Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-11-04
2001-10-16
Trost, William (Department: 2683)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S062000, C455S434000
Reexamination Certificate
active
06304756
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to cordless telephones and more particularly to channel selection in cordless telephone systems.
2. Description of the Related Art
Cordless telephones include a wall powered base station and a cordless battery powered handset which communicate with each other over a predetermined frequency band through the use of radio transmissions. The frequency band is broken up into a number of channels that may be used by the handset and base station. The handset and base station may both be sharing the same channel (time division duplex) or transmitting on one channel and receiving on another (frequency division duplex). There may be, e.g., forty channels over which communication takes place between the base station and the handset. Having more than one channel increases the likelihood of finding a clear channel on which to communicate.
There can be multiple handsets and base station pairs in relatively close proximity to each other. It is necessary to prevent the multiple handsets and base stations from erroneously or illegally communicating with one another. Therefore, the handset and base station exchange unique security IDs so that they communicate only with the correct partner. The higher the number of bits in the security ID, the less likely the chances of a handset or base station being able to communicate with the incorrect unit. The unique pair of security IDs is created using a random number generator, which is typically implemented in software in both the handset and the base station. When the handset and the base station register with one another, e.g., when the handset is mated in the cradle of base station
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, they exchange security IDs and henceforth use these IDs to establish communications with each other. The random security IDs are stored in nonvolatile memory, so neither a power outage nor an electrostatic discharge event destroys them.
In a standby mode of operation in which most of the power of the handset is off, the handset periodically wakes-up to scan all the channels in the appropriate frequency band to see if there is an incoming call. In the U.S., telephones ring in a pattern of two seconds on and four seconds off. That means that the handset should scan all forty channels at least every two seconds to ensure that the handset does not miss the ring indication being transmitted from the base station over one of the channels. Scanning for incoming calls requires the handset to be actively powered on.
Thus, scanning all forty channels at least every two seconds costs power, which reduces the battery life of the handset, an obviously undesirable result.
In a time division duplex system, where the handset and base station transmit and receive on the same channel, the handset and base station may not always agree on which channels are free and which are not acceptable for communication because of interference. That is especially true if they are located a significant distance from one another, as interference levels may vary in different locations. In a frequency division duplex system, the handset and base station are not capable of measuring whether their counterpart's receive channel has interference or not. To speed initial linkup time between the handset and base station, it would be desirable for both the handset and the base station to know which channels are clear in both the transmit and receive direction. It would also be desirable to achieve longer battery life while still having the advantage of multiple channels.
SUMMARY OF THE INVENTION
Accordingly, a handset and a base station of a cordless telephone system both periodically scan all channels to determine which channels are clear. The handset clear channel information is transferred to the base station and the base station clear channel information is transferred to the handset. The clear channel information from the base station and handset are logically combined to generate an array or list of available channels. The array of available channels indicates those channels which both the handset and the base station consider clear. A subset of the available channels is then selected. A channel in the array of available channels is randomly chosen as a first channel in the subset based on a security ID exchanged between the handset and the base station. The remaining channels in the subset are selected according to how many channels are predetermined to be scanned by the handset for incoming calls. The base station and handset then use only those channels in the subset to communicate. That reduces the power used by the handset since the handset only scans a subset of the available channels in standby mode. In addition, initial link up time between the base station and the handset is improved since only clear channels are scanned for incoming calls.
In one embodiment of the invention, a cordless telephone system according to the present invention includes a base station including a first control processor and associated first memory, first RF transceiver transmitting and receiving over a plurality of channels in a frequency band and a line interface, the base station responsive to an incoming call from the line interface to send an indication of the incoming call over one of a subset of the channels via the RF transceiver. The telephone system also includes a handset having a second RF transceiver and a second control processor and associated second memory. The handset is responsive to a wake-up signal to periodically scan the subset of the channels for the indication of the incoming call. At least one of the handset and base station include a stored array of available channels determined to be clear by both the handset and the base station, the subset of channels over which communications are initiated being a subset of the available channels.
In another embodiment, a method of communicating is provided for a cordless telephone system that includes a base station and a handset, which communicate over a frequency band having a plurality of channels. The handset scans the channels to determine which of the channels, according to the handset, are clear and which channels have radio activity on them. The handset provides the base station an indication of which channels the handset determined to be clear. The base station also scans the channels to determine which of the channels, according to the base station, are clear and which channels have radio activity on them. The base station provides the handset with an indication of which channels the base station determines to be clear. Both the base station and the handset determine the available channels to be those channels determined to be clear by both the base station and the handset. A subset of the available channels is selected on which to initiate calls.
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Alton Ken
Hebeler Steven E.
Gesesse Tilahun B
Legerity Inc.
Trost William
Zagorin O'Brien & Graham LLP
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