Abstract:
A method for use in a receiver in a time division multiple access (TDMA) wireless communication system to determine whether a channel is occupied and whether the transmitter occupying the channel is internal or external to the system. The method comprises the steps of receiving a signal, on a channel determining that the channel is unoccupied when the signal strength of the channel signal is below a threshold, determining that the transmitter is internal to the system when the received signal is highly correlated to a signal that is known to contain the proper predefined field.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of wireless communication, and, more specifically, to the area of determining whether a transmitter transmitting on a channel of the wireless system is internal to the wireless communication system. 
     BACKGROUND OF THE INVENTION 
     A wireless communications system is designed to cover a large geographic area that supports numerous mobile stations. Recently, some of these large systems are being used as a foundation for smaller systems, such as a wireless office (herein “indoor system”). One such indoor system employs time-division multiple access (TDMA) technology to deliver an indoor communication environment that is flexible (especially in comparison to wireline networks) and compatible with existing digital wireless networks (herein “outdoor system”). 
     An outdoor TDMA system achieves economy of scale by dividing the geographic coverage region into “cells.” In each cell, each operational frequency is divided into timeslots, whereby multiple users share the same frequency by broadcasting only during the assigned timeslot. A frequency and a timeslot are reused from cell to cell over the geographic region—carefully engineered using distance and topography to keep wireless units in different cells from interfering with each other. Some overlap does occur, however, even with such careful planning. Therefore, the TDMA system assigns a “color code” to each mobile station. The transmitter of the mobile station encodes and sends this color code with other content in every timeslot. The receiver can then uniquely identify the transmitting mobile station when it decodes the content of the timeslot. 
     To achieve the capacity necessary to support the communication needs of an indoor system, both frequency reuse and TDMA technology are employed. An indoor network may be conceptually one large “cell” with only one control channel. Speech quality is improved by having a number of radio heads distributed throughout the coverage area. The radio heads use low-power transmitters, so that other radio heads throughout the indoor network can reuse channels. 
     In one example, indoor networks allocate channels by setting up a dynamic list of the “best” frequencies and timeslots within each frequency. “Best” is defined as those channels that have the least interference from sources outside of the indoor network (generally the nearest outdoor system). 
     Further, as a mobile station moves around in the coverage area, the indoor system changes the radio head that the mobile station is communicating with by doing handoffs. The indoor system tries to use the same channel after the handoff as before, thus eliminating the requirement that the mobile station re-tune, as is commonly done during handoffs. This type of handoff is seamless to the mobile station, and is often referred to in literature as Soft Handoff. A prerequisite for soft handoff is that the radio heads are synchronized with each other. 
     The scanning receivers, potentially at each radio head, provide measurements to be used both for channel allocation and handoff decisions. The measurements could consist of received signal strength, and some sort of identity of the signal (if it is from an external interferer or from a mobile station within the office system) for each measured channel. For handoff purposes, the system verifies that the “new” radio head&#39;s scanning receiver is receiving the correct mobile (signal is from the mobile station involved in the handoff, and not some other signal) by checking the identity of the signal. For channel allocation purposes, the source of a signal is used to see if a channel is in use within the system or not. 
     Current scanning receivers, however, cannot accurately identify whether the source of a signal on a channel is internal or external to the system without decoding the content of each timeslot and examining the color code to determine whether the color code belongs to the indoor system. Completely decoding the content of a time slot requires demodulation, data detection and color code decoding. This requires fast, complex processors, program memory space and memory data space; all of which are contrary to the requirements of a small, inexpensive indoor system. On the other hand, energy at or above a threshold at synchronization (or other) symbols in the timeslot can not indicate whether the source of the signal is internal or external to the system. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of this invention, a method is disclosed for use in a wireless communication system having a plurality of internal transmitters transmitting on a plurality of channels. The method determines whether a signal on a channel is being transmitted by a transmitter internal to the system. The method comprise the steps of receiving the signal being transmitted by a transmitter on a channel and determining a degree of correlation between a predefined field in the signal and a model of a predefined field in a signal transmitted by a transmitter synchronized the system. A determination is made that the transmitter of the signal is internal to the system if the degree of correlation is high. 
     According to a further aspect of this invention, the step of determining the degree of correlation comprises determining the degree of correlation between the predefined field and the signal over a predefined frequency range and a model of a predetermined field of the signal transmitted by a transmitter synchronized to the system. According to a further aspect of this invention, the step of determining the degree of correlation comprises determining the degree of correlation between the predefined field and the signal over a predefined time offset and a model of a predefined field as if the predefined field were transmitted by a transmitter synchronized the system. 
     According to a further aspect of this invention, a scanning radio in a wireless telephone system measures a signal&#39;s strength and phase of symbols and determines whether the signal&#39;s transmitter is internal or external to the system. The signal has a plurality of timeslots each having one or more predefined fields. The scanning receiver comprises a controller communicating with the wireless system and a variable frequency tuner. The variable frequency tuner includes a tunable receiver to receive a timeslot of a signal on one of a plurality of frequencies selected responsive to the controller. The receiver changes the frequency of the signal to an intermediate frequency and measures the signal&#39;s strength. 
     This aspect of the invention also includes a quantitizer configured to sample the signal strength measurement and to quantitize each sample of the signal strength and a sampling device configured to sample the intermediate frequency signal and to derive a mathematical representation of the phase of the signal in each sample. The inventive receiver also includes a memory containing a stored list of mathematical representations of the phases and signal strengths of samples of the predefined fields. 
     This invention also includes a comparator that determines whether the transmitter of the signal is internal or external to the system by determining whether there is a high correlation between the mathematical representation of the phase of the signal in each sample and one of the stored list of mathematical representation of the phases. The comparator includes means for comparing the mathematical representation of the phase and the signal strength of a predetermined plurality of signal samples to the stored list of mathematical representations of phases and signal strengths of the unique synchronization words, wherein the transmitter of the signal is internal to the system if there is a high correlation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of this invention may be obtained from a consideration of the following detailed description in conjunction with the drawings, in which: 
     FIG. 1 is a block diagram of a wireless office system that employs a scanning receiver according to an exemplary embodiment of this invention; 
     FIG. 2 is a block diagram of a radio head of the wireless office of FIG. 1; 
     FIG. 3 is a block diagram of a scanning receiver according to an exemplary embodiment of this invention; and 
     FIGS. 4 and 5 are flowcharts of operation of a scanning receiver according to an exemplary embodiment of this invention. 
    
    
     DETAILED DESCRIPTION 
     All mobile stations within a control area are synchronized to the same clock. The present invention makes use of this synchronization to improve the efficiency of determining whether a received signal originated from a wireless communications device controlled by the system or whether the signal originated from a different system, without having to fully decode the content of the signal in order determine the color code. 
     FIG. 1 is a block diagram of a wireless office system  10  that uses a scanning receiver according to an exemplary embodiment of this invention. A wireless office of this nature is discussed in S. Broddner, M. Lilliestrale and K. Wallstedt,  Evolution of Cellular Technology for Indoor Coverage,  presented at the International Symposium on Subscriber Loop Services, 1996, which is incorporated herein by reference in its entirety. Of course, a scanning receiver in accordance with the principals of this invention may be employed in any wireless system (indoor or outdoor) that measures signal strength and/or determines whether a transmitter is internal or external to the system. 
     The wireless office system  10  generally includes a private branch exchange (PBX)  12  as a switching center. The PBX  12  receives calls from, and sends calls to, the public switched telephone network (PSTN)  14 , as known in the art. The PBX  12  is connected to a hub  16  by a plurality of lines  18 . The hub  16  controls and coordinates the wireless connections among a plurality of radio heads  20 - 29  and various wireless communications devices, represented by wireless telephones  40  and  42 , wireless fax machine  44  and wireless modem  46 . 
     The hub  16  provides communications over the area covered by its radio heads  20 - 29  as if it were one cell with one common control channel from the outdoor wireless paradigm. Two or more radio heads  20 - 29  can be used for the downlink transmission and uplink reception when possible, because improved reception is gained on the traffic channels. An adaptive antenna selection scheme decides which radio heads are best suited to serve each mobile station, based on received signal quality, and the signal is not transmitted where it is not needed. The radio heads do not have to be the same for uplink and downlink. For example, the radio heads  20  and  21  are used for downlink transmission from the wireless telephone  40 , and the radio heads  21  and  24  are used for uplink transmission. 
     The hub  16  switches the active radio heads as a mobile station moves around in the cell area. Preferably, a soft handoff is used to change the radio heads where possible. A soft handoff is one without any change of channels, without informing the mobile station about the event, and without being detectable by the user. A conventional “hard” handoff will also occur if the same channel is not available after an inter-radio head handoff (e.g., due to limitations in hardware availability). 
     An adaptive channel allocation scheme determines which traffic channel is best for each connection based on measurements of co-channel interference. It is then possible to assign a traffic channel (one frequency and one time slot) to multiple mobiles in the cell, further increasing the system capacity by frequency reuse. In the example of FIG. 1, if wireless telephone  40  and wireless telephone  42  (and their serving radio heads) are distant enough not to interfere with each other, they can use the same channel although they are within the same cell. If one of the mobiles moves towards the other, the interference increases, and one of the mobiles is ordered by the hub  16  to change to another channel. 
     Turning now to FIG. 2, a radio head  200  having a scanning receiver according to the exemplary embodiment of this invention is shown. The radio head  200  includes, in this exemplary embodiment, five transmitters  202 ,  204 ,  206 ,  208  and  210 , and receivers  212 ,  214 ,  216 ,  218  and  220 . A scanning receiver  240  according to the exemplary embodiment of this invention is connected to a controller  250 . The controller  250  is connected to the hub  16  by a control line  252  and a plurality of two-way communication channels  253 . Transmitters  202  and  204  are connected to antenna  254  by way of multiplexer  256 . Transmitters  206  and  208  are connected to antenna  258  by way of multiplexer  260 . Receivers  212 ,  214 ,  216   218 ,  220  and scanning receiver  240  are connected to duplexer  262 . Transmitter  210  is also connected to duplexer  262 . Duplexer  262  facilitates transmitter  210  to transmit and receivers  212 ,  214 ,  216 ,  218   220 , and  240  to receive over antenna  270 . 
     Radio head  200  provides connections to wireless communications devices via transmitters  202 ,  204 ,  206 ,  208  and  210  and receivers  212 ,  214 ,  216 ,  218  and  220 . The  20  radio signals are translated to/from an analog or digital stream usable in switching in controller  250 . Scanning receiver  240  provides information regarding channel occupancy and noise to the hub  16 , to aid in decisions regarding channel assignment, channel handover and the like. 
     Turning now to FIG. 3, the scanning receiver  240  according to an exemplary embodiment of this invention includes a controller  302  that first receives a command from the system controller ( 250 , FIG. 2) to determine the signal strength on a frequency and determine whether the signal is from a user of the system or whether the frequency is being used by a system outside of the wireless office (such as an outdoor wireless network, or a wireless network from an adjoining office building). Processing starts when the controller  302  causes a radio frequency (RF) tuner  304  to tune to a specific frequency. The RF tuner  304  captures a signal at the requested frequency and timeslot of the signal. The RF tuner  304  changes the frequency of the signal to an intermediate frequency. By changing the frequency of the signal from the original frequency to an intermediate frequency the scanning receiver can perform its functions on all signals at a common frequency, so that it does not have to operate at all of the possible frequencies of the wireless system. 
     The RF tuner  304  delivers the signal strength measurement to an Analog-to-Digital converter (A/D converter)  306 . The A/D converter  306  acts as a quantitizer in that it samples the signal strength and derives a magnitude measurement for each of the signal strength samples. The RF receiver  304  delivers the content of the signal at the intermediate frequency to a phase digitizer  307  that is configured to sample the intermediate frequency signal and derive a mathematical representation of the phase of the signal in the sample. In this exemplary embodiment, each sample is described in terms of phase angle of the signal. 
     A digital signal processor (DSP)  308  receives the quantitized signal strength samples and the phase angle samples of the signal. The DSP  308  acts as a comparator to determine if the signal was transmitted from within the system or outside of the system. To this end, since the internal system is a low power system, it is assumed that all transmitters internal to the system are nearby. As a result, there should be little or no frequency shift or time shift in signals transmitted by internal transmitters. Therefore, the system looks for a predefined field, such as a synchronization word, in the timeslot of interest to determine if the predefined field was received within a frequency range and time period in which it was expected (i.e., the common air frame synchronization reference in the system). The time period is measured in a number of symbols within which the predefined field should start. 
     The DSP  308  contains a stored list of mathematical representations and signal strengths of samples of the expected synchronization words for each time slot. A comparator then determines whether the transmitter of the signal is internal or external to the system by determining whether, in this exemplary embodiment, the synchronization word is present in the signal, by comparing the signal strength and the mathematical representation (preferably in complex IQ representation) of the phase of a predetermined plurality of signal samples from the timeslot of the signal to the stored list of mathematical representations and signal strengths of the unique synchronization words, wherein the transmitter of the signal is internal to the system if one of the proper synchronization words is present. 
     To this end, DSP  308  performs the comparison several times. It varies the phase angle offset and tests for the presence of the synchronization word in all of the allowed time offset position. The maximum value after the subtraction, if it is above a threshold, is deemed an inside transmitter. If the synchronization word is not found, then the transmitter is an outside transmitter. This information is delivered to the controller  302 , which sends it on to the hub  16 . 
     Turning now to FIGS. 4 and 5, processing according to an exemplary method as practiced in scanning receiver  240  is described. Processing starts in a box  400 , where the RF receiver captures the signal strength and the samples for one timeslot. Processing proceeds to the box  402 , where the phase offset for the allowed frequency drift is obtained. In a box  404 , the samples are converted from polar to complex representation incorporating the phase offset. Processing continues in box  406 , where the correlation start position is initialized to the earliest possible time (symbol) that could have been received. 
     Processing proceeds to box  408 , where the received IQ data is correlated against the stored samples of the IQ representation of the expected sync word. In decision diamond  410 , a determination is made whether the correlation made in box  408  is greater than any earlier correlation. If it is, then processing proceeds to box  412 , where the result is stored. If the correlation is not greater than any earlier correlation and from box  412 , then processing proceeds to decision diamond  414  where a determination is made whether correlations have been made for the entire time window. If not, then processing moves to box  416 , where the correlation start position is set to the next sample in the buffer. 
     If, in decision diamond  414 , correlations have been made for the entire time window, then processing proceeds to decision diamond  418 , where a determination is made whether correlations have been made for all frequency offsets. If not, then processing proceeds to box  420 , where the phase offset for the next presumed frequency offset is obtained. Processing loops back to box  404 . 
     If, in decision diamond  418 , correlations have been performed for all of the frequency offsets, then processing proceeds through connector A to FIG. 5, In FIG. 5, processing begins in decision diamond  500 , where a decision is made whether the maximum correlation result is greater than a threshold. If it is, then an internal transmitter has been found and processing moves to box  502  where a sync status is set to true. If, in decision diamond  500 , the maximum correlation result is not greater than a threshold, then the signal is deemed to be from outside the system, and the sync status is set to false in box  506 . Processing continues from both boxes  502  and  506  to box  508 , where the average received signal strength is calculated. The sync status and the average received signal strength is reported to the controller in box  510 , where processing ends. 
     It will therefore be apparent that the invention economically, simply and rapidly provides a determination of whether a signal originated from a wireless communications device controlled by the system or originated from another system. Consequently, the need to fully decode a signal is avoided and a scanning receiver for an indoor wireless system does not require high speed processors and large amounts of memory. It will further be appreciated that the invention may be used in any receiver in any wireless system where all or most of the transmitters are time synchronized. 
     It is to be understood that the above-described embodiment is to illustrate the principles of this invention, and that those skilled in the art may devise many variations without departing from the scope of the invention. It is, therefore, intended that such variations be included within the scope of the appended claims.