Method of exchanging communicated signals between a remote base site and a central site in a communication system

A communication system employs a method for exchanging communicated signals between a remote base site (e.g., 63) and a central site (205) on a communication resource (e.g., 77) assigned to the remote base site (63). The central site (205) or the remote base site (63) determines that a signal must be exchanged between the remote base site (63) and the central site (205). The remote base site (63) then allocates the communication resource (77) for exchanging the signal from those communication resources (e.g., 76 and 77) assigned to the remote base site (63) for exchanging signals with communication units (e.g., 18) in the service coverage area (53) of the remote base site (63).

FIELD OF THE INVENTION 
This invention relates to communication systems and more specifically to 
cellular communication systems. 
BACKGROUND OF THE INVENTION 
Cellular communication systems are known. Such systems are, typically, 
comprised of a number of cells, each having a service coverage area, and a 
number of cellular telephones (communication units). The service coverage 
areas of adjacent cells may be arranged to partially overlap in such a 
manner as to provide a substantially continuous coverage area in which a 
communication unit receiving service from one cell may be handed off to an 
adjacent cell with no interruption in service. The Groupe Special Mobile 
(GSM) Pan-European digital cellular system, as specified in GSM 
recommendations available from the European Telecommunications Standards 
Institute (ETSI) and incorporated herein by reference, is an example of 
just such a system. 
A cell's radio coverage is provided by a base transceiver station (BTS). 
Each BTS may contain one or more transceivers (TRX) which can 
simultaneously receive on one frequency and transmit on another. 
Communication between a BTS and a mobile communication unit (or mobile 
station) (MS) typically occurs using a portion of a pair of frequencies 
(transmit and receive) temporarily assigned in support of the 
communication transaction at the BTS. 
The pair of frequencies assigned for use at the remote site are typically 
referred to as a radio channel. Downlink transmissions (from BTS to MS) on 
the radio channel occur on a first frequency of the pair of frequencies. 
Uplink transmissions (from MS to BTS) on the radio channel occurs on the 
second frequency of the pair of frequencies. 
The GSM system is a TDM/TDMA system providing eight full duplex signal 
paths (8 TDM slots per TDM frame) on each radio channel. A single, primary 
radio channel assigned to a BTS, by virtue of its being time multiplexed, 
can support up to seven full rate duplex traffic users (speech or data) in 
addition to a multiplexed common control channel within the eight TDM 
slots. Additional, secondary radio channels assigned to the same cell can 
provide a full complement of eight full rate traffic users (in the 8 TDM 
slots) per radio channel, since the control channel within the primary 
radio channel can control allocation of communication resources on 
secondary radio channels. 
Transmissions (control or speech and/or data traffic) from a BTS to an MS, 
on the downlink, occupy a first TDM slot (downlink slot) on a first 
frequency of a radio channel and transmissions from a communication unit 
to a BTS, on the uplink, occupy a second TDM slot (uplink slot) on the 
second frequency of the radio channel. The uplink slot on the second 
frequency is displaced in time three TDM slot positions following the 
downlink slot on the first frequency. The uplink slot on the second 
frequency is offset 45 MHz lower in frequency than the downlink. The 
uplink slot and downlink slot (together providing a two-way signal path 
for a single user)may be referred to as a "communication resource", 
allocated by the BTS to an MS for exchanging signals. The term 
"communication resource" also typically includes an associated signalling 
channel, as for example the GSM specified slow associated control channel 
used with traffic channels. 
Exchanges of paging and setup control information within GSM between MSs 
and BTSs typically occurs on the common control channel (CCCH) which 
occupies at least one slot of a primary channel of the BTS. Transmitted by 
the BTS on the CCCH are distinctive identification signals as well as 
synchronization and timing information common to all other frequencies and 
slots of the BTS. CCCH information allows an MS to differentiate between 
primary and non-primary channels. 
System control attributes of a GSM-like system are quite complex and 
demanding. The operation is hinged to the existence of a primary channel 
being present for each BTS. Key system control functions such as cell 
selection and handover are based upon the primary channel. Mobiles select 
a serving BTS based upon signal measurements of primary channels of nearby 
BTSs. Handovers are achieved, in part, based upon primary channel signal 
measurements performed by a mobile and transferred to a serving BTS. 
Upon activation, an MS scans a set of frequencies in search of CCCH 
identification signals transmitted from proximate BTSs. Upon detecting a 
CCCH identification signal the communication unit measures a signal 
quality factor (such as signal strength and/or bit error rate) of the 
identification signal as a means of determining relative proximity of the 
BTS. Upon completing the scan of frequencies within the set, the MS 
generally selects the BTS providing the largest relative signal quality 
factor, as a serving BTS. Upon identifying, and locking onto a suitably 
strong signal (and registering if necessary) the communication unit 
monitors the selected CCCH for incoming calls. Should the communication 
unit desire to initiate a call, an access request may be transmitted using 
the CCCH of the serving BTS. 
During normal operation (including during active calls), the MS monitors 
for, identifies, and measures primary channels of nearby BTSs. If involved 
in an active call, the MS relays measurement information back to the base 
site on the SACCH. Through such a process, it is possible for the MS to 
maintain an association with the most appropriate BTS. During an inactive 
state the process may entail an autonomous switching by the MS to a 
different BTS, causing perhaps a re-registration by the MS with the system 
indicating that such a switch has occurred. 
Alternatively, during an active communication exchange, the MS may be 
commanded by the system to handover to a more appropriate BTS. 
Access by an MS to a local BTS may allow the MS telephony access to a 
communication target, such as another MS, served by the same, or another 
BTS, or to a subscriber within a public switched telephone network (PSTN). 
Access by the MS to a local BTS may also provide the MS access to a 
diversity of other data services. 
In general, communication access is provided to the MS through a cellular 
infrastructure system which, in the case of a PSTN target, may include the 
BTS, a base station controller (BSC), a mobile switching center (MSC), and 
the PSTN network. Under GSM, a BSC may control a number of BTSs. An MSC, 
connected to the PSTN network, may control a number of BSCs. 
The exchange of information within the infrastructure network of the GSM 
cellular system generally occurs over high speed (2.048 mb/s) transmission 
links, based upon CCITT standardized exchange protocol. Physical mediums 
used to facilitate these high speed links include wireline, coax, fiber 
optic, or microwave. Such links may be utilized, for example, to 
interconnect remote BTSs with a controlling BSC. 
The use of high speed transmission links within the cellular infrastructure 
has been justified, in the past, by high capacity requirements due, in 
part, to the relatively large geographic areas covered by BTSs. High speed 
links, in other cases, have been justified by the highly variable nature 
of communication traffic through BTSs and by a desire for standardization. 
While, in the past, high speed transmission links have worked well, the 
cost has not been justified in all applications. In some cases the cost of 
a high speed transmission link between a BTS and BSC is not justified by 
the communication traffic. BTSs serving rural areas may be lightly loaded 
and spaced at relatively large distances thereby increasing cost in areas 
least likely to need the capacity of such links. There is often also some 
difficulty providing such conventional links due to certain logistical 
problems. (e.g. a remote BTS site at the top of a light pole). 
In urban areas, on the other hand, as cellular traffic has increased, the 
trend has been to divide cells into increasingly smaller cells 
(microcells). The use of microcells reduces the average distance between 
MS and BTS allowing for reduced transmission power (thereby increasing the 
viability of hand-held portables). Reducing average transmission power of 
communication exchanges allows for a greater number of users within a 
system by allowing closer reuse of communication resources. 
Increasing the capacity of the system by reducing cell size may reduce the 
average traffic through microcell BTSs. Use of high speed links to 
interconnected BSCs with microcell BTSs, in such cases, may result in a 
significant mismatch of need to capacity with sever economic consequences. 
As with the use of high speed links at the BTS, it has likewise been 
traditional in the past to include the capability for radio test 
diagnostic subsystems at each BTS site. The test subsystem provides MS 
emulation and is used, under control of an operations and maintenance 
center (OMC), to test the radios of the BTS, thereby insuring the 
integrity of the BTS and, through loopback testing, the overall network. 
The test subsystem, in the past, has been limited to internal test 
routines within the BTS and has been provided through the high speed link. 
Because of the importance of microcells and of cellular service in rural 
areas, with little communication traffic, a need exists for an alternative 
to the high speed data links typically used to interconnect remote BTS 
sites to the remainder of the cellular infrastructure. Such an alternative 
should have the potential for working equally well in rural areas as in 
the microcell environment of urban or other relatively high traffic areas. 
Such an alternative should also provide a convenient and economical means 
of testing remote BTS sites. 
SUMMARY OF THE INVENTION 
A backbone communication system is offered for exchanging, on a TDMA 
channel, a number of communicated signals with a plurality of remote base 
sites. The system comprises at least one mobile function transceiver, 
transceiving on the TDMA channel and means for synchronizing a transceiver 
at each of the number of base sites to the at least one mobile function 
transceiver.

BRIEF DESCRIPTION OF A PREFERRED EMBODIMENT 
The solution to the problems caused by wireline or microwave interconnects 
between remote base sites and the BSC lies, conceptually, in sharing 
communication resources assigned to a remote base site in support of 
communication transactions between the remote site and the BSC. In 
accordance with the invention the communication transactions occur on a 
backbone system comprised of the shared communication resources and a 
central controller. 
The central site controller interconnects with the BSC and provides 
transceivers and control functions for requesting access from, and 
exchanging signals with, the remote base site on the shared communication 
resources. The shared resources are allocated from communication resources 
that otherwise would be available to user communication units located 
within the service coverage area of the remote base site. 
Shown in FIG. 4 are seven cells (50, 51, 52, 53, 54, 55, and 56) of a 
communication system under the invention. Included within each cell (50, 
51, 52, 53, 54, 55, and 56) is a remote base site (60, 61, 62, 63, 64, 65, 
and 66) providing communication services within the service coverage area 
of each cell (50, 51, 52, 53, 54, 55, and 56). 
Remote sites (60, 61, 62, 63, 64, 65, and 66) are constructed as shown in 
FIG. 8. FIG. 8 includes a site controller (301, FIG. 8) and transceivers 
(302 through 305). 
FIG. 3 is a block diagram of a central site controller (central site) 
generally (205) under one embodiment of the invention. Included within the 
central site is a central site controller (200) and transceivers (201, 
202, 203 and 204). The transceivers (201, 202, 203 and 204) are 
constructed to transmit as mobile function transceivers, under the same 
format as communication units (16 and 17) (transmit as communication unit 
transmitters and receive as communication unit receivers). The central 
site controller (200) is shown having an interconnect with a BSC. 
Under one embodiment of the invention the central site (205) may be 
interconnected by wireline to a BSC (105) and MSC (106) and co-located 
with a remote site (66, FIG. 4). Under such an embodiment a means for 
exchanging control information with the remote base sites (60, 61, 62, 63, 
64, or 65), such as a dedicated control link, may be necessary. A 
dedicated control link from central site (205) to each remote site (60, 
61, 62, 63, 64, or 65) provides a means for communicating with remote base 
sites (60, 61, 62, 63, 64, or 65). 
The dedicated control link selected, under the invention, is determined by 
the volume of control traffic required to support remote base site 
operation. The dedicated control link may be a TDM timeslot (0 through 7) 
on a primary or secondary channel assigned to a remote base site (60, 61, 
62, 63, 64, or 65) or a portion of a TDM timeslot, such as a SDCCH channel 
or combination of SDCCH channels. 
The control link is continuously maintained through a dedicated transceiver 
(201, 202, 203, or 204) for the exchange of communicated signals such as 
paging requests and authorizations of resource allocations to the remote 
sites (60, 61, 62, 63, 64, or 65). The dedicated transceiver (201, 202, 
203, or 204), in turn, monitors the dedicated control link of the remote 
sites (60, 61, 62, 63, 64, or 65) for communicated signals such as 
resource requests and paging responses. 
Message flows for resource requests and allocation of communication 
resources between communication units (16, 17, and 18) and remote base 
sites (60, 61, 62, 63, 64, 65, and 66), under the invention, may be 
processed consistent with ETSI GSM standards for intra-cellular 
communication transactions. 
Inter-cellular communication transactions on a backbone system, under one 
embodiment of the invention, occur on at least one communication resource, 
such as a TCH, or DCCH, or SDCCH, assigned to a remote base site that is, 
in turn, shared between the remote base site (60, 61, 62, 63, 64, or 65) 
and the central site (205). Shared communication resources are requested 
by the remote base sites (60, 61, 62, 63, 64, or 65) or central site 
(205), with the central site controller (200) acting as a means for 
determining that a signal must be exchanged between the remote base site 
and the central site. 
Shown in FIG. 9 is a flow chart of central site operation under the 
invention. Reference shall be made to FIG. 9 as required for understanding 
of the invention. 
As an example of the invention the remote site (63) receives a resource 
request (76) from a requesting communication unit (18) for access to a 
PSTN subscriber (not shown). The remote site (63) transmits the request 
(77) to the central site (205) on the dedicated control link between the 
requesting remote base site (63) and the central site (205). The request 
(77) may contain an ID of the originating remote site (63), an ID of the 
requesting communication unit (18), and an ID of the PSTN target. 
The central site controller (205) receives the request (77) (501) through a 
transceiver (203). The access request (77) is transferred by wireline to 
the BSC (105) and MSC (106) for processing. The request is processed 
within the BSC (105) and MSC (106), under the invention, consistent with 
prior art GSM standards. The BSC (105) and MSC (106), in due course, 
communicate the access request to the PSTN system. 
The PSTN system responds by providing an interconnect authorization to the 
MSC (106) and an interconnect path to the PSTN target. The MSC (106) 
transfers the interconnect authorization to the BSC (105). The BSC (105), 
in response, generates a set of two resource allocations and transfers the 
resource allocations to the central site (205) for transfer to the remote 
base site (63). The central site (205) transmits (504) the set of resource 
allocations to the remote base sites on the dedicated control channel. The 
first resource allocation allows the requesting communication unit (18) to 
exchange a communicated signal with the remote base site (63) and the 
second resource allocation allows the central site (205) to exchange the 
communicated signal with the remote base site (63). The central site (205) 
allocates a transceiver (201) and a signal path (506) between the 
transceiver (201) and the PSTN subscriber. 
In the case of a PSTN subscriber requesting access to a communication unit 
(16) a similar process is used. In this case an access request is 
transferred by wireline from the PSTN network to the MSC (106). The MSC 
(106), upon validating the request, generates and transfers a paging 
request to the BSC (106). The BSC (106) transfers the paging request to 
the remote base sites (60, 61, 62, 63, 64, or 65) via the dedicated 
control links. The remote base site (64) where the target communication 
unit (16) is located responds, also via the dedicated control link, with 
an acknowledgement (73). The acknowledgement is transferred to the MSC 
(106). The MSC (106) responds with an interconnect and interconnect 
authorization which is transferred both to the PSTN and to the BSC (105). 
The interconnect between PSTN subscriber and communication unit (16) is 
completed as above. 
In another embodiment of the invention, control links between the central 
site (205) and remote base sites (60, 61, 62, 63, 64, or 65) are 
established on an as-needed basis, existing only for so long as control 
traffic exists between the remote base sites (60, 61, 62, 63, 64, or 65) 
and the central site (205). Under such an embodiment, and upon receipt of 
a resource request from a communication unit (16 or 17), the site 
controller (100) immediately transmits a page containing an ID recognized 
by the central site (205). The central site (205), upon recognizing the 
page, responds with a central site request for a set-up channel. The 
remote base site (60, 61, 62, 63, 64, or 65), upon receiving the central 
site request transmits a SDCCH channel grant on the CCCH. The central site 
(205) upon receipt of the channel grant tunes to the SDCCH channel and 
receives the resource request information and transmits resource 
authorizations, as above. The SDCCH channel is released by the central 
site (205) upon receiving a communicated signal resource allocation for 
the inter-cell communicated signal as above. 
Paging requests from the central site (205) are transferred to the remote 
base sites (60, 61, 62, 63, 64, or 65) by first transmitting a resource 
request to the remote base site (60, 61, 62, 63, 64, or 65) followed by 
the exchange of control information on a SDCCH granted for such exchange. 
SDCCH channels granted in support of paging may be retained by the remote 
base site (60, 61, 62, 63, 64, or 65) for a time period pending a response 
from a target unit or immediately released. If immediately released, then 
a remote site (60, 61, 62, 63, 64, or 65) receiving a response from a 
target communication unit would transmit a page to the central site (205) 
followed by a SDCCH grant on the CCCH re-establishing the control link for 
channel set-up. 
The means for determining the need for a signal exchange within the site 
controller (301) comprises a software routine providing a search algorithm 
for interrogating communication units (16, 17, or 18) within the service 
coverage area (50 through 56) served by the site controller (301) and 
determining the need for a signal exchange based upon an absence of a 
response from a target communication unit (16, 17, or 18). 
In another embodiment of the invention the communication resource allocated 
by the remote site (63) for communication with the central site (205) is 
operated as a high speed link exchanging information between the remote 
site (63) and the central site (205) at a rate that is a multiple of a 
base transfer rate used between the remote site and the communication unit 
(18). The high speed link is then used to service a number of 
communication transactions, simultaneously, from the remote site (63) on 
the same communication resource, which number of transactions is equal to 
the multiple of the high speed link divided by the base transfer rate. 
The high speed data link between remote sites (60, 61, 62, 63, 64, or 65) 
may be achieved by a suitable encoding technique for a backbone 
communication network involving inter-cell communications between remote 
base sites (60, 61, 62, 63, 64, or 65) and the central site (205). The 
unique and relaxed requirements for encoding information for inter-cell 
slots used for point-to-point (central site to remote site) allows for 
considerable flexibility in the slot's actual composition in comparison to 
GSM uplink and downlink requirements. For example, it is not necessary 
that inter-cell traffic be error correction coded since the degree of 
error protection needed over the relatively high quality, stationary 
backbone link is reduced. In GSM, the difference in error corrected coding 
and non-error corrected coding relates to 22.8 kb/s versus 13 kb/s. Other 
sources of reduced inter-cell traffic (increased capacity) include 
increasing the modulation of an encoded signal, through the elimination of 
tail bits, some sync bits, flag stealing bits, etc. As such it is possible 
to encode more than a single traffic channel and or to combine traffic and 
signalling information into fewer backbone, inter-cell slots. 
In one embodiment of the invention, the high speed link for inter-cell 
communications uses a high order modulation technique to enhance the 
channel capacity for communicating backbone information. The high order 
modulation technique allows an increase in the subscriber capacity 
improving the economics and viability of the technique. In the case of 
inter-cell communication eight-level multi-shift frequency keying (FSK) 
may be used since it is easy to generate and decode and, being a constant 
envelope modulation technique, is compatible with typical system operation 
and prior-art GSM equipment. 
The ability to utilize such an enhanced modulation technique (such as eight 
level FSK) requiring a stronger signal level for backbone slots is 
justified because the point-to-point, backbone link is not subject to the 
rapid fade normally associated with an uplink channel. Fixed site 
locations can also be easily selected and/or constructed to have a 
line-of-site transmission paths thereby providing additional signal to 
noise advantage. In this way, the particular modulation used for each 
function is tailored for its particular radio path application--fixed or 
mobile. Selection based upon function allows maximizing the capacity 
throughput (thereby minimizing cost) for the non faded stationary, 
point-to-point carrying capacity of inter cell communications. 
Inter-cell timing within the communication system may be controlled by the 
central site (205). In such an embodiment control links and/or 
communicated signals may be exchanged between the central site (205) and 
the remote base sites (60, 61, 62, 63, 64, or 65) on successive time slots 
and be serviced from a single transceiver (201, 202, 203, or 204). 
Shown (FIG. 5) is a TDM timing diagram for the above examples (a requestor 
(16) in remote site 64 in contact with a target (17) in remote site 65 and 
requestor (18) in contact with a PSTN subscriber (not shown)). Remote site 
64 occupies slots 0 and 5 for transmission and reception, remote site 63 
occupies slots 1 and 6, and remote site 65 occupies slots 2 and 6. 
Remote sites (63, 64, and 65) synchronize (FIG. 5) to transmissions from 
the central site (205). The reference in FIG. 5 (Central Site Tx) is a 
transmission (Tx) in each time slot, in phase with the time slot. As shown 
(FIG. 5) a transmission (Tx) from the central site (205) in slot 0 is 
received (Rx) at remote site 64 after a transmission time delay of t1. 
Similarly a transmission (Tx) in time slot 1 and 2 to remote site 63 and 
65 is received (Rx) after time delays t2 and t3. Transmissions received at 
the central site (205) from the remote sites (60, 61, 62, 63, 64, or 65) 
are similarly displaced by transmission delays. Total transmission time 
delay on a signal received from remote site 64 is t1 x2. Total time delay 
for remote sites 63 and 65 is t2 x2, and t3 x2, respectively. 
Remote sites (60, 61, 62, 63, 64, or 65) adjust the timing of transmissions 
to the central site (205) through the use of interslot timing commands 
transmitted by the central site (205) to the remote sites (60, 61, 62, 63, 
64, or 65) in a manner similar to the ETSI GSM algorithm used by GSM 
mobile communication units. The ETSI GSM algorithm reduces (advances) the 
interslot timing (time between receive and transmit) at remote sites based 
upon timing information transmitted from the central site (205) to the 
remote sites (64, 65, and 63). 
In the case of remote site 64 the central site (205) measures a time offset 
(in slot 5) of t1 x2 on a signal received from remote site 64 in response 
to a signal transmitted in slot 1. Upon measuring the offset of t1 x2 the 
central site (205) transmits commands to remote site 64 reducing the 
interslot timing between receive (slot 1) and transmit (slot 5) by a time 
interval of t1 x2 to compensate for transmission delays. The central site 
(205), likewise, transmits commands to remote site 63 and 65 reducing 
interslot timing by t2 x2 and t3 x2, respectively. As shown (FIG. 6) use 
of the ETSI GSM algorithm allows for reduced incidence of signal collision 
at the central site (205). 
In another embodiment of the invention the central site (205) functions as 
a system tester of remote base sites (60, 61, 62, 63, 64, or 65). The 
central site (205) may test remote base site (60, 61, 62, 63, 64, or 65) 
operation by emulating the transmissions of a communication unit (16, 17, 
and 18) thereby testing and verifying remote site operation. In such a 
case the central site (205) may transmit a resource request to a remote 
site (60, 61, 62, 63, 64, or 65) using a fictional ID of a test 
communication unit (not shown) to verify remote site (60, 61, 62, 63, 64, 
or 65) call set-up procedures. The central site (205) may also transmit a 
fictional paging message to a remote base site (60, 61, 62, 63, 64, or 65) 
and then monitor the CCCH of the remote site (60, 61, 62, 63, 64, or 65) 
for a transmitted page. 
The central site may also test remote site (60, 61, 62, 63, 64, or 65) 
operation through the transmission of executive commands initiating test 
routines over an allocated resource requested for that purpose. Such 
commands may initiate specialized test routines or capabilities provided 
within the remote base site (60, 61, 62, 63, 64, or 65). For example, the 
remote base site (60, 61, 62, 63, 64, or 65) could be requested to enter a 
loop around mode, report local status or measurements, and/or enter other 
diagnostic modes.