Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2001-02-09
2004-07-27
Urban, Edward F. (Department: 2685)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S442000, C455S438000, C370S331000
Reexamination Certificate
active
06768907
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to cellular mobile communication networks, for example Code Division Multiple Access (CDMA) cellular networks.
2. Description of the Related Art
FIG. 1
of the accompanying drawings shows parts of a cellular mobile telecommunication network according to the Telecommunication Industries Association (TIA)/Electronic Industries Association (EIA) Standard TIA/EIA/IS-95 of October 1994 (hereinafter “IS95”). Each of three base transceiver stations (BTSs)
4
(BTS
1
, BTS
2
and BTS
3
) is connected via a fixed network
5
to a base station controller (BSC)
6
, which is in turn connected to a mobile switching centre (MSC)
7
. The BSC
6
serves to manage the radio resources of its connected BTSs
4
, for example by performing hand-off and allocating radio channels. The MSC
7
serves to provide switching functions and coordinates location registration and call delivery.
Each BTS
4
serves a cell
8
. When a mobile station (MS)
10
is in a so-called “soft hand-off” (SHO) region
9
where two or more cells overlap, a mobile station can receive transmission signals (downlink signals) of comparable strength and quality from the respective BTSs of the overlapping cells. Transmission signals (uplink signals) produced by the mobile station (MS) can also be received at comparable strengths and qualities by these different BTSs when the mobile station is in the SHO region
9
.
FIG. 2
of the accompanying drawings shows a situation where the MS
10
is located within the SHO region
9
, and is transmitting such uplink signals that are being received by plural BTSs
4
. According to the IS95 standard, a BTS
4
that receives such an uplink signal from the MS
10
relays the signal to the BSC
6
via a dedicated connection line of the fixed network
5
. At the BSC
6
, one of the relayed signals is selected based on a comparison of the quality of each of the received signals, and the selected signal is relayed to the MSC
7
. This selection is referred to as Selection Diversity.
Similarly,
FIG. 3
of the accompanying drawings shows a situation where the MS
10
is located within the SHO region
9
and is receiving downlink signals from plural BTSs
4
. According to the IS95 standard, downlink signals received by the BSC
6
from the MSC
7
are relayed to all BTSs
4
involved in the soft hand-off via respective connection lines of the fixed network
5
, and subsequently transmitted by all the BTSs
4
to the MS
10
. At the MS
10
the multiple signals may be combined, for example, by using maximum ratio combination (MRC), or one of them may be selected based on the signal strength or quality, i.e. using Selection Diversity as for the uplink case.
In contrast to, for example, Global System for Mobile Communication (GSM) networks, in CDMA networks each BTS
4
transmits at the same frequency. Consequently, careful control of transmission power must be maintained to minimise interference problems.
Signals are transmitted as a succession of frames according to the IS95 standard. As
FIG. 4
of the accompanying drawings shows, each frame is of duration 20 ms, and comprises sixteen 1.25 ms time slots. In each time slot several bits of user data and/or control information can be transmitted.
Power control of transmissions from the MS
10
to the BTSs
4
(uplink power control) in IS95 is achieved as follows. When a BTS
4
receives a signal from the MS
10
it determines whether a predetermined property of the received signal (for example absolute signal level, signal to noise ratio (SNR), signal-to-interference ratio (SIR), bit error rate (BER) or frame error rate (FER)) exceeds a preselected threshold level. Based on this determination, the BTS
4
instructs the MS
10
either to reduce or to increase its transmission power in the next time slot.
For this purpose, two bits in every time slot of a pilot channel (PCH) from the BTS
4
to the MS
10
are allocated for uplink power control (see FIG.
4
). Both bits have the same value, and accordingly will be referred to hereinafter as the “power control bit” (or PCB) in the singular. The power control bit is assigned a value of zero by the BTS
4
if the MS
10
is required to increase transmission power by 1 dB, and a value of one if the MS
10
is required to decrease transmission power by 1 dB. The BTS
4
is not able to request directly that the MS
10
maintain the same transmission power; only by alternately transmitting ones and zeros in the power control bit is the transmission power maintained at the same level.
When the MS
10
is in a SHO region
9
, the MS
10
is required to make a decision on whether to increase or to decrease uplink transmission power based on a plurality of power control bits received respectively from the BTSs
4
involved in the soft hand-off. Consequently, an OR function is performed on all the power control bits. If the result of this OR function is zero then the MS
10
will increase power on uplink transmissions, and if the result is one then the MS
10
will decrease power on uplink transmissions. In this way, uplink transmission power is only increased if all BTSs
4
ask for an increase.
Power control of transmissions from the BTS
4
to the MS
10
(downlink power control) in IS95 is achieved as follows. When the MS
10
receives a downlink signal from a BTS
4
(or from each of a plurality of BTSs
4
in soft hand-off operation) via a traffic channel (TCH), the FER of that signal is calculated by the MS
10
which reflects the degree to which the traffic-channel signal has been corrupted by, for example, noise. This FER is then relayed by the MS
10
to the BTS
4
which transmitted the downlink signal concerned, and the BTS
4
uses this FER to decide whether to make any change to its downlink transmission power.
The soft hand-off system described above is effective in improving signal transmission between the MS
10
and the network when the MS
10
is located in regions of cell overlap near the boundaries of the individual cells. Signal quality in these regions when using a single BTS
4
may be relatively poor, but by making use of more than one BTS
4
the quality may be substantially improved.
However, the IS95 soft hand-off system has the disadvantage of increasing signal traffic (“backhaul”) in the fixed network
5
since it is necessary to transmit signals carrying the same data and/or control information between the BSC
6
and every BTS
4
involved in the soft hand-off for both the uplink and downlink cases described above. This duplication of information is undesirable for two main reasons. Firstly, it leads to more traffic congestion in the fixed network. Secondly, higher costs are experienced by the mobile service provider (and consequently the mobile service user), who may not own the fixed network infrastructure.
This disadvantage is addressed in our co-pending UK patent application No. 9810424.3, in which a soft hand-off method is provided where it may be determined, in certain circumstances, that it is not necessary to transmit the same data between the BSC
6
and every BTS
4
. For the uplink case, for example, a BTS
4
may decide that it does not need to transmit an uplink signal received from the MS
10
to the BSC
6
if it may be ascertained that another BTS
4
involved in the soft hand-off operation is receiving the same signal more strongly. For the purpose of making this decision, in each time slot (or frame), each BTS
4
receives from the MS
10
a power control message containing all of the uplink power control bits last sent to the MS
10
by the BTSs involved in the soft hand-off operation.
Therefore the decision of whether or not to transmit the uplink signal from the BTS
4
to the BSC
6
is made based on uplink signal quality in the previous, not the current, time slot (or frame). This is sufficient for a relatively slow-moving mobile station where the fading characteristics of the air interface between MS
10
and BTS
4
change slowly so that the received signal power at the BTS
Fujitsu Limited
Jackson Blane J.
Katten Muchin Zavis & Rosenman
Urban Edward F.
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