Time and frequency slot allocation system and method

Radio communication systems employing time division multiple access (TDMA) on several frequency channels are disclosed. The base station transmitting powers can be reduced when communicating with nearby mobile stations while still permitting the base station to employ a constant transmitting power in all time slots. This can be accomplished by, for example, grouping mobile stations having similar transmit power requirements together and allocating such groups to time slots on a same frequency. In this way, the base station can transmit at a constant power for each time slot on a frequency and provide acceptable communication quality, but without unnecessarily wasting base station transmitter power by transmitting to mobile stations having widely divergent power requirements.

BACKGROUND 
The user capacity of mobile radio communication systems is limited by the 
width of the frequency spectrum available for signal transmission. In 
order to maximize a system's capacity, therefore, it is desirable to 
utilize the available frequency band in the most efficient manner 
possible. 
Cellular telephone systems in operation today commonly use an access 
technique known as Frequency Division Multiple Access (FDMA) to permit a 
base station to communicate with a plurality of mobile stations. In FDMA 
systems, each communication link is allocated a unique frequency slot of 
channel in the radio spectrum. 
Newer systems use Time Division Multiple Access (TDMA), in which a base 
station communicates with a plurality of mobiles on the same frequency 
channel by dividing up a time cycle into time slots. The European GSM 
standard is an example of a system using FDMA and TDMA to allocate both 
frequency and time slots to mobile calls. The system uses 200 KHz wide 
frequency slots in each of which a 4.6 mS transmission cycle is divided 
into eight, 560 uS time slots, with short guard periods between each. 
The guard periods in GSM are provided because base station transmission 
during a time cycle is not held at a constant power for all time slots, 
but instead changes the power level for each time slot based on the 
distance of the mobile station using that time slot from the base station. 
Moreover, for transmissions which employ frequency hopping, wherein the 
frequency channel employed for each 4.6 mS time cycle changes, a guard 
period of zero transmission power is provided whenever power or frequency 
is changed discontinuously to avoid spectral splatter into other frequency 
channels. 
Another example of a system employing both TDMA and FDMA is the US 
Telecommunications Industry Association standard IS54. The IS54 standard 
describes a system having 30 KHz wide time slots, in each of which a base 
station employs a 20 mS transmission cycle divided into three, 6.6 mS time 
slots with no guard period between. The base station transmission in this 
system is actually just a continuous transmission of time-multiplexed data 
to three mobile stations. There is no guard period provided in TIA IS54 
because frequency hopping is not employed, on the contrary, the system 
anticipates that the power level will be the same in all time slots. 
U.S. Pat. No. 4,866,710 to Schaeffer describes a method of allocating 
frequencies and time slots to mobile stations such that all the time slots 
on a given frequency are filled first before allocating time slots on 
another frequency. By packing mobile stations preferentially in this way, 
the transmitters and frequencies that have not as yet allocated time slots 
can be switched off completely, reducing interference. This would reduce 
wasted capacity in the IS54 system arising from the requirement that base 
stations continually transmit on all three time slots even when only one 
is needed. However, it will be noted that the base station still transmits 
at one maximum power level for each frequency in use, irrespective of the 
power needs of each particular mobile, resulting in a higher net level of 
interference than if the power needs of each mobile were taken into 
account. 
SUMMARY 
Accordingly, it is an object of the present invention to achieve reduction 
of interference by a more effective strategy that works even when all time 
slots are filled. Exemplary methods according to the present invention 
allocate mobile stations to time slots on the same frequency as other 
mobile stations requiring similar base station transmitter power levels. 
In this way, mobiles which are allocated time slots on a given frequency 
channel will likely lie at similar distances from the base station. The 
base station transmitter power can then be chosen to be just sufficient 
for the mobile station on that frequency that needs the greatest power 
level. This provides a greater power margin than needed for the other 
mobiles on that frequency, but nevertheless allows a lower base station 
power than if mobiles had been allocated time slots and frequencies 
without regard to power needs. Thus, each frequency channel will serve a 
group of mobiles with similar base power transmission needs, and the base 
power can be correspondingly reduced on each frequency channel so as to be 
just sufficient for good signal transmission for the group. The cumulative 
reductions in power on every channel, therefore, will significantly reduce 
interference in the system. 
According to an exemplary embodiment of the present invention, when the 
first mobile link with a given base is set up, the base chooses a 
frequency and time slot containing minimum interference. Commands are then 
issued to the mobile station to adjust its power level to a level 
sufficient for good received signal quality at the base. The mobile 
station in turn reports signal strength or quality received from the base 
station and the base station chooses a power level sufficient to provide 
good signal quality at the mobile. 
When a second mobile link with the same base is set up, the base estimates 
the power level to be transmitted to that mobile and allocates to the 
second mobile another time slot on the same frequency if the power level 
to be transmitted is close to that used for the first mobile. If the 
required power level is slightly higher than that for the first mobile, 
the base smoothly increases the power transmitted to the higher level. If 
the second mobile requires a power sufficiently lower than the first 
mobile, it is allocated a time slot on a second frequency. The base then 
adapts its power and commands the mobile power to appropriate levels to 
maintain adequate signal quality in both directions. 
According to exemplary embodiments, when a new mobile link is to be 
established with a base station already having a plurality of ongoing 
communications, the base station first estimates the power level that is 
appropriate for transmitting to that mobile. This is compared to the power 
level of all ongoing transmissions on frequencies that have at least one 
empty time slot. The mobile is then allocated a time slot on that 
frequency where the transmission power is greater than but closest to the 
estimated power. If no existing transmitter is of high enough power, the 
highest power transmission is smoothly increased to the estimated 
requirement for the new mobile, and the new mobile allocated an unused 
time slot on that frequency.

DETAILED DESCRIPTION 
In order to fully appreciate systems and methods according to the present 
invention, a more detailed description of conventional systems will first 
be provided. 
FIGS. 1 and 2 illustrate conventional allocation schemes whereby the base 
station transmits at maximum power to the mobiles, irrespective of their 
power requirements. In FIG. 1 (a), mobiles are assigned frequencies 
(F1-F4) and time slots (Ts1-Ts3) essentially at random. Regardless of the 
power level required for each mobile, the base station transmits at the 
same maximum power level on all time slots as seen in FIG. 1(b). FIG. 2(a) 
illustrates allocating frequency and time slots to new mobiles so as to 
concentrate the mobiles on as few frequencies as possible in order to 
eliminate transmission on other frequencies. Note that all of the time 
slots on frequencies F1 and F2 and two of the three time slots on F3 have 
been filled. It can be seen in FIG. 2(b), however, that all base stations 
having at least one active time slot transmit at the same maximum power 
level according to this conventional scheme while those that have no 
active time slot are switched off. Moreover, neither conventional 
allocation scheme adjusts the power level transmitted by the base to be 
commensurate with that required by the mobiles. 
FIG. 3(a) shows mobiles having the same power requirements as used in FIGS. 
1(a) and 2(a) being allocated to time slots (Ts) and frequencies (F) 
according to an exemplary embodiment of the present invention. Note that 
the three mobiles (1, 7 and 4) requiting the most power are allocated time 
slots on frequency F1, the next highest three mobiles (8, 2 and 5) are 
allocated on frequency F2 and the mobiles requiting the lowest base 
transmit power (6 and 3) are allocated to frequency F3, illustrating that 
many transmitters transmit at lower than maximum power while those that 
have no active time slots do not transmit at all. Although the number of 
transmitters which have been switched off (one) is the same as in FIG. 
2(b), an additional benefit is obtained by operating those transmitters 
that are active at reduced power levels. 
According to an exemplary embodiment of the present invention, when the 
first mobile link with a given base is set up, the base either chooses a 
frequency and time slot at random, or chooses the frequency and time slot 
containing minimum interference. Commands are issued to the mobile station 
over the air to adjust its power level to a level sufficient for good 
received signal quality at the base. According to one embodiment, this 
power level can be that which is just high enough to provide good received 
signal quality at the base. The mobile station reports signal strength or 
quality received from the base station and the base station chooses a 
power level sufficient to provide good signal quality at the mobile. 
Again, this power level may be that which is only just sufficient for this 
purpose. 
When the second mobile link with the same base is set up, the base 
estimates the power level to be transmitted to that mobile and, if, for 
example, within the range 6 dB higher to 10 dB lower than that used for 
the first mobile, the base allocates to the second mobile another time 
slot on the same frequency as the first mobile, preferably the time slot 
containing the lowest level of interference. Note in this regard the 
similarity in power requirements for each mobile on each frequency channel 
F1, F2 and F3 in FIG. 3(a). If the required power for the second mobile 
link level is, for example, 0 to 6 dB higher than that for the first 
mobile, the base smoothly increases the power transmitted to the higher 
level. If the second mobile requires a power more than, for example, 10 dB 
lower, or 6 dB higher, than the first mobile, it is allocated a time slot 
on a second frequency, preferably the time slot which contains the minimum 
level of interference. The base then adapts its power and commands the 
mobile power to appropriate levels to just maintain adequate signal 
quality in both directions, as before. 
When the third mobile link with the same base is set up, the base estimates 
the power it will need to transmit to the third mobile. Assuming the first 
two mobiles are already using the same frequency, if the third mobile 
requirement is within the range of, for example, 12 dB greater than the 
weaker of the first two mobiles to 12 dB lower than the stronger of the 
first two mobiles, the third mobile is allocated another time slot on the 
same frequency and power levels are adapted appropriately as before. 
Otherwise, the third mobile is allocated a time slot on another frequency, 
preferably that having the lowest level of interference. 
When a new mobile link is to be established with a base station already 
having a plurality of ongoing communications, the base station first 
estimates the power level that is appropriate for transmitting to that 
mobile. This is compared to the power level of all ongoing transmissions 
on frequencies that have at least one empty time slot. The mobile is then 
allocated a time slot on that frequency for which the transmit power is 
greater than but closest to the estimated power. If no existing 
transmitter is of high enough power, the highest power transmission is 
smoothly increased to the estimated requirement for the new mobile, and 
the new mobile allocated an unused time slot on that frequency, preferably 
that containing the least interference. The transmit power levels are then 
adjusted appropriately as before. Similarly, the transmission power can be 
ramped down for frequencies in which a highest power time slot becomes 
idle after a connection serviced on that time slot becomes disconnected. 
FIG. 4 shows an exemplary network block diagram according to the present 
invention. A mobile switching center (MSC) 40 is connected by landline or 
other communication links to a number of base station sites referenced by 
numerals 41,42. Each base station site contains a number of TDMA 
transmitters, receivers and antennas for communicating with mobile 
stations M. The operating frequencies of each transmitter and receiver may 
be fixed according to a so-called cell plan or frequency-reuse pattern, 
but are preferably programmable to any channel in the allocated frequency 
band. The base station site may also contain a base station controller 43. 
The optional base station controller can be provided when it is desired to 
separate the intelligence for implementing the current invention from 
those functions normally performed by the MSC. When the MSC 40 is able to 
perform the functions required, the base station controller 43 may simply 
be a concentrator to funnel communications between the transceivers and 
the MSC. 
As a further option, an interference assessment receiver 44 can be used to 
provide information via the base station controller to assist in the 
allocation of frequency and time slots to mobiles. The interference 
assessment receiver can be a scanning receiver, spectrum analyzer or 
multichannel device adapted to determine the interference energy levels in 
each of the presently unused frequencies and time slots at that base 
station site. This can be supplemented by measurements from the traffic 
receivers in unused time slots on their own frequencies. 
The base station normally also contains a calling channel transmitter and 
random access receiver 45. The calling channel transmitter broadcasts 
information about the status of the base station to mobiles that may wish 
to establish communication. The random access receiver receives 
transmission from mobiles attempting to establish communication, before a 
traffic channel is allocated to the mobile according to this exemplary 
embodiment of the present invention. In the IS54 system, the calling 
channel is presently a non-TDMA transmission employing continuous 
transmission on a special frequency. The random access receiver operates 
on a corresponding frequency 45 MHz lower. Calling channel broadcasts and 
random access take place using Manchester code frequency modulated data 
transmission as in the US AMPS cellular system. At a later date it is 
probable that a TDMA calling channel will be introduced, together with a 
TDMA random access channel. If the TDMA calling channel uses, for example, 
only one out of three time slots while traffic is transmitted in the other 
two, then traffic requiring full power should be assigned to the remaining 
time slots on the calling channel frequency which typically requires full 
power. 
It will be appreciated that the functions of the MSC and the base station 
controller as described above can be implemented conveniently with the aid 
of one or more microprocessors or computers and appropriate software e.g., 
processor 46 in FIG. 4. The processor or computer receives data messages 
transmitted by mobile stations requesting call set up or, for already 
existing communications, reporting signal strength or quality levels 
received from the base station. The computer or processor also receives 
data from the base station receivers which provides information pertaining 
to the signal strength or quality received from the mobiles, as well as 
interference levels in unused time slots. 
According to this exemplary embodiment of the present invention, the 
computer processes this data to determine an appropriate frequency and 
time slot for communicating with a given mobile station, and sends control 
signals to the chosen base station transmitter-receiver so that it expects 
the mobile signal. The computer generates a message for transmission to 
the mobile to command it to operate in the chosen frequency and time slot. 
Messages are also generated for transmission to the mobile to command it 
to adjust its power level according to the received signal strength or 
quality at the base station receiver. Similar control signals are also 
sent to the base station transmitter so as to control its power level to 
be, for example, the minimum necessary to maintain signal quality as 
reported by the mobile on that frequency receiving the lowest quality. 
Alternatively, the power level can be selected to be some margin higher 
than this minimum necessary power. 
When a base station maintains a large number of ongoing conversations with 
a multiplicity of mobile stations, there can arise reasons to change the 
frequency and timeslot allocations between mobile stations even when no 
old calls are terminating and no new calls are being initiated. Due to 
mobile motion, a mobile previously requiring high power may now be 
satisfied by lower base station power or vice versa. A simple systematic 
means to reshuffle frequency and timeslot allocations is for the network 
to maintain a list of ongoing conversations sorted by order of signal 
strength received from the mobiles, or, more accurately, sorted in order 
of radio propagation loss between the base station and the mobiles. The 
radio propagation loss may be computed from a knowledge of the received 
signal strength and the power level the mobile was previously commanded to 
adopt. A second check on this value may be computed from a knowledge of 
the signal quality reported back by the mobile and the transmitter power 
the network is transmitting to it. All such information may be utilized 
and averaged over a period of a few seconds to obtain a smoothed estimate 
of propagation loss. 
Using the sorted list, the network ensures to the best of its ability that 
the top three mobiles on the list are allocated timeslots on the highest 
power carrier frequency; the next three mobiles in the list are allocated 
timeslots on the next strongest carrier frequency and so forth. If a 
Digital Control Channel is in use and transmitted on the strongest 
carrier, then the top two mobiles in the list are allocated the same 
carrier, the next three the second strongest carrier and so-on. The 
network may, if required, swap two mobiles between two carriers to achieve 
this. For example, if the highest power mobile X on carrier B due to 
relative movement now has a higher power requirement than the lowest power 
mobile Y on a stronger carrier A, then X and Y are caused to change 
frequency and timeslot allocations by issuing them with hand-off commands. 
Such hand-offs within the same base station area are called "internal 
handovers", and are made purely to achieve a more optimum 
frequency/timeslot packing that minimizes created interference with 
neighboring bases. 
It has already been indicated above that an exception to the packing rule 
may be desirable if there is a large dB difference (e.g., &gt;10 dB) between 
the carrier power and that needed by a mobile next on the list. It may be 
desirable to allocate that mobile to a lower power carrier together with 
the next two mobiles below it in the list. This results in an apparently 
unnecessary higher power transmission on a timeslot that is not allocated, 
but this departure from the absolute tightest packing algorithm has the 
advantage that a few unoccupied timeslots are distributed throughout the 
signal strength range and are thus available for allocating to new calls 
without having first to disturb a large number of ongoing conversations. 
It can even be adopted as a deliberate strategy, to leave a "hole" every 
15 dB or so of propagation loss range, depending on the loading of the 
system, in order more rapidly to be able to accommodate new calls. If 
because of this coarse power step between "holes", a mobile has to be 
allocated to a "hole" on a carrier that is unnecessary, this will be 
corrected by the systematic resorting procedure that takes place on a 
slower timescale. Such a continuous resorting procedure also handles the 
event of a mobile call terminating. In principle all mobiles below it in 
the power/propagation loss list can be moved up, resulting in the highest 
of three perhaps receiving an internal handover to the next highest power 
carrier. This does not however take place all at once necessarily but 
gradually. The rate of handovers can be restricted so that no mobile 
receives a handover more often than, for example, say once per ten 
seconds. If a mobile has received an internal handover or handoff within 
the last ten seconds for example, it is not allowed to be a candidate for 
a handoff until ten seconds have passed. When the strongest of three 
mobiles on the three timeslots on a given carrier terminates its call, the 
power of the carrier may of course be regulated down to the stronger of 
the two remaining, thus reducing created interference levels. 
The above-described exemplary embodiments are intended to be illustrative 
in all respects, rather than restrictive, of the present invention. Thus 
the present invention is capable of many variations in detailed 
implementation that can be derived from the description contained herein 
by a person skilled in the art. All such variations and modifications are 
considered to be within the scope and spirit of the present invention as 
defined by the following claims.