Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-09-29
2001-09-18
Le, Thanh Cong (Department: 2749)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S039000, C455S455000, C455S515000
Reexamination Certificate
active
06292660
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to wireless communications and, more particularly, to site scanning in wireless communications.
BACKGROUND OF THE INVENTION
Communication networks typically include at least one sender and one receiver. In either a wired or wireless network, a signal transmitted between the sender and receiver must be of sufficient magnitude (or strength) to allow the information contained within the signal to be discriminated from the noise which is generally present in the communication network. This may be a greater problem with a wireless network and mobile terminals, which typically are more susceptible to noise and multi-path fading effects.
An example of a wireless network is a radio network such as a cellular network commonly utilized for voice and/or data communications between a fixed base station covering a geographic region and mobile devices such as cellular telephones or private radios present in the covered region. A cellular telephone typically includes a radio receiver including an antenna for receiving signals and an amplifier/detector for generating a measure of the strength of received signals or noise. A signal strength measure, commonly known as Received Signal Strength Indication (RSSI), may be expressed as a logarithmic measure of received signal strength and may be converted to a digital form by an analog to digital converter.
It is known in the prior art that radio signal strength measurements can be useful in determining which base station should serve a cellular telephone during a call. In the U.S. AMPS system, the mobile telephone would typically use such signal strength measurements to determine the strongest base station to which it should listen for calls during standby (idle) mode. This determination of which base station to communicate with may be referred to as site scanning as periodically the radiotelephone samples the RSSI from other base station sites. To sample the RSSI from other sites typically involves the radio or telephone switching frequency to the new sites control channel and then switch back to the currently selected site to listen for control channel update messages. When the radio is scanning another site to determine its RSSI level the radio can miss control channel information and when it returns to the selected site. The penalty incurred is that the call may be entered late and information may be missed. Hence there may be a significant penalty paid in scanning off the current control channel to look for a better site.
Site scanning is typically employed because, as described above, mobile radios move through different base station coverage regions. Thus, a low RSSI measurement is typically utilized to initiate site scanning in low signal level environments. This use of RSSI generally requires that each device be calibrated so that the RSSI threshold levels are consistent from device to device.
In light of the above discussion, improvements are needed in radiotelephone site scanning so that only a minimum amount of time is spent away from the current control channel frequency and calibration between radios is not required for site scanning purposes.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide improved site scanning performance in land mobile radios and/or cellular telephones.
These objects are provided according to the present invention by controlling site scanning of a wireless device by adjusting the rate at which the wireless device scans adjacent sites based on whether the wireless device is in motion. Furthermore, mobile velocity determination may be made by estimating a fade rate and then basing the rate of site scanning on this estimated fade rate. The rate at which the wireless device site scans is related to the estimated fade rate. Fade rate is the rate at which a wireless device's signal level goes through a deep fade in signal strength (for example, a deep fade may occur where the signal level drops below −120 dBm) resulting in audible clicks for analog voice or resulting in bit error bursts when sending digital information. These audible clicks or bit error bursts result even in strong signal environments when severe multi-path fading is present. The fade rate (or level crossing rate) that is estimated is proportional to the velocity of the device, the frequency of the transmitted signal and the threshold value used.
Thus, the present invention may reduce the time away from the currently selected sites control channel by using a very low rate of scanning when the mobile is stationary and increasing the scan rate as the velocity of the mobile increases, i e. adaptively controlling the site scanning. By adjusting the site scanning rate of a wireless device based on an estimate of level crossing or fade rate the site scanning can be tailored to both the motion of the terminal and the signal strength seen by the terminal. For a given threshold and velocity the level crossing rate will decrease as the signal level increases. If the device is moving quickly, the site scanning rate may be increased. If the device is moving slowly, the site scanning rate may be decreased. If the device is not moving, then site scanning may be kept to a very low value. If the velocity of the terminal is slow and the signal level is getting lower, then the site scanning rate will increase because the level crossing rate is preferably dependent on the signal level for a fixed threshold.
Preferably, the site scanning rate is established based on an estimate of fade rate of a received signal. Such a fade rate may be used because, typically, the fade rate is proportional to the velocity of the device. Thus, wireless devices may only need minor software modifications to allow for the adaptive site scanning techniques of the present invention. Furthermore, the determination of fade rate may be carried out without disturbing the normal operation of the device. This adaptive site scanning rate may be used to reduce the time that the device is away from a control channel as well as reduce power drain by eliminating unnecessary operations when the device is in standby mode. Also, in an embodiment based on fade rate, because fade rate increases with decreased signal strength (the level of noise at the discriminator output increases as signal level decreases), the present invention allows for changes in site scanning rates without requiring calibrated RSSI measurements. Thus, the calibration of devices to obtain similar RSSI thresholds may be avoided, i.e. relative RSSI measured in each radio is still used to select the best site but no absolute reference between radios needs to be used to turn on the site scanning function at a calibrated signal level.
In a further embodiment of the present invention, the rate at which the wireless device site scans may be set to a first rate if the fade rate is greater than a first fade rate threshold value. The rate at which the wireless device site scans is set to a second rate, less than the first rate, if the fade rate is less than the first fade rate threshold value. Furthermore, the rate at which the wireless device site scans may be set to a third rate, less than the second rate, if the fade rate is less than a second fade rate threshold value. In such a case, the rate at which the wireless device site scans is set to a second rate, less than the first rate, if the fade rate is less than the first fade rate threshold value and greater than the second fade rate threshold value.
In a particular embodiment of the present invention for an 800 MHz land mobile radio system, the first fade rate threshold value is about 25 fades/s and the second fade rate threshold value about 5 fades/s. If the fade rate exceeds the first fade rate threshold the radio scans a site every 250 ms if the fade rate is between 25 and 5 fades per second the radio scans a site every second. If the fade rate is less 5 fades per second the radio scans a site every 4 seconds
In yet another embodiment of the present invention, the fade rate is determined by hi
Hartless Mac
Janky William O.
Cong Le Thanh
Ericsson Inc.
Myers Bigel & Sibley & Sajovec
Tran Pablo
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