Patent Publication Number: US-6708032-B2

Title: Base station hand-off mechanism for cellular communication system

Description:
This application is a continuation of copending application Ser. No. 09/262,462 filed on Mar. 4, 1999, now U.S. Pat. No. 6,240,290. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to cellular communication systems, and is particularly directed to a new and improved communication control mechanism for controlling the hand-off of frequency channels through which communications are conducted between base stations of adjacent cells and a mobile transceiver as the mobile transceiver moves between those cells. 
     BACKGROUND OF THE INVENTION 
     Wireless (cellular) communication service providers customarily supply wireless communication capability to (mobile) subscribers located within a geographic area, through the use of a relatively limited number of communication channels. In order to optimize coverage within the geographical area of interest, the service provider typically subdivides the area into a cluster or multiple clusters of base stations. In addition, in order to minimize interference from adjacent or nearby cells, the service provider may employ some form of frequency reallocation (or reuse) scheme, such as that described in the U.S. Pat. No. 4,144,496, as a non-limiting example. 
     In such a spatially distributed or ‘cluster’ network architecture, a fixed number of sectors (i) are served by a cluster of (k) base stations. This has the effect of subdividing the number of available channels N by the product of i and k, namely by (i*k). Unfortunately, with today&#39;s expanding traffic, particularly in densely populated urban areas, service providers face the eventuality of running out of channels to meet demand. 
     One solution is to construct more base stations and reduce power levels—which is both hardware intensive and expensive. Another scheme is to reuse channels in time (TDMA) or in frequency (CDMA). Other approaches, such as described in the above-referenced patent, include dynamic allocation of frequencies or channels to accommodate channel demand. Initially, the relatively poor efficiency of frequency allocation schemes was not a significant problem as the demand was small and the number of available channels was more than adequate. However, as demand increased, new channel assignment and frequency reuse strategies were developed. 
     Such schemes have included sectorization of cells to minimize interference, and dynamic allocation or ‘borrowing’ of channels from other cells with a cluster, to meet unbalanced demand within the cluster. A new and promising approach is to spatially separate channels using switched or steered antenna beams. The overall objective of any strategy is to maximize the number of channels available, subject to an acceptable carrier (C) to interference (I) ratio, with the current industry standard being a figure of merit (or C/I ratio) of 18 dB. 
     Sectorization is a technique that uses fixed beams formed by directional antenna (phased) arrays installed at the base stations to divide the cell into an integral number of smaller cells. This technique serves to reduce interference to the base station, by attenuating channel interference to those mobile subscribers who are not located in that sector&#39;s beam. It also reduces interference to the mobile subscriber, by attenuating channel interference from base stations transmitting in a direction that is predominately away from the location of the mobile subscriber. However, as the number of sectors increases, the number of channels per sector necessarily decreases, thereby reducing the figure of merit. Ideally, at the time of system installation, there would be no sectorization, which would greatly increase system capacity. 
     Regardless of the channel allocation mechanism employed, whenever a mobile subscriber moves from one cell to another, it is necessary to change the frequency channel used to conduct communications with the base station in the ‘old’ cell from which the mobile transceiver is departing to a new frequency channel used to conduct communications with the base station in the ‘new’ cell which the mobile transceiver is entering. 
     Techniques using steered beam antennas have unique problems accomplishing this handoff between cells. In particular, the ‘new’ cell has the problem of where to point its narrowbeam antenna. The mobile subscriber is waiting on transmission from the new base station to transmit on the new frequency. If the new base station points the beam in the wrong direction, then the mobile subscriber sees no signal, does not synchronize and does not transmit. After the elapse of a prescribed period of time with no communication, the call will be dropped. The problem then is for the new base station to determine the correct beam to the mobile subscriber. 
     One mechanism for performing such frequency channel reuse/reallocation (or hand-off from the previous base station to the new base station) is described in the U.S. patent to Forssen et al, U.S. Pat. No. 5,615,409. This scheme involves the base station using an ‘intermediate’ channel to determine the direction of the mobile transceiver relative to it. It then assigns the mobile transceiver to an available narrowbeam channel. Because this technique requires what could otherwise be used for a regular communication channel be employed as an intermediate construct channel to determine the direction of the mobile transceiver, it necessarily reduces the number of available precious resources (channels). 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, this drawback is effectively obviated by a channel hand-off communication control mechanism that uses the very channels that are employed for communications between base stations of adjacent cells and a mobile transceiver as the mobile transceiver moves between those cells, to locate the mobile transceiver relative to the base stations, so that the acquiring base station may readily place a narrowbeam channel on the mobile transceiver at hand-off. Each base station employs a phased array antenna, which allows the base station to controllably define its antenna coverage pattern with respect to any mobile transceiver, so as to minimize interference from one or more other transceivers, and thereby reduce frequency reuse distance. 
     Pursuant to a first embodiment of the invention, when the quality of a narrowbeam link between the mobile subscriber and an already acquired cell base station indicates that the mobile transceiver is approaching a cell boundary with a new cell, the already acquired base station will initiate a hand-off sequence with the acquiring base station in the new cell to which the mobile subscriber is moving. For this purpose, the current base station will forward a message to the new base station that a channel hand-off is to commence. This hand-off initiating message will contain the identification of the communication channel currently employed by the mobile transceiver. 
     In response to this message, the acquiring base station employs one of the antenna elements of its phased array antenna to transmit an omnidirectional burst on a new communication channel to which mobile transceiver is to tune itself for conducting communications with the acquiring base station at hand-off, when the mobile transceiver enters cell. In response this burst signal on the new channel, the mobile transceiver transmits a reply signal on the new channel, which is processed by the acquiring base station to derive a steering vector representative of the direction of the mobile transceiver relative to the new base station. 
     The new base station employs this derived steering vector to adjust the directivity pattern of its phased array antenna, so as to place a narrowbeam pattern of the new communication channel in the direction of mobile transceiver, completing the hand-off. The new base station proceeds to conduct narrow beam communications with the mobile transceiver on the new communication channel. Using its ability to control the directivity of the narrowbeam lobe by way of its phased array antenna, the new base station continues to communicate with and track the mobile transceiver as long as the mobile transceiver is located in the new cell. 
     In a second embodiment of the invention, the acquiring base station a ‘sectorized’ burst in place of an omni burst of the first embodiment to locate the mobile subscriber. This sectorized burst is confined to a prescribed spatial sector sourced from the new base station toward the current cell in which the mobile subscriber is currently located. 
     In a third embodiment of the invention, the base station of the current cell from which the mobile transceiver is about to depart into the new cell determines the direction of the mobile transceiver relative to the already acquired base station, and generates a first steering vector associated with this direction. This steering vector is conveyed as part of the hand-off initiating message to the new or acquiring base station. In response to this first steering vector message, the new base station generates a second steering vector, representative of the direction of the mobile transceiver relative to that base station for the new communication frequency channel to be used between the mobile transceiver and the new base station at channel hand-off. Using this second steering vector, the new base station adjusts the directivity pattern of its phased array antenna, so as to place a narrowbeam pattern of the new communication channel in the direction of the mobile transceiver. 
     When the mobile subscriber responds on the new channel, hand-off is complete between the base stations, and the mobile transceiver thereafter communicates with the second base station as it enters into and travels through the new cell. The new base station then proceeds to conduct narrow beam communications with the mobile transceiver on the new communication channel, using its phased array antenna to track and communicate with the mobile transceiver as long as the mobile transceiver is located in the new cell. 
     In a fourth embodiment of the invention, the new or acquiring base station ‘pretunes’ its transceiver to the ‘old’ or ‘pre hand-off’ frequency employed by the current or already acquired base station, in order to determine the direction of the mobile subscriber, prior to channel hand-off. In response to a pre hand-off message, the new base station uses its phased-array antenna to place a narrowbeam pattern for the current channel being employed by the already acquired base station in the general direction of the cell in which the mobile subscriber is currently located. 
     The new base station then monitors the current channel to derive a steering vector representative of the direction of the mobile transceiver relative to the new base station. At hand-off, the new base station employs the steering vector derived for the previous channel to place a narrowbeam lobe for the new channel in the direction of the mobile subscriber. When the mobile subscriber responds on the new narrowbeam channel, hand-off is complete between the base stations, and the mobile transceiver thereafter communicates with the new base station as it enters into and travels through new cell. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 diagrammatically illustrates adjacent cells of a cellular communication network, and associated base stations of which employ phased array antennas for narrowbeam communications with a mobile subscriber; 
     FIG. 2 is a cellular system diagram associated with a first ‘omni’ burst embodiment of the present invention; 
     FIG. 3 sets forth a communication flow sequence associated with the ‘omni’ burst embodiment of FIG. 2; 
     FIG. 4 is a cellular system diagram associated with a second ‘sectorized’ burst embodiment of the present invention; 
     FIG. 5 is a communication flow sequence associated with the ‘sectorized’ burst embodiment of FIG. 4; 
     FIG. 6 sets forth a communication flow sequence associated with a third embodiment of the invention, in which the base station of the cell from which the mobile transceiver is about to depart determines the direction of the mobile transceiver and transmits a steering vector associated with this direction to the new base station; and 
     FIG. 7 sets forth a communication flow sequence associated with a fourth embodiment of the present invention, in which the new base station ‘pretunes’ its transceiver to the ‘old’ frequency to determine the direction of the mobile subscriber, prior to hand-off. 
    
    
     DETAILED DESCRIPTION 
     Before describing the frequency hand-off control mechanism in accordance with the present invention, it should be observed that the invention resides primarily in what is effectively a prescribed arrangement of conventional communication circuits and associated digital signal processing components and an attendant base station control program, that controls the operations of such circuits and components. Consequently, the configuration of such circuits and components and the manner in which they are interfaced with other communication system equipment have, for the most part, been illustrated in the drawings by readily understandable block diagrams, which show only those specific details that are pertinent to the present invention, so as not to obscure the disclosure with details which will be readily apparent to those skilled in the art having the benefit of the description herein. Thus, the block diagram illustrations and associated flow charts of the supervisory base station control program are primarily intended to show the major components of the system in a convenient functional grouping and processing sequence, whereby the present invention may be more readily understood. 
     As described briefly above, and as diagrammatically illustrated in FIG. 1, pursuant to a first embodiment of the invention, each cell base station  10  employs a phased array antenna  12  to which transceiver equipment  14  of the base station is coupled. The use of a phased array antenna allows the base station to controllably define its antenna coverage pattern with respect to any mobile transceiver  21 , which minimizes interference from one or more other transceivers, and thereby reduces frequency reuse distance. 
     Omni Burst (FIGS. 2 and 3) 
     A first embodiment of the frequency allocation control mechanism of the present invention will now be described with reference to the mobile transceiver cellular system diagram of FIG.  2  and the communication flow sequence of FIG.  3 . As shown in FIG. 2, a mobile transceiver  21  traveling through a first cell  20  is approaching the boundary  23  between cell  20  and a second (immediately adjacent) cell  30 . As mobile transceiver  21  travels through the cell  20 , it communicates with that cell&#39;s base station  25  over an assigned frequency channel. 
     The narrowbeam directivity pattern  27  of this channel is continually updated in the direction of the mobile transceiver  21  by means of an adjustable steering vector that controls the operation of the base station&#39;s phased array antenna  29 . As a non-limiting, but preferred embodiment, each base station&#39;s phased array antenna may be configured as described in co-pending U.S. patent application Ser. No. 09/081,476, filed May 19, 1998, by R. Hildebrand et al, entitled: “Circular Phased Array Antenna Having Non-Uniform Angular Separations Between Successively Adjacent Elements,” assigned to the assignee of the present application and the disclosure of which is incorporated herein. 
     In addition, although the steering vector algorithm is not limited to any particular implementation, non-limiting examples of the algorithm include the PSF algorithm described in U.S. Pat. No. 4,255,791 to P. Martin, entitled: “Signal Processing System,” issued Mar. 10, 1981, and the “Maximum SNR Method,” described in the text “Introduction to Adaptive Arrays,” by R. Monzingo et al, published 1980, by Wiley and Sons, N.Y. 
     Alternatively, and in accordance with a preferred embodiment of the invention, the steering vector algorithm may execute a directivity pattern control mechanism of the type described in co-pending U.S. patent application Ser. No. 09/081,287, filed May 19, 1998, by Karen Halford et al, entitled: “Bootstrapped, Piecewise-Optimum Directivity Control Mechanism for Setting Weighting Coefficients of Phased Array Antenna”, assigned to the assignee of the present application and the disclosure of which is incorporated herein. 
     In the course of base station  25  within cell  20  monitoring the quality of the link, when the quality measurement on the mobile transceiver  21  indicates that the transceiver is in the vicinity of the cell boundary  23 , base station  25  initiates the communication sequence shown in FIG. 3 with base station  35  in the ‘acquiring’ cell  30 . At step  301 , using a dedicated inter base station communication control link, base station  25  forwards a message to the base station  35  that a channel hand-off is to commence. 
     This message will contain the identification of the communication channel currently employed by the transceiver  21 . In response to this message, at step  303 , base station  35  uses one of the antenna elements of its phased array antenna  39  to transmit an omnidirectional burst on the new communication channel to which mobile transceiver  21  is to tune itself for conducting communications with the base station  35 , when the mobile transceiver enters cell  35 . In a preferred embodiment, this broadbeam energy burst is set at a transmission power level of NP, where N is ratio of the average gain of the phased array antenna to the gain of a single antenna element of the phased array antenna, and P is the power at a single element required for the energy burst to close the communication link with the mobile transceiver  21  at cell boundary  23  at the full gain of the phased array antenna. 
     At step  305 , in response to receipt of this signal, mobile transceiver  21  transmits a reply signal on the new channel. In step  307 , base station  35  monitors the (omnidirectional) burst from the mobile transceiver by means of the full reception capability of its phased array antenna  39  base station  35 . In step  309 , it processes the received signal to determine the direction of the mobile transceiver  21  relative to the second base station  35 . For this purpose, a steering vector signal processing mechanism of the type described above may be employed. 
     Having determined the direction of the mobile transceiver  21  relative to base station  35 , in step  311 , the base station  35  adjusts the directivity pattern  37  of its phased array antenna  39 , so as to place a narrowbeam pattern of the new communication channel in the direction of mobile transceiver  21 . The average power allocated to each element of the phased array antenna is equal to the above-described value of P. 
     Once it has completed hand-off of the mobile transceiver from the base station  25 , and acquired the mobile transceiver  21 , in step  313 , base station  35  proceeds to conduct narrow beam communications with the mobile transceiver  21  on the new communication channel. Using its ability to control the directivity of the narrowbeam lobe by way of its phased array antenna, base station  35  continues to communicate with and track the mobile transceiver  21  as long as the mobile transceiver is located in the cell  30 . 
     Sectorized Burst (FIGS. 4 and 5) 
     A modification of the first embodiment of the frequency allocation control mechanism of the present invention, which employs a sectorized beam, will be now described with reference to the mobile transceiver cellular system diagram of FIG.  4  and the communication flow sequence of FIG.  5 . As in the embodiment shown in FIG. 2, as a mobile transceiver  21  traveling through a first cell  20  approaches the boundary  23  between cell  20  and a second (immediately adjacent) cell  30 , the link quality measurement conducted by base station  25  or the mobile transceiver  21  will indicate that the mobile transceiver  21  is in the vicinity of the cell boundary  23 , and base station  25  will initiate the communication sequence shown in FIG. 5 with base station  35 . 
     At step  501 , using the system backhaul, base station  25  forwards a message to base station  35  that a channel hand-off is to commence. As in the first embodiment, this message will contain the identification of the communication channel currently employed by the transceiver  21 . In response to this message, at step  503 , base station  35  employs its phased array antenna  39  to transmit a ‘sectorized’ burst on the new communication channel to which mobile transceiver  21  is to tune itself for conducting communications with the base station  35 . As shown in FIG. 4, this sectorized burst is confined to a prescribed spatial sector  41  sourced from the base station  35  toward the cell  20 . As a non-limiting example, sector  41  may subtend an angle of 120°, which is effective to encompass the geographical region containing the mobile transceiver  21 . 
     The power level of this sectorized transmission is set at a transmission power level of NP/M, where N is ratio of the average gain of the phased array antenna to the gain of one antenna element of the phased array antenna, P is the power required for the energy burst to establish a communication link with the mobile transceiver  21  at the cell boundary  23  at the full gain of the phased array antenna, and M is the ratio of the angle subtended by the spatial sector to 360°. 
     At step  505 , as in the first embodiment, in response to receipt of this signal, mobile transceiver  21  transmits a reply signal on the new channel. In step  507 , base station  35  monitors the (omnidirectional) response from the mobile transceiver and then processes the received signal in step  509  to derive a steering vector representative of the direction of the mobile transceiver  21  relative to the second base station  35 . 
     Having determined the direction of the mobile transceiver  21  relative to it, in step  511 , the base station  35  adjusts the directivity pattern of its phased array antenna  39 , so as to place a narrowbeam pattern of the new communication channel in the direction of mobile transceiver  21 , just as in the first embodiment. In step  513 , with channel hand-off completed, the second base station conducts narrow beam communications with the mobile transceiver  21  on the new communication channel, and continues to communicate with and track the mobile transceiver  21  as long as the mobile transceiver is located in the cell  30 . 
     Steering Vector Passed to New Base Station (FIG. 6) 
     A third embodiment of the handoff control mechanism of the present invention, shown in the channel hand-off flow sequence of FIG. 6, begins at step  601 , in which the base station  25  of the current cell  20  from which the mobile transceiver is about to depart into adjacent cell  30  determines the direction of the mobile transceiver  21  relative to the base station  25 , and generates a first steering vector associated with this direction. In step  603 , this steering vector is conveyed as part of the hand-off initiating message conveyed from the current base station  25  to new base station  35 . 
     In response to this first steering vector message, at step  605 , the new base station  35  generates a second steering vector, representative of the direction of the mobile transceiver relative to the base station  35  for a new communication frequency channel to be used between the mobile transceiver  21  and the new base station  35 , at channel hand-off. Using this second steering vector, in step  607 , base station  35  adjusts the directivity pattern of its phased array antenna, so as to place a narrowbeam pattern of the new communication channel in the direction of mobile transceiver  21 . 
     When the mobile subscriber  21  responds on the new channel, hand-off is complete between the base stations, and the mobile transceiver  21  communicates with the second base station  35  as it enters into and travels through cell  30 . In step  609 , having completed hand-off and acquired the mobile transceiver  21 , the new base station  35  proceeds to conduct narrow beam communications with the mobile transceiver  21  on the new communication channel. Using its phased array antenna, base station  35  then continues to tune its narrowbeam channel, so as to track and communicate with the mobile transceiver  21  as long as the mobile transceiver is located in the cell  30 . 
     New Base Station Pretunes to Mobile Subscriber (FIG. 7) 
     A modification of the third embodiment of the frequency allocation control mechanism of the present invention, in which the new base station  35  ‘pretunes’ its transceiver to the ‘old’ or ‘pre hand-off’ frequency employed by the current base station  25 , in order to determine the direction of the mobile subscriber, prior to channel hand-off, is shown in the channel hand-off flow sequence of FIG.  7 . At step  701  (in anticipation of hand-off), the base station  25  of the current cell  20  from which the mobile transceiver is about to depart into adjacent cell  30  transmits an orderwire message to the new station  35 , that a hand-off is to take place. 
     In response to this pre hand-off message, at step  703 , the new base station  35  in cell  30  uses its phased-array antenna to place a narrow beam pattern (for the current channel being employed by the base station  25  and the base station  35 ), in the direction of cell  20 . In step  705 , the new base station  35  monitors the current (‘old’) channel to derive a steering vector representative of the direction of the mobile transceiver  21  relative to the base station  35 . When hand-off occurs, in step  707 , the new base station generates a steering vector for the new channel in accordance with that derived for the previous channel. In step  709 , acquiring base station  35  uses this new steering vector to place a narrowbeam lobe for the new channel in the direction of the mobile subscriber  21 . 
     When the mobile subscriber  21  responds on the new narrowbeam channel, hand-off is complete between the base stations, and the mobile transceiver  21  communicates with the second base station  35  as it enters into and travels through cell  30 . In step  711 , having completed hand-off and acquired the mobile transceiver  21 , the new base station  35  proceeds to conduct narrow beam communications with the mobile transceiver  21  on the new communication channel. Using its phased array antenna, base station  35  then continues to tune its narrowbeam channel, so as to track and communicate with the mobile transceiver  21  as long as the mobile transceiver is located in the cell  30 . 
     As will be appreciated from the foregoing description, the undesirable usurping of what could otherwise be used for a regular communication channel, as an intermediate construct channel to determine the direction of the mobile transceiver, is effectively obviated by the hand-off mechanism of the present invention, which employs the very same channels for communications between base stations and the mobile transceiver, as the mobile transceiver moves between adjacent cells, to locate the mobile transceiver relative to the base stations, so that the acquiring base station may readily place a narrowbeam channel on the mobile transceiver at hand-off. Each base station employs a phased array antenna, which allows the base station to controllably define its antenna coverage pattern with respect to any mobile transceiver, so as to minimize interference from one or more other transceivers, and thereby reduce frequency reuse distance.