Abstract:
A method of mitigating phase nulls in a local wireless telephone system comprises inserting one or more random bits into a frame of a simulcast signal. The introduction of the random bits into the frame causes subsequent bits in the frame to be modulated differently, thereby reducing phase cancellation nulls. While particularly suited to localized wireless telephone systems, the methodology may be equally adapted to any mobile network wherein geographically proximate antennas broadcast the same signals.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to wireless communications and, more particularly, to a method for simulcast transmissions. 
     In recent years, there has been increased interest in building localized, private wireless telephone systems used by corporations or other organizations. Private wireless telephone systems employ many concepts and features used by conventional cellular networks and may be thought of as a scaled-down cellular network. A plurality of radio heads are positioned throughout the building or facility to provide radio coverage within the facility. The radio heads are connected to a central controller which, in turn, connects the private wireless telephone system with other networks, such as a public land mobile network or the public-switched telephone network. 
     One advantage of a private wireless telephone system is that it allows customers to use mobile terminals in an office or other localized area rather than a desk phone with a fixed connection to the telephone system. Thus, customers have freedom to move about within the facility, yet remain accessible by phone. The private wireless telephone system can also interface with the public land mobile network, allowing users of the private wireless telephone system to use the same mobile terminal in both networks. Thus, the user can move seamlessly between the private wireless telephone system and public land mobile network. 
     Many commercial deployments of private wireless telephone systems use a technique known as simulcasting on the downlink frequencies carrying the control channel. That is, multiple radio heads simultaneously transmit the same data from two or more base stations whose coverage areas may overlap. Simulcasting simplifies cell planning in the private wireless telephone system. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention relates to a simulcasting method that is useful in wireless communication networks, such as a private wireless telephone system. Simulcast signals are transmitted from two radio heads whose coverage area overlaps. The simulcast signals comprise a plurality of frames, each frame comprising a plurality of bits. One or more bits in at least one of the simulcast signals is randomized in order to change the phase relationship of the simulcast signals from one frame to the next. By varying the phase relationship of the simulcast signals from one frame to the next, long nulls where the overlapping signals are 180° out-of-phase are avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a schematic view of a communication system amenable to use with the present invention; 
     FIG. 2 illustrates a schematic view of a mobile terminal, such as may be used in the communication system of FIG. 1; 
     FIG. 3 illustrates a schematic view of a Local Wireless Telephone System adapted for use with the present invention; 
     FIG. 4 illustrates a flow chart embodying the present invention; 
     FIG. 5A illustrates two simulcast signals without a randomized bit; 
     FIG. 5B illustrates two simulcast signals with one randomized bit; and 
     FIGS. 6A and 6B show exemplary test results using the present method. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to an improvement on a network having proximate antennas engaged in simulcast activity. While especially useful in localized wireless telephone systems, the invention is applicable in other communication networks subject to nulls from simulcasting. While the following discussion is couched in terms of a TIA/EIA-136 communication system, it should be appreciated that the present invention is equally applicable to other differential encoding schemes. 
     Turning now to FIG. 1, a communication system  10  is illustrated. In particular, the communications system  10  includes the Public Switched Telephone Network (PSTN)  20  and the Public Land Mobile Network (PLMN)  30 , which may, in turn, be connected to one or more Localized Wireless Telephone Systems (LWTS, only one shown)  60 . LWTS  60  may be proprietary or public. While not shown, satellites may be used as needed either within the PSTN  20  or the PLMN  30  to provide remote communication links, such as across oceans or the like. The operation of the PSTN  20  is well established and subject to extensive documentation beyond the scope of the present invention and therefore a more detailed discussion is omitted. 
     PLMN  30  may include a plurality of proprietary mobile networks  40 . Each mobile network  40  comprises one or more base stations  50  supporting over-the-air communications with mobile terminals  100 . Each base station  50  is located in a geographic region, referred to as a cell. In general, there is one base station  50  per cell. Base station  50  includes a plurality of transceivers for communicating with mobile terminals  100  within the cell. Each base station  50  connects to a Mobile Switching Center (MSC)  42 . At least one MSC  42  in the PLMN  30 , and more likely one MSC  42  in each mobile network  40 , is connected via a gateway to the PSTN  20 . Some MSCs  42  may also serve as gateways connecting the various mobile networks  40  within the PLMN  30 . Gateway functions may be all consolidated at a single MSC  42  within a mobile network  40  or dispersed amongst a plurality of MSCs  42  within a mobile network  40 . The function of the MSCs  42  is to route calls and signals in the mobile network  40  to the appropriate destination. To perform this function, a mobile network  40  relies on the HLR  44  and the VLR  46 . HLR  44  and VLR  46  store information concerning the whereabouts of mobile terminal users. This information typically includes the user&#39;s name and address for billing purposes, the serial number of the user&#39;s mobile terminal  100 , and the services that the subscriber is entitled to receive. Together, the HLR  44  and the VLR  46  provide the information needed by the MSCs  42  to locate mobile terminals  100 . The exact protocols and communication regimens between the various entities in a mobile network  40  are well documented, such as in TIA/EIA-136, which is hereby incorporated by reference in its entirety. 
     LWTS  60  is typically a private network installed in a building or group of buildings. LWTS  60  may be used, for example, by a large corporation. LWTS  60  allows employees or other persons working in the building to use a mobile terminal  100  as an office telephone. LWTS  60  connects with an MSC  42  in the PLMN  30 . Thus, users of the LWTS  60  may move seamlessly between the PLMN  30  and the LWTS  60 . LWTS  60  comprises a control and radio interface (not shown) and a plurality of transceiver stations, as will be explained in greater detail with reference to FIG.  3 . 
     FIG. 2 is a functional block diagram of an exemplary mobile terminal  100 . The term mobile terminal as used herein comprises a cellular radiotelephone with or without a multi-line display; a Personal Communications System (PCS) terminal that may combine a cellular radiotelephone with data processing, facsimile and data communications capabilities; a PDA that can include a radiotelephone, pager, Internet/intranet access, Web browser, organizer, calendar and/or a global positioning system (GPS) receiver; a conventional laptop and/or palmtop receiver, or other appliance that includes a radiotelephone transceiver and processing capabilities. Mobile terminals  100  may also be referred to as “pervasive computing” devices or telephones. 
     Mobile terminal  100  typically includes a controller  122 , an operator interface  126 , a transmitter  138 , a receiver  150 , and an antenna assembly  158 . Operator interface  126  typically includes a display  128 , keypad  130 , interface control  132 , microphone  134 , and a speaker  136 . Display  128  allows the operator to see dialed digits, call status, and other service information. Keypad  130  allows the operator to dial numbers, enter commands, and select options. Interface control  132  interfaces the display  128  and keypad  130  with the controller  122 . Microphone  134  receives acoustic signals from the user and converts the acoustic signals to an analog electrical signal. Speaker  136  converts analog electrical signals from the receiver  150  to acoustic signals that can be heard by the user. 
     The analog electrical signal from the microphone  134  is supplied to the transmitter  138 . Transmitter  138  includes an analog to digital converter  140 , a digital signal processor  142 , and a phase modulator and RF amplifier  148 . Analog to digital converter  140  changes the analog electrical signal from the microphone  134  into a digital signal. The digital signal is passed to the digital signal processor (DSP)  142 , which contains a speech coder  144  and channel coder  146 . Speech coder  144  compresses the digital signal and the channel coder  146  inserts error detection, error correction and signaling information. DSP  142  may include, or may work in conjunction with, a DTMF tone generator (not shown). The compressed and encoded signal from the digital signal processor  142  is passed to the phase modulator and RF amplifier  148 , which are shown as a combined unit in FIG.  2 . The modulator converts the signal to a form that is suitable for transmission on an RF carrier. RF amplifier  148  then boosts the output of the modulator for transmission via the antenna assembly  158 . 
     Receiver  150  includes a receiver/amplifier  152 , digital signal processor  154 , and a digital to analog converter  156 . Signals received by the antenna assembly  158  are passed to the receiver/amplifier  152 , which shifts the frequency spectrum, and boosts the low-level RF signal to a level appropriate for input to the digital signal processor  154 . 
     Digital signal processor  154  typically includes an equalizer to compensate for phase and amplitude distortions in the channel corrupted signal, a demodulator for extracting bit sequences from the received signal, and a detector for determining transmitted bits based on the extracted sequences. A channel decoder detects and corrects channel errors in the received signal. The channel decoder also includes logic for separating control and signaling data from speech data. Control and signaling data are passed to the controller  122 . Speech data is processed by a speech decoder and passed to the digital to analog converter  156 . Digital signal processor  154 , may include, or may work in conjunction with, a DTMF tone detector (not shown). Digital to analog converter  156  converts the speech data into an analog signal that is applied to the speaker  136  to generate acoustic signals that can be heard by the user. 
     Antenna assembly  158  is connected to the RF amplifier of the transmitter  138  and to the receiver/amplifier  152  of the receiver  150 . Antenna assembly  158  typically includes a duplexer  160  and an antenna  162 . Duplexer  160  permits full duplex communications over the antenna  162 . 
     Controller  122  coordinates the operation of the transmitter  138  and the receiver  150 , and may for instance take the form of a typical microprocessor. This microprocessor may be a dedicated or shared microprocessor and may be a single processor or multiple parallel processors as needed or desired. This coordination includes power control, channel selection, timing, as well as a host of other functions known in the art. Controller  122  inserts signaling messages into the transmitted signals and extracts signaling messages from the received signals. Controller  122  responds to any base station  50  commands contained in the signaling messages, and implements those commands. When the user enters commands via the keypad  130 , the commands are transferred to the controller  122  for action. Memory  124  stores and supplies information at the direction of the controller  122  and typically includes both volatile and non-volatile portions. 
     FIG. 3 illustrates an exemplary LWTS  60  in greater detail. LWTS  60  comprises one or more base stations  50 . A plurality of radio heads  210  are connected by a transmission cable  202  to the base station  50 . Transmission line  202  may be a coaxial cable, a waveguide, a strip transmission line, a twisted pair, or the equivalent. Each radio head  210  is coupled to an antenna  200 . Radio head  210  comprises a data buffer  212 , a modulator  214 , a power amplifier  216 , a random bit generator  218 , and a local controller  220 . Data to be transmitted through the antenna  200  is initially stored in data buffer  212 . Local controller  220  causes random bit generator  218  to generate random bits as needed for insertion into transmitted signals, as will be hereinafter described. Modulator  214  modulates the data to the desired RF channels. Power amplifier  216  amplifies the modulated signal in a conventional manner. Each random bit generator  218  may include a different seed. 
     In the exemplary LWTS  60 , the technique of simulcasting is used on downlink frequencies carrying the control channel. That is, multiple radio heads  210  transmit simultaneously the same data on the same frequency from different locations. The simulcast signals may overlap. 
     In most LWTSs  60 , the propagation channel is essentially static, without Rayleigh fading typically seen in cellular networks. When a mobile terminal  100  is located in a region where the power from two simulcasting radio heads  210  is roughly equivalent, deep phase nulling of the signal can occur. That is, if the two simulcast signals arrive at the mobile terminal  100  at the same level and are 180° out-of-phase, then the simulcast signals will cancel each other out, making it difficult or impossible to demodulate the signal. The phase nulls also raise the noise floor in the area around the phase null. The increase in the noise floor may cause the bit error rate (BER) to increase and degrade performance of the mobile terminal  100 . 
     In a null, a mobile terminal  100  may repeatedly fail to detect and demodulate the control channel. These repeated failures may cause the mobile terminal  100  to drop off the original control channel and perform a control channel reselect function. Needless channel reselection unnecessarily drains battery power from the mobile terminal  100 , shortening its effective operation time. Further, placement in a null may mean that the mobile terminal  100  may not receive an incoming call or be able to secure service for an outgoing call. 
     In conventional simulcasting techniques, the exact same bits are being modulated in the same manner for all simulcast signals. The coherent signals thus being transmitted at each antenna  200  will maintain the exact same phase relationship from frame to frame. If, however, one or two bits within the simulcast signals are sacrificed and independently randomized at the radio head  210 , the phase relationship between the coherent symbols of the individual downlink simulcast signals will likewise change from frame to frame. The differential encoding scheme of TIA/EIA-136 ensures that a phase change at a point t o  will carry forward in time, not just locally around the change. 
     If the phase relationship of the symbols of the individual downlink simulcast signals changes randomly from frame to frame, then the phase relationship of the individual simulcast signals will also change from frame to frame. Consequently, if a null is formed by two simulcast signals being 180° out of phase in one frame, then most likely, due to the independent random differential symbol created in each simulcast signal by the present invention, the simulcast signals will not be out of phase on the next frame. If two bits are used, a 90° phase shift may be put on each frame, so the probability of going from a null on one frame to a null on the next frame is only 25 percent. This is sufficient to break up the repeated pattern of CRC failures of a sleeping mobile terminal  100  in a phase null. 
     FIG. 4 illustrates the simulcasting method of the present invention. Initially, information is received at the base station  50  for transmission (block  300 ). This information is in the form of an incoming signal, perhaps wireless, but more likely wire-based. This signal is split amongst the plurality of radio heads  210  (block  302 ) over transmission line  202  by conventional splitters (not shown). At each radio head  210 , the signal is input frame-by-frame into buffer  212  (block  304 ). Local controller  220  instructs the random bit generator  218  to generate at least one random bit per frame (block  306 ). In the exemplary embodiment, two random bits per frame are generated. Those skilled in the art will realize, however, that random bits do not have to be generated in every frame. 
     Regardless of number, the random bit(s) are “inserted” into the frame (block  308 ) at predetermined location(s). Note that insertion, as used herein, includes adding a bit to the data in the buffer, as well as randomly changing an existing bit at a predetermined location in the frame. When there is more than one random bit, these random bits may be offset or colocated within the frame. In a first embodiment, the bit(s) are inserted at reserved locations within a frame. Reserved bits presently serve no purpose, but are set to a default value of a logical “one” for lack of a better setting. In a second embodiment, the random bit(s) are inserted in place of information bearing symbols. In the event that the random bit is inserted in place of an information bit, error correction coding may correct any errors so introduced. One induced bit error in a block of coded data may have minimal impact, especially in the generally benign LWTS environment. 
     An induced bit error would also have implication in terms of BER measurement by the mobile terminal  100 , since successfully error corrected, decoded bits are re-encoded and used to estimate BER against raw demodulated bits. In this case, one or two bits, depending on the number of random bits inserted, will have an error rate of 50%. The bias rate in this measurement caused by the introduction of known errors can be adjusted out. 
     It is noted that for breaking up repeated CRC failure on the DCCH, only one or two sacrificial bits are needed per 1.28 second superframe. To help Frame Error Rate (FER) performance on a Digital Traffic Channel (DTC) only one or two sacrificial bits are needed per 20 ms frame. 
     After random bit creation, the simulcast signal is modulated by modulator  214  (block  310 ) and amplified by power amplifier  216  (block  312 ). After amplification, the simulcast signals are transmitted by respective radio heads  210  to the mobile terminal (block  314 ). As used herein, the term simulcast signals means two or more signals simultaneously broadcast from different locations, wherein the two signals are identical or nearly identical. In the present invention, the simulcast signals are identical except for the few randomized bits. 
     In an alternate embodiment, random bit generators  218  may be located in the base station  50  proper or even in the MSC  42  if so desired. These random bits would be sent to the radio heads  210  with addressing information or the like and incorporated into the signal in the buffers  212 . The present invention works best when the random bits are independently generated. This may be done by taking consecutive bits off a random string of bits, or by pulling bits from different random bit generators  218 . To maximize the likelihood that random bit generators  218  generate different strings of random bits, different seeds A and B may be used. 
     FIGS. 5A and 5B illustrate how the simulcast method of the present invention helps prevent phase nulls. FIG. 5A shows two simulcast signals, denoted A and B. The simulcast signals are differentially modulated. Thus, a transition from “1” to “0” or vice versa represents a binary “1”. In FIG. 5A, the simulcast signals A and B are identical. Assuming that these simulcast signals A and B are 180° out-of-phase when received by the mobile terminal, the two signals A and B will cancel, as shown on the right side of FIG.  5 A. Thus, the mobile terminal may not be able to demodulate the resulting signal, which is denoted as C. 
     FIG. 5B shows two simulcast signals, denoted D and E, which have been differentially modulated. One bit in each of the simulcast signals D and E has been randomized, as indicated by being blocked. The randomization has resulted in different bits appearing in the two signals. Again, assuming that the signals are received 180° out-of-phase at the mobile terminal, the simulcast signals D and E do not cancel. The resulting signal, denoted as signal F, is shown. 
     The efficaciousness of the present invention is illustrated in FIGS. 6A and 6B. It is known in the field of mobile communications to maintain a counter, known as the Monitoring Radio Link Quality (MRLQ), which indicates a Radio Link Failure Condition. The MRLQ counter is basically a “leaky bucket” counter, which is incremented when the CRC or other signal strength indicator is good, but decremented when the CRC or other signal strength indicator is bad. The counter includes an upper limit of, and starts at, ten. If the counter reaches zero, then Radio Link Failure has been reached. Upon reaching this condition, the mobile terminal  100  is forced to scan for another channel upon which to camp. In FIGS. 6A and 6B, the mobile terminal  100  is within ˜0.04 wavelengths of a null, such that the phase offset between two equal power received signals is 165 degrees out of phase. A static channel is used for each individual simultaneously broadcast signal. The Error Vector Magnitude (EVM) of each transmitter is 10%, raising the effective noise floor for the demodulator significantly. The solid line, in both FIGS. 6A and 6B, indicates the MRLQ counter where no random bits are used. As can be seen, it rapidly deteriorates to zero, forcing channel reselection. In contrast, the dotted line in FIG. 6A represents the MRLQ for the same phone with one random bit used. The counter never dips below eight. The dotted line in FIG. 6B represents the MRLQ counter with the use of two random bits. The counter is decremented even fewer times than in FIG.  6 A and has much better performance than the prior art. 
     This technique will certainly place other mobile terminals  100  in temporary nulls that otherwise would not exist in a clear simulcast reception field. This is a calculated trade-off. Consider the following example. Four mobile terminals  100  are positioned in the same region, where the individual signals are of relatively the same power, and one is in a null. If the present methodology is not performed, the mobile terminal  100  in the null remains in the null, and will drop off the control channel. However, with the present methodology, the null will potentially hop from mobile terminal  100  to mobile terminal  100 , causing occasional CRC failures, yet none of the mobile terminals  100  will drop off the control channel. The occasional CRC failure is acceptable since control channel data may be repeated from superframe to superframe. 
     The present methodology also provides the benefit that since these bits are randomized independently at each antenna  200 , there is no need for cell planning or “phase offset reuse” within the LWTS  60 . This may promote the efficient use of LWTS  60  in environments heretofore thought unsuitable for such a system. 
     In an alternate embodiment, only some of the radio heads  210  or base stations  50  utilize the randomization scheme of the present invention. However, to maximize the efficaciousness of the present invention, at any potential null, at least one of the signals contributing to the null should be randomized according to the present invention. 
     The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and the essential characteristics of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.