Multiplex communications – Diagnostic testing – Determination of communication parameters
Reexamination Certificate
1997-09-26
2001-05-01
Nguyen, Chau (Department: 2663)
Multiplex communications
Diagnostic testing
Determination of communication parameters
C455S226200, C370S332000
Reexamination Certificate
active
06226271
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to communication networks and, more particularly to the determination of received signal strength in communications networks.
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, which typically is more susceptible to noise from various interference sources.
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 terminals (or phones) present in the covered region. A cellular phone 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 Radio 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 phone during a call. Cellular phones using a Time Division Multiple Access method conforming to either the European cellular standard known as GSM or either of the American TDMA standards known respectively as D-AMPS or PCS1900 use spare time between transmit and receive timeslots to change frequency and monitor the signal strengths of other base stations. Several measurements of signal strength are typically averaged for the same base station. The averages are reported to the currently serving base station, which determines if a handoff should be made to another, stronger base station. Determination of average signal strength can comprise performing a series of measurements that are digitized at the sampling rate of an analog to digital convertor which is used for demodulating traffic symbols, i.e., the sampling rate may be one or more times the traffic symbol rate. The measurements are preferably averaged over a TDMA timeslot or other measurement window in order to reduce the effects of fading and noise. Thus, it is known to be useful to average RSSI measurements.
In the prior art, RSSI measurements are typically obtained on a logarithmic scale in order to encompass a large dynamic range of possible signal values, see for example U.S. Pat. No. 5,048,059, which is hereby incorporated by referenced herein in its entirety. In conventional systems, when RSSI measurements obtained in logarithmic form were to be averaged, it was typically considered acceptable to average the logarthmic values because the effort to perform an antilogarithm, which is needed to obtain a true power average, was considered excessive. There is however, an inherent inaccuracy in averaging logarithmic values by this method. Average logarithmic RSSI values could be obtained by analog means, using a resistor-capacitor smoothing filter, or by using a capacitor to integrate an RSSI-dependent current over an averaging window in an analog integrate-and-dump circuit. The latter method is useful when the signal strength has to be averaged over a rectangular measurement window such as a TDMA timeslot, and was used in the first GSM cellular telephone sold by L. M. Ericsson AB in Europe. In co-pending application Ser. No. 08/768,975 (Dent, Filed Dec. 18, 1996) entitled “Method of Obtaining Initial Synchronization,” a method to obtain synchronization with a communications satellite signal received at extremely low signal to noise ratios is described. This application is also incorporated by reference herein in its entirety. The disclosed method can require the summation of very large number of RSSI measurements made during disjointed time periods.
A problem with these previously known methods for measuring signal strength is the processing load required for averaging a large number of values taken over disjointed time periods. This processing load may particularly be a problem for mobile devices such as cellular phones which typically operate for extended periods on battery power. Excessive processing load may reduce operating life available to the user of the cellular phone.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an improved and more energy efficient method and system for calculating a received signal strength indication.
It is a further object of the present invention to provide a method and system which calculates a received signal strength indication by efficiently adding and thereby averaging a large number of logarithmically represented signal strength values.
The present invention determines a signal strength of a received signal by averaging a large number of signal strength measurements (or RSSI's) of a signal taken over a disjoint period of time. This averaging is accomplished by counting the number of occurrences of measurements of each of a discrete number of possible values which are monotonically related, preferably logarithmic values. The counts are then iteratively reduced according to the present invention until only a single non-zero value remains which represents the accumulated value of all of the samples taken during the sampling period chosen for determining the signal strength. By utilizing counts of discrete values instead of the actual logarithmic values, the present invention beneficially decreases processing load in comparison to logarithmic addition of actual logarithmic values. The present invention thereby reduces the power consumed in determining the signal strength by utilizing the novel methods and apparatus of the present invention which, in part, utilize memory in place of processing power for generation of the summation of the samples. Moreover, a result that is a more accurate representation of the signal power average is obtained. This may provide greater battery life in devices such as battery-operated cellular phones.
According to one embodiment of the present invention, a large number of signal strength measurement values are optionally collected in a first memory and then retrieved from memory to construct a histogram. Alternatively, the values are directly collected in a histogram memory indicating the number of times each possible discrete value was observed. The number of histogram bins is only of the order of the number of possible quantized RSSI values (e.g. 256 in the case of 8-bit quantization) which is much smaller than the number of values typically received and processed. In one embodiment, the present invention calculates the logarithm of the sum of the values whose logarithms are given by the coarsely digitized RSSI values by starting with a first bin (or counter) corresponding to the lowest RSSI value, the content indicating the number of times that value was observed is read out and used as the address for a look-up table to obtain the logarithm of that number, to the same base i.e. on the same logarithmic scale as the RSSI values themselves. The log of the bin count is read from the look-up table and added to the address of the bin to obtain a value indicative of the log of the sum of the energy values collected in that histogram bin. The logsum value is then the address of a new, higher-number-addressed histogram bin corresponding to the new, summed energy and the new, higher-numbered bin count is incremented by one, while the original, first bin optionally has its contents reset to zero. The method or apparatus then proceeds to process the next highest histogram address in a similar way with any bin which contains a count of zero being skipped. If a bin contains a count of one, the bin number
Ericsson Inc.
Myers Bigel & Sibley & Sajovec
Nguyen Chau
Trinh Dung
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