Pulse or digital communications – Spread spectrum – Frequency hopping
Reexamination Certificate
2000-06-27
2003-07-01
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Frequency hopping
Reexamination Certificate
active
06587498
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to levelling out of interference in a mobile network using the hopping method.
BACKGROUND OF THE INVENTION
In mobile communications systems, mobile stations and base stations are capable of setting up connections using the so-called radio interface channels. Various requirements, depending on the type of data involved, are imposed on such connections relating to the data transmission rate, the accuracy of the data, and transmission delay.
A specific frequency range is always allocated for use by the mobile network. This frequency range is subdivided into channels whose transmission capacity is optimised to match the services provided by the mobile network. To ensure sufficient capacity within the limited frequency range allocated for the mobile network, the channels available must be re-used. For this purpose, the system coverage area is divided into cell consisting of the coverage areas of the individual base stations, which is why such systems are often also referred to as cellular radio systems.
Through the radio connection, mobile stations have access to the services provided by the mobile network.
FIG. 1
outlines the structure of a known mobile network system. The network includes a number of inter-connected Mobile Services Switching Centres MSC. A mobile services switching centre MSC is capable of setting up connections with other mobile services switching centres MSC or other telecommunications networks, such as the Integrated Services Digital Network ISDN, the Public Switched Telephone Network PSTN, the Internet, the Packet Data Network PDN, the Asynchronous Transfer Mode ATM and the General Packet Radio Service GPRS. Each mobile services switching centre has several base station controllers BSC connected to it. Similarly, each base station controller is connected to several base stations. The base stations are capable of setting up connections with mobile stations MS. The Network Management System NMS is used for collecting data on the network and re-programming the network elements.
The air interface between the base stations and mobile stations can be divided into channels in a number of different ways. Known methods include at least Time Division Multiplexing TDM, Frequency Division Multiplexing FDM, and Code Division Multiplexing CDM. In TDM systems, the allocated bandwidth is divided into sequential time-slots. A specific number of sequential time-slots constitute a periodically recurring time frame. The channel is defined by the time slot used in the time frame. In FDM systems the channel is defined by the frequency used, and in CDM systems by the frequency-hopping pattern or hashing code. Various combinations of the division methods described above can also be used.
FIG. 2
provides an example of a known FDM/TDM division. In the figure, the vertical axis represents frequency and the horizontal axis time. The allocated frequency range is divided into six frequencies denoted by F
1
through F
6
. In addition, the frequency channel consisting of each individual frequency is sub-divided into recurring time frames made up of 8 sequential time-slots. The channel is always defined by the pair (F, TS), where F is frequency and TS is the time-slot, used in the time frame.
To maximize capacity, the channels must be re-used in cells that are located as close to one another as possible, providing, however, that the quality of the connections using the channels remains adequate. The quality of the connection is affected by the sensitivity of the transmitted information to the transmission errors occurring in the radio channel and the quality of the radio channel. Resilience against signal transmission errors depends on the properties of the information being transferred and can be improved by processing the information by means of channel coding and interleaving before the data are sent and by using re-transmission of erroneous transmission frames.
The quality of the radio channel is, in particular, affected by the extent of mutual interference caused by the connections, which, in turn, depends on the channels used by the connections, the geographical distribution of the connections, and the transmission power used. These factors can be influenced by a systematic allocation of the channels to the various cells with due regard to such interference, by regulating the transmission power, and by averaging the interference experienced by the various connections.
Even if channel allocation is successful, different connections are exposed to different levels of interference. As a result, some connections may suffer from interference that severely affects their quality while other connections could, at the same time, tolerate a higher level of interference. A channel may be allocated, if the signal-to-noise ratio achieved by the connections set up for the channel involved falls below a predefined limit for only a small percentage (e.g. 5 percent), of the connections. If the fluctuations in the level of interference between various connections can be reduced, the said quality of connection can be achieved at a denser re-use rate of the channel, which increases system capacity.
Known methods for levelling out relative interference between connections include frequency hopping, used in the FDM systems, and time-slot hopping, used in the TDM systems. These and other methods based on channel alteration will be collectively referred below as channel hopping methods. In CDM systems, differences in interference between connections can be suppressed by using hashing codes of sufficient diversity. However, in this method, all the connections make use of the same frequency, which increases average cross-interference considerably.
With frequency hopping, the frequency used by the connection keeps changing at short intervals. Thus, the transmission frequency serves as the hopping quantity. The methods can be divided into slow and fast frequency hopping. In fast frequency hopping, the connection frequency is changed more often than the carrier wave frequency. In slow frequency hopping, the connection frequency is changed less often than the carrier wave frequency.
For example, in the known GSM system, frequency hopping is implemented so that an individual burst is always transmitted at one frequency and the burst in the following time-slot at another. As a result, an individual burst can be subjected to a high level of interference. Thanks to channel coding and interleaving, the required quality of connection can be achieved by ensuring that a sufficiently high percentage of the bursts are transmitted free of significant interference. With frequency hopping, this requirement can be satisfied specifically for each individual connection, even if some of the bursts were subjected to major interference.
FIG. 3
provides an illustration of a frequency-hopping arrangement with the frequencies used for the various bursts. Four frequencies, F
1
through F
4
, are allocated for use by the cell. The hopping pattern is cyclic in that the cell transmits the sequential bursts at the frequencies F
4
, F
2
, F
3
, and F
1
in that particular order and that this cycle is repeated once completed. Because the length of the cycle is 4 bursts, a single connection in a system using eight time-slot frames shown as an example in
FIG. 2
uses the same frequency only for every fourth burst. As a result, the fadings occurring in the connection between the mobile station and the base station are averaged over the individual connections. With frequency hopping, the best levelling-out performance for interference is achieved when the frequency-hopping patterns used by cells close to one another are mutually independent. This is achieved by employing carefully selected cyclic or pseudo-random frequency-hopping patterns.
In time-slot hopping, the hopping quantity is the TDMA frame time-slot used for the connection.
FIG. 4
illustrates a time-slot hopping pattern where the signal is transmitted in sequential frames in time-slots
1
,
4
,
0
, and
6
, after which the c
Nokia Corporation
Pham Chi
Squire Sanders & Dempsey L.L.P.
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