HK1023897B - Radio communications systems and methods for jittered beacon transmission - Google Patents

Description

Wireless communication system and method for jittered beacon transmission

Technical Field

The present invention relates to wireless communication systems, and more particularly, to uncoordinated dedicated wireless communication systems and methods for beacon channel transmission.

Background

Commercial use of cellular radio communication for cordless and mobile phones has shown tremendous growth over the past few decades. Typically, these wide area cellular networks may be divided into two parts: i.e. the fixed part of the interconnected network including radio base stations, and the mobile or portable part including mobile terminals accessible to the network, e.g. radio telephones. Each base station transmits control information on a control channel that the mobile terminal can utilize to access the network. Each radio base station in the network covers a limited area, called a cell. Different base stations in the network are coordinated by a Base Station Controller (BSC). Frequency reuse patterns (fixed or adaptive) are applied to avoid interference from different base stations in transmission. Examples of such cellular systems include AMPS, D-AMPS, and GSM.

In a private macrocell system, the network part is different from an equivalent wide area cellular network. Dedicated systems must typically be much cheaper (since system costs are shared among fewer users). In addition, dedicated systems typically cover indoor environments, which are more difficult to predict than outdoor environments (e.g., walls, opening and closing of doors, corridors functioning as waveguides, etc.). Therefore, typically, the radio base stations in indoor systems operate in a more autonomous manner in order to decide which channels to use for traffic and control (or beacon) information.

In commercial or office cordless telephone systems, such as DECT systems, there can still be a degree of interactivity between the base stations of the individual indoor networks. Although wireless base stations in commercial systems (such as DECT) are as autonomous as possible, they are still loosely synchronized in time within the network to allow hand-off from one base station to another. The network functions are performed in the base station controller. For handover purposes it is important that beacons from different base stations arrive at the mobile terminal within a limited time window in order to be scanned during idle frames of the communication. In private residential systems, such as cordless telephone systems, the wireless base stations of cordless telephones form only a single private network connected to the PSTN, and typically do not communicate or synchronize with other private residential base stations (e.g., neighbor base stations). In an indoor radio system, the radio base station itself looks for a channel for operation. These channels should preferably not interfere with other nearby radio base stations. Therefore, the radio base station looks for the channel of the lowest amount of interference (the quietest channel) before it starts transmitting. Periodic measurements may be made to ensure that the base station remains on the least interfering channel.

Conventional analog radiotelephone systems typically employ a system known as Frequency Division Multiple Access (FDMA) to create a communications channel. In fact, as is well known to those skilled in the art, radiotelephone communications signals (which are modulated waveforms) are typically transmitted over a predetermined frequency band in a carrier frequency spectrum. These separate frequency bands are used as channels for cellular radiotelephones (mobile terminals) to communicate with a cell via a base station or satellite serving the cell. In the United states, for example, the Federal government has allocated cellular communications a segment of the UHF spectrum that IS divided into pairs of narrow frequency bands, known as systems of EIA-553 or IS-19B. Channel pairing is required due to a frequency duplex arrangement in which the transmit and receive frequencies in each pair of channels differ by 45 MHz. In the united states, 832 radio frequency channels 30kHz wide are now allocated to cellular mobile communications.

As the number of users increases, the limitation on the number of frequency bands available presents several challenges. Increasing the number of users in a cellular radiotelephone system requires more efficient use of the limited available spectrum to provide more total channels while maintaining communication quality. This challenge is enhanced because the users are not necessarily evenly distributed among the cells of the system. At any given time, more channels may be needed for a particular cell to handle a potentially higher local user density. For example, a cell in an urban area could conceivably contain hundreds or thousands of users at any one time, which could easily exhaust the number of frequency bands available in the cell.

To this end, conventional cellular systems employ frequency reuse to increase the potential channel capacity within each cell, as well as to increase spectral efficiency. Frequency reuse involves allocating a frequency band to each cell, and cells using the same frequency are geographically separated to allow radiotelephones in different cells to use the same frequency simultaneously without interfering with each other. By doing so, thousands of users can be served by a system with only hundreds of bands.

Another technique that may further improve channel capacity and spectral efficiency is Time Division Multiple Access (TDMA). A TDMA system can be implemented by dividing the frequency band employed in a conventional FDMA system into sequential time slots. While communication on the various frequency bands typically occurs over a common TDMA frame comprising a plurality of time slots, communication on each frequency band may occur according to a unique TDMA frame, the time slots of which are unique to that frequency band. Examples of systems employing TDMA are the dual analog/digital IS-54B standard employed in the united states, in which each original frequency band of EIA-553 IS divided into 3 time slots, and the european GSM standard, which divides each frequency band into 8 time slots. In these TDMA systems, each user communicates with the base station by using bursts of digital data transmitted during time slots assigned to the user.

A channel in a TDMA system typically comprises one or more time slots on one or more frequency bands. As discussed above, traffic channels are used to communicate voice, data, or other information between users, such as between wireless telephones and landline telephones. In this case, each traffic channel forms one direction of a duplex communication link established by the system from one user to another. Traffic channels are typically allocated dynamically by the system when and where it is needed. In addition, some systems, such as the european GSM system, cause the traffic channel to "hop", i.e., randomly switch the frequency band used to transmit the particular traffic channel. Frequency hopping can reduce the probability of interference events between channels, it uses the dispersion of interferers and averaging to improve the overall communication quality.

Included among the dedicated control channels transmitted within the cell are forward control channels that are used to broadcast control information in the cell of the radiotelephone system to radiotelephones that may seek access to the system. The control information broadcast on the forward control channel may include information such as an identification of the cell, an identification of the associated network, system timing information, and other information needed to access the radiotelephone system from the radiotelephone.

Forward control channels, such as the Broadcast Control Channel (BCCH) of the GSM standard, are typically transmitted in dedicated frequency bands in each cell. A radiotelephone seeking access to the system typically "listens" to the control channel in a standby mode and does not synchronize with the base station or satellite until it acquires the control channel of the base station or satellite. To prevent excessive interference between control channels in adjacent cells, frequency reuse is typically employed, with different dedicated frequency bands being used for control channels in adjacent cells, in a frequency reuse pattern to ensure minimum separation between co-channel cells. Frequency hopping, while it may be more intensive to reuse the control channel band, is generally not employed because unsynchronized radiotelephones often have difficulty acquiring frequency hopped control channels due to lack of reference points to the hopping sequence employed. Moreover, for dedicated uncoordinated wireless communication systems, the frequency reuse pattern cannot be used because each system operates independently of other systems that may generate interference.

In general, in radio communication control communication, a downlink (from a base station to a portable station) and an uplink (from a portable station to a base station) for a forward control channel are defined. The radio base station listens to the uplink information of the portable station with its uplink receiver. In order to listen to downlink information transmitted by other base stations, the base stations typically also require a downlink receiver. The uplink and downlink may be distinguished by different frequencies, so-called Frequency Division Duplex (FDD), or by different time slots, so-called Time Division Duplex (TDD). Cellular systems typically use FDD as a downlink control channel as described above. To measure other base stations, downlink receivers may be built into the base stations, which may increase costs. With the TDD scheme, the downlink may be located in only another time slot, so that the reception of the downlink and uplink can be done by the same receiver structure. For example, DECT uses the TDD scheme.

In some applications, FDD is preferred over TDD for a number of reasons. The TDD scheme typically results in mutual interference between the uplink and downlink when the base stations are not synchronized in time. In addition, because the radio base station is preferably located relatively high in order to reach within the line of sight of the portable station, interference from base stations (to portable stations and other base stations) may be dominant. In FDD, the uplink and downlink are completely separated in frequency and do not typically interfere with each other.

Additionally, if the private system is considered to be based on a cellular air interface standard (e.g., GSM, or D-AMPS), FDD may be employed for compatibility reasons. Therefore, in a private wireless communication system that employs FDD to distinguish between uplink and downlink, it is typically determined by a base station on which channel to operate without knowing the transmission from other nearby wireless base stations.

This problem relates specifically to the control or beacon channel of the base station being periodically transmitted for contact with the portable station. For traffic channels, the system may use a downlink receiver in the portable station to locally learn about the interference. Downlink measurements made at the portable station may then be communicated to the wireless base station, which may then select the best (duplex) traffic channel. For the beacon channel, this approach is not generally used because the presence of a portable station when there is no service is not verifiable.

In uncoordinated private wireless communication systems, the mobile terminal and the base station may not even be able to establish communication access if radio beacon interference occurs. Such interference may occur between radio beacon transmissions of uncoordinated private wireless communication systems that are located within interference distance and that transmit radio beacons at overlapping times and frequencies. In particular, since the radio frequency beacon transmissions are transmitted at fixed time intervals, they can interfere with each other for significant time intervals, severely preventing mobile terminals from accessing uncoordinated systems.

Disclosure of Invention

It is therefore an object of the present invention to address the problem of beacon channel interference in private wireless communication systems that are uncoordinated and cannot hear each other, but share the same frequency spectrum. To address the problem of interference between beacon signals from different uncoordinated and uncoordinated private wireless communication base stations, the present invention provides base stations that employ short radio frequency beacon bursts whose positions are dithered in a pseudo-random manner in time. Collisions may occur between radio beacons, but considering the longer time window most likely will not cause all radio beacons to be affected, since radio beacons from different base stations are independently jittered. More specifically, multiple consecutive collisions sufficient to lose synchronization will be unlikely to occur. However, because the jitter for a given base station is performed in a pseudo-random manner, the mobile station can predict when the next beacon from the identified base station will arrive, reducing the risk of losing synchronization with the base station due to beacon jitter.

In one aspect of the beacon transmission of the present invention, the time intervals between adjacent radio frequency beacon bursts in the same radio base station are dithered in a pseudo-random manner around the average. In another embodiment, the frame number between adjacent radio frequency beacon bursts of the same radio base station is constant; only the time (slot) position at which the radio beacon occurs within the frame varies in a pseudo-random manner.

In one embodiment of the present invention, a private radiotelephone base station is provided that includes radio frequency transmitting means for transmitting radio frequency beacon bursts for establishing wireless communication access with a mobile terminal. The base station comprises beacon transmission control means for controlling the transmission of the radio frequency beacon periodically at jittery time intervals to avoid repeated collisions with beacon transmissions from other uncoordinated private base stations. The jitter generator generates for each beacon transmission a current beacon jitter value whose magnitude is limited to a maximum beacon jitter value and which is generated by a predetermined function whose average output value is zero. A beacon transmission initiating device initiates a beacon transmission after a determined time interval that is a function of the current beacon jitter value.

In another aspect of the invention, a mobile terminal is provided that includes a receiver for receiving radio frequency beacon transmissions from uncoordinated private radiotelephone base stations. The mobile terminal includes a beacon readout device for deriving an identification value of the transmitting base station from the received radio frequency beacon. The mobile terminal further comprises determining means for determining a predetermined jitter generating function based on the identification value and synchronizing means for synchronizing the mobile terminal to the time interval of the jittered radio frequency beacon transmission based on the predetermined jitter generating function. For example, the base station identification value may be a parameter in a jitter generation function that allows the mobile terminal to predict subsequent individual jitter values.

Methods for jittering transmission of beacons are also provided. A radio frequency beacon is transmitted by the base station, the beacon including an associated identification value and status information. The base station then waits a determined time before initiating transmission of the next subsequent radio beacon. This time is determined by calculating the current beacon jitter value and waiting a time that is a function of the jitter value and the desired average time between transmissions of the rf beacon. The process of initiating this cycle of transmission and waiting will then repeat each subsequent radio frequency beacon transmission. The mobile terminal receives the transmitted beacon. The mobile station derives status information from the received radio frequency beacons and determines the likelihood that the base station will provide communications to the mobile terminal. The mobile terminal also derives a base station identification value and synchronizes to the base station beacon timing according to a predetermined jitter generating function associated with the identified base station.

The jittered radio frequency beacon transmission of the present invention thus solves the problem of beacon collisions between uncoordinated private wireless communication systems by dithering the beacon transmission time to reduce the potential for repetitive collisions to occur. The present invention also provides a pseudo-random beacon dithering pattern that is associated with a base station identification value transmitted to a mobile terminal in a radio frequency beacon. The mobile station derives an identification value and can later determine a beacon jitter pattern for the identified base station to maintain synchronization even when multiple sequential beacon collisions occur.

Drawings

FIG. 1 diagrammatically illustrates three private wireless communication systems that are uncoordinated and have overlapping transmission ranges;

FIG. 2 graphically illustrates radio frequency beacon burst collisions between uncoordinated private wireless communication systems;

FIG. 3 graphically illustrates jittered transmission of radio frequency beacon bursts to prevent repetitive collisions of the radio frequency beacon bursts;

FIG. 3a schematically illustrates a modular shift register generator that may be used to generate a beacon jitter function in accordance with the present invention;

FIG. 4 is a schematic block diagram of a wireless personal communication base station in accordance with the present invention;

FIG. 5 graphically illustrates one embodiment of beacon dithering in accordance with the present invention;

fig. 6 graphically illustrates another embodiment of beacon dithering in accordance with the present invention;

FIG. 7 graphically illustrates beacon jitter in accordance with the present invention in a multi-frame TDMA wireless communication environment;

FIG. 8 is a schematic block diagram of a mobile terminal according to the present invention;

fig. 9 is a flowchart illustrating the operation of a private wireless communication base station according to the present invention;

fig. 10 is a flowchart illustrating an operation of a mobile terminal according to the present invention.

Detailed Description

The present invention will now be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

Reference is now made to FIG. 1, which schematically illustrates the operating environment of the present invention. Fig. 1 shows base stations 10A, 10B, 10C, which are uncoordinated and unsynchronized radio base stations that are not likely to hear each other due to incompatibility between the uplink and the downlink. As shown in fig. 1, the base stations 10A, 10B, 10C are home-specific wireless personal communication systems located in private homes. An example of such a wireless personal communication system is described in U.S. patent No.5,428,668, which is hereby incorporated by reference in its entirety. Each base station 10A, 10B, 10C is only connected to the PSTN and typically does not communicate directly with each other. The transmission range of each base station is shown by the dashed circle, and as shown, they overlap. Therefore, interference may occur between the base stations 10A, 10B, 10C.

Each base station 10A, 10B, 10C periodically transmits short radio frequency beacon bursts that may contain status information and associated identification values for the base stations 10A, 10B, 10C. The mobile terminal 14, if within the transmission range 12A, 12B, 12C of the base station 10A, 10B, 10C, may receive the radio beacon and determine whether it should be in contact with the base station 10A, 10B, 10C.

Referring now to fig. 2, there will be described problems in beacon base station communications due to interference caused by collisions between beacons of uncoordinated base stations 10A, 10B, 10C. Shown at 16, 18 and 20 are wireless beacon transmissions for base station a10A, base station B10B, and base station C10C, respectively. Each base station 10A, 10B, 10C periodically transmits a burst of radio frequency beacons. Thus, as shown in fig. 2, each radio frequency beacon signal consists, in time, of an infinitely long series of beacon bursts spaced by a fixed period "T". Since the radio beacon bursts are not synchronized, they may, as shown, have transmissions 16 of base station a and 20 of base station C randomly aligned and colliding at times 22, 24, 26, 28. Since the time interval T is fixed, once a collision occurs at the time interval 22, subsequent beacons 24, 26, 28 will typically also collide. Drift in the clock systems of base station a10A and base station C10 may cause the beacons to no longer collide with each other. However, when the clock drift is small, the base station a10A and the base station C10C may have colliding beacons for a long time.

The present invention intentionally dithers the beacon radio frequency transmission to solve this problem, as graphically shown in fig. 3. Figure 3 shows graphical representations of radio frequency beacon burst transmissions at 30, 32, and 34 for base station a10A, base station B10B, and base station C10C, respectively. As shown in fig. 3, the time interval between successive radio beacon bursts for each radio base station 10A, 10B, 10C is jittered within a window of predetermined average time intervals. The average time interval between radio frequency beacons is still a fixed period T in time, as in the system of fig. 2. Even with jittered transmitted radio beacons, there may still be collisions between the radio beacons, as shown at time reference point 36, when a collision occurs between base station B10B and base station C10C. However, as further illustrated in fig. 3, the next transmissions of radio beacons from base station B10B and base station C10C will not collide due to the jitter of the radio beacon transmissions.

Mathematically, it can be shown that the probability that M successive radio beacon transmissions experience collisions decreases exponentially as M increases. Thus, while the jittered beacon transmissions of the present invention may not affect the likelihood of a single collision between the radio frequency beacon transmissions of uncoordinated wireless base stations, the likelihood of successive collisions will decrease exponentially. Because the likelihood of loss of synchronization between the mobile terminal 14 and the base stations 10A, 10B, 10C increases the longer the time interval between receiving radio frequency beacons, the present invention provides an improved means of maintaining synchronization between the mobile terminal 14 and the unsynchronized base stations 10A, 10B, 10C.

In implementing the present invention, for jittered beacon transmissions, it is preferred that: the dither patterns are determined in a pseudo-random manner. As used herein, pseudo-random means that the determination of jitter may result from the respective base stations 10A, 10B and 10C providing appropriate uncoordinated jitter, while at the same time the jitter provided in a jitter pattern may be temporally predictable to any one of the base stations 10A, 10B, 10C. For example, in a preferred embodiment, the jitter pattern depends on an associated identification value of the radio base station and can be predicted from this value. This pseudo-random jitter still has a random jitter pattern for the different base stations 10A, 10B, 10C, so that the base stations 10A, 10B, 10C will not jitter in a common pattern, since this would cause problems with repeated collisions. However, having a predictable dithering pattern for any particular base station 10A, 10B, 10C associated with the base station's identification value will help the mobile terminal 14 to maintain synchronization with the dithered radio beacon transmissions from the base stations 10A, 10B, 10C. Because the mobile terminal 14 is provided with the base station's associated identification value in the radio beacon, the jitter pattern for that base station 10A, 10B, 10C can be determined.

For example, in one embodiment, the associated identification value of the base station 10A, 10B, 10C may specify a pseudo-random jitter pattern. Once the mobile terminal 14 knows the phase of the base stations 10A, 10B, 10C in this mode, the mobile terminal 14 can predict the location of all future beacons from the identified base stations 10A, 10B, 10C. The phase may be communicated explicitly by the base stations 10A, 10B, 10C in the beacon signal or at the first fix when the mobile terminal 14 is in contact with the base stations. Thereafter, the mobile terminal 14 may remain locked on the identified base station 10A, 10B, 10C without losing synchronization to the radio frequency beacon even if multiple successive radio frequency beacon bursts are lost due to collisions from respective different adjacent uncoordinated base stations 10A, 10B, 10C.

More specifically, examples of pseudo-random techniques based on correlated identification values suitable for use with the present invention are described below. The dithering function of the present invention may be compared to the encryption function in a wide area cellular network operating under, for example, the GSM protocol. The encrypted word is generated by a function whose inputs are the key and a "counter" number, which is incremented by one increment per encryption. Typically, the counter number is the frame number of the TDMA channel. The counter number is incremented modulo N, where N is the repetition period of the encryption algorithm. The counter number in fact determines the phase of the algorithm (the algorithm is constantly accumulating). The key specifies a particular algorithm.

A similar approach may be used for beacon dithering in accordance with the present invention. The inputs to the dithering algorithm (encryption algorithm) may be a base station identification number and a counter number, which may be the frame number transmitted within a frame for each beacon signal. Optionally, a key may also be added. The dithering algorithm produces an n-bit word whose lower m LSBs can (optionally) be used to derive 2m different dither values. For each new jitter interval, the frame number is incremented by one increment. The base station identification number is fixed and determines how the frame number is mapped to the jitter value at the output. In addition to the base station identification number, a key may optionally be added which, together with the base station identification number, determines a mapping from the frame number to the jitter value. The key may be given to the mobile terminal 14 at initialization, for example. This means that only the mobile terminal 14 has a key corresponding to the received base station identification number, which allows it to synchronize with the base station.

To synchronize with the base station, the mobile station 14 knows the base station identification number and frame number (and possibly the key). The base station identification number may be transmitted in the beacon itself. The frame number may also be sent in a beacon (e.g., a frame number in the current broadcast channel of the wide area cellular network) or it may be provided to the mobile terminal 14 when the mobile terminal 14 first registers with the transmitting base station. The key may be derived from a look-up table that maps base station identification numbers to keys (the look-up table may be generated during initialization). The encryption algorithm that may be used in the present invention preferably provides jitter values that are evenly distributed over its range. One approach is to use a pseudo-random binary sequence (PRBS) generator with a linear feedback register (LFSR) or to use a Modular Shift Register Generator (MSRG). An example of a modular shift register circuit is shown in fig. 3 a.

The hardware in the example of fig. 3a consists of a series of shift registers 37, which add feedback according to a specific function. The multiplier 38 multiplies the feedback signal by a coefficient a _ i (i ═ 0 to N), and adds the result to a modulo-2 adder (EXOR) 39. The coefficients a _0 to a _ N determine the feedback function, which is actually the jitter function. a _ i may be 0 or 1, where 1 is establishing a feedback connection and 0 means not connected. The base station identification number (possibly together with a key) determines the value of a _ i. To determine the new jitter value, the current frame number is loaded into the shift register. The information is then fed into a clock signal once (or a fixed number of times), after which the jitter values are derived from the respective (or some) outputs of the shift register. For the next jitter value, the frame number is added by an increment, loaded into the shift register, and the circuit is again clocked in.

The mapping from the base station identification number together with the key can be done in various different ways. For example, it is possible to use a look-up table that corresponds base station identification numbers to specific combinations of a _ i. Many variations are possible for implementing a dithering function suitable for obtaining the various benefits of the present invention, as will be appreciated by those skilled in the art.

Referring now to fig. 4, an embodiment of a base station 10A, 10B, 10C according to the present invention will be described. The base stations 10A, 10B, 10C include a base station controller 40 or means for controlling the operation of the base stations 10A, 10B, 10C, including the wireless communication between the base stations 10A, 10B, 10C and the mobile terminals 14. Although the base station controller 40 serves a variety of different functions, for purposes of the present invention, specific functions of the base station controller 40 include providing base station identification and status information to the beacon transmission controller 42 either directly through the electrical connection means 44 or through the shared memory 46. Both the base station controller 40 and the beacon transmission controller 42 are connected to a memory 46 by buses 48 and 50. Base station controller 40 also coordinates beacon transmissions controlled by beacon transmission controller 42 with other wireless communication transmissions initiated by base station controller 40 through transmitter 52.

A memory 46 or other storage device operatively connected to the beacon transmission controller 42 and the base station controller 40 stores a predetermined dithering function associated with each base station 10A, 10B, 10C. For example, the predetermined dithering function may be a plurality of functions whose coefficients may be specified based on the associated identification values of the base stations 10A, 10B, 10C. In this case, the coefficients of the function may be stored in the memory 46. The base station status information and associated identification values may also be stored in the memory 46.

A transmitter 52 or other wireless transmission means for transmitting wireless communications is operatively connected to the beacon transmission controller 42 and the base station controller 40. Although the wireless transmitting means 52 only requires one transmitter for the purpose of radio frequency beacon transmission, it may also be a transceiver providing both transmit and receive functions to support uplink and downlink communications between the base stations 10A, 10B, 10C and the mobile terminal 14.

A beacon transmission controller 42 or other beacon transmission control means for controlling the base station to transmit radio frequency beacons at jittery time interval periods is operatively connected to the transmitter 52. Beacon transmission controller 42 includes a jitter generator 54 or other jitter generating device for generating a current beacon jitter value having an amplitude no greater than a predetermined maximum beacon jitter value according to a predetermined function stored in memory 46 and having an average output of almost zero. The almost zero average jitter output provides the benefit of keeping the average period between rf beacon transmissions at the same fixed period T. The predetermined maximum value is provided for jitter values for frame timing considerations, which are important when implementing the present invention in a TDMA-based wireless communication environment, as will be discussed further below.

Beacon transmission controller 42 also includes a transmission initiation circuit 56 or other beacon transmission initiation means for initiating transmission of a radio frequency beacon at a time that is a function of the current beacon jitter value generated by jitter generator 54. Therefore, the transmission initiation circuit 56 is responsive to the jitter generator 54 and is physically electrically connected to the transmitter 52 by the electrical connection means 58. The beacon transmission controller 42 prepares a radio frequency beacon comprising an identification value relating to the base station 10A, 10B, 10C and status information from the base station controller 40, the beacon being transmitted by the transmitter 52 in response to the transmission initiation circuitry 56 which triggers the transmission of bursts of radio frequency beacons at pseudo-randomly jittered time intervals.

Fig. 4 also shows a timer 60 or other timing means for providing a clock timing reference to the beacon transmission controller 42 for timing the transmission of the rf beacon. The timer 60 is electrically connected to the beacon transmission controller 42 by electrical connection means 62.

Although beacon transmission controller 42 may have a variety of different functions, two specific embodiments will be described with reference to fig. 5 and 6, respectively, for repeatedly transmitting radio frequency beacons in jittered time intervals. Fig. 5 shows an embodiment of transmission initiation based on a desired average time interval between the radio frequency beacon T and the jitter value from the jitter generator. Specifically, the exact time between subsequent successive radio frequency beacon transmissions is based on the time of transmission of the most recently transmitted radio frequency beacon.

As shown in FIG. 5, the first RF beacon 64 is followed by a time T + Δ₁An initiated second radio frequency beacon 66, where T is the desired average time of each beacon transmission, and Δ₁Is the current beacon jitter value from jitter generator 54. Time T + Δ of the third beacon 68 after initiating transmission of the second beacon 66₂Is transmitted. Likewise, the fourth beacon 70 is at a time T + Δ after the initiation of transmission of the third beacon 68₃Is transmitted. Delta₂And Δ₃Respectively, the current beacon jitter value from jitter generator 54 for each cycle thereafter. By defining the jitter generator function as a pseudo-random function that is determined based on the associated identification values of each identified base station 10A, 10B, 10C, the mobile terminal 14 can predict all subsequent Δ once any one of the radio beacons 64, 66, 68 is received by the mobile station 14_iAnd is synchronized with beacon transmissions from the identified base stations 10A, 10B, 10C.

In other words, in the embodiment of fig. 5, the reference for jitter depends on the location of the previous radio beacon burst transmission. For example, assume that the first radio beacon is at T₁Is transmitted, then the second burst is at T₂＝T₁+T+Δ₂Is transmitted. Similarly, the third burst is based on the position of the second burst, which will be at T₃＝T₁+T+Δ₂+T+Δ₃＝T₁+2T+Δ₂+Δ₃Is reached, where₂Is the jitter of the second beacon instant, and Δ₃Is the jitter of the third beacon time instant. As previously mentioned, the average jitter Δ_iIs zero.

Referring now to fig. 6, another embodiment for determining the exact time at which the transmission initiation circuit 56 initiates transmission of the radio frequency beacon is shown. In the embodiment of fig. 6, the dithering of the transmission timing of the radio frequency beacon is based on a constant time reference, rather than an offset in the transmission time of a previous beacon burst. As shown in fig. 6, the beacon transmissions are dithered with respect to a predetermined time reference 72, 72 ', 72 ", 72'". In the embodiment of fig. 6, the initiation of beacon transmission is based on a time reference 72, 72 ', 72 ", 72'" for each radio frequency beacon burst transmission. The actual transmission is initiated by transmission initiation circuit 56 offsetting the fixed initiation times 72, 72 ', 72 ", 72'" by the current beacon jitter value generated by jitter generator 54. For example, assume that the first radio beacon burst is at time T₁＝0+Δ₁I.e., initiated at 74 shown in fig. 6. The zero reference of time is simply referred to as the time reference 72, which is illustrated for simplicity only. As shown at FIG. 76, the second radio frequency beacon burst is at time T₂＝T+Δ₂And (4) initiating. Subsequently, as shown at fig. 78, a third beacon burst is at time T₃＝2T+Δ₃And (4) arriving.

For the embodiment of fig. 5, the exact time between the initiation of each radio frequency beacon burst transmission is equal to the average time interval T between base station beacon transmissions plus the current beacon jitter value calculated from jitter generator 54. In contrast, in the embodiment of fig. 6, the exact time to initiate each rf beacon burst transmission, with jitter around the predetermined reference 72, 72 ', 72 ", 72'", is equal to the average time interval between each base station beacon transmission plus the calculated value of the current beacon jitter minus the calculated value of the beacon jitter of the last previously transmitted rf beacon burst. Thus, in the embodiment of fig. 6, beacon transmission jitter relative to a predetermined reference 72, 72 ', 72 ", 72'" can be accomplished by timing the initiation of transmission relative to a previous radio beacon transmission while simultaneously maintaining the current beacon jitter value and the most recent beacon jitter value of the last transmitted radio beacon burst. It can be shown mathematically that by monitoring the base stations 10A, 10B, 10C over a relatively short period of time, the deviation from the mean time T in the embodiment of fig. 5 can be much greater than the deviation from the mean time T in the embodiment of fig. 6 for a given maximum beacon jitter value.

The embodiments of fig. 5 or 6 may be used with private radiotelephone base stations 10A, 10B, 10C located in private populated areas. The embodiment of fig. 5 is more random in nature, so the probability of successive collisions will be lower than the embodiment of fig. 6. On the other hand, in the embodiment of fig. 6, the probability of the beacons of the mobile terminal 14 losing synchronization is small when a large number of successive beacon collisions really occur. The embodiment of fig. 6 may be preferred for multi-user dedicated radiotelephone base stations 10A, 10B, 10C in which there are multiple base stations forming an associated private local network. This is particularly true where the private network utilizes a TDMA communications standard that only allows the mobile station 14 to listen for radio frequency beacon bursts during idle frames.

For example, in a private wireless communication network using a GSM compatible air interface, the mobile terminal 14 can only look for base station beacon transmissions during idle frames 80, 80' (fig. 7), which occur every 26 TDMA frames. The radio beacon burst must arrive at the idle frame 80, 80' in order to be monitored by the mobile terminal 14. As shown in fig. 7, the mobile terminal 14 (graph 82) has a call in progress with the base station 10B (graph 84) over the wireless communication connection. In idle frames 80, 80', the base stations 10A, 10B will transmit radio frequency beacons and the mobile terminal 14 will listen for beacon transmissions. As shown in fig. 7, the radio frequency transmission beacon period T is a multiple of a 26-frame multiframe; jitter must occur over one frame or 8 slots. This jitter prevents collisions of the radio beacon transmissions between base stations 10A and 10B (compare graphs 84 and 86).

Those skilled in the art will appreciate that the aspects of the present invention described above with respect to fig. 4 may be provided by hardware, software, or a combination of both. Although the different components of the base stations 10A, 10B, 10C are shown as discrete elements in fig. 4, they may in fact be implemented by a microcontroller comprising input and output ports and running software code, by a custom or hybrid chip, by discrete elements or by a combination thereof. For example, beacon transmission controller 42, memory 46, and base station controller 40 may all be implemented using a single programmable device.

Referring now to fig. 8, the mobile terminal 14 of the present invention will be described. The mobile terminal 14 includes a mobile terminal controller 88 or other means for controlling operation of the mobile terminal 14, including controlling wireless communication between the mobile terminal 14 and the base stations 10A, 10B, 10C. The mobile terminal 14 also includes a receiver 90 or other mobile radio receiving device connected to the mobile terminal controller 88 by a line 89 for receiving radio communications including radio frequency beacons from the base stations 10A, 10B, 10C. The receiver 90 need only be a single receiver for radio beacon reception purposes, but it may also be a transceiver providing both transmit and receive functions to support uplink and downlink communications between the mobile terminal 14 and the base stations 10A, 10B, 10C.

The mobile terminal 14 includes an ID derivation circuit 92 or other beacon readout device for deriving the transmitted base station identification value from the received radio frequency beacon. The ID derivation circuit 92 is actually electrically connected to the receiver 90 through a line 91. The mobile terminal also includes a function determination circuit 94 or other determining means for determining a predetermined function to be used by the identified base station 10A, 10B, 10C for jitter of the radio frequency beacon transmissions based on the base station identification value derived by the ID derivation circuit 92. The function determination circuit 94 is actually electrically connected to the ID derivation circuit 92 as represented by line 96. The function determination circuit 94 is also electrically connected by line 97 to a synchronization circuit 98 or other means for synchronizing the reception of beacon transmissions by the mobile station 14 with the time intervals of jittered radio frequency beacon transmissions from the identified base stations 10A, 10B, 10C according to a predetermined dithering function. The synchronization circuit 98 is also physically connected to the receiver 90, and may also be physically connected to the mobile terminal controller 88, as shown by line 101.

Also shown in fig. 8 is a timer 100 or other timing means for providing a clock time reference to the mobile terminal 14 for use in timing the reception or transmission of the radio frequency beacons. The timer 100 is actually electrically connected to the synchronization circuit 98 by electrical connection means 102.

The memory 104 is physically connected to the ID derivation circuit 92 and the mobile terminal controller 88 by electrical connection means 106 and 108, respectively. The memory 104 may provide means such as a look-up table with cross-reference information between the identification value of the transmitting base station and a predetermined jitter function.

Those skilled in the art will appreciate that the above-described aspects of the present invention in FIG. 8 may be provided by hardware, software, or a combination thereof. Although the various components of the mobile station are shown as discrete elements in fig. 8, they may in fact be implemented by a microcontroller including input and output ports and running software code, by custom or hybrid chips, by discrete elements or by a combination thereof. For example, the mobile terminal controller 88, the memory 104, the ID derivation circuit 92, the function determination circuit 94, and the synchronization circuit 98 may all be implemented in a single programmable device.

Fig. 9 shows a method of operation of the beacon channel transmission timing by the private radiotelephone base stations 10A, 10B, 10C, including a method of repeatedly transmitting radio frequency beacons at jittery time intervals. Radio frequency beacon transmission timing operations begin at block 110 with the base stations 10A, 10B, 10C initiating beacon transmissions. The transmitted radio frequency beacon contains an identification value associated with the transmitting base station 10A, 10B, 10C and may further contain base station status information. In block 112, the base station 10A, 10B, 10C calculates the current beacon jitter value. As previously described, the jitter value is limited to a maximum beacon jitter value amplitude, which is generated by a predetermined function related to the base station identification value, and which has an average output value of zero. In block 114, the base station 10A, 10B, 10C calculates the exact time to initiate a subsequent radio beacon transmission based on a function of the beacon jitter value and the average period T between each transmission of a radio beacon of the private wireless communication system. In block 116 the base station 10A, 10B, 10C waits for the exact time calculated and then returns to block 110 to initiate another radio beacon transmission and repeats the steps of blocks 112, 114 and 116, i.e. calculating and waiting for the jittered time interval until the next transmission.

Fig. 10 illustrates the operation of the mobile terminal 14 in one embodiment of the method of the present invention. At block 120, the mobile station 14 receives the radio frequency beacon transmitted from the base station 10A, 10B, 10C. The mobile terminal 14 derives base station status information from the received radio frequency beacon at block 122. The mobile terminal 14 then determines whether the identified base station can be provided for communication with the mobile terminal 14 based on the received base station status information at block 124. The base station may not be able to provide communications, for example, if the mobile terminal 14 is not an authorized user for the base station. If the status information indicates that the associated base station 10A, 10B, 10C is unable to provide communications with the mobile terminal 14, the mobile terminal 14 returns to block 120 and continues to receive the radio frequency beacon transmitted from the base station 10A, 10B, 10C.

If the received status information indicates that the transmitting base station 10A, 10B, 10C can provide communication with the mobile terminal 14, the mobile terminal 14 derives a base station identification value from the received radio frequency beacon at block 126. A predetermined jitter function associated with the identified base station is determined at block 128. At block 130, the mobile terminal 14 synchronizes beacon timing with the identified base station 10A, 10B, 10C based on the predetermined dithering function associated with the identified base station derived from the operation at block 128. Thereafter, the mobile terminal 14 predicts the timing of the jittered radio frequency beacons from the identified base stations 10A, 10B, 10C and maintains synchronization until the mobile terminal moves outside the transmission range 12A, 12B, 12C of the identified base stations 10A, 10B, 10C.

As shown in fig. 10, the state is derived and then the base station identification is performed. It should be appreciated that the benefits of the present invention can also be achieved by first making the identification. The mobile terminal 14 then determines from the identification value whether it is a permitted base station and, if so, derives status information. If the base station identification value is not on the list of base stations that the mobile terminal 14 is allowed to use, then no state information need be derived.

In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.

Claims (11)

1. A method for beacon channel transmission timing by a private radio telephone base station, the base station having an associated base station identification value and a transmission range, the base station repeatedly transmitting radio frequency beacons including the base station identification value to a mobile terminal located within the transmission range of the base station, and receiving by the mobile terminal one of the transmitted radio frequency beacons and deriving the base station identification value from the received radio frequency beacon, said method characterized by the steps of:

repeatedly transmitting a radio frequency beacon containing a base station identification value at jittery time intervals;

calculating a time interval of jitter according to a predetermined function related to the base station identification value; and

wherein mobile terminals within the transmission range of the base station perform the step of synchronizing to the base station beacon timing according to a predetermined function associated with the identified base station.

2. The method of claim 1, wherein the radio frequency beacon contains base station status information, and wherein said deriving step comprises the step of deriving the base station status information from the received radio frequency beacon, and wherein the mobile terminal performs the determining step after said deriving step of determining whether the mobile terminal is an authorized user of the base station based on the received base station status information.

3. A method for beacon channel transmission timing by a private radiotelephone base station transmitting radio frequency beacons, said method characterized by the steps of:

initiating transmission of a radio frequency beacon;

waiting an exact time after said initiating transmission step, the exact time being a function of the beacon jitter value; and then

Repeating the initiating and waiting steps; and

wherein the waiting step comprises the steps of:

calculating a current beacon jitter value having an amplitude not greater than a maximum beacon jitter value by using a predetermined function having an average output value of almost zero; and

wherein said exact time of said waiting step is equal to the average period between transmissions of the private radiotelephone base station beacon plus the calculated current beacon jitter value.

4. The method of claim 3 wherein the base station has an associated identification value and wherein said predetermined function is associated with the base station.

5. The method of claim 4, wherein the radio frequency beacon contains an associated identification value and wherein mobile terminals within transmission range of the base station perform the steps of:

receiving the transmitted radio frequency beacon;

deriving a base station identification value from the received radio frequency beacon; and

is synchronized with base station beacon timing according to a predetermined function associated with the identified base station.

6. The method of claim 5, wherein the radio frequency beacon contains base station status information, and wherein said deriving step comprises the step of deriving base station status information from the received radio frequency beacon and wherein after said deriving step is performed by the mobile terminal, the step of determining whether the base station can provide communication with the mobile terminal based on the received base station status information.

7. A method as claimed in claim 3, wherein said exact time of said waiting step is equal to the average period between transmissions of the private radiotelephone base station beacon plus the calculated current beacon jitter value minus the beacon jitter value of the radio beacon transmitted most recently.

8. Private radio telephone base station comprising

Radio frequency transmitting means for transmitting radio frequency communications; and

beacon transmission control means connected to said radio frequency transmission means for controlling the periodic transmission of radio frequency beacons by said base station, said base station characterized by:

wherein said beacon transmission control means provides for periodic transmission of radio frequency beacons by said base station at jittery time intervals, the beacon transmission control means comprising

Jitter generating means for generating a current beacon jitter value having an amplitude not greater than a maximum beacon jitter value according to a predetermined function having an average output value of almost zero; and

beacon transmission initiating means connected to said radio frequency transmitting means and for initiating said radio frequency beacon transmission at a time that is a function of said current beacon jitter value in response to said jitter generating means.

9. The private radio telephone base station of claim 8 further comprising storage means, coupled to said beacon transmission control means, for storing said predetermined function.

10. The private radiotelephone base station of claim 9 wherein said base station has an associated identification value and wherein said storing means comprises means for storing said associated identification value and wherein said radio frequency beacon contains an associated identification value.

11. Synchronous private radio communication system with jittery time intervals between transmissions of radio beacons from a private base station to a mobile terminal, said base station comprising

Base station radio frequency transmitting means for transmitting radio frequency communications; and

a beacon transmission control device connected to the base station radio frequency transmission device for controlling the periodic transmission of the radio frequency beacon by the base station, the mobile terminal comprises

The mobile radio frequency receiving device is used for receiving the radio frequency beacon;

beacon readout means coupled to said mobile radio frequency receiving means for deriving said base station identification value from said received radio frequency beacon; the system is characterized in that:

wherein said beacon transmission control means provides for periodic transmission of radio frequency beacons by said base station at jittery time intervals, the beacon transmission control means comprising

Jitter generating means for generating a current beacon jitter value having an amplitude not greater than a maximum beacon jitter value according to a predetermined function having an average output value of almost zero; and

beacon transmission initiating means connected to said radio frequency transmitting means and for initiating said radio frequency beacon transmission at a time that is a function of said current beacon jitter value in response to said jitter generating means; and

wherein the mobile terminal comprises

Determining means connected to said beacon reading means for determining said predetermined function in dependence upon said base station identification value; and

synchronization means coupled to said determining means for synchronizing said mobile terminal and said time interval of said radio frequency beacon transmission in accordance with said predetermined function.