Pulse or digital communications – Spread spectrum – Frequency hopping
Reexamination Certificate
1997-05-30
2001-10-02
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Frequency hopping
Reexamination Certificate
active
06298081
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of radio communications, and then particularly to a method of channel hopping between different channels in a radio communications system. The invention also relates to apparatus in a radio communications system for implementing the method. The proposed method can be applied with frequency-divided and time-divided systems, such as FDMA and TDMA systems, and also in CDMA systems.
BACKGROUND ART
The term channel hopping is used in this document as a collective term for making hops between different information transmission channels, such as hops solely between frequencies, hops solely between time slots and hops between both frequencies and time slots in a radio communications system, for instance.
As is well known to the person skilled in this art, frequency hopping may be applied in a radio communications system to improve the performance of the radio system or as a security measure against unauthorised listening to radio communications. Frequency hopping is carried out in a predetermined order in such systems, without taking the prevailing quality of the connection into account. Frequency hopping in radio communications systems is thus not adaptive.
There can be established between a transmitter and a receiver of a radio communications system a radio connection over which radio communication can take place. The connection is two-directional, i.e. includes a downlink which forms the connection in a direction away from a base station of the system to a mobile station, and an uplink which forms the connection in the opposite direction, i.e. from the mobile station to the base station. Transmission and reception of radio traffic on different connections is effected on channels that can be defined by a given frequency in an FDMA system (Frequency Division Multiple Access), or by a combination of a given frequency and a given time slot in a system that utilizes TDMA (Time Division Multiple Access). A channel may be defined by a code in a CDMA system (Code Division Multiple Access). Seen generally, the channels that are available in a radio communications system may be disturbed significantly by other radio traffic, including radio signals on the same channels as those used for other connections, wherewith each channel in the system has a certain interference level. Thus if each connection were to use solely one channel, the connections would obtain different interference levels. The interference experienced by some connections may be so heavy as to deny an acceptable call quality. The differences in call quality of the connections can be evened-out, by jumping between different channels, wherein the connections use channels of mutually different interference levels. This spreads the use of the channels between the various connections and, with the aid of interleaving and error correction coding, more connections can obtain an acceptable call quality when the system is observed in its entirety.
Each connection may be allocated a plurality of channels, wherein the system controls the connections during ongoing communication, such as to cause the channels to hop between the channels in accordance with a given hopping rule.
This rule may, for instance, be a predetermined pseudorandom series, in which case the connections would appear to hop randomly between all available channels; cf. in this regard European Patent Application EP 93905701-4. However, this type of channel hopping may result in an unnecessarily high interference level, since the channels are not always allocated to the connections in an optimal manner when a pseudorandom series is used.
Another type of channel hopping is cyclic channel hopping. In cyclic channel hopping, a connection hops between a number of channels in accordance with a cyclically repeated channel hopping sequence.
It is well known that channel hopping can be applied in a GSM system. The GSM system is a TDMA system, meaning that each frequency is divided into a plurality of time slots that form a TDMA frame. In the GSM system a TDMA frame is comprised of eight time slots. When establishing a connection between base station and mobile station in a GSM system, the connection is allocated one of these time slots in each TDMA frame. Channel hopping is then effected, by hopping the connection between channels that have the same time slot, wherein in practice the connection hops solely between different frequencies. According to the GSM specification, a particular frequency may occur only once in a channel hopping sequence, and the frequencies in a channel hopping sequence will always occur in a rising order. On the other hand, the duration of the channel hopping sequences may vary between different base stations.
A radio communications system will normally include a plurality of channels that can be used for establishing connections between a given base station and mobile stations. It is then important that the same channel is not used simultaneously for two or more connections between the base station and mobile, stations. If two base station transmitters transmit to their respective receivers different signals simultaneously on the same channel, it is very likely that at least one receiver will be disturbed by interference deriving from the transmission to the other receiver. When this cannot occur, i.e. when only one of the base station connections can be transmitted on a channel at each point in time, it is said that the base station has orthogonality.
When a connection in a radio communications system is excessively poor, so that acceptable speech quality cannot be achieved, it may be due to an excessively low ratio between signal strength and interference. By signal strength is meant the strength of the received desired signal. By interference is meant the sum of the signal strength of all received undesirable signals on the channel used. These undesired signals arrive primarily from other connections that use the same channel in nearby cells in the radio communications system. The received undesirable signals may also arrive from those connections within the own cell that use an adjacent frequency or time slot.
The strength of the desired received signal will depend on transmitter power and on the extent to which the desired signal is attenuated in its path from the transmitter to the receiver. Attenuation is determined, among other things, by distance, direction and topology between transmitter and receiver. Other terms used in parallel with attenuation are path gain and path loss, as the skilled person is well aware.
Channel hopping in a radio communications system is described in the International Patent Application WO 96/02979. Channel hopping is effected between a plurality of channels that are each allocated a respective connection. A signal attenuation parameter, e.g. path gain, is measured in respect of the connections, which are thereafter ordered with respect to the signal attenuation parameter. The described method includes measuring a mean value of a channel quality parameter, such as interference, on individual channels. The channels are thereafter ordered with respect to the measured channel quality parameter. Only those channels that will provide the best channel quality are used for establishing the connections.
When allocating channel hopping sequences to connections, attention is paid to the connection quality of respective connections and to the channel quality of the channels in respective channel hopping sequences. A low quality connection is allocated a channel hopping sequence in which the channels used have a high quality, and a high quality connection is allocated a channel hopping sequence in which the channels used have a low (poorer) channel quality. This division of channel hopping sequences to connections ensures that orthogonality is achieved in each base station. A channel hopping sequence may include a different number of channels in different base stations. The number of channels used in a channel hopping sequence is fixed within a base station.
Swedis
Almgren Knut Magnus
Olofsson Håkan Gunnar
Persson Yngve Bengt
Burd Kevin M
Burns Doane Swecker & Mathis L.L.P.
Pham Chi
Telefonaktiebolaget LM Ericsson (publ)
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