Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-08-28
2004-12-28
Tran, Shinh (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S134000, C455S438000, C455S161100, C455S161200, C455S161300, C370S328000
Reexamination Certificate
active
06836660
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to high capacity mobile communications systems, and more particularly to digital transport of radio frequency signals in a microcellular communication system.
BACKGROUND
A conventional cellular phone system
5
is shown in FIG
1
A. Such systems are currently in widespread use in the United States. As illustrated in FIG
1
A, system
5
has a fixed number of channel sets distributed among the base stations
12
,
13
serving a plurality of cells
11
,
16
arranged in a predetermined reusable pattern. Typical cell areas range from 1 to 300 square miles. The larger cells typically cover rural areas and smaller cells cover urban areas. Cell antenna sites utilizing the same channel sets are spaced by a sufficient distance to assure that co-channel interference is held to an acceptably low level.
A mobile unit
10
in a cell
11
has radio telephone transceiver equipment which communicates with similar equipment in base stationisites
12
,
13
as the unit moves from cell to cell. Each base station
12
,
13
relays telephone signals between mobile units
10
and a mobile telecommunications switching office (MTSO)
17
by way of communication lines
18
. The lines
18
between a cell site, and the MTSO
17
, typically T1 lines, carry separate voice grade circuits for each radio channel equipped at the cell site, and data circuits for switching and other control functions. The MTSO
17
is also connected through paths
19
to a switched telephone network
15
including fixed subscriber telephone stations as well as various telephone switching offices.
MTSO
17
in FIG
1
A includes a switching network for establishing call connections between the public switched telephone network
15
and mobile units
10
located in cell sites
11
,
16
, and for switching call connections from one cell site to another. In addition, the MTSO
17
includes a dual access feeder for use in switching a call connection from one cell site to another. Various handoff criteria are known in the art and utilize features such as phase ranging to indicate the distance of a mobile unit from a receiving cell site, triangulation, and received signal strength to indicate the potential desirability of a handoff. Also included in the MTSO
17
is a central processing unit for processing data received from the cell sites and supervisory signals obtained from the network
15
to control the operation of setting up and taking down call connections.
A conventional base station
12
is illustrated in
FIG. 1B. A
radio controller unit
22
provides the interface between the T1 lines from the MTSO and the base station radio equipment. Transmitters
23
, one for each channel serviced by the base station, are driven by circuit
22
, which supplies each transmitter with an analog voice signal. Next, the signals are passed to a separate nonlinear power amplifier for each channel, or the signals may be combined and applied to a single linear power amplifier
24
as shown in FIG
1
B. The output of power amplifier
24
is applied through duplexer
25
to antenna
26
, to be broadcast into the cellular area serviced by the base station.
Signals received in antenna
26
are applied through duplexer
25
to multi-coupler
27
. Multi-coupler
27
applies the wideband signal to receivers
28
(one for each channel), and scanning receiver
28
b.
The analog voice signal outputs of receivers
28
are applied to circuit
22
. Base station
20
may optionally include a diversity antenna
26
′, corresponding diversity filter
25
′, multi-coupler
27
′, and a plurality of diversity receivers
28
′, one for each associated main receiver
28
. Where implemented, the outputs of diversity receivers
28
′ are applied to circuit
22
, which would thus include circuitry for selecting the strongest signal as between corresponding receivers
28
and
28
′ using known techniques. Scanning receiver
28
b
monitors the strength of signals in neighboring cells to identify mobiles which are potential candidates for being handed into its own cell.
In densely populated urban areas, the capacity of a conventional system
5
is severely limited by the relatively small number of channels available in each cell
11
,
16
. Moreover, the coverage of urban cellular phone systems is limited by blockage, attenuation and shadowing of the RF signals by high rises and other structures. This can also be a problem with respect to suburban office buildings and complexes.
To increase capacity and coverage, a cell area can be subdivided and assigned frequencies reused in closer proximities at lower power levels. Subdivision can be accomplished by dividing the geographic territory of a cell, or for example by assigning cells to buildings or floors within a building. While such “microcell” systems are a viable solution to capacity and coverage problems, it can be difficult to find space at a reasonable cost to install conventional base station equipment in each microcell, especially in densely populated urban areas. Furthermore, maintaining a large number of base stations spread throughout a densely populated urban area can be time consuming and uneconomical.
AT&T has proposed a system to solve the problem of coverage in urban areas without having to deploy a large number of conventional base stations. The system is shown and described with respect to
FIG. 1
of AT&T's European Patent Application No. 0 391 597, published on Oct. 10, 1990. A system of the type described in the AT&T application is shown in
FIG. 1C
of the present application. In the system of
FIG. 1C
, a grid of antenna sites
40
is placed throughout the microcellular system. An optical fiber network
42
interconnects the antennas with the base station
44
. The base station
44
is connected to a mobile telecommunications switching office (MTSO)
52
by way of communication lines
54
. Optical wavelength carriers are analog modulated with RF mobile radio channels for transmission through the optical fiber network
42
to the antenna sites
40
. A detector circuit is provided for each antenna site
40
to receive the modulated carrier and reconstruct an RF signal to be applied to the antenna sites
40
for transmission into the microcell area
50
. RF signals received at antenna sites
40
from mobile units are likewise modulated onto a fiber and transmitted back through optical fiber network
42
to base station
44
. All of the channels transmitted from base station
44
are distributed to all antenna sites
40
. Also, all the channels transmitted from the base station
44
can be received from the mobile units in any microcell area
50
and transmitted via optical fiber base station
44
.
The above-described system of
FIG. 1C
has certain limitations. The ability to analog modulate and demodulate light, the limitations imposed by line reflections, and path loss on the fiber all introduce significant distortion and errors into an analog modulated signal and therefore limit the dynamic range of the signals which can be effectively carried via an analog system, especially in the uplink direction. These factors limit the distance from the base station to the antenna sites.
Moreover, in amplitude modulated optical systems an out-of-band signal is required to transmit control and alarm information to and from the antenna sites, again adding to the expense of the modulation and demodulation equipment.
The problems associated with analog optical modulation are addressed in a system described in commonly assigned U.S. patent application Ser. No. 08/204,660, now U.S. Pat. No. 5,627,879. In this system, a composite RF signal occupying the entire 12.5 MHz cellular band is sampled and digitized by a wideband digitizer at a rate greater than twice the bandwidth of the composite signal (in this case the sample rate is 30.72 MHz). The digitized signal is transported serially at 552.96 Mbps to a mote site over an optical fiber using digital optical modulation. The digitized signal is then converted back into a replica
ADC Tolocommunications, Inc. and ADC Mobile Systems, Inc.
Afshar Kamran
Fogg & Associates LLC
Tran Shinh
LandOfFree
Methods and systems for communicating in a cellular network does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Methods and systems for communicating in a cellular network, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Methods and systems for communicating in a cellular network will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3282410