WO1993023963A1

WO1993023963A1 - Method and circuit for selecting tuning of a radio receiver

Info

Publication number: WO1993023963A1
Application number: PCT/US1993/003692
Authority: WO
Inventors: Christopher F. Bussan; Patrick J. Marry; Duane C. Rabe; Charles P. Binzel; Arvind S. Arora
Original assignee: Motorola Inc.
Priority date: 1992-05-08
Filing date: 1993-04-19
Publication date: 1993-11-25
Also published as: RU2123771C1; FR2691027B1; ITRM930277A0; SE9400018D0; CN1082786A; DE4392213T1; FR2691027A1; HU9400043D0; SE9400018L; CN1035588C; CA2112809A1; JPH06508973A; IT1261466B; GB9326218D0; GB2274750B; GB2274750A; DE4392213C2; ITRM930277A1; HUT69370A; JP3196211B2

Abstract

A circuit and method for selecting tuning of a radio receiver to a frequency channel of a set of frequency channels. The radio receiver scans selected control channels and measures the power levels of signals generated upon control channels. Only when the measured power levels (684) of the signals generated upon the respective ones of the control channels are in excess of a certain power level is a communication link attempted to be effectuated with a transmitter of such data signals (696). When a communication link cannot be effectuated, the radio receiver powers down (708) for a predetermined time period, then powers on again (710), and the power levels of signals transmitted upon the control channels are again measured (710). Effectuation of a communication link with a transmitter which transmits a signal upon one of the control channels is attempted if the measured power level on such control channel increases beyond a certain amount.

Description

METHOD AND CIRCUIT FOR SELECTING TUNING OF A RADIO

RECEIVER

Background of the Invention

The present invention relates generally to radio receivers, and, more particularly, to a method for selecting tuning of a radio receiver to a frequency channel of a set of frequency channels, and associated circuitry for implementing such. A communication system is operative to transmit information between two or more locations, and includes, at a minimum, a transmitter and a receiver interconnected by a communication channel. A radio communication system is a communication system in which the communication channel comprises a radio frequency channel wherein the radio frequency channel is defined by a range of frequencies of the communication spectrum.

The transmitter which forms a portion of the radio communication system includes circuitry for converting the information into a form suitable for transmission thereof upon a radio frequency channel. Such circuitry includes modulation circuitry which performs a process referred to as modulation. In such a process, the information which is to be transmitted is impressed upon a radio frequency electromagnetic wave, commonly referred to as a carrier signal. The resultant signal iβ commonly referred to as a modulated signal. Such resultant signal is also referred to as a communication signal as the modulated signal includes the information which is to be communicated between the transmitter and the receiver.

Various modulation schemes are known for impressing the information upon the carrier signal to form thereby the communication signal. For instance, amplitude modulation, frequency modulation, phase modulation, and combinations thereof are all modulation schemes by which information may be impressed upon a carrier wave to form the communication signal.

Radio communication systems are advantageous in that no physical interconnection is required between the transmitter and the receiver; once the information signal is modulated to form a modulated signal, the modulated signal may be transmitted over large distances.

Additionally, numerous modulated signals may be simultaneously transmitted at different frequencies of the electromagnetic frequency spectrum. Transmission of communication signals on frequency channels defined upon certain frequency bands of the electromagnetic frequency spectrum are regulated by regulatory bodies.

A two-way, radio communication system is a radio communication system, similar to the radio communication system above-described, but which further permits both transmission of information to a location, and transmission of information from that location. Each location of such a two-way, radio communication system contains both a transmitter and a receiver. The transmitter and the receiver positioned at a single location typically comprise a unit referred to as a radio transceiver, or, more simply, a transceiver.

A cellular communication system is a type of two-way radio communication system in which communication is permitted with a radio transceiver positioned at any location within a geographic area encompassed by the cellular communication system. A cellular communication system is created by positioning a plurality of fixed-site radio transceivers, referred to as base stations, at spaced-apart locations throughout the geographic area. The base stations are connected to a conventional, wireline, telephonic network. Each base station has associated therewith a portion of the geographic area located proximate to each of such base stations. Such portions are referred to as cells. The plurality of cells, each defined by corresponding ones of the base stations of the plurality of base stations together define the coverage area of the cellular communication system.

A radio transceiver, referred to in a cellular communication system as a radiotelephone, positioned at any location within the coverage area of the cellular communication system is able to communicate with a user of the conventional, wireline, telephonic network by way of a base station. Communication signals generated by the radiotelephone are transmitted to a base station, and then, by way of the conventional, wireline, telephonic network to a desired wireline location to effectuate thereby telephonic communication therewith. Telephonic communication may also be effectuated with the radiotelephone upon initiation at the wireline location.

A conventional protocol of operation of most cellular communication systems determines to which of the base stations of the cellular communication system the radiotelephone transmits, and receives, communication signals. Effectuation of transmission of communication signals by a base station to a radiotelephone is referred to as a "communication down-link;" effectuation of transmission of communication signals by a radiotelephone is referred to as a "communication up-link."

The frequency channels into which the frequency band allocated for cellular communication are divided are further divided into control channels and traffic channels. In conventional, cellular communication systems, the control channels and traffic channels are defined to be of differing frequencies. In systems utilizing time division multiplexing techniques, the control and traffic channels may also be defined to be of similar frequency channels, but defined to be of dissimilar time slots therein.

The control channels are allocated for transmission of communication signals, here referred to as control signals, by the base stations. The radiotelephones are operative to scan the control channels (both during powering on of the radiotelephone and periodically during operation thereof). The power levels of the signals transmitted upon the control channels are ascertained, and responsive to such measured values, the radiotelephone attempts to ascertain the information content of the control signal generated upon one of the control channels. During such stage of signal transmission, if the information content of the control signal transmitted upon the selected control channel may be ascertained, and the information content of the control signal transmitted upon such control channel meets pre-defined criteria, a communication down-link is effectuated. The receiver circuitry of the radiotelephone remains tuned to the selected control channel. In essence, the radiotelephone tunes-in to the selected control channel, and "listens" to the information being transmitted thereupon. Such operation of the radiotelephone is sometimes referred to as "camping" of the radiotelephone, and once a communication down-link is effectuated between a base station and the radiotelephone, the radiotelephone is sometimes referred to as being "camped-to a base station (or cell site)." Once the communication down-link has been effectuated between the radiotelephone and a base station, receiver circuitry of the radiotelephone maybe powered-down, and only periodically powered-on again in a desired duty cycle. During periods in which the receiver circuitry is again powered, the information content of the control signal transmitted upon the selected control channel (i.e., the control channel to which the radiotelephone is "camped") is monitored. (It is noted that, at times, control signals may also be transmitted upon the traffic channel. Such control signals are typically transmitted, however, after a communication link has already been effectuated between the radiotelephone and the base station.)

Because the receiver circuitry is powered-on only periodically, energy consumption of such circuitry is minimized once a communication down-link has been effectuated with a base station. When a call is being made to the radiotelephone, the information content of the control signal directs the radiotelephone to certain ones of the traffic channels whereupon voice communication may then commence. Because of the increased popularity of use of such cellular communication systems, many of such cellular communication systems, at times, are operated at full capacity. That is to say, at times, every available traffic channel of the frequency band allocated for cellular communications is used. To provide access to greater numbers of users of such cellular communication systems, schemes have been developed to utilize more efficiently the frequency bands allocated for such use.

In particular, to increase capacity, cellular communication systems are being introduced which utilize more efficient modulation schemes. Modulation schemes which make use of discretely-encoded information signals more efficiently utilize the frequency channels allocated for cellular communications. Hence, use of such modulation schemes permit greater numbers of users to utilize simultaneously a cellular communication system. Such modulation schemes generate communication signals which are amenable to transmission utilizing the time-division multiplexing techniques noted briefly hereinabove. Conventional, cellular communication systems and cellular communication systems of increased capacity are generally incompatible systems. That is to say, radiotelephones operative in a conventional, cellular communication system are not operative in a cellular communication system of increased capacity. (While dual mode radiotelephones are operative in either of the cellular communication systems, such dual mode radiotelephones include dual circuitry portions, with one circuitry portion operative in one system and another circuitry portion operative in the other system, whereby one or the other of the circuitry portions is operative depending upon the communication system in which the radiotelephone is to be operated.) Two or more cellular communication systems are sometimes installed in certain geographical areas. For instance, both conventional and increased-capacity, cellular communication systems are installed in certain geographical areas providing cellular coverage in either of such systems. In other geographical areas, only conventional, cellular communication systems are installed. (A geographic area may, analogously, have only an increased-capacity system installed.) And, in still other geographical areas, no cellular communication systems are installed.

As noted previously, when the receiver circuitry of a radiotelephone is powered on, the radiotelephone searches control channels to detect the presence of a control signal transmitted thereupon. When the radiotelephone is positioned in a geographic area not having cellular coverage, such search of the control channels does not result in the detection of a signal transmitted by any base station. Also, a radiotelephone constructed to be operative in a particular one of the cellular communication systems (i.e., a conventional, cellular communication system, or a cellular communication system of increased capacity), but positioned at a location having cellular coverage by another, but incompatible, cellular communication system also does not detect the presence of a control signal transmitted upon the defined control channels of the cellular communication system in which the radiotelephone is operative. Because the frequency bands of conventional cellular communications systems and cellular communication systems of increased capacity are operative in -frequency bands having overlapping frequencies, a radiotelephone, when searching the control channels, may detect the presence of a control signal transmitted by the other of the cellular communication systems. However, because the systems are incompatible, the information content of such a detected signal cannot be ascertained by such radiotelephone. Many radiotelephones are constructed to be operated by battery power supplies. Because battery power supplies are of finite energy storage capacity, such radiotelephones are operative for only a limited period of time. Searching by the radiotelephone of the control channels for control signals transmitted thereupon requires powering of the radiotelephone. The radiotelephone searches not only for the presence of signals upon the control channels, but, additionally, also to determine the information content of any detected signal. If the radiotelephone is positioned at a location having no cellular coverage, or having cellular coverage of an incompatible cellular communication system, continued searching by the radiotelephone of the control channels results in depletion of the stored energy of the battery power supply which powers the radiotelephone. Such continued searching, therefore, can result in severe limitation in the operational period of the radiotelephone to communicate therethrough, as an excessive amount of stored energy is depleted searching the control channels. What is needed, therefore, is means for limiting continual searching by a radiotelephone of control channels when a communication link cannot be established with a base station.

Summary of the Invention

The present invention, accordingly, advantageously provides a method, and associated circuitry, for overcoming the limitations of the existing art.

The present invention further advantageously provides means for limiting continual searching by a radiotelephone of control channels when a communication link cannot be established with a base station.

The present invention includes further advantages and features, the details of which will become more apparent by reading the detailed description of the preferred embodiments hereinbelow.

In accordance with the present invention, a method, and associated circuit for implementing such, for selecting tuning of tuning circuitry of a radio receiver to a frequency channel of a set of frequency channels defined upon a frequency band is disclosed. Each frequency channel of the set of frequency channels is suitable for transmission thereupon of a communication signal transmitted by a transmitter of a group of remotely-positioned transmitters. The tuning circuitry of the radio receiver is tuned to each frequency channel of the set of frequency channels defined upon the frequency band. Power levels of communication signals transmitted upon individual ones of the frequency channels of the set of frequency channels are measured. Frequency channels upon which communications signals of measured power levels beyond a predetermined minimum power level are selected to form a first subset of frequency channels. A determination is made whether a communication link may be effectuated between the radio receiver and individual ones of the transmitter which transmits communication signals upon the frequency channels of the first subset of frequency channels. Tuning of the tuning circuitry of the radio receiver to a frequency channel upon which a communication signal is transmitted by a transmitter to which a communication link with the radio receiver may be effectuated is selected when such communication link is possible.

Brief Description of the Drawings

The present invention will be better understood when read in light of the accompanying drawings in which:

FIG. 1 is a partial schematic, partial block diagram of a cellular communication system in which the method and circuit of the preferred embodiment of the present invention is operative;

FIG. 2-1 is a schematic representation of a portion of a frequency band allocated for cellular communications;

FIG. 2-π is a schematic representation of a single frequency channel illustrating time slots defined thereupon as utilized in a time-division multiplexing technique;

FIG. 3 is a logical block diagram of a radio receiver including the circuit of the preferred embodiment of the present invention; FIG. 4-1 is a representation of the relationship between the power levels of control signals measured during operation of the preferred embodiment of the present invention and a dynamically- established noise floor derived therefrom; FIG. 4~π is a representation of the relationship between the power levels of control signals measured during operation of the preferred embodiment of the present invention and an absolute-valued noise floor;

FIG. 5 is a representation illustrating the relationship between the power level of a previously-measured signal and the power level of such signal upon re-measurement thereof; a comparison of such power levels is utilized during operation of the preferred embodiment of the present invention;

FIG. 6 is a logical flow diagram of an algorithm embodying the method of the preferred embodiment of the present invention;

FIG. 7 is a logical flow diagram listing the method steps of a preferred embodiment of the present invention; and

FIG. 8 is a block diagram of a radio transceiver including the circuitry of the preferred embodiment of the present invention.

Description of the Preferred Embodiments

Referring first to the schematic illustration of FIG. 1, a cellular communication system, referred to generally by reference numeral 100, is shown. As mentioned hereinabove, a cellular communication system is formed by positioning numerous base stations at spaced-apart locations throughout a geographical area. Such base stations are indicated in FIG. 1 by points 104, 108, 112, 116, 120, 122, 126, and 130. While eight base stations are illustrated in the figure, it is to be understood, of course, that a typical cellular communication system, represented by cellular communication system 100, is conventionally comprised of significantly greater numbers of base stations. Each base station 104-130 contains circuitry permitting transmission of communication signals transmitted by the base station to a plurality of radiotelephones when such radiotelephones are positioned at locations within the vicinity of respective ones of the base stations, and to receive communication signals transmitted by such plurality of radiotelephones.

Each base station 104-130 is coupled to a conventional, wireline, telephonic network. Such connection is represented in the figure by line 134 interconnecting base station 130 and wireline network 138. Connections between wireline network 138 and other ones of the base stations 104-126 may be similarly shown.

The positioning of each of the base stations 104-130 forming cellular communication system 100 is carefully selected to ensure that at least one base station is positioned to receive a communication signal transmitted by a radiotelephone positioned at any location throughout the geographical area encompassed by system 100, thereby defining the cellular coverage area of the system. That is to say, at least one base station 104-130 must be within the transmission range of a radiotelephone positioned at any such location throughout the geographical area. (Because the maximum signal strength, and hence, maximum transmission range, of a signal transmitted by a base station is typically greater than the maximum signal strength, and corresponding maximum transmission range, of a signal generated by a radiotelephone, the -maTi-miim transmission range of a signal generated by a radiotelephone is a primary factor which must be considered when positioning the base stations of the cellular communication system.)

Because of the spaced-apart nature of the positioning of the base stations, portions of the geographical area throughout which base stations 104-130 are located are associated with individual ones of the base stations. Portions of the geographical area proximate to each of the spaced-apart base stations 104-130 define "cells" which are represented in the figure by areas 144, 148, 152, 156, 160, 162, 166, and 170. Cells 144- 170 together form the geographical area and define the coverage area encompassed by cellular communication system 100. A radiotelephone positioned within the boundaries of any of the cells 144-170 of system 100 may transmit, and receive, modulated signals to, and from, at least one base station 104-130.

Communication system 100 is representative of a conventional, cellular communication system or a cellular communication system of increased capacity. As also mentioned previously, the general protocol of operation of initiation of communication in a cellular communication system involves detection by a radiotelephone of communication signals, here referred to as control signals, transmitted by various ones of the base stations on various ones of the control channels defined in the communication system. Each base station of the cellular communication system transmits control signals on predefined control channels to identify the presence of such base station, and thereby to effectuate a communication down-link with a radiotelephone once the radiotelephone ascertains the information content of the control signal transmitted upon a selected control channel. (Again, when time- division multiplexing techniques are utilized, control and traffic channels may be defined at similar frequencies, but in dissimilar time slots.)

Once a communication down-link has been effectuated between the radiotelephone and a base station, portions of the receiver circuitry of the radiotelephone may be powered-down, to be re-powered thereafter in a desired duty cycle to ascertain, during resultant periodic intervals, the information content of the control signal transmitted upon the control channel. When a call is made to the radiotelephone, the information content (i.e., the data) of the control signal instructs the radiotelephone to be tuned to particular traffic channels defined upon the cellular communication system to permit two-way communication between the radiotelephone and the base station. Otherwise, portions of the receiver circuitry of the radiotelephone is powered-down according to the desired duty cycle. However, when the radiotelephone is positioned beyond the coverage area of a cellular communication system, a search of the control channels of the communication system does not result in the detection of a signal permitting effectuation of a communication link. For instance, a radiotelephone, indicated in the figure by radiotelephone 174, is positioned beyond the cellular coverage area of cellular communication system 100. A search of the control channels by the radiotelephone when positioned beyond the cellular coverage area does not result in effectuation of a communication link with any of the signals transmitted by any of the base stations 104-130 of system 100. Repeated searching of each of the control channels of the communication system, to detect the presence of a signal transmitted thereupon, and the information content of any such detected signal, is both time-consuming and energy-consumptive. As a radiotelephone is, in many instances, powered by a battery power supply, energy consumption of the radiotelephone is desired to be limited to maximize the period of operability of the radiotelephone with such battery power supply.

As also mentioned previously, due to the introduction of cellular communication systems of increased capacity, cellular coverage areas of two or more cellular communication systems may overlap both geographically and in frequency. In FIG. 1, points 204 and 208 represent base stations of a second cellular communication system, and cells 214 and 218, shown in hatch, represent coverage areas of the cells of the second cellular communication system. Shaded area 222 represents overlapping, geographically, of coverage areas of the two cellular communication systems. A radiotelephone located in area 222 is able to detect control signals transmitted by both base station 122 of system 100, and base station 208 of the second communication system. During a search of control channels of the cellular communication system in which the radiotelephone is operative, the control signal generated by a base station of the other of the communication systems may be detected by the radiotelephone. A communication down-link with the base station of such other cellular communication system cannot be effectuated as the two communication systems are incompatible. The detected control signal on such control channel should, accordingly, be ignored by the radiotelephone. Additionally, when the radiotelephone is initially positioned beyond the cellular coverage area of the communication system in which the radiotelephone is operative, (here, for purposes of illustration, cellular communication system 100), but is subsequently relocated to be -within the coverage area of the cellular communication system, the radiotelephone must be able to detect the presence of control signals transmitted upon the control channels of the communication system once positioned within the coverage area thereof. However, again, such continual searching of the control channels of the communication system to detect the presence of control signals thereupon can be quite energy-consumptive.

FIG. 2-1 is a schematic representation of a frequency band, referred to generally by reference numeral 300, allocated for cellular communications. Frequency band 300 is divided into numerous frequency channels, some of which are designated to be control channels, here designated by Di, D2, . . . D_n, and traffic channels Vi, _2» V3, . . . V_n. Upon powering-on of a radiotelephone, the general protocol of operation of initiation of establishment of a communication link with a base station, the radiotelephone searches each of the control channels defined upon frequency band 300. FIG. 2-π is a schematic representation of a single frequency channel, here designated by reference numeral 306, and the time slots defined thereupon over a period of time. Frequency channel 306 represents time slots defined upon a single frequency channel when time-division multiplexing techniques are utilized. Frequency channel 306 defines a plurality of time slots thereupon. Using the designations of FIG. 2-1, time slots Vi through V_n are defined to be traffic channels, and other time slots Di through D_n.are defined to be control channels. FIGs. 2-1 and 2-H utilize similar designations, as the method and circuit of the preferred embodiments of the present invention are operable to search control channels defined either conventionally or in a time-division multiplexed system. Tuning circuitry of the receiver portion of such radiotelephone tunes the radiotelephone to each of the control channels Di, D₂, . . . D_n to detect the presence of control signals transmitted upon any of the control channels. The power levels of any detected control signals on any of the control channels are measured, and a communication link is attempted to be effectuated with the transmitter which transmits the detected signal of greatest power level (by ascertaining the information content of such detected signal).

A communication down-link may be effectuated with such transmitter when the information content of the control signal is ascertainable by the radiotelephone. That is to say, when an attempt is made to establish a communication down-link with a base station of an incompatible cellular communication system, the information content of such control signal cannot be ascertained by the radiotelephone. Such data signal should be ignored by the radiotelephone in subsequent searches of the control channel when attempting to effectuate a communication link with one of the base stations of a cellular communication system.

It is noted that the power levels of signals detected by receiver circuitry may be quickly and efficiently (i.e., with little energy consumption) determined, whereas determination of the information content of an encoded signal detected by the receiver typically requires operation of signal processing circuitry. Such circuitry requires relatively significant amounts of energy for operation. The present invention advantageously minimizes the times in which such processing circuitry must be operated.

Turning next to the logical block diagram of FIG. 3, a circuit, referred to generally by reference numeral 400 comprised of the elements encompassed by the block shown in hatch, of the preferred embodiment of the present invention is shown. Circuit 400, as illustrated in FIG. 3, forms a portion of a radio receiver, referred to generally by reference numeral 406. Radio receiver 406 may, for example, comprise the receiver circuitry portion of a radio transceiver, such as a radiotelephone, operative in a cellular communication system.

Radio receiver 406 is operative to search predefined control channels, such as control channels Dι-D_n shown in FIG. 2-1 or FIG. 2- II to detect the presence of control signals transmitted upon individual ones of the control channels.

Control channels, of which lines 412 are indicative, transmitted by transmitters, such as transmitter 416, are detected by antenna 418 of receiver 406. Signals representative of the signals detected by antenna 418 are generated on line 420 and supplied to programmable tuning/down-conversion circuitry 422. Circuitry 422 tunes the receiver to certain frequency channels, here initially the control channels, responsive to signals supplied thereto on lines 436.

Circuitry 422 generates signals on line 448 representative of the control signals within particular ones of the frequency channels to which circuitry 422 tunes the receiver 406. The signals generated on line 448 are supplied to demodulator 458 and signal strength measuring circuitry 468. Demodulator 458 generates a demodulated signal on line 464 which is supplied to decoder 470. Decoder 470 generates signals on line 476 which are applied to processing device 482. Decoder 470 is further operative to generate a decoded signal on line 486 which is coupled to transducer 488, such as a speaker. Signal strength measuring circuitry 468 generates a signal on line 492 which is coupled to an input of processing device 482 to supply thereby processing device 482 with information relating to the power level of the signal generated on line 448. As the signal generated on line 448 is indicative of a signal detected by antenna 418, the level of the signal generated on line 492 is similarly indicative of the power level of the signal detected by antenna 418.

Memory element 496 is further illustrated in the figure connected to processing device 482 bylines 498. In operation, radio receiver 406 first searches each channel of a predetermined set of the control channels defined upon a communication system to detect the presence of control signals transmitted thereupon.

Processing device 482 generates signals on line 436 to cause tuning of circuitry 422 to each of the control channels in sequence.

When the receiver 406 is tuned to each of the control channels, signals indicative of the detected signal on the respective ones of the control channels are generated by circuitry 422 on line 448. Signal strength measuring circuitry 468 measures the amplitudes of the signals generated on line 448 and generates signals on line 492 indicative of such power levels. Processing device 482 stores such measured values in memory element 496. Processing device 482 is operative to correlate the measured power levels and the frequency channels on which such signals are transmitted. Once the power levels of the data signals transmitted upon the various control channels have been measured, processing device 482 is operative to determine whether an attempt should be made to establish a communication down-link with a transmitter which transmits a control signal upon one of the control channels. In order to make such determination, processor 482 forms a first subset of control channels of the set of control channels. The control channels of which the first subset is comprised are selected responsive to the power levels of the control signals transmitted thereupon.

In the preferred embodiment, only control channels upon which data signals having power levels beyond either 1.) a dynamically-established "noise floor" level; or 2.) an absolute-valued noise floor are selected to form the channels of the first subset of control channels.

The dynamically-established noise floor is established by comparing the power levels of each of the control signals measured by signal strength measuring circuitry 468. The noise floor is established at a power level at a certain value above a measured power level of lowest magnitude.

FIG. 4-1 is a representation of the dynamically-established noise floor, designated in the figure by line 550. Each arrow 560 designates a signal detected upon a control channel by receiver 406 of FIG. 3. The height of each arrow 560 is indicative of the power level thereof. Line 550 designating the dynamically-established noise floor is established at a certain power level, indicated by bracket 570, above the power level of the signal of the lowest power level, such signal being represented by the arrow 560 positioned at the right-most side of the figure. Frequency channels upon which data signals of power levels greater than this noise floor are selected to form the first subset of control channels. As the value of the dynamically-established, noise floor is dependent upon the measured power level values, the value of such noise floor so established is variable, depending upon the actual, measured power levels of the control signals.

The absolute-valued noise floor is a power level of a selected magnitude. FIG. 4-ϋ is a representation of the absolute-valued, noise floor, designated in the figure by line 605. Each arrow 610 designates a signal detected upon a control channel. The signals represented by arrows 610 correspond to signals represented by arrows 560 of FIG. 4-1, and the heights of each arrow are indicative of the power level thereof. Frequency channels upon which control signals of power levels greater than this noise floor are also selected to form the first subset of control channels.

Hence, in the preferred embodiment, control channels are selected to form the first subset of control channels if a control signal transmitted thereupon is greater than either the dynamically- established noise floor or the absolute-valued noise floor.

An attempt to effectuate a communication link with a transmitter which transmits a control signal to receiver 406 is made only with transmitters which transmits signals upon control channels of the first subset of control channels.

A communication link is effectuated only when decoder 470 indicates the presence of valid information of the control signal once demodulated by demodulator 458, and supplied thereto on line 464. (That is, decoder 470 generates signals on line 476 when the information- content of the control signal transmitted upon the selected control channel may be ascertained.) The signal generated by decoder 470 on line 476 and supplied to an input of processing device 482 indicates proper decoding of the information content of such a control signal, and, hence, an indication that a communication link may be effectuated with the transmitter which transmits such a control signal.

If a communication link may not be effectuated with any of the transmitters which transmit the control signals upon the frequency channels of such first subset of frequency channels, the radio receiver pauses for a certain time period prior to again commencing to attempt to establish a communication link with a transmitter. During such time period, portions of the circuitry of the receiver may be powered-down to limit energy usage of the receiver 406 during such period. (For instance, decoder 470, typically comprised of a digital signal processor, may be powered-down during such time period.) In a preferred embodiment, the time period during which portions of the receiver power down is the lesser of a fixed amount of time (e.g., fifteen seconds) and the time period given by the equation:

TP = (x+(y * the number of frequency channels of the first subset)) * z% where:

TP is the time period (in seconds); x is the time period required to measure the power levels of the data signals transmitted upon any of the control channels; y is the time required, once a decision has been made to attempt to establish a communication link with one of the base stations, of circuitry 422 to tune receiver 406 to a particular control channel; and z is a desired duty cycle of the radio receiver.

After the time period during which the receiver powers- down elapses, power is again supplied to all the circuit components of the receiver 406 and the circuitry 422 is again operative to tune the receiver to each of the con channels of the frequency band. The power levels of each of the signals transmitted upon each of the respective ones of the control channels are again measured (i.e., re- measured) by circuit 468, and signals indicative of such measured power levels are supplied on line 492 to processing device 482. Processing device 482 is operative to compare the re-measured power levels of the control signals transmitted upon the respective ones of control channels, and to compare such re-measured power levels with corresponding, previously-measured power levels of the signals transmitted upon the frequency channels of the first subset of frequency channels.

Processing device 482 is operative to form a second subset of control channels. Only control channels upon which data signals having power levels which: 1.) were previously less than the dynamically-established noise floor, and which, upon re-measurement are above such dynamically-established noise floor; 2.) were previously less than the absolute-valued noise floor, and which, upon re- measurement are above such absolute-valued noise floor; or 3.) exhibit a significant rise in power level (for instance, an increase in power level of six decibels) are selected to form the channels of the second subset of control channels And, a communication link is attempted to be effectuated only with transmitters which transmits signals upon frequency channels of this second subset. Decoder 470 is again operative to generate signals on line 476 to indicate whether a communication link may be effectuated. (It is noted that other methods of selecting the second subset of frequency channels may also be utilized. For instance, in substitution for steps 1.) and 2.) above, control channels upon which data signals having power levels which were previously less than both the dynamically-established noise floor and the absolute-valued noise floor, and which, upon re-measurement are above either one of the noise floors may be utilized to form the second subset of control channels.) More particularly, processing device 482 is operative to determine upon which frequency channels are signals which were previously below either of the noise floors — i.e., below either the dynamically-established noise floor or the absolute-valued noise floor - and which as re-measured, are above the respective one of the noise floors, or are of a significantly increased magnitude.

Line 499 is further illustrated in the figure providing a line for an external input to processing device 482 to reset operation of receiver 406 at any time. Such a signal may be generated, for example, by actuation of an actuation switch connected to line 499.

FIG. 5 is a representation of operation of processing device 482 to determine which frequency channels to select to form the second subset of frequency channels. Again, attempts are made to effectuate a communication link with only those transmitters which transmit signals upon control channels of such second subset. Horizontal line 650 represents the dynamically-established noise floor, and horizontal line 655 represents the absolute-valued noise floor. (It is noted that the relative values of the noise floors could also be reversed, viz., the absolute-valued noise floor may be of a value greater than the dynamically-established noise floor.) Arrow 606 represents a signal transmitted upon a frequency channel having an initially- measured power level beneath that of noise floors 650 and 655. (It is noted that noise floors 650 and 655 correspond to noise floors 550 and 605 of FIGs. 4-1 and 4-II, respectively.) The frequency channel upon which signal 606 is transmitted does not form a portion of the first subset of frequency channels as the power level of signal 606, upon initial measurement, was beneath the noise floors.

However, after the receiver pauses for the time period determined by the above-noted algorithm, and the power levels of the signals transmitted on the various ones of the control channels are re- measured, the signal generated on die same control channel, and here indicated by arrow 644, shown in hatch, is above at least one of the noise floors. Such control channel is selected to form a control channel of the second subset of frequency channels. And, a communication down-link is attempted to be effectuated with a transmitter which transmits a control signal upon a selected control channel of the second subset of control channels. A control channel having a control signal transmitted thereupon of significantly increased power levels (irrespective of whether the re-measured value exceeds a noise floor) is also selected to form the second subset of control channels. Such a signal is indicated in FIG. 5 by arrow 670, shown in hatch.

If, after the power levels of the data signals transmitted upon the respective ones of the control channels are re-measured, no communication down-link can be effectuated, the radio receiver 406 again pauses for a time period, and the circuitry of the receiver powers- down once again. In a preferred embodiment, the predetermined time period is a time period corresponding to the greater of a fixed time period, or the previously-noted algorithm.

That is, the time period during which portions of the receiver power down is the greater of a fixed amount of time (e.g., fifteen seconds) and the time period given by the equation:

TP ^**** (x+(y * the number of frequency channels of the first subset)) * z% where: TP is the time period (in seconds); x is the time period required to measure the power levels of the data signals transmitted upon any of the control channels; y is the time required, once a decision has been made to attempt to establish a communication link with one of the base stations, of circuitry 422 to tune receiver 406 to a particular control channel; and z is a desired duty cycle of the radio receiver. With reference again to FIG. 1, when the radiotelephone is located beyond the area of cellular coverage of cellular communication system 100, signals transmitted by various ones of the base stations upon the control channels are beneath the noise floors. A radiotelephone incorporating circuitry similar to radio receiver 406 will not attempt to establish a communication down-link with any of the base stations transmitting any of such signals on any of the frequency channels. However, when the radiotelephone is repositioned to be located within the cellular coverage area of cellular communication system 100, and the power levels of the data signals transmitted on the various ones of the control channels by the base stations are re-measured, at least one of the control signals will now be of a power level in excess of the noise floors or will be of a significantly increased power level. Control channels upon which such data signals are transmitted are selected to form the control channels of the second subset of frequency channels, and an attempt is made to effectuate a communication link with the base βtation which transmits such signal upon one of such control channels. As the radio receiver circuitry powers-down for a predetermined time period after a determination is made that a communication link cannot be established with any of the transmitters which transmit control signals upon various ones of the control channels, energy usage of the radio is minimized.

Such a process of selecting control channels to form the second subset of control channels is repeated, communication down¬ links are attempted to be effectuated, and powering-down of the receiver circuitry occurs if no down-link can be effectuated continues. After a selected time period, such as ten minutes, if a communication down¬ link cannot be effectuated, the system is re-initialized to determine again a first subset of control channels.

Turning next to the logical flow diagram of FIG. 6, an algorithm, referred to generally by reference numeral 680, incorporating the method of the preferred embodiment of the present invention is shown. First, and as indicated by block 684, the power levels of signals transmitted upon each of the control channels is measured. The first subset of control channels is formed, as indicated by block 688, and an attempt is made to establish a communication down-link with a transmitter which transmits a control signal upon one of such channels, as indicated by block 692.

At decision block 696, a determination is made whether a communication down-link may be effectuated. If a down-link may be effectuated, the yes branch is taken to block 700, and the tuning frequency of the receiver tuning circuitry is maintained at the frequency of the selected control channel. Otherwise, the no branch is taken, the measured power levels are saved, as indicated by block 704, and portions of the receiver power-down for a time period, as indicated by block 708. After the time period elapses, the portions of the receiver power-on again, and the power levels of the control signals transmitted upon the control channels are re-measured, as indicated by block 708. The second subset of control channels is formed, as indicated by block 712, and an attempt to establish a down link with a transmitter which transmits a control Pignnl upon one of such channels is made, as indicated by block 716. At decision block 720, a determination is made whether a communication down-link may be effectuated. If a down-link may be effectuated, the yes branch is taken to block 724, and the tuning frequency of the receiver tuning circuitry is selected at the frequency of the selected control channel. Otherwise the no branch is taken, the measured power levels are saved to update the stored power levels thereby, as indicated by block 728, and the portions of the receiver power- down for a time period, as indicated by block 732.

After the time period elapses, a determination is made, indicated by block 736, as to whether or not an extended time period since first measuring the power levels of the control signals (at block 684) has transpired. If so, the yes branch is taken to block 684, and the process is repeated Otherwise, the no branch is taken to block 768, and the power levels of the control signals are re-measured.

Turning next to the logical flow diagram of FIG. 7, the method steps of the preferred embodiment of the method, referred to generally by reference numeral 750, of the present invention are listed. Method 750 selects tuning of tuning circuitry of a radio receiver to a frequency channel of a set of frequency channels defined upon a frequency band. Each frequency channel of the set of frequency channels is suitable for transmission thereupon a communication signal transmitted by a transmitter of a group of remotely-positioned transmitters.

First, and as indicated by block 756, the tuning circuitry of the radio receiver is tuned to each frequency channel of the set of frequency channels defined upon the frequency band. Next, and as indicated by block 762, the power levels of communication signals transmitted upon individual ones of the frequency channels of the set of frequency channels are measured.

Next, and as indicated by block 768, the frequency channels upon which communication signals are transmitted of measured power levels beyond a -m- i -min power level to form a first subset of frequency channels are selected.

Next, and as indicated by block 774, a determination is made as to whether a communication link may be effectuated between the radio receiver and individual ones of the transmitters which transmits communication signals upon the frequency channels of the first subset of frequency channels.

Then, either, as indicated by blocks 778 and 782, timing of the tuning circuitry of the radio receiver is selected to a frequency channel upon which a communication signal transmitted by a transmitter to which a communication link with the radio receiver may be effectuated is selected, or 1.) the tuning circuitry of the radio receiver is re-tuned to each frequency channel of the set of frequency channels, 2.) the power levels of the communication signals are re-measured, 3.) a second subset of frequency channels is formed, and 4.) tuning of the radio receiver to a frequency channel of the second subset of frequency channels is selected when a communication link may be effectuated therewith.

Finally, turning to the block diagram of FIG. 8, a radio transceiver, such as a radiotelephone, referred to generally by reference numeral 800, is shown. Transceiver 800 includes circuit 806, comprising the elements within the block shown in hatch, of a preferred embodiment of the present invention. Circuit 806 corresponds to circuit 400 of FIG. 3. The receiver portion of radio transceiver 800 is similar to radio receiver 406 shown in the block diagram of FIG. 3, and operation thereof will not again be described in detail. Examination of the receiver portion of radio transceiver 800 indicates that radio transceiver 800 includes antenna 818 to which line 820 is connected for interconnecting antenna 818 to programmable tuning/down conversion circuitry 822. Operation of circuitry 822 is controlled by input signal supplied thereto on lines 836. Circuitry 822 generates a signal on line 848 which is coupled to demodulator circuitry 858 and signal strength measuring circuitry 868. Demodulator circuitry 858 generates a demodulated signal on line 864 which is supplied to decoder 870.

Decoder 870 generates a signal on line 876 which is coupled to an input of processing device 882. Decoder 870 further generates a decoded signal on line 886 which is supplied to transducer 888, such as a speaker. Signal strength measuring circuit 868 generates a signal which is supplied to processing device 482 by way of line 892. Memory element 896 is further coupled to processing device 882, thereby way of lines 898, and line 899 provides an external input to the processing device 882.

Radio transceiver 800 is further shown to include a transmit portion havin component portions shown generally to include a transducer, such as a microphone, 908, encoder 912, modulator 916, and programmable tuning-up-conversion άrcuity 922. Operation of circuitry 922 is controlled by input signals supplied thereto by way of line 928.

Circuitry 922 generates a signal on line 934 which is coupled to antenna 818 to permit transmission of information signals therefrom.

While the present invention has been described in connection with the preferred embodiments shown in the various figures, it is to be understood that other similar embodiments may be used and modifications and additions may be made to the described embodiments for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.

Claims

1. A circuit for selecting a tuning frequency of tuning circuitry of a radio receiver to a frequency channel of a set of frequency channels defined upon a frequency band, each frequency channel of the set of frequency channels being suitable for transmission thereupon of a communication signal transmitted by a transmitter of a group of remotely-positioned transmitters, said circuit comprising:

means for tuning the tuning circuitry of the radio receiver to each frequency channel of the set of frequency channels defined upon the frequency band;

means for measuring power levels of communication signals transmitted upon individual ones of the frequency channels of the set of frequency channels and for storing values of measured power levels measured thereat;

means for selecting frequency channels upon which communication signals of measured power levels beyond a predetermined minimum power level to form a first subset of frequency channels;

means for determining when a communication link may be effectuated between the radio receiver and individual ones of the transmitters which transmit communication signals upon the frequency channels of the selected subset of frequency channels; and

means for selecting tuning of the frequency of the tuning circuitry of the radio receiver to a frequency channel upon which a communication signal is transmitted by a transmitter to which a communication link with the radio receiver may be effectuated when a determination is made by said means for determining that a communication link may be effectuated.

2. The circuit of claim 1 wherein said means for tuning comprises means for tuning the radio receiver sequentially to each frequency channel of the set of frequency channels defined upon the frequency band.

3. The circuit of claim 1 wherein said means for measuring power levels comprises means for measuring amplitudes of signals representative of the communication signals upon the individual ones of the frequency channels.

4. The circuit of claim 1 wherein the measured power levels measured by said means for measuring are stored in a memory element.

5. The circuit of claim 1 wherein the minimum power level utilized by said means for selecting to select frequency channels forming the selected subset of frequency channels comprises a predetermined, absolute ^•m_- τnnτm power level.

6. The circuit of claim 1 wherein the minimum power level utilized by said means for selecting to select frequency channels forming the selected subset of frequency channels is of a level related to the measured power levels measured by said means for measuring.

7. The circuit of claim 6 wherein the ^•m-mii- im power level is of a value corresponding to a predetermined level greater than a measured power level of the measured power levels of a smallest value.

8. The circuit of claim 1 wherein said means for deterπiining when a communication link may be effectuated comprises: means for decoding the communication signals transmitted upon the frequency channels of the selected subset of frequency channels to form decoded signals thereby; and means responsive to the decoded signals for comparing portions of the decoded signal with data stored at the radio receiver.

9. The circuit of claim 1 further comprising: means for re-tuning the tuning circuitry of the radio receiver to each frequency channel of the set of frequency channels; means for re-measuring power levels of communication signals transmitted upon the individual ones of the frequency channels of the set of frequency channels; means for selecting frequency channels upon which communication signals exhibiting increased power levels beyond a certain level to form a second subset of frequency channels; means for determining whether a communication link may be effectuated between the radio receiver and individual ones of the transmitters which transmit communication signals upon the frequency channels of the second subset of frequency channels; and means for selecting tuning of the radio receiver to a frequency channel of the second subset of frequency channels when a communication l nk may be effectuated.

10. The circuit of claim 9 further comprising means for delaying operation of said means for re-tuning power levels for a predetermined time period after the determinations have been made by the means for determining.