Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1996-07-24
2001-04-17
Hunter, Daniel S. (Department: 2749)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S434000, C455S425000, C455S426100, C370S347000, C370S442000
Reexamination Certificate
active
06219549
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to portable radio telephones and in particular to such telephones for communication with base stations in a digital cellular radio telephone system. The invention also relates to a method of operation of such radio telephones, frequently called handsets. The invention is concerned with ‘locking on’ to a digital cellular radio telephone system to allow communication with the system.
System information is typically transmitted over at least one channel at all times for users to obtain the information necessary to select a channel for communication. This may be one or more dummy signals transmitting at all times or just when there are no traffic channels. A handset wishing to lock on to a system must be able to find a channel on which the information required is being transmitted. In digital systems, in order to receive this information the handset must bit synchronise with the signal transmitted.
Every handset will have specific access rights to specific radio telephone systems. In order to set up a call on a channel (that is a combination of a carrier frequency and time slot), the handset must have the right of access to the particular system providing the channel. A handset is provided with one or more Access Right Keys (ARKs) that relate to a particular radio telephone system. The handset is able to lock on to any system that recognises one of the ARKs.
In prior art methods of locking on to a system, the handset assumes a reference frame having the same duration as a frame of the system and which is divided into the requisite number of time slots to provide a complete set of reference time slots. The reference time slots are not synchronised to the time slots of the system being monitored as the handset needs to find a signal being transmitted before synchronisation can occur. The handset monitors the RSSI of each of the reference channels by monitoring each timeslot of a carrier in turn. Once the RSSIs of all the reference channels have been determined the handset selects the reference channel with the highest RSSI, synchronises with the signal and reads the information being transmitted in that channel. The handset is then in a position to determine whether the access rights of the handset allow it to lock on to the system it is now monitoring.
The channel with the highest RSSI is likely to be the channel on which the system is transmitting a dummy bearer or a traffic channel. This is not, however, necessarily the case. A high RSSI could result from noise which may occur because of interference between channels of different systems. The handset will not, therefore, be able to lock on to the system. Even if the highest RSSI does indicate a transmission channel, it is not necessarily a channel on a system to which the handset has access rights. So in the same way that the handset cannot lock onto a noise channel selected as a result of a comparison of the RSSIs, the handset will be unable to lock on to a transmission channel of a system that does not allow the handset access.
If the handset is not able to lock on to the selected channel for any reason a problem arises. Because the reference channels are not synchronised with the transmission time slots of the system, as soon as the handset synchronises with the system to read information on the selected channel, the recorded RSSI information is no longer relevant to the new frame of reference of the handset. If the handset is reading noise it has lost its frame of reference altogether. If the handset is reading a signal from a system to which it is not allowed access, it has adopted the timing of that system. Under either set of circumstances, the handset cannot select a further channel based on the reference information it has previously recorded.
To make a new channel selection all the RSSI information must be gathered afresh. This is both time consuming, and power inefficient. In addition, if there is more than one system in the vicinity of the handset, once the handset has locked on to the one with the strongest signal and found it is not allowed access to that system, it will be difficult to disregard signals resulting from transmissions on channels resulting from that system in order to obtain information on signals from other systems that might also be in the vicinity.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the present invention there is provided a radio handset for operating under a cellular radio telephone protocol supported by respective cellular radio systems in which the handset is operable for communication on a channel of the system if the handset and system are compatible, wherein the radio handset comprises processing means operative to cause signal channels defined by the protocol to be scanned for detecting signals indicative of access rights to the respective systems, the processing means being adapted to cause interruption of the scan in response to detection of a signal, and to cause the scan to continue in response to a failure to determine that the handset and the system transmitting the signal are compatible.
In accordance with a second aspect of the present invention there is provided a method for selecting a particular celluar radio telephone system compatible with a handset operable under a cellular radio protocol supported by respective cellular radio systems in which the handset is operable for communication on a channel of a particular cellular radio system if the handset and system are compatible comprising, scanning signal channels of the system for signals indicative of access rights to respective systems, interrupting scanning in response to detection of a signal, and continuing to scan in response to a failure to determine that the handset and system transmitting the detected signal are not compatible.
By a method or apparatus in accordance with the present invention, a determination regarding the compatibility between the handset and the system transmitting the detected signal is made each time a signal is detected. This is efficient for locking on as only relevant information is collected, reducing the time and power expended in finding a suitable system.
The handset and the system are compatible for the purposes of this application if the handset has a right of access to the system. This may be the result of an agreement between the handset user and the system provider.
The handset is understood to be ‘locked on’ for the purposes of this application when it has a right of access to the base station.
All the information necessary for locking on need not be read on a first scan of the available channels. Only a certain subset of information providing a first indication of the compatibility of the handset and the system may be considered. This has the advantage that the process can be faster than would otherwise be the case whilst still giving a high likelihood of successfully locking on. In those situations in which the initial scan does not establish that the systems are compatible, a second scan may be undertaken to look more closely at the signals transmitted by the system to determine if the two are in fact compatible.
Under the DECT protocol two access rights are transmitted by the system, primary and secondary access rights. In one embodiment of the invention, the handset determines if it is compatible with the system transmitting signals as a result of the primary access rights on the first scan or on the secondary access rights on a subsequent scan.
This allows the handset to determine relatively quickly if it is compatible with the primary access rights of any of the systems transmitting signals in its vicinity. The longer process of determining if it is compatible with the secondary access rights of the transmitting systems can be deferred until the search for a system with suitable primary access compatibility has been exhausted.
If the protocol defines transmission on a plurality of carrier frequencies in frames each comprising a predetermined number of time slots, the processing means may scan a carrier f
Hunter Daniel S.
Nokia Mobile Phones Limited
Perman & Green LLP
Zewdu Meless N.
LandOfFree
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