Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-12-15
2003-04-08
Vo, Nguyen T. (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C370S332000
Reexamination Certificate
active
06546252
ABSTRACT:
BACKGROUND
This invention relates to cellular radio communication systems and more particularly to increasing system capacity. System capacity is increased by providing handover mechanisms that substantially assist in keeping the load within each of two or more frequency bands balanced. Thus, interference within each of these frequency bands can be kept at a level that does not lead to severe performance degradation.
Continuing growth in telecommunications is placing increasing stress on the capacity of cellular systems. The limited frequency spectrum available for cellular communications demands cellular systems having increased network capacity and adaptability to various communications traffic situations. Although the introduction of digital modulation to cellular systems has increased system capacity, these increases alone may be insufficient to satisfy added demand for capacity and radio coverage. Other measures to increase capacity, such as decreasing the size of cells in metropolitan areas, may be necessary to meet growing demand.
Another method of increasing capacity is through the use of spread spectrum modulation and code division multiple access (CDMA) techniques. In typical direct sequence CDMA systems, an information data stream to be transmitted is superimposed on a much-higher-symbol-rate data stream, sometimes known as a spreading sequence. Each symbol of the spreading sequence is commonly referred to as a chip. Each information signal is allocated a unique spreading code that is used to generate the spreading sequence typically by periodic repetition. The information signal and the spreading sequence is typically combined by multiplication in a process sometimes called coding or spreading the information signal. A plurality of spread information signals are transmitted as modulations of radio frequency carrier waves and are jointly received as a composite signal at a receiver. Each of the spread signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. By correlating the composite signal with one of the unique spreading sequences, the corresponding information signal can be isolated and decoded. Since signals in CDMA systems overlay one another in frequency and time, they are frequently referred to as being self-interfering.
The coverage area of a mobile communication system may be subdivided into cells, depending on the system. A cell may be defined as the area that is covered by one base station. The base station is generally located in the center of the cell. Each cell might be an omnicell covering 360-degrees or the cell might be split up into several sectors, e.g., three sectors that cover a 120-degree angle each, which is referred to as physical sectors.
The base station serves as an interface between the mobile station (MS) and the fixed network. In a call situation, a MS may be connected via one or more logical sectors of a frequency band to one (or more) base stations (BTSs). Logical sectors used by the MS to communicate with the BTS are called active sets.
In a mobile communication system, a downlink (DL) and an uplink (UL) are used to transmit data to and from the BTS and the MS. The BTS transmits data to the MS via the DL, while data is transmitted from the MS to the BTS via the UL. Both the UL and DL may utilize two frequency bands. Often, in a mobile communication system, one of the two frequency bands of the UL or DL may be used more often than the other frequency band. Thus, there would be unbalanced usage of the two available frequency bands and the corresponding system resources. For example, most of the information might be transmitted on one frequency band while only a small transmission load is on the other frequency band. Hence, the system wastes its capacity.
Thus, there is a need to avoid unbalanced usage of the frequency bands and system resources. Accordingly, it would be desirable for each MS to be capable of performing a handover from one frequency band to the other in order to equally split the transmission load of the whole system between two available frequency bands.
FIG. 1
illustrates a typical cell
10
. The cell
10
is divided into three physical sectors
12
,
14
and
16
. Physical sector
12
is assigned logical sectors
18
and
20
. Logical sector
18
is assigned a frequency band f
1
. Logical sector
20
is assigned a frequency band f
2
. Similarly, physical sector
14
is assigned logical sectors
22
and
24
. Logical sectors
22
and
24
are assigned frequency band f
1
and frequency band f
2
, respectively. Physical sector
16
is assigned logical sector
26
and
28
, which are assigned frequency band f
1
and frequency band f
2
, respectively.
Transmission is accomplished on frequency band f
1
and frequency band f
2
of each physical sector
12
,
14
, and
16
via logical sectors
18
,
20
,
22
,
24
,
26
and
28
. Logical sectors that are located in the same physical sector are referred to as siblings. For example, logical sectors
18
and
20
are siblings. It would be appreciated by those of ordinary skill in the art that it is possible in some situations that only one logical sector of a physical sector might support traffic channels on a single frequency band. Thus, in these situations, a logical sector may transmit a signal that is only used for support of measurements. This is often referred to as Beacon.
It would be further appreciated by those of ordinary skill in the art that the cell
10
may be divided into any number of physical sectors having any number of logical sectors and any number of frequency bands.
There are shortcomings with traditional handover procedures. For example, due to hardware limitations of a MS, the MS might be able to only perform measurements on one of two or more frequency bands at a time. As a consequence, for any physical sector the MS can accomplish quality measurements for one of the two logical sectors, but not for its sibling. In addition, the coverage area of a logical sector and its sibling might be different.
Accordingly, it would be desirable for the MS to be able to perform transmission quality measurements on multiple frequency bands at a time instead of a single frequency band as currently known. As a consequence for any physical sector, the MS should be able to determine transmission quality measurements for multiple logical sectors, including its siblings. In addition, it would be desirable to perform such measurements when the coverage area of a logical sector and its sibling are different. Further, it would be desirable to get information regarding the transmission quality on other frequency bands to enable a MS to carry out an interfrequency handover.
SUMMARY
These and other problems associated with cellular communications are solved by the present invention, wherein a mobile station measures the transmission quality Q
F1
on frequency band f
1
and estimates the quality Q
F2
on frequency band f
2
by adding an offset Q
F1,2
to the measured transmission quality Q
F1
. Once the estimated transmission quality Q
F2
exceeds a quality threshold, an interfrequency handover from frequency band f
1
to frequency band f
2
may be accomplished. The quality threshold may be determined by the system operator or by other means.
A quality offset is used by the mobile station to estimate the quality of a logical sector instead of using actual measurements on the logical sector. In addition, applying the quality offset enables the mobile station to carry out interfrequency handover, even if the mobile station can only perform transmission quality measurements on one frequency band. Furthermore, depending on the number of active mobile subscribers in one physical sector, system capacity may be substantially increased since it is possible to have a more balanced load on the two frequency bands used by the system.
REFERENCES:
patent: 5230081 (1993-07-01), Yamada et al.
patent: 5353332 (1994-10-01), Raith et al.
patent: 5420864 (1995-05-01), Dahlin et al.
patent: 5430720 (1995-07-01), Larsson et al.
patent: 55090
Blomberg Petter
Butovitsch Peter
Jetzek Ulrich
Johansson Lars B
Müller Walter
Telefonaktiebolaget LM Ericsson (publ)
Vo Nguyen T.
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