Telecommunications – Transmitter and receiver at separate stations – Optimum frequency selection
Reexamination Certificate
1998-12-30
2001-05-29
Nguyen, Lee (Department: 2683)
Telecommunications
Transmitter and receiver at separate stations
Optimum frequency selection
C455S450000, C455S512000, C455S513000, C370S332000, C370S333000
Reexamination Certificate
active
06240275
ABSTRACT:
BACKGROUND OF THE PRESENT INVENTION
1. Field of the Invention
The present invention relates generally to telecommunications systems and methods for providing speech quality to mobile stations within a cellular network, and specifically to assigning channels to mobile stations based upon uplink interference levels and channel quality.
2. Background and Objects of the Present Invention
Mobile communications, especially cellular radio, is one of the fastest growing and most demanding telecommunications applications ever. Today it accommodates a large and continuously increasing percentage of all new telephone subscriptions around the world with the increasing service requirements. Cellular networks have evolved into two different networks within Time Division Multiple Access (TDMA) technology. The European cellular network uses the Global System for Mobile Communication (GSM) standard as the digital cellular system. In the United States, cellular networks have traditionally been primarily analog, but recent advances have been incorporating digital systems within the analog networks. One such North American cellular network is the D-AMPS network, which is described hereinbelow.
With reference now to
FIG. 1
of the drawings, there is illustrated a D-AMPS Public Land Mobile Network (PLMN), such as cellular network
10
, which in turn is composed of a plurality of areas
12
, each with a Mobile Services Center (MSC)
14
and an integrated Visitor Location Register (VLR)
16
therein. The MSC/VLR areas
12
, in turn, include a plurality of Location Areas (LA)
18
, which are defined as that part of a given MSC/VLR area
12
in which a mobile station (MS)
20
may move freely without having to send update location information to the MSC/VLR area
12
that controls the LA
18
.
Each Location Area
12
is divided into a number of cells
22
. Mobile Station (MS)
20
is the physical equipment, e.g., a car phone or other portable terminal, such as a laptop, used by mobile subscribers to communicate with the cellular network
10
, each other, and users outside the subscribed network, both wireline and wireless. The MSC
14
is in communication with a Base Station (BS)
24
. The BS
24
is the physical equipment, illustrated for simplicity as a radio tower, that provides radio coverage to the geographical part of the cell
22
for which it is responsible.
With further reference to
FIG. 1
, the PLMN Service Area or cellular network
10
includes a Home Location Register (HLR)
26
, which is a database maintaining all subscriber information, e.g., user profiles, current location information, and other administrative information. The HLR
26
may be co-located with a given MSC
14
, integrated with the MSC
14
, or alternatively can service multiple MSCs
14
, the latter of which is illustrated in FIG.
1
.
The VLR
16
is a database containing information about all of the Mobile Stations
20
currently located within the MSC/VLR area
12
. If a MS
20
roams into a new MSC/VLR area
12
, the VLR
16
connected to that MSC
14
will request data about that Mobile Station
20
from the HLR database
26
(simultaneously informing the HLR
26
about the current location of the MS
20
). Accordingly, if the user of the MS
20
then wants to make a call, the local VLR
16
will have the requisite identification information without having to reinterrogate the HLR
26
. In the aforedescribed manner, the VLR and HLR databases
16
and
26
, respectively, contain various subscriber information associated with a given MS
20
.
The radio interface between the BS
24
and the MS
20
can utilize, for example, Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA) to transmit information between the BS
24
and the MS
20
. In TDMA, as shown in
FIG. 1
of the drawings, one TDMA frame
24
is assigned per carrier frequency. Each frame
24
consists of six timeslots or physical channels
35
. Depending upon the kind of information sent, different types of logical channels can be mapped onto the physical channels
35
. For example, speech is sent on the logical channel, “Traffic Channel” (TCH)
37
, and signaling information is sent on the logical channel, “Control Channel” (CCH)
38
.
Currently, speech and data are transmitted from the BS
24
to the MS
20
on a downlink channel
30
and from the MS
20
to the BS
24
on an uplink channel
32
. Interference on either the downlink channel
30
or uplink channel
32
can significantly reduce the quality of the signal transmitted on these channels. Two types of interference of interest are co-channel interference and adjacent channel interference. Co-channel interference is the interference caused by the usage of the same frequency within two different clusters (not shown) of cells
22
. Adjacent channel interference is caused by the usage of adjacent frequencies between adjacent cells
22
within the same cluster or within two different clusters.
In analog systems, the carrier-to-interference (co-channel or adjacent-channel) (C/I) ratio is one of the most important radio network performance criteria in evaluating an analog cellular network
10
, such as the AMPS network. In order to reduce interference within the cellular network
10
, both co-channel and adjacent channel interference must be minimized. Therefore, by increasing the C/I ratio, e.g., by reducing the interference with respect to the carrier (level) of the desired signal, the co-channel or adjacent channel interference can be reduced and the signal quality received by MSs
20
within the cell
22
can be improved.
The speech quality in digital cellular systems
10
, such as the Global System for Mobile (GSM) Communication network or the D-AMPS network, is measured via quantities such as frame erasure, which is the percentage of TDMA frames that cannot be perceived, and the bit error rate (BER), which is an estimate of the number of coded bits in error. In order to measure the BER, the encoded bits that are transmitted in each burst or frame of data across the downlink channel
30
or uplink channel
32
are received by a receiver (not shown) and decoded, using, for example, a convolutional decoding algorithm. The algorithm also estimates how many errors were induced by the channel. This estimate of the BER can be referred to as the raw BER. It should be understood that the number of errors estimated by the convolutional decoder is just an estimate of the actual BER. However, this estimate can be considered reliable to a certain degree, and since convolutional codes are usually the most efficient coding mechanisms employed, the BER can be considered as the best estimate of the deterioration in speech quality for digital cellular networks
10
.
In order to ensure adequate speech quality for MS's
20
, the assignment of a traffic channel
37
to an MS
20
involved in a call connection has traditionally been based upon the C/I or BER uplink measurements. Many different approaches to channel assignment have been proposed to date. For example, two commonly used channel assignment methods include the traditional fixed channel assignment (FCA) strategy, and the distributed minimum interference scheme. In the minimum interference scheme, the MS
20
is assigned the traffic channel
37
of the nearest BS
24
with the minimum uplink
32
interference.
Another type of channel assignment method is the multi-channel assignment (MCA) algorithm. In this approach, various C/I constraints are guaranteed to various subscriber services. As these services require different BER performance levels, different C/I values are used to meet these requirements. The cell
22
is typically divided into concentric zones and the C/I performance is traded-off according to the subscriber service requirements of each user.
A further type of channel assignment method is discussed in U.S. Pat. No. 4,794,635 to Hess. This method includes determining the channel
37
and sector activity and establishing a minimum quality factor for each active channel in order to establish an eligible channel
37
H'Mimy Hossam H.
Shah Ali R.
Ericsson Inc.
Jenkens & Gilchrist P.C.
Nguyen Lee
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