Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-08-24
2003-08-12
Urban, Edward F. (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S446000, C455S436000, C375S132000
Reexamination Certificate
active
06606500
ABSTRACT:
This invention relates to a method for improving the capacity of a GSM base station in accordance with the preamble of claim
1
, and a device to embody the method.
FIELD OF THE INVENTION
The present invention relates generally to the field of cellular radio communication, and in particular to base stations in a radio communication network.
BACKGROUND
The usage of mobile radio telephony has increased enormously during the past decade. As the load on the existing mobile radio systems is continuously increasing, it becomes more and more important to efficiently utilise the scarce frequency resources. A lot of effort is today put into optimising, given a certain amount of bandwidth, the number of connections with acceptable channel quality.
One way of improving the capacity of a mobile radio network is to introduce so called frequency hopping. When using frequency hopping in a network, the frequency used for transmission on a certain connection is changed at regular intervals. This results in an increased quality of the radio connection due to both the frequency diversity and the interference diversity hereby obtained (see e.g. H. Olofsson et al. “Interference diversity as means for increased capacity in GSM”, published EMPCC '95). Frequency hopping is used mainly because radio signals are subjected to multi-path fading, which is space and frequency selective, but also to avoid interference with strong signals from neighbour cells transmitting on or close to the actual carrier frequency. Since the quality of the ongoing connections is generally increased as a result of the introduction of frequency hopping, a quality decrease caused by an increased number of connections in the system can be accepted. Thus, the quality increase can be traded for increased capacity.
Frequency hopping is introduced to a system by assigning to each connection a frequency hopping sequence that defines which frequency the connection will use at different points in time. Such a frequency hopping sequence may be defined by two different parameters:
a hopping sequence number, which defines the hopping sequence according to how the frequencies will vary, and
a frequency offset number, which defines where in the hopping sequence the connection in question will be at a particular point in time (see e.g. Global System for Mobile Communication (GSM) Technical Specification 05.02).
In GSM, all transceivers in a cell are assigned the same hopping sequence number, while each transceiver in a cell is assigned a cell unique frequency offset number.
Hereby is achieved that a connection will not experience any co-channel interference from other connections within the same cell. It has been shown that in order to achieve the desirable interference diversity gain by introducing frequency hopping, the number of frequencies to hop between should be at least three or four. In many situations there are not that many frequencies available for each cell. This problem can be solved by applying one of at least two different methods (see T. Toftegård Nielsen et al., “Slow frequency hopping solutions for GSM networks of small bandwidth”, published VTC '98).
1. The first method: By letting neighbouring cells form a pool of the frequencies allocated to each cell, where each transceiver being a member of the pool, utilises all frequencies available in the pool, the number of frequencies available for frequency hopping increases for each cell. To avoid co-channel interference within these neighbouring cells, the same hopping sequence is applied to each cell, but a unique frequency offset number is assigned to each transceiver. This solution, however, requires, that the cells are synchronised with each other.
2. The second method: The number of frequencies available for frequency hopping could be increased by using a small frequency reuse distance and thus obtain a high number of frequencies in each cell. In order to avoid an unacceptable level of co-channel interference the load on each frequency has to be limited.
Another way of increasing the capacity in a mobile radio network is to introduce so called adaptive antennas. Conventional antennas, which have an antenna lobe form which is static, are replaced by adaptive antennas, which can vary the form of the antenna lobe as well as the direction in which the antenna lobe is transmitted. This is provided by having an antenna array having a ½ wavelength distance between each other co-operating to form a lobe if different signal shifts are provided on the different antenna elements adapted to the wished lobe-form and lobe-direction.
A narrow antenna lobe can thus be directed towards the particular mobile station, which the base station is presently serving, instead of having an antenna lobe which covers the entire cell as is the case when a conventional antenna is used. Hereby is achieved that the overall interference level in the system is reduced, since each base station transceiver on the down-link transmits with narrow lobes in more concentrated geographical regions. Each base station receiver on the up-link rejects signals from other directions than the direction that it is presently configured for. For more detailed information regarding adaptive antennas, see e.g. S. Anderson et al. “Adaptive Antennas for GSM and TDMA Systems”, published IEEE Personal Communications June 1999.
The effects of combining the use of adaptive antennas and frequency hopping in the same network has been investigated by F. Kronestedt et al. in “Adaptive Antennas in Frequency hopping GSM”, published ICUPC 1998. It has been found that in such networks the frequency reuse plan can be very tight. As mentioned in the CONCLUSIONS in the publication it is possible to carry full load in a ⅓ cell reuse case without DTX (Discontinuous Transmission) or power control. Even a {fraction (1/1)} cell reuse (all frequencies are used in every cell) might be possible as long as the channel utilisation is kept below 70%.
The problem discussed above of having too few frequencies available for hopping in each cell does not appear if the channel utilisation is kept on a low level. However, a limitation may be set for adjacent channel interference effects. With a tight reuse, such as ⅓ or {fraction (1/1)}, adjacent frequencies may be assigned to a cell. Thus, adjacent frequencies may be used simultaneously in a cell and adjacent interference from own cell will occur. This is very severe since the interfering signal (the adjacent interference) arises from the same base station as the desired signal.
In GSM, the frequency hopping procedure is described by two parameters in combination, i.e. MAIO (Mobile Allocation Index Office) and HSN (Hopping Sequence Number). In a cell each transceiver (TRX) is assigned the same HSN as the other transceivers in the cell, but a unique MAIO. The table below shows an example of this procedure for a cell A:
Cell A
HSN
MAIO
TRX1
1
5
TRX2
1
7
TRX3
1
9
In this way, two transceivers in a cell will never use the same frequency simultaneously. Further, also exemplified in the table above, allocating MAIO in such a way that its value increments by at least 2 between TRXs in a cell results in that adjacent channel interference from the own cell is completely avoided. Adjacent frequencies are never used simultaneously.
If two TRXs in a cell have consecutive MAIO, adjacent channel interference will occur at every burst. This undesired behaviour would occur in the example below for a cell B:
Cell B
HSN
MAIO
TRX1
0
5
TRX2
0
6
This implies that consecutive MAIOs for a cell could not be used in the Prior Art networks. However, a drawback with the state of the art MAIO is, that allocation is only possible as long as the number of frequencies in the hopping sequence is twice as many as the number of installed transceivers in the cell. The reason is that the MAIO can only take as many values as the number of frequencies in the hopping sequence. In order to avoid adjacent channel interference only every second MAIO can be utilised. This criterion for Prior Art networks with conventi
Gary Erika A.
Nixon & Vanderhye P.C.
Telefonaktiebolaget LM Ericsson (publ)
Urban Edward F.
LandOfFree
Method and a device for improving the capacity of a GSM base... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and a device for improving the capacity of a GSM base..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and a device for improving the capacity of a GSM base... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3092555