immediately adjacent to that which it is using. 
This invention aims to provide a portable radio telephone, and a method of 
operation, employing a dynamic channel assignment algorithm that gives 
very robust performance, regardless of the availability and accuracy of 
the blind slot information. In addition to this, if an identical algorithm 
is deployed on every portable radio telephone in the system, further 
improvements in call blocking and call quality are likely. 
SUMMARY OF THE INVENTION 
According to one aspect of the invention there is provided a portable radio 
telephone for communication with base stations in a digital cellular radio 
telephone system employing transmission by a plurality of carrier 
frequencies in frames each consisting of a predetermined number of time 
slots, wherein the portable radio telephone comprises processing means 
operative to select for the radio telephone a communication channel, that 
is a combination of carrier frequency and time slot, by excluding any 
channel in a time slot where a channel selection attempt at a base station 
has failed. 
It will be appreciated that the exclusion of a time slot will be temporary 
and that a particular slot will be "unblinded" by subsequent events. 
Preferably, the exclusion of a slot prevails for a number of unsuccessful 
channel selection attempts. 
Preferably, on call handover from a current channel to a new channel the 
processing means additionally exclude any channel in the three adjacent 
time slots centered on the current channel. 
The portable radio telephone may comprise storage means for storing a blind 
slot mask representative of the slots in each frame which are excluded, 
the remaining slots being available for selection of a channel. 
Once channel selection has been unsuccessfully attempted in each of the 
remaining slots, the blind slot mask can be reversed so that those slots 
that have not previously been tried are rendered available for possible 
selection of a channel. 
The processing means may be operative to check the slots for availability 
in a predetermined sequence which may correspond to the temporal sequence 
of the slots or to the reverse of the temporal sequence of the slots, 
until an available slot is found and a channel therein is selected. 
In the preferred embodiment to be described, slots are checked for 
availability in sequence, starting from the first slot in the frame 
allocated for handset transmission and progressing to the final slot, 
before returning to the first slot. 
According to another aspect the invention provides a method of 
communicating between a portable radio telephone and base stations in a 
digital cellular radio telephone system employing transmission by carrier 
frequencies in frames each consisting of a predetermined number of time 
slots, comprising selecting for the radio telephone a communication 
channel, that is a combination of carrier frequency and time slot, by 
excluding any channel in a time slot where a channel selection attempt at 
a base station has failed. 
According to a yet further aspect there is provided a portable radio 
telephone for communication with base stations in a digital cellular radio 
telephone system employing transmission by a plurality of carrier 
frequencies in frames each consisting of a predetermined number of time 
slots, wherein the portable radio telephone comprises processing means 
operative to select for the radio telephone a communication channel, that 
is a combination of carrier frequency and time slot, such that successive 
attempts at channel selection are made in respective ones of the 
predetermined number of time slots.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, the portion of the radio system depicted comprises two 
fixed base stations 1 and 2 serving respective cells 3 and 4 representing 
geographical areas of coverage which may be inside or outside buildings. A 
user or subscriber to the system carries a portable handset 5 which is 
capable of two-way radio communication with one or other of the base 
stations 1, 2, and with other base stations (not shown). 
In the described example shown in the drawings, the radio telephone system 
conforms to the DECT (Digital European Cordless Telecommunications) 
standard and uses ten carrier frequencies, separated by 1.728 MHz, within 
a frequency band from 1880 MHz to 1900 MHz. This system divides time into 
TDMA frames, with each frame having a time duration of 10 ms. Each frame 
is divided into 24 time slots, numbered from 0 to 23. Each frame is 
divided into two halves, the first half (slots 0 to 11) being reserved for 
the transmission of the base station and the second half (slots 12 to 23) 
being reserved for the transmission of the handset. 
The handset 5 has processing means programmed with an algorithm which is 
operative to select for the handset a communication channel, i.e. a 
combination of carrier frequency and time slot, taking into account the 
quietness of each channel, i.e. the degree of signal interference in that 
channel. To achieve this, the numbered signal strength of all channels is 
monitored at regular time intervals and the measured signal strengths are 
stored in the handset in a channel list, an example of which is shown in 
FIG. 3 in which the time axis extends horizontally and the frequency axis 
extends vertically. 
Referring to FIG. 3, the twelve slots 1-12 of a representative frame are 
shown as columns, with the ten carrier frequencies being represented by 
horizontal rows. Hence, the array of FIG. 3 has 120 boxes, each 
representing a particular channel, i.e. a particular combination of time 
slot and carrier frequency. Signal strength is allocated a level from 0 to 
12 (typically 6 dB bands are used), with 0 being the quietest (i.e. least 
interference) and 12 being the least quiet (i.e. most interference). 
Representative quietness levels for each channel are shown in FIG. 3 and 
the channels that do not satisfy the quietness requirement are shown 
shaded. FIG. 3 also shows the base station scan. The base station scans 
one carrier every time frame and works through all ten carriers in ten 
frames. 
The algorithm fulfills three theoretical principles that all improve the 
probability of the portable handset assigning itself an available channel. 
These are: 
1. Once a quiet channel has been attempted on a particular base station it 
can be assumed that this slot is blind, and no other channel on this slot 
should be attempted until all other slots have been tried first. 
2. Once all the timeslots on the strongest base station have been attempted 
it can be assumed that it is full to capacity, and the same procedure can 
be applied to the next strongest base station. 
3. Channels that fulfill the quality criteria for use (in DECT the least 
interfered channels are assumed to fulfill this criterion) should then be 
sorted for assignment in slot order. Then if all handsets use the same 
ordering for channels, slots will be blinded at approximately the same 
rate on all base stations if roughly even numbers of calls exist in each 
cell. This would have the effect of matching blind slot messages from all 
the base stations in the system. 
Using the quietness bands to specify signal strength, the quality criteria 
for channel assignment is as follows: 
1. The quietest available channel 
2. If this quietest channel cannot be attempted within the next three 
frames (i.e. it is within three carriers of the current primary scan of 
the base station) a channel that can be attempted within three frames can 
be selected if it is within 2 bands of the quietest, otherwise the 
quietest is selected. 
FIG. 4 shows at 6 the blind slot message received by the handset 5 from the 
base station 1 on call set-up. This indicates that the first three time 
slots are blind. Since the handset 5 imposes no blind slots, the blind 
slot mask 7 on the handset corresponds to the blind slot message 6 from 
the base station. Hence, the first three time slots in the channel list 8 
stored in the handset are blind. The algorithm is operative to check 
availability of time slots in a sequence which proceeds from the first 
time slot allocated for base station transmission to the last time slot 
allocated for base station transmission, before returning to the first 
slot. Hence, quiet channels are checked for availability progressing from 
left to right in the channel list 8 in FIG. 4, so that there is a tendency 
for channels to be filled in an orderly sequence which, if all handsets 
operate the same system, will promote efficient channel allocation at the 
base stations. This check could be in reverse order, and could commence 
with any time slot. 
The algorithm attempts set-up in a channel at "X" in the immediate slot 
after the blind slots in the mask, if one of these channels is one of the 
quietest. Since the blind slot information on the portable is correct, 
set-up succeeds. 
Next, referring to FIG. 5, it is assumed that an intracell handover is 
required because the current channel degrades in quality. In this case the 
blind slot message 9 from the base station indicates that the first three 
slots remain blinded, together with the fourth slot because this is 
occupied by the transmission of the handset 5. The channel currently 
occupied by the handset, together with a channel either side thereof, is 
blinded by the handset, so the blind slot mask on the handset is a shown 
at 10, the resulting channel list being shown at 11. 
This time the channel assigned, channel X, is slightly out of slot order on 
the base station. However, this vacant slot will soon be filled by another 
handset, due to the channel assignment procedure being in slot order. This 
type of algorithm (if used on every handset in the system) will tend to 
have the effect of assigning adjacent slots at the base station. This also 
decreases the chance of blind slots at the handset not actually being 
blind at the base station. 
Next, referring to FIG. 6, it is assumed that intercell handover is 
required, and that the blind slot message 12 at the new base station 2 is 
different from the blind slot message 13 at the current base station 1. 
This might not be the case if the proposed algorithm is used on every 
portable terminal in the system. 
On the handset, the current transmission channel and the two immediately 
adjacent channels are blinded so, referring to FIG. 7, the blind slot 
message from the currently connected base station 1 is shown at 14 and the 
blind slot mask on the handset at 15, the resulting channel list on the 
handset being as illustrated at 16. The handset tries channels in the 
fourth time slot but as this is blinded at the new base station (as shown 
by the blind slot message 12) channel assignment fails. 
Once the handset fails assignment in a channel, it then marks this slot as 
blind and tries channels in the next slot. As shown in FIG. 8, the handset 
fails in this slot (the eighth) also, marks it as blind and tries the next 
slot (the ninth), as shown in FIG. 9. This time, channel assignment is 
successful and handover occurs. 
To ensure all possible slots are attempted at least once, the following 
procedure is used. Once all the slots in the blind slot mask have been 
marked blind, an inverse of the blind slot message is used to reset the 
blind slot mask, and the process continues as before. This is shown in 
FIG. 10. 
This ensures that if an available slot exists on the base station (and it 
is not one of those thought to be blind by the handset) that it will be 
found. Once all the available slots at a single base station have been 
attempted, then an alternative base station can be tried. In general the 
number of attempts necessary to try all the available slots is the number 
of time slots in a frame less the number of slots blinded by the handset 
itself. In this particular case the number of attempts necessary is nine. 
To avoid additional unnecessary attempts at set-up or handover, reducing 
the efficiency of the set-up or handover process, it is desirable to make 
the number of attempts before handover/setup is discontinued for a 
specified period of, say, two to three seconds correspond to the number of 
attempts necessary to try all the available time slots. 
The first base station tried is usually the strongest, and the second tried 
will be the second strongest. Once a new base station is decided upon, the 
blind slot mask is reset and the procedure starts again. The important 
features of the algorithm are: 
1. The slot ordering of channels that meet a defined quality criteria for 
channel assignment, particularly on every handset in the system. This has 
two beneficial effects: 
Blind slots on the base stations are more likely to be the same, thus 
improving the probability that portable handsets using the current base 
station blind slot information can perform intercell handover to an 
available slot at other base stations. 
Channels will be used at base stations that are generally immediately 
adjacent to each other, thus the likelihood of handsets (that have blind 
slots adjacent to the one in use) missing available slots at the base 
station is reduced. 
2. The marking of the entire slot as blind (in addition to those already 
blind) once assignment has failed at a single channel in this time slot. 
3. When using (2), the limiting of channel assignment attempts at a single 
base station to the maximum number of available slots minus the number of 
blind slots at the handset. 
4. Using (3) to detect a "busy" base station and then starting the channel 
assignment procedure anew with a different base station. 
FIG. 2 shows the steps followed by the algorithm in call set-up. Commencing 
at start, box 20, the next step, box 22, is to measure signal strengths, 
accept the blind slot message from a base station (if available) and load 
into the storage means in the handset the blind slot mask. Channel 
selection (box 23) then proceeds from the first time slot. If an available 
time slot is found, channel set-up is attempted until all nine channels 
are attempted, after which set-up with an alternative base station is 
attempted. Each time set-up in a channel fails, the time slot containing 
that particular channel is marked blind. If the blind slot mask is full 
(decision block 24), the original blind slot message is inverted (box 25) 
and the process repeated (box 26).