Method and system for autonomously allocating transmit power levels for communication between a cellular terminal and a telephone base station

A private radio system within a serving cell of a cellular communications system periodically monitors the broadcast information transmitted on the control channels of the serving and surrounding cells. From this monitoring, all those carrier frequencies serving as control channels are derived for the cell areas around the serving cell and a control channel list constructed. The private radio system then carries out signal strength measurements on the downlink control channels, and orders the control channel list accordingly. Alternatively, a mobile station that is attached to a private radio system can perform the downlink measurements and forward the measurements to the private radio system. In either event, the private radio system selects a communication channel from the ordered list and derives, from the received signal strength values of the control channels, the transmit power level required to both overcome interference caused by more distant cellular base stations and, at the same time, not interfere with those stations.

FIELD OF THE INVENTION 
The present invention relates generally to private radio communication 
systems, which typically cover local indoor residential or business areas. 
Particularly, the present invention relates to radio communication systems 
which employ an air-interface compatible to an existing cellular digital 
Time Division Multiple Access (TDMA) standard like the Global System for 
Mobile Communication (GSM) or the Digital Advanced Mobile Telephone 
Service (D-AMPS). More particularly, the present invention relates to 
automatic transmit power level allocation in such private radio systems 
providing levels that are low enough not to disturb the cellular system 
but are at the same time large enough to overcome the interference from 
the cellular system, and methods and communication systems for 
effectuating the same. 
BACKGROUND OF THE INVENTION 
The past decades have seen a considerable rise in the deployment of mobile 
telephony. After the slow start of analog standards like AMPS, Nordic 
Mobile Telephone (NMT) and the Total Access Communication System (TAS), 
mobile telephony has become recently quite popular in the consumer markets 
with products employing advanced digital standards like GSM and D-AMPS. In 
addition to other developments in mobile phone features, like smaller size 
and longer battery life, much progress has been made at the network side 
as well, particularly, in frequency reuse schemes to avoid co-channel 
interference between adjacent cells. Increasingly, dense cell reuse plans 
have been complemented with hierarchical cell structures, where macrocells 
cover entire districts, microcells cover smaller parts like streets, and 
picocells cover very small areas the size of a few rooms. 
Although with the more dense base station planning the average distance 
between the base station and the mobile stations decreases, along with 
propagation loss, the requisite transmit powers have not decreased 
proportionally. This is primarily because better indoor coverage is 
required, which can only be guaranteed by relatively high transmit power 
levels, even if the overall distances between base stations and mobile 
stations decrease. 
Recently, private networks for residential and business areas have been 
developed, which although using the same air-interface and the same 
spectrum as the cellular system, are not integrated with the overlaying 
public cellular network. In this sense, these private systems cannot be 
considered as micro or pico networks since there is no direct connection 
between these private systems and the cellular system. For example, for 
residential usage, private base stations can be used, such as described in 
U.S. Pat. Nos. 5,428,668 and 5,526,402, which only connect to the Public 
Switching Telephone Network (PSTN). 
If, however, such a private radio communication system is placed into an 
area covered by the cellular system with which the private system has to 
share frequencies, a problem arises since the private base stations are 
not coordinated with the cellular network. In a related patent application 
by the inventor herein, entitled "Method and System for Autonomously 
Allocating a Cellular Communications Channel for Communication between a 
Cellular Terminal and a Telephone Base Station," filed concurrently 
herewith and incorporated herein by reference, a method and communications 
system are disclosed which allow a private radio system to autonomously 
determine those frequencies it can use with minimal disturbance to and 
from the overlaying cellular system. This is accomplished by having the 
private radio system analyze the uplink and downlink received signal 
strength measurements. The aforementioned related patent application, 
however, does not address the separate albeit related problem addressed in 
the present application, i.e., how the transmit power levels on those 
private radio systems should be chosen. 
In a sparsely populated communications area with cellular base stations 
widely spaced apart, private radio systems within an overlaying cellular 
system can use low transmit power levels since the interfering cellular 
base station will generally be far away. However, in a more dense signal 
environment, a cellular base station reusing the same frequencies may be 
considerably closer and can, therefore, produce much interference with the 
private radio systems therein. This interference can only be overcome by 
the private base station using greater transmitter (TX) power. However, 
since the private base station is not coordinated with the cellular 
system, it is a non-trivial problem for the private radio system to 
determine what TX power to use in its particular environment so as not to 
cause interference itself. Furthermore, the requisite TX power levels may 
change over time when cellular base stations are added or removed. 
It is, accordingly, an object of the present invention to provide a method, 
and an associated communications system, in which transmit power level 
allocation occurs in a private radio system autonomously. 
It is a further object of the present invention that the method be 
adaptive, particularly, that the private radio system adapt autonomously 
and automatically to transmit power level and frequency changes within the 
overlapping cellular network. 
It is a more particular object of the present invention that the method 
automatically determines a suitable transmit power level for a given 
private radio system within the cellular network which not only overcomes 
interference from the cellular network but is also not so strong as to 
cause interference itself. 
SUMMARY OF THE INVENTION 
The present invention advantageously provides an allocation method and 
communication system for autonomously allocating transmit power levels to 
a private radio system. 
The private radio system periodically monitors the broadcast information 
transmitted on the control channels of the serving and surrounding cells 
of a cellular system or systems. From this monitoring, all those carrier 
frequencies serving as control channels are derived for the cell areas 
around the serving cell. The private radio system then carries out signal 
strength measurements on the downlink control channels. Alternatively, a 
mobile station attached to a private radio system can perform the downlink 
measurements and forward the measurements to the private radio system. In 
either event, the private radio system derives, from the received signal 
strength values of the control channels, what transmit power level to a 
respective private radio system is required to overcome interference 
caused by more distant cellular base stations. 
A more complete appreciation of the present invention and the scope thereof 
can be obtained from the accompanying drawings which are briefly 
summarized below, the following detailed description of the 
presently-preferred embodiments of the invention, and the appended claims.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
The present invention will now be described more fully hereinafter with 
reference to the accompanying drawings, in which preferred embodiments of 
the invention are shown. This invention may, however, be embodied in many 
different forms and should not be construed as limited to the embodiments 
set forth herein; rather, these embodiments are provided so that this 
disclosure will be thorough and complete, and will fully convey the scope 
of the invention to those skilled in the art. 
Referring now to FIG. 1, a conceptual diagram of a radio personal 
communications system according to the present invention is shown. Such a 
system operates within a cellular communications network which allocates 
portions of a plurality of frequencies within a spectrum to separate 
geographic cells. Thus, the network encompasses a wide area wireless 
communications network having the capacity to provide high quality 
wireless communications to a large number of users with a limited number 
of frequencies allocated to the wide area cellular network. As shown in 
FIG. 1, a wide area cellular network includes at least one radio network 
cell station 10, such as a cellular telephone cell station, for 
transmitting and receiving messages in a network cell range indicated by 
12, via cell antenna 14. The range 12 of radio network cell station 10 is 
typically represented graphically as a hexagon, as illustrated in FIGS. 1 
and 2. Radio network cell station 10 also interfaces with a Public Land 
Mobile Network (PLMN) 16. 
It will be understood by those having skill in the art that a wide area 
cellular network 18 typically includes many radio network cell stations 10 
to cover a large area as illustrated in FIG. 2. In such a system, each 
radio network cell station 10 covers a cell (range) 12 within the wide 
area cellular network 18 and may interface with Base Station Controllers 
(BSCs, not shown) and Mobile Switching Centers (MSCs, not shown). The MSC 
may provide the connection to the PLMN 16 for all of the network cell 
stations 10 that make up the wide area cellular network 18. 
With further reference to FIG. 1, one or more private or personal telephone 
base stations 20 are located within the cell (range) 12 of the network 
cell station 10 of the wide area cellular system or network 18. Base 
station 20 includes a low power transceiver for transmitting and receiving 
signals via base station antenna 22, over a limited base station range 24, 
typically on the order of tens of meters. Thus, a base station 20 may be 
used for transmission and receipt of radio personal communications in a 
home or office. Base station 20 is electrically connected to a wire 
network 21, such as the Public Switched Telephone Network (PSTN). PSTN 21 
is the regular "wire line" telephone system supplied by, for example, the 
regional Bell Operating Companies, and may use copper wire, optical fiber 
or other stationary transmission channels. Base station 20 may be wired 
directly to the PSTN 21 or connected to a Public Access Branch Exchange or 
PABX (not shown). 
Still referring to FIG. 1, a mobile terminal 26 is shown for radio 
communications with both base station 20 and radio network cell station 10 
via an antenna 28 using basically the same air-interface and the same 
spectrum. Terminal 26 includes a radio telephone such as a cellular phone. 
Terminal 26 may also include, for example, a full computer keyboard and 
display, a scanner, and have full graphics and multimedia capabilities. As 
illustrated in FIG. 1, when terminal 26 is in the range 24 of the base 
station 20, terminal 26 attaches to base station 20 and a radio link 30 
may be established. 
It will be understood by those having skill in the art that a complete 
private radio communications system, referenced herein for convenience by 
the numeral 24, will typically include a private base station 20 and a 
plurality of terminals 26. It will also be understood by those having 
skill in the art that conventional communications and handoff protocols 
may be used with the present invention, and need not be described further 
herein. 
Today's wide area cellular networks, such as network 18 in FIG. 2, utilize 
cell reuse methodologies in order to reduce co-channel interference. Cell 
reuse guarantees that a channel used in one cell, such as cell (range) 12, 
is not reused in a cell nearby but only in cells at a sufficient distance 
away to avoid interference problems. In particular, the interfering 
signals experience a propagation loss sufficiently large such that with 
respect to the received (carrier) level of the desired signal, the 
resulting carrier-to-interference ratio in the considered cell is high 
enough for acceptable radio operation. Cell reuse provides the capability 
to cover large areas with only a limited amount of frequency spectrum. 
An example of a cellular reuse pattern is illustrated with reference to 
FIG. 2, which depicts a 3-site/9-sector reuse plan within the network 18. 
This means that in a cluster L of nine sectors (indicated by the thick 
line) containing sectors A1, A2, A3, and B1, B2, B3, and C1, C2, C3, all 
frequencies are unique. Conventionally, the frequencies start to be reused 
in a structured manner outside the cluster L. The frequency planning is 
fixed in most cellular systems, but can be somewhat adaptive in more 
advanced cellular systems. In the latter case, the system can slowly adapt 
to changes in the network and can therefore be considered semi-fixed. 
As discussed, private or personal communications systems 24, such as those 
serviced by base station 20, have been described and developed that make 
use of the same frequency spectrum as the cellular system. In this way, a 
cellular terminal 26 can access a private base station 20 without the need 
for modifications in the radio hardware design. These private base 
stations 20 provide a direct connection between the cellular terminal 26 
and the PSTN 21 without the interaction of an overlaying cellular system 
or a cellular operator, such as at the radio network station 10. However, 
this also means that these private base stations 20 are not controlled by 
the cellular operator and thus are not integrated into the cellular system 
18. 
It should be understood, therefore, that since there is no coordination 
between the overlaying cellular system 18 and the private base stations 20 
therein, the private systems 24 cannot be part of the reuse scheme 
employed in the cellular system. Consequently, problems of co-channel 
interference result since the private base stations 20 can use the same 
frequencies as the cellular system even though not being a part of the 
cellular reuse plan. In addition, private base stations 20, even though 
being in close proximity to each other, have no direct communication with 
each other, which is also a cause for co-channel interference between 
different and adjacent private radio systems 24. 
As discussed in the aforementioned U.S. Pat. Nos. 5,428,668 and 5,526,402 
and in U.S. patent application Ser. No. 08/517,710, a private radio system 
overlaid by a cellular system should not use those frequencies used by the 
cellular system in the cell (or in adjacent cells) where the private radio 
system is located. However, unlike a conventional reuse pattern described 
in connection with FIG. 2 where all frequencies within the cluster L are 
unique, frequencies used in distant cells within the cluster L may be 
reused, provided the frequencies used in the distant cells within the 
cluster L and in the neighboring cells of contiguous clusters are unique. 
For example, if a private radio system 24, i.e., base station 20 and 
terminal 26 in FIG. 1, is located in cell A1, i.e., cell (range) 12, in a 
cellular system 18 with a 3/9 reuse scheme, as illustrated in FIG. 2, the 
private radio system 24 should not use those frequencies in serving cell 
A1 or in the contiguous surrounding cells A2, A3 or B2 (in a contiguous 
cluster), B3 or C2, C3. Rather, those frequencies in the more distant, 
non-contiguous cells B1 and C1 would be used, since those frequencies 
would produce the least amount of co-channel interference to the private 
radio system 24 and vice versa. Accordingly, private radio systems 24 
located in cell Al would preferably choose radio frequencies for reuse 
from a set of frequencies formed from the set of frequencies used in the 
more distant and non-contiguous cells B1 and C1. Although the experienced 
interference from cells B1 and C1 is higher than interference from 
corresponding A1 cells outside the cluster (which are the normal 
co-channel cells for cell A1), because of the small distance between the 
mobile terminal 26 and the private base station 20 and thus higher 
received carrier power, still an acceptable carrier-to-interference ratio 
can be obtained in the private radio system 24. Methods to derive this 
optimal set of frequencies to be used in the private system 24 are set 
forth in U.S. patent application Ser. No. 08/517,710 and in said related 
co-pending application of the present inventor, filed concurrently 
herewith. 
The allocation of frequencies that permit a cellular system and a private 
radio system therein to co-exist and share the same communicating spectrum 
is one problem to consider, as addressed in said related co-pending 
application. The allocation of allowable transmit power levels to the 
private radio system, however, is another, which is addressed herein. 
In accordance with the present invention, a method and a communications 
system are described by which a private radio system 24 autonomously 
determines a fixed transmit power level to use in a fixed power 
embodiment. In other words, the TX power level selected in accordance with 
the method and associated system of the present invention must be high 
enough so as to overcome interference from the overlaying cellular network 
18, but small enough so as to not interfere with the cellular network 18. 
Consequently, an upper and a lower transmission level are determined, and 
the preferred TX value is within this range. Since a primary concern is to 
not cause interference to the cellular network 18, something unknown to 
private radio system 24 owners, the selected TX power level should be set 
at the low end of the range. This value represents the minimum required 
level to obtain acceptable Carrier-to-Interference (C/I) under average 
conditions, and the maximum transmit power level for use in the private 
radio system 24 when power control is applied. In the latter case, it is, 
of course, understood that if conditions allow lower TX power levels, 
i.e., the C/I are met, then the current regulated TX power value can be 
lower than the selected TX power level. 
The first step of the method of the present invention is for the private 
radio system 24 in question to receive broadcast control information sent 
by the surrounding cellular base stations 10, i.e., those cell stations 10 
in the serving cell and the surrounding and neighboring cells. Among other 
things, this downlink information includes a list of frequencies that the 
mobile station, such as terminal 26, is assumed to monitor in idle and 
connection modes. This list, referred to as a Broadcast Control Channel 
(BCCH) allocation list or BA list, contains the Absolute Radio Frequency 
Channel Numbers (ARFCN) of the control channels used in the neighboring 
cells, and is normally used for producing measurement reports to decide on 
cell re-selection and inter-cell handovers. By not only reading the 
broadcast information of the serving cell, such as cell A1, but also the 
broadcast information of the surrounding or contiguous or adjacent cells 
(whose carrier frequencies have been provided by the broadcast channel of 
the serving cell), a good indication can be obtained which broadcast 
control channel frequencies are used in the cells surrounding the private 
radio system, i.e., cells A2, A3, B2, B3, C2 and C3 surrounding A1. 
For example, in FIG. 2, the serving cell A1, using the frequency ARFCN 
f.sub.-- A1 for broadcasting information, will transmit a cell list, which 
includes the control channel frequencies (going counter-clockwise around 
cell A1) f.sub.-- A2, f.sub.-- B3, f.sub.-- C2, f.sub.-- C3, f.sub.-- B2 
and f.sub.-- A3, respectively, used by the surrounding cells. However, 
when listening to broadcast information on, for example f.sub.-- C2, 
another cell list having frequencies (going counter-clockwise around cell 
C2) f.sub.-- B3, f.sub.-- B1, f.sub.-- A3, f.sub.-- C1, f.sub.-- C3 and 
f.sub.-- A1, respectively, will be received. Similarly, when listening to 
the broadcast channels f.sub.-- A2, f.sub.-- B3, etc., more control 
channel frequencies will be identified (although for this specific 3/9 
case, the two above broadcast messages revealed all control frequencies). 
In this way, the private radio system 24 can obtain a complete list of all 
control channel frequencies that are used in the cellular area neighboring 
the private radio system, e.g., a control channel list 40 as shown in 
FIGS. 3 and 4. 
The aforedescribed broadcast listening can either be performed within the 
private radio system 24 itself, or the private radio system 24 or the base 
station 20 can order the radio personal communications terminal 26, such 
as a mobile station, to gather the downlink broadcast information of the 
neighboring control channels and subsequently transfer this information to 
the private base station 20 for further processing, as will be discussed 
herein. 
After all control channel frequencies have been so identified, downlink 
measurements are then carried out (in the private base station 20 or the 
mobile station 26) to obtain received signal strength indications (RSSI) 
on each of the afore-identified control channels. After averaging these 
measurements, the private radio system 24 can then order the measured 
control channel frequencies according to their averaged RSSI, i.e., RXLEV, 
values from largest to smallest, forming an ordered list 42. If the 
(adaptive) frequency allocation scheme (as set forth in said related 
co-pending application) operates correctly, then the private radio system 
24 will probably not select those traffic channels corresponding to the N 
strongest control channels, where N depends on the cellular pattern and 
other considerations (reuse factor, sectorization). For example, in FIG. 
2, N is 7, indicating the serving cell (Al) and the adjacent six cells 
surrounding the serving cell in the first ring (i.e., A2, B3, C2, C3, B2 
and A3). The control channel values for these stronger signals are 
preferably removed from the ordered list 42, forming a shorter candidate 
list 44. 
Even after removing the N strongest control channels from the top of the 
list 42 to form candidate list 44, however, those control channels and 
their corresponding traffic channels that remain may nonetheless interfere 
with the private radio system 24. In order to overcome and minimize the 
effect of this interference, it is therefore necessary to set the TX power 
level in the private radio system to such a level that a certain 
Carrier-to-Interference (C/I) ratio is guaranteed over the range the 
system is supposed to operate, such as range 24. 
With further reference to FIGS. 3 and 4, where a situation as in FIG. 2 is 
assumed, an example of the RSSI levels or RXLEV in the situation where the 
inter-Base Transceiver Station (BTS), i.e., network cell stations 10, 
distance is about 5 km is shown in FIG. 3. It should be understood that 
the RXLEV values depend on the BTS 10 TX power level, the distance between 
the BTS 10 and the measuring device, the orientation of the BTS antenna 
14, shadowing and other factors. The RXLEV values on the control 
frequencies belonging to cells B1 and C1 (not adjacent cell A1) will be 
lowest due to the increased distance from the private radio system 24 in 
the serving cell A1 and the antenna orientation, as is shown in FIGS. 3 
and 4. If the inter-BTS 10 distance is increased from about 5 km to about 
25 km, the control channel list 40 may, for example, look like the one in 
FIG. 4. 
Comparing the relative differences between the RXLEV of different control 
frequencies within either of the respective control channel lists 40 in 
FIGS. 3 and 4 does not show that much difference, but the absolute levels 
have nonetheless decreased by more than 20 dB when the inter-BTS 10 
distance is increased, as can be seen from a comparison between the two 
lists. If the private radio system 24 measures a list 40 like that 
depicted in FIG. 3, then it will probably use frequency channels belonging 
to f.sub.-- E1 and f.sub.-- C1, as shown in the candidate list 44. 
Therefore, one can expect interference levels as high as -110 dBm. 
Depending on the desired range, the private base station 24 can then 
determine the minimum required TX power level which provides acceptable 
C/I. For example, if a private base station range of about 100 m is 
desired, having a nominal propagation loss of say 120 dB, the required TX 
power in order to provide nominal C/I of 10 dB is Ptx=10+120-110=2O dBm. 
In other words, the required TX power is the sum of the nominal C/I 
required, the nominal propagation loss of a desired private base station 
range and the received signal level (maximum interference level) for the 
frequency in question. If instead a list like in FIG. 4 was measured, the 
maximum interference level to expect is -138 dBm, and therefore, the 
required TX power can be lowered by 28 dBm to -8 dBm in order to fulfill 
the same requirements specified previously. 
The aforedescribed measurement procedure is preferably repeated 
periodically in order to adapt to changes in the cellular network 18. For 
example, if more cellular base stations 10 are added, the RSSI values of 
the control channels in co-channel cells will increase, and so will the 
required TX power levels in the private base station 24. Conversely, if 
base stations 10 are removed, the requisite TX power levels will decrease. 
Thus, the transmit power level allocation scheme of the present invention 
is done once at installation and updated periodically, albeit with a large 
time constant, such as on the order of days or even weeks. 
As noted, since there is no coordination between the private radio system 
24 and the overlaying cellular system or network 18, the private radio 
system 24 is not part of the reuse scheme of the cellular system. 
Consequently, the value of N, i.e., the number of strongest control 
channels to be removed from the list 42, N being a parameter dependent 
upon the reuse factor in the cellular system, is not generally available 
to the private radio system 24, particularly in view of the fact that 
reuse is only an ideal representation. However, the value for N may 
nonetheless be ascertained by examining the BCCH allocation list, i.e., 
the BA list, transmitted by each cellular base station 10. Upon reading 
the BA lists of the serving cell and the surrounding cells, an indication 
of the reuse is obtained by counting the number of unique ARFCNs when all 
of the BA lists are put together. For example, if a perfect reuse scheme 
is used and only the BA lists of the serving cell and the six strongest 
surrounding cells are taken into account, then for a 3/9 reuse, there are 
9 unique ARFCNs. For a 4/12 reuse, there are 12 unique ARFCNs, and for a 
7/21 reuse, there are 19 unique ARFCNs. Accordingly, the number of unique 
ARFCNs gives an indication of the reuse factor, albeit not a perfect 
indication. 
The problem in selecting a suitable value for N is shown in the case where 
M unique ARFCNs are found in the BA list. The difference M-N is the number 
of cells the channel allocation scheme is supposed to steal, and N is the 
number of strongest frequencies that are hopefully not used for channel 
allocation, i.e., are removed from the list 42. If N is too small and the 
estimate is pessimistic, then the private system 24 will have too much TX 
power. Conversely, if N is too large, then the assumption is that the 
number of cells in the channel allocation scheme is small. Problems may 
therefore arise when the density of private systems 24 in the cell(s) 
increases and use is made of channels from cells whose BCCH were excluded 
in the power allocation routine. In any event, an algorithm has been found 
in which N is a parameter which can be derived from M by the following 
relationship: 
##EQU1## 
Thus, if the reuse factor is small (smaller than 11 but bigger than 7), M 
is small and only the 7 strongest cellular bases (the serving cell and the 
six surrounding cells) are excluded. If the reuse factor is large, 
however, M is large, and more cellular base stations 10 are excluded, in 
which case the channel allocation scheme can still steal from 4 cells. The 
second condition, i.e., N=1, must be used in systems with very high reuse 
(a reuse factor smaller than or equal to 7), in which case only the 
serving cell is excluded. 
The previous description is of a preferred embodiment for implementing the 
invention, and the scope of the invention should not necessarily be 
limited by this description. The scope of the present invention is instead 
defined by the following claims.