Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1999-11-08
2003-03-11
Maung, Nay (Department: 2681)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S442000, C370S331000, C455S436000, C455S552100
Reexamination Certificate
active
06532226
ABSTRACT:
TECHNOLOGICAL FIELD
The invention relates generally to the scheduling of run-time measurements and signalling reception in a cellular radio system. Especially the invention relates to the optimization of measurement and reception schedules in a situation where a mobile terminal is able to operate on several frequency bands and perform handovers between the frequency bands.
BACKGROUND OF THE INVENTION
At the time of filing of this patent application a decision has been made to accept WCDMA (Wideband Code Division Multiple Access) as one of the multiple access schemes of an universal third generation cellular radio system known as the UMTS (Universal Mobile Telecommunication System), where a terrestrial base station subsystem is known as the UTRA (UMTS Terrestrial Radio Access). In CDMA, the transmission on a certain channel is generally continuous in contrast to TDMA systems (Time Division Multiple Access) where each channel reserves a certain cyclically repeating time slot. However, both in CDMA and TDMA it is customary to arrange the transmission into consecutive frames of constant duration.
In order for a mobile terminal to fully exploit the available cellular resources it will be very advantageous if it can communicate with both third and second generation base stations and move from the cell of one to the cell of another according to the momentary availability and pricing of services at a reasonable connection quality. In order to continuously look for the optimal base station to communicate with, a mobile terminal must perform measurements that reveal the signal strength it is able to receive from each candidate base station. Additionally the terminal must receive certain signalling messages from the candidate base stations in order to plan ahead for a cell reselection or a handover. It has been widely recognised that if a terminal is currently communicating with a CDMA base station, it needs some short time intervals during which the otherwise continuous downlink CDMA transmission is interrupted for the measurements and signalling reception to be possible.
Recently it has been shown that it is advantageous to interrupt also the uplink CDMA transmission for the duration of measurements and signalling reception. We will briefly describe the reasons behind this observation by reference to
FIG. 1
, where the GSM1800 system (Global System for Mobile telecommunications at 1800 MHz; also known as the DCS1800 or Digital Cellular System at 1800 MHz) is considered as an exemplary cellular radio system of the second generation. The uplink frequencies of the GSM1800 system lie between 1710 and 1785 MHz and the corresponding downlink frequencies between 1805 and 1880 MHz. From the downlink frequency range upwards there is a narrow range for DECT frequencies (Digital European Cordless Telephone) and another narrow range for UTRA TDD frequencies (Time Division Duplex). The UTRA FDD uplink frequencies (Frequency Division Duplex) lie between 1920 and 1980 MHz and the corresponding downlink frequencies between 2110 and 2180 MHz. Therebetween is also another relatively narrow range for UTRA TDD frequencies from 2010 to 2025 MHz.
If a mobile terminal is considering a handover from an UTRA FDD cell to a GSM1800 cell, it should be able to receive and decode signalling messages on the GSM1800 downlink frequency range. The UTRA FDD uplink frequency range is so close to the GSM1800 downlink frequency range that a simultaneous uplink transmission on the former is very likely to cause a RF leakage through the duplex filter of the terminal to the receiver chain, thus interfering with or even disabling any measurements or reception for decoding. There are potential hardware solutions to this problem, such as using a different antenna for different frequency ranges and very high quality duplex filters, but they typically require considerable complication of the terminal structure and are therefore undesirable for the manufacturing point of view. It is much easier to arrange for suitable interruptions also in the uplink transmission, especially if such interruptions have already been specified for the corresponding downlink.
The accepted form of interrupting a CDMA transmission in an UTRA system for measurements and signalling reception is known as the slotted mode. It means generally that a predetermined part of a certain frame period will be left empty with no ongoing transmission. The frame including the information contents destined for transmission during such a frame period will be transmitted during the remaining part of the frame period in a compressed form, using for example a slightly higher transmission power. Three types of slotted mode have been suggested for use; additionally we may regard a combination of two of them as a fourth type.
FIG. 2
illustrates the different types of slotted mode in a coordinate system where the horizontal axis represents time divided into frame periods (e.g. 10 ms) and the vertical axis represents transmission power in some arbitrary units. Frame
201
illustrates an idle period at the end of the frame period, frame
202
illustrates an idle period in the middle of the frame period and frame
203
illustrates an idle period at the beginning of the frame period. Mutually consecutive frames
204
and
205
are of the first and third types described above, whereby it is immediately obvious that there will be a relatively long idle period bridging the separation line between these consecutive frame periods. The “double length” idle period may be regarded as a fourth type of slotted mode, even if it is actually just a suitable coordinated arrangement of slotted modes of the first and third type.
In principle it would be possible to have a fixed allocation of simultaneous slotted frames for all mobile terminals in the cell of a CDMA base station. However, due to the large number of potential connections simultaneously active such a common slotted mode is not advisable, because it would cause large jumps in the amount of emitted transmission power. In practice the base station will compose a time schedule for the use of slotted mode in its cell and use terminal-specific signalling to allocate a unique or nearly unique pattern of slotted frames to each terminal so that the overall effect of the slotted frames to the average transmission power within the cell will be negligible.
Next we will describe briefly the known arrangement of channels which a GSM1800 base station will use for transmitting those common channel messages which the mobile terminal operating in a nearby cell should receive in order to prepare for a cell reselection or a handover to the cell in question. A complete description of the GSM1800 common channels (which follow the arrangement of the corresponding channels in the conventional 900-MHz GSM system) is available to the public from the GSM specifications published by the ETSI (European Telecommunication Standards Institute) and e.g. from the book Michel Mouly, Marie-Bernadette Pautet: The GSM System for Mobile Communications, published by the authors, ISBN 2-9507190-0-7, Palaiseau 1992. In the following description we will emphasize the timing aspects, because these are of importance to the present invention.
Each GSM base station will regularly transmit, on a certain common channel frequency, so-called FCCH and SCH messages (Frequency Correction CHannel; Synchronisation CHannel). The transmission schedules of all GSM channels have been determined in relation to the concept of a frame, which contains 8 consecutive time slots or BPs (Burst Periods) each having the length of 15/26 ms (approximately 0.577 ms). On said common channel frequency we may take a period of 51 frames and designate the frames therein from 0 to 50; in such arrangement the first time slot of the 0th, 10th, 20th, 30th and 40th frame will contain an FCCH message and the first time slot of the 1st, 11th, 21th, 31th and 41th frame will contain an SCH message. In other words we may say that a GSM base station will transmit on said common channel frequency FCCH messages with reg
Lehtinen Otto-Aleksanteri
Toskala Antti
Maung Nay
Nokia Mobile Phone Ltd.
Perman & Green LLP
Trinh Sonny
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