Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1999-05-03
2001-07-24
Chang, Vivian (Department: 2682)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S502000
Reexamination Certificate
active
06266536
ABSTRACT:
BACKGROUND OF THE PRESENT INVENTION
FIELD OF THE INVENTION
The present invention relates generally to telecommunications systems and methods for simultaneously broadcasting a signal from multiple transmitters in a mobile radio network, and specifically to compensating for overlap between simulcast signals in a simulcast radio network.
BACKGROUND AND OBJECTS OF THE PRESENT INVENTION
Simulcast, or Simultaneous Broadcast, is a mobile radio system architecture that uses two or more transmitters operating on a single radio frequency, in which the same information is carried by all of the transmitters, and the coverage area of two or more of the transmitters overlap. Simulcast provides some significant advantages including wide area communications with a limited number of channels without the use of a multisite switch. In addition, a simulcast system provides more efficient use of channels in situations where groups operate in multiple locations. Furthermore, simulcast systems offer seamless roaming within the total simulcast coverage area, provide efficient coverage in areas with difficult terrain and provide improved in-building coverage in some cases due to the multiple transmitter concept.
However, there can be performance issues in simulcast systems, especially when digital voice or digital data is being transmitted. With reference now to
FIG. 1
of the drawings, a simulcast system
10
having only two transmitters
24
a
and
24
b
is shown. Both transmitters
24
a
and
24
b
are connected to a central control point
23
, or master base station, which utilizes special circuitry to transmit a signal to each transmitter
24
a
and
24
b
for simultaneous broadcast of the signal in a cell
22
a
and
22
b
associated with each transmitter
24
a
and
24
b,
respectively, using the same radio frequency (RF). Each transmitter
24
a
and
24
b
is connected to the control point
23
via a dedicated, phasestable microwave or optic fiber backbone system.
Typically, there is a delay introduced by the control point
23
in the sending of the signal to the various transmitters
24
a
and
24
b
depending upon the distance between the control point
23
and the transmitters
24
a
and
24
b.
For example, if a first transmitter
24
a
is 10 kilometers away from the control point
23
, while a second transmitter
24
b
is 20 kilometers away from the control point
23
, the control point
23
will delay sending the signal to the first transmitter
24
a,
so that the signal will arrive at both transmitters at the exact same time. This difference in transmission times is generally referred to as the timing differential.
Theoretically, if a Mobile Station (MS)
20
, which is the physical equipment, e.g., a car-mounted mobile radio or other portable radio, used by mobile subscribers to communicate with the mobile radio network
10
, each other, and users outside the subscribed network, both wireline and wireless, is located exactly between the two transmitters
24
a
and
24
b,
the signal transmitted from each of the transmitters
24
a
and
24
b
would be received by the MS
20
at exactly the same time, e.g., digital bits received by both transmitters
24
a
and
24
b
would line up exactly. As the MS
20
moves towards the edge of the overlap zone
25
, the MS
20
captures the transmission from one of the transmitters, such as transmitter
24
b.
This “capture zone” can be defined as the area in which the carrier signal (signal strength) of the closer transmitter
24
b
exceeds the signal strength of the farther transmitter
24
a
by at least approximately 10 decibels (dB).
However, as the MS
20
moves through the overlap zone
25
from one of the transmitters
24
a
towards the other of the transmitters
24
b,
the interference increases. This is due to the fact that a bit transmitted from the closer transmitter
24
b
would be received by the MS
20
at an earlier time than the same bit would be received by the MS
20
from the farther transmitter
24
a.
If this time difference (hereinafter referred to as the delay spread) becomes too large, the bits begin to interfere with each other, and the MS
20
may demodulate the bit in error. The bit errors caused by this self-imposed interference manifest themselves as problems such as no access to the system, retransmissions of the signal, loss of audio and/or loss of data.
For example, in the Enhanced Digital Access Communication System (EDACS) system, the two transmitter overlap design parameters allows approximately 30-40 microseconds (usec) of delay spread with capture ratio ranges of 8-12 dB. The system is theoretically designed so that the MS
20
can always receive the signal without significant error. Unfortunately, in reality, most simulcast systems have some overlap regions where the rules are exceeded and the system coverage is severely degraded or unusable. In addition, in some simulcast systems, more than two transmitters overlap, which can amplify this problem. Furthermore, this overlap problem is more severe for higher data rates because the ratio of the size of the overlap zone to the capture zone increases. Thus, for digital radio transmissions such as control channel, digital voice, or data, a simulcast system almost always provides non-uniform coverage.
It is, therefore, an object of the present invention to dynamically shift the delay spread in the overlap zone to reduce interference between simulcast signals.
SUMMARY OF THE INVENTION
The present invention is directed to telecommunications systems and methods for deliberately producing synchronous timing jitters in order to dynamically shift the delay spread in the overlap zone in a simulcast system. At a synchronous periodic rate, the timing differential between transmission of the signal from the control point to each of the transmitters can be continuously adjusted to continuously change the amount of delay spread in the overlap zone. Alternatively, the timing adjustments can be performed only upon reception of a retransmission request. In either case, the timing adjustments are performed so as to not be noticed by the mobile subscriber. In addition, in preferred embodiments of the continuously adjusting embodiment, the timing jitters cycle above and below the optimal timing values determined from the system coverage analysis. Advantageously, the dynamic shifting of the delay spread in the overlap zone can improve signaling, voice quality perception and data transmissions in an otherwise unusable overlap area.
REFERENCES:
patent: 4255814 (1981-03-01), Osborn
patent: 4696052 (1987-09-01), Breeden
patent: 5172396 (1992-12-01), Rose, Jr. et al.
patent: 5423059 (1995-06-01), LoGalbo et al.
patent: 5594761 (1997-01-01), Brown
patent: 5734985 (1998-03-01), Ito et al.
patent: 5873044 (1999-02-01), Goldberg et al.
patent: 5963868 (1999-10-01), Back
patent: 6011977 (2000-01-01), Brown et al.
Chang Vivian
Ericsson Inc.
Jenkens & Gilchrist P.C.
Nguyen Duc
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