Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-09-04
2002-06-11
Marcelo, Melvin (Department: 2733)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S348000, C370S443000
Reexamination Certificate
active
06404753
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a method and apparatus for controlling access to a communication channel. In particular, the present invention provides method and apparatus by which multiple users can coordinate their use of a shared transmission medium.
BACKGROUND
It is well known in the art to provide a transmission medium to be shared by a plurality of communication devices. One example of an arrangement which relies on such sharing of communication resources is a wireless communication system. An -One commonly known wireless communication system comprises wireless mobile networks wherein users having communication devices may move throughout wireless communication service areas and conduct communications via over the air links. Yet another wireless communication configuration has been proposed as an alternative to wireline communications that provide local access to telephone switching networks. For example, it has been proposed to provide an arrangement referred to as “fixed wireless” by which homes or locations could be provided with a remote unit or terminal that enables the transmission and receipt of wireless communications, as an alternative or an adjunct to a wireline connection to a local exchange carrier. The remote unit would communicate with a base station via an air link to provide access to a local telephone network.
An example of a fixed wireless configuration is illustrated in block diagram form in FIG.
1
. In this arrangement a base station
100
is coupled to a local switch which can include, for example, a No. 5 ESS provided by Lucent Technologies and/or other components such as a local switch
120
. The local switch could be a voice switch or a data switch. The base station includes a transmitter, a receiver and a switch interface. The transmitter and receiver act as an air link interface to a plurality of remote units for example remote units
111
,
112
,
113
,
114
and
115
. The base station also includes certain processing and memory capabilities which control the operation of the base station in accordance with the methods and techniques described below. The remote units can be coupled to various communication devices such as telephones in the case of remote units
111
,
113
and
114
, a computer and modem in the case of remote unit
112
and a fax machine in the case of remote unit
115
. The remote units also include transmission and receiver elements which act as an air link interface to the base station. Furthermore, a remote unit includes a processor and a memory which together control the air link interface in accordance with the type of communication device connected to the remote unit as well as in accordance with the methods and techniques of the present invention as described below.
In a configuration known prior to the present invention the base station communicated to remote units using a forward channel comprising a plurality of time slots such as that illustrated in FIG.
3
. In the illustration, eight of those time slots, T
A
to T
H
, are shown as consecutive time slots along the forward channel. When the base station transmits these time slots over the air link, all of the remote units serviced by the base station receive these time slots and can process them accordingly. In addition to the forward channel of
FIG. 3
, the air link that carries communications to the base station from the remote units is a shared resource referred to as the reverse channel. This shared resource also utilizes a plurality of time slots, some of which are shown in
FIG. 2
as time slots T
1
to T
8
, for carrying information from various remote units to the base station.
A problem arises in this configuration in that as remote units detect that the reverse channel is free, multiple remote units may attempt to seize the reverse channel by transmitting data in the same time slots on the reverse channel. This problem will be better understood with reference to FIG.
4
.
The time slots on the forward channel in
FIG. 4
contain at least two bits of information. One bit labeled the B/I (busy/idle) bit provides status information as to whether the base station is capable of receiving data in a time slot along the reverse channel. Equivalently, this is an indication that the reverse channel is free for use by one of the remote units. In one example if the B/I bit is “0” this indicates that the base station is idle or the reverse channel is free while if the B/I bit is equal to “1” then this indicates that the base station is busy or the reverse channel is not available. A second bit, the D bit, is directed to the status of whether the base station has successfully decoded the data in a received time slot. A failure to decode received data may result from either a collision of two remote units inserting data into the same time slot or from other transmission errors. Again, for purposes of this application when the D bit equals “0” this means that the base station has not successfully received the data in the time slot and when the decode bit equals “1” this means that the base station has successfully decoded and hence successfully received data from one of the remote units. In the example shown in
FIG. 4
time slots T
B
and T
C
have bits B/I and D equal to zero. If a remote unit notes that the B/I bit is zero, then it understands that the base station is accessible. In this instance remote unit one (RU
1
) attempts to transmit data in a time slot T
3
. This time slot must be propagated to the base station, processed by the base station and then a responding time slot transferred back along the forward channel to the remote units.
The time between a remote unit's transmission of a data packet in a time slot on the reverse channel and receipt of a response to that transmission by the remote unit is referred to as a “round-trip” time interval. From system to system the “round trip” time interval may vary depending on the hardware and software implementations. Within a given system however, it is presumed that this delay interval is fixed and is known to the base station and the associated remote units.
An example of the round trip time interval is illustrated in
FIG. 4
by the fact that the B and D bits in the time slots on the forward channel remain as zero for two time slots (T
D
and T
E
) beyond the first time slot in which RU
1
transmits data, namely time slot T
3
. It is not until T
F
that there is any indication that data from the remote unit has been received at the base station. In this instance time slot T
F
indicates that the data in time slot T
3
has been successfully decoded.
Problems can arise because of the round trip delay. Despite the fact that RU
1
has attempted to seize the reverse channel in time slot T
3
, remote unit
2
(RU
2
) believes, based on the contents of time slots T
D
and T
E
, that the resources, (namely the reverse channel and the base station) are available to receive data packets from RU
2
. As a consequence, RU
2
may elect to transmit its data package starting in time slot T
4
. If that occurs then data from both remote units RU
1
and RU
2
may appear in time slots T
4
and T
5
for instance. If one of those sets of data is not received with significantly more power than the other then it is likely that the data in time slots T
4
and T
5
will not be successfully decoded. This fact is reflected in time slots T
G
and T
H
where the B/I bit is set to “1” but the D bit is set to “0”.
Additional delay can appear in the systems as well if the time necessary for the remote unit to process a response is not negligible in comparison. For example, assume that three reverse channel time slots elapse before the remote unit can act on information received in a forward channel time slot. In such a case, referring to
FIG. 4
, if RU
1
had data available for transmission at time T
1
and T
A
had the B/I at “0”, RU
1
would not transmit data until slot T
4
because it would take RU
1
time slots T
1
to T
3
to process the information in slot T
A
.
It would be beneficial to provide the a
Chien Herman
Leung Kin K.
AT&T Corp.
Marcelo Melvin
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