Abstract:
Briefly, the present invention provides a dynamic channel re-assignment capability between mobile units, base stations and sectors within base station coverage areas. The wireless devices used in the present invention may include impulse radio communication devices such as, for example ultra-wideband radio (also known as digital pulse wireless) communication devices. Ultra-wideband bandwidth and channel allocation can be effectively managed, even though link quality generally deteriorates near the outer boundary of the base station. By maintaining dual communications with an adjoining base station, the present invention reduces the bit error rate and maintains signal strength (e.g., RF signal strength). This procedure is termed a “soft-handoff”.

Description:
Priority is claimed from U.S. provisional patent application Ser. No. 60/255,469, filed on Dec. 14, 2000, entitled “Ultra-Wideband Communication System and Method”, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of ultra-wideband communication systems. More particularly, the present invention relates to hand-offs between cell sites in an ultra-wideband communication system. 
     BACKGROUND OF THE INVENTION 
     Wireless communication systems are changing the way people work, entertain, and communicate. For example, portable phones and other mobile devices have enabled highly mobile individuals to easily communicate. Such devices can transmit and receive both voice and data signals. As more features are added to these mobile wireless devices, users are able to receive a wider variety of information. This enhances the user&#39;s entertainment and more efficiently solves the user&#39;s business problems. 
     Data, such as computer files, graphics, video, and music may be sent from a remote location and received by mobile wireless devices located throughout a large (or “wide”) area. Such wide area uses generally require a series of fixed transceivers arranged to communicate with the mobile wireless devices. The wireless device is able to communicate only as long as it remains in contact with at least one of the transceivers. 
     While the use of such wide area systems is expanding, the use of local wireless communication systems is also growing. A local wireless communication system, for example, may configure the wireless devices in a single building, such as a residence, to share information. Such local wireless communication systems may enable computers to control peripherals without physical connections, stereo components to communicate, and almost any appliance to send and receive information to the Internet. 
     The amount of data being sent on both wide area and local communication systems is mushrooming, and it may quickly exceed the bandwidth available in the traditional communication bands. A relatively new communication technology (termed “ultra-wideband” technology) may provide assistance in meeting the ever-increasing bandwidth demands. An example of ultra-wideband technology is the communication system using an impulse radio system that is disclosed in U.S. Pat. No. 6,031,862, entitled “Ultra-Wideband Communication System and Method”. Impulse radio uses individually pulsed monocycles emitted at fractions of nanosecond intervals to transmit a digital signal. For many applications, the pulses are transmitted at extremely low power density levels, for example, at less than −30 dB. The generated pulses are so small that they typically exist in the noise floor of other more traditional communication systems. 
     Ultra-Wide band communication systems enable communication at a very high data rate, such as 100 megabits per second or greater, when operated in a small local area. Ultra-Wideband systems, however, must operate at extremely low power, typically transmitting signals at the noise level. These systems must operate at low power because they need to avoid interfering with the more established communication frequencies. The low power requirement restricts the size of each ultra-wideband cell. Thus, ultra-wideband cells generally are smaller than the cells in the more traditional continuous wave or carrier based systems. 
     The relatively small size of a cell in an ultra-wideband communication system necessitates a relatively dense placement of base station antennas. This high density of antennas may, under some circumstances, lead to cross-talk between the channels assigned to different users. This is especially true if the users are highly mobile. In this case, they will often travel across cell boundaries where the signals of two or more base stations overlap. Since this event will be relatively frequent with such small cells, user channels must be geographically separated to minimize the occurrence of channel interference. For example, if a particular channel is used in a cell, that channel should not be used in any other cell within several miles. Accordingly, since only relatively few of the communication channels can be allocated to each cell, the reuse distance determines the total capacity of the overall cell communication system. 
     The utilized bandwidth in conventional cells varies as a function of user demand. Since user demand can vary greatly from one time period to another, there are likely to be times when a particular cell is greatly under-utilized. There are also likely to be other times when that same cell is saturated, thereby causing undesirable drops in transmissions, connection refusals, and quality degradation. When a cell&#39;s bandwidth utilization exceeds system quality standards in a conventional communication system, the system operator typically will add another cell in the area to move some of the user traffic from the over-utilized cell to the new cell. Adding cells and antennas, however, can be a costly and time-consuming process. 
     Although ultra-wideband technology has the ability to decrease the impact of multipath interference, it is still subject to attenuation of the received signal as the signal passes between transmitter and receiver. For a point RF source, received signal strength varies as the inverse of the squared distance for open line of sight communications. In cluttered and mobile environments, the attenuation is more closely proportional to the inverse of the fourth power of the distance. This is due to multipath cancellation, which is present even in ultra-wideband signals. In either scenario, the attenuation of the signal can decrease the signal level to a value that is unsuitable for reliable data transfer. 
     Due in part to the deficiencies described above, conventional ultra-wideband communication systems risk poor quality of service, especially as a mobile unit moves from one location to another. Such systems also do not enable entirely efficient utilization of bandwidth and system resources. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide mobile ultra-wideband methods and devices for effectively linking and maintaining an acceptable level of service and coverage while simultaneously handling multiple data streams and multiple users. It is also an object of the present invention to efficiently utilize bandwidth and system resources. To meet the stated objectives, and to overcome or greatly alleviate the disadvantages in known ultra-wideband units, the present invention provides methods, systems, software and related devices for performing a “soft hand-off” between and within ultra-wideband cells. 
     Briefly, the present invention provides a dynamic channel re-assignment capability to mobile units, base stations and sectors within base station coverage areas. The wireless devices may include impulse radio communication devices such as ultra-wideband radio (also known as digital pulse wireless) communication devices. The bandwidth and channel allocation of these devices and sectors can be effectively managed with the present invention despite the fact that link quality generally deteriorates near the outer boundary of the base station. This effective management is achieved by having the mobile unit maintain dual communication with a linked base station and an adjoining base station. The present invention thereby reduces the bit error rate and maintains signal strength (e.g., RF signal strength). This dual communication procedure is termed a “soft-handoff”. 
     In the present invention the mobile units and base stations constantly monitor both signal strength and the bit error rate in order to determine whether there is a need for a hand-off. When the data integrity of a mobile unit drops below a minimum acceptable bit error rate (BER), and/or the signal strength drops below a pre-determined minimum acceptable level, a soft hand-off that maintains acceptable service will be initiated.  FIG. 1  lists the typical minimum acceptable bit error rates for video, audio and data. 
     Advantageously, the present invention efficiently insures that a soft hand-off is performed for a mobile device as it moves from one location to another. This greatly enhances the desirability of the associated ultra-wide band system by minimizing or eliminating interruptions in communication. High quality communication is thus maintained and at the same time the ability to accommodate additional traffic is provided. 
     In one aspect the present invention features a method for performing a soft hand-off in a cellular communication system (preferably a code-based cellular communication system) and a corresponding computer program product. The method involves the steps of: (a) monitoring signal strength and the bit error rate from a primary source (preferably an ultra-wideband primary source); (b) monitoring signal strength and the bit error rate from a secondary source (preferably an ultra-wideband secondary source); (c) comparing the strength of the signal and the bit error rate from the primary source to the strength of the signal and the bit error rate from the secondary source; and (d) transferring data reception and transmission from the primary source to the secondary source when the strength of the signal from the secondary source is greater than the strength of the signal of the primary source, or when the bit error rate of the secondary source is less than the bit error rate of the primary source, or when either signal strength or bit error rate is below a pre-determined level. Monitoring signal strength may involve determining signal strength and storing the information in memory. 
     In one embodiment, the hand-off is from a first base station to a second base station to a mobile unit. In this case: (a) the first base station is linked to the mobile unit and selects an adjoining second base station; (b) the first base station contacts the second base station to request initial hand-off sequence; (c) the second base station acknowledges the request, provides a channel assignment to the mobile unit and links to the mobile unit; (d) the mobile unit transmits a hand-off release to the first base station; and (e) the first base station releases the mobile unit and completes the soft hand-off. 
     In another embodiment, the soft hand-off is from a mobile unit to a first base station to a second base station. In this case: (a) the mobile unit is linked to the first base station and detects an increase in bit error rate and/or a reduction in signal strength; (b) the mobile unit sends a request to the first base station for a hand-off; (c) the first base station receives the request, selects the second base station and contacts the second base station to request an initial hand-off sequence; (d) the second base station acknowledges the request for an initial hand-off sequence; (e) the second base station contacts the mobile unit, provides a channel assignment to the mobile unit and links to the mobile unit; (f) the mobile unit transmits a hand-off release request to the first base station; and (g) the first base station releases the mobile unit, and thereby completes the soft hand-off. 
     In still another embodiment, the soft hand-off involves dynamic power range linking. In this embodiment: (a) a mobile device is linked to a first base station and requests the position of a plurality of base stations; (b) the plurality of base stations reply; (c) the mobile unit determines and stores the location of each of the base stations; (d) each of the base stations transmits an associated rating to the mobile device; (e) the mobile device calculates the data integrity of each base station and establishes a link with a base station having the highest data integrity; and (f) the mobile device transmits a link curtailment to the first base station. 
     In another aspect, the invention provides a method for performing a soft hand-off in a code-based cellular communication system. The soft hand-off is from a first mobile unit to a second mobile unit to a base station. The method involves the steps of: (a) monitoring signal strength and the bit error rate from a first base station and determining that either in unacceptable; (b) attempting to locate an adjacent base station with an acceptable signal strength and bit error rate and determining that no adjacent base station has an acceptable signal strength and bit error rate; (c) transmitting a hand-off request from a first mobile device that is linked to the first base station to a second mobile device; (d) receiving a response from the second mobile device; and (e) using the second mobile device as a temporary repeater to pass data to a second base station. 
     In yet another aspect, the present invention provides an adaptive link controller. The adaptive link controller includes: (a) logic for monitoring the signal strength and bit error rate of a mobile unit and a plurality of base stations; (b) logic for performing dual link coordination and maintenance with a linked base station and a hand-off base station; and (c) logic for performing hand-off initiation and link curtailment. 
     For each of the methods of the invention described above, a corresponding computer program product is also provided. The invention also features ultra-wideband code based cellular communications systems capable of performing each of the methods of the invention. The present invention also features mobile units and base stations that are configured and structured to operate in such systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The nature, goals, and advantages of the invention will become more apparent to those skilled in the art after considering the following detailed description when read in connection with the accompanying drawing in which like reference numerals identify like elements throughout, wherein: 
         FIG. 1  lists typical minimum acceptable bit error rates for video, audio and data in accordance with the present invention; 
         FIG. 2  shows base station architecture with overlapping coverage in accordance with the present invention; 
         FIG. 3  shows a single base station with connectivity to six other base stations for handoff and channel coordination in accordance with the present invention; 
         FIG. 4  shows sectorization at an ultra-wide band base station in accordance with the present invention; 
         FIG. 5  is a flowchart for scenario one (a soft hand-off from base station to new base station to mobile unit) in accordance with the present invention; 
         FIG. 6  is a flowchart for scenario two (a soft hand-off from mobile unit to base station to new base station) in accordance with the present invention; 
         FIG. 7  is a flowchart for soft handoff scenario number three that performs dynamic power range linking in accordance with the present invention; 
         FIG. 8  is a power range linking model for soft handoff with a mobile unit leaving the coverage of base station  2  in accordance with the present invention; 
         FIG. 9  shows scenario four (a soft hand-off from mobile unit to mobile unit to base station with emergency geo-locating) in accordance with the present invention; 
         FIG. 10  is a flow chart for soft hand-off procedures for scenarios one through four in accordance with the present invention; and 
         FIG. 11  depicts an adaptive link controller in accordance with the present invention. 
     
    
    
     It will be recognized that some or all of the figures may be schematic representations for purposes of illustration and do not necessarily depict the actual relative sizes or locations of the elements shown. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following paragraphs, the present inventions will be described in detail by way of example with reference to the attached drawings. Throughout this description, the preferred embodiment and examples shown should be considered as exemplars, rather than as limitations on the present invention. As used herein, “the present invention” refers to any one of the embodiments of the invention described herein. 
     I. Soft Hand-Off and Mobile Ultra-Wideband Dynamic Linking Architecture 
       FIG. 2  illustrates a preferred base station  10  hexagonal topology  100  that will provide overlapping coverage  30  for ultra-wideband base stations  10 . Other configurations may also be used, such as a micro pico-network on a daisy chain backbone, placed like emergency call boxes along a highway, for separate routing of signals, but this configuration will allow an effective soft hand-off, which is essential for code-based cellular communications. Base station topologies can be hexagonal or linear. The hexagonal topology covers disperse user density environments whereas the linear topology mainly covers linear features such as roads. The concept is the same but the number of sectors per base station differs. Given this preferred arrangement, or other suitable arrangements, the present invention may be used in conjunction with the methods, devices, and systems described in U.S. patent application, number to be assigned, entitled “Ultra Wideband Communication System And Method”, filed Dec. 13, 2000, which is incorporated herein by reference in its entirety. 
     The overall base station  10  architecture  100  includes a plurality of base stations  10 . Each base station  10  has an associated coverage area  20 , for example a substantially circular coverage area  20  as shown in FIG.  2 . The coverage areas  20  of adjacent base stations  10  may overlap, thereby creating overlapping coverage areas  30 . The spacing and configuration of the base stations  10  and the size and shape of the coverage areas  20  will determine the size and shape of the overlapping coverage areas  30 . In the hexagonal configuration shown in  FIG. 2 , each base station  10  on the interior of the architecture  100  has six adjacent base stations  10 , while each base station  10  on the exterior of the architecture  100  has three adjacent base stations  10 . 
       FIG. 3  shows a single base station  10  with connectivity  40  to six other adjoining base stations  10 . The station in the center  60  links with the other base stations  10  to coordinate allocated channels so that adjoining base station  10  sectors  50  do not use the same channels. The suitability of the channels may be managed using the methods and devices described in U.S. patent application Ser. No. 09/746,348 entitled “System for Pre-testing and Certification of Multiple Access Codes”, filed Dec. 21, 2000, which is incorporated herein by reference in its entirety. 
     The linked architecture  200  also provides an inter-connected communications system that is necessary to provide a “soft hand-off” as a mobile unit  70  moves from the coverage area of one base station  10  to the coverage area of another. Mobile unit  70  may be a handheld-type mobile device. Alternatively, it may be an ultra-wideband component in a mobile phone, a mobile internet device, a portable radio, a personal data assistant, a desktop computer or appliance located in a home, an automobile, or office environment or a device for similar applications. 
     During the “soft hand-off”, the mobile unit  70  will maintain a link with both base stations  10  until the hand-off is complete.  FIG. 3  also demonstrates the “sectorization” within the coverage area  20  of the base station  10 . Each base station  10  is sub-divided into six coverage sectors  50 . The sub-division provides greater bandwidth management in the base station&#39;s coverage area  20 . 
       FIG. 4  demonstrates that when a mobile unit  70  passes from one sector  50  to another  80  within the coverage area  20  of a single base station  10 , the base station  10  will complete a “soft hand-off” as channel re-assignment is accomplished. In this case, the base station  10  will allocate a new channel for the mobile unit  70  as it moves into another sector  80 , and will maintain the current allocated channel until the hand-off is complete. 
     II. Soft Hand-Off Scenarios 
     A. Scenario #1: Base Station to Receiving Base Station to Mobile Unit 
       FIG. 5  illustrates a preferred soft hand-off technique  500 . In step  510 , a base station  10  detects an increase in bit error rate and/or a reduction in signal strength (e.g., RF signal strength) using known methods. The base station  10  that is linked to the mobile unit  70  monitors the bit error rate and signal strength between all of it&#39;s linked mobile units  70 , as well as their relative geo-locations. When the bit error rate exceeds the acceptable bit error rate limit or the signal strength drops below the predetermined acceptable level, the base station  10  initiates the hand-off with an adjoining base station  10 . The hand-off procedure begins with the linked base station  10  selecting the most suitable adjoining base station  10 . This selection is based on the calculated relative geo-positional data (obtained using known methods or as described herein) that the currently linked base station  10  has for the mobile unit  70  in relation to the best suited base station  10  within the hexagonal coverage scheme. 
     In step  520  the currently linked base station  10  (base station  810 ) contacts the selected adjoining base station  10  (base station  820 ) to request an initial hand-off sequence. Step  530  determines if the first adjoining base station  10  has replied using known methods. If not, then in step  540  base station  810  contacts the next available base station  10  in the hexagonal coverage scheme. Alternatively, if the first adjoining base station  10 , here base station  820 , has replied, then the initial hand-off sequence is given by base station  820  in step  550 . This initial hand-off sequence consists of the acknowledgment to base station  810  for requested hand-off, the channel assignment selection for the mobile unit  70  and the initial contact with the mobile unit  70  by base station  820 . At this time the mobile unit  70  is in communication with both base station  810  and base station  820 . 
     After base station  820  links with the requesting mobile unit  70  in step  560  and both calculate and “acceptable” bit error rate and signal strength, the mobile unit  70  will transmit in step  570  a hand-off release request to base station  810 . In step  580 , base station  810  then releases the mobile unit  70 , and in step  590  the soft hand-off is complete. 
     B. Scenario #2: Initiation by Mobile Unit in Contact with Base Station 
       FIG. 6  illustrates that when a mobile ultra-wideband unit  70  moves from one coverage area  20  to another, the hand-off process happens in multiple steps.  FIG. 6  illustrates the process  600  when a base station  10  or mobile unit  70  detects, in step  620 , an increase in bit error rate and/or a reduction in signal strength. In step  630 , the mobile unit  70  initiates the hand-off request to the linked base station  810  if the mobile unit  70  detects the quality of service changes. When the bit error rate has exceeded the acceptable limit (e.g., see  FIG. 1 ) or the signal strength drops below the predetermined acceptable level, the mobile unit  70  sends a request in step  640  to the linked base station  10  for a hand-off to an adjoining base station  10 . 
     After the linked base station  10  has received the request, the hand-off procedure starts with the linked base station  10  selecting the most suitable adjoining base station  10 . This selection is based on the base station  10  sector  50  occupied by the mobile user or, as in step  650 , the known geo-location of the mobile unit  70  in relation to the best suited base station  10  within the hexagonal coverage scheme. Thus, at this time the base station  10  has, from step  650 , an up-to-date geo-location on the mobile unit  70 , and the six nearest neighbor base stations  10 . 
     In step  640 , the currently linked base station  10  (base station  810 ) contacts the selected adjoining base station  10  (base station  820 ) that it predicts to be within range of the mobile unit  70  (based on the mobile&#39;s position and direction) to request an initial hand-off sequence. Step  660  determines if the new base station  10  is able to take the mobile unit  70 . If not, then in step  665  base station  810  contacts the next closest base station  10  in the hexagonal coverage scheme. 
     If base station  820  can accept the mobile unit  70 , it sends an acknowledgement in step  670  to base station  810  and proceeds to link with the mobile unit  70 . In step  670 , the initial hand-off sequence by base station  820  includes the acknowledgment to base station  810  for requested hand-off, the channel assignment selection for the mobile unit  70  and the initial contact with the mobile unit  70  by base station  820 . At this time the mobile unit  70  is data linked to both base station  810  and base station  820 . The mobile unit  70  will remain linked to base station  810  until a confirmed Quality of Service (QOS) link with base station  820  or another base station  10  is established. 
     After base station  820  links with the requesting mobile unit  70 , a dialogue ensues that leads to a calculation of a bit error rate and signal strength. If this bit error rate and signal strength are better than that achieved through base station  810 , as determined in step  675 , then in step  680  base station  820  will link with the mobile unit  70  and establish a data channel. Then, the mobile unit  70  will, in step  685 , transmit a hand-off release request to base station  810 . Base station  810  then releases the mobile unit  70  in step  690 , and, as shown in step  695 , the soft hand-off is complete. 
     C. Scenario #3: Mobile Ultra-Wideband Dynamic Power Range Linking 
       FIG. 7  demonstrates the principles of the ultra-wideband dynamic power range linking and soft hand-off technique  700 . In this technique a mobile ultra-wideband unit  70  can determine and select a base station  10  that will provide optimum signal integrity. The process includes not only the initial “handshake” with the base station  10  providing the mobile unit  70  with optimum capability, but also provides a coordinated “soft hand-off” as required with the previously linked base station  10 . 
     In step  710  a mobile device  70  is linked to a first base station  10  and requests the position of a plurality of base stations  10 . In steps  715  and  725  it is determined if the plurality of base stations  10  reply. Once the plurality of base stations  10  reply, then in steps  730  and  735  the mobile unit  70  determines and stores the location of each of the base stations  10 . The determination of the location may be done by conventional triangulation or it may be done using the methods and/or devices described in U.S. patent application Ser. No. 09/745,498, entitled “Establishing Geopositional Coordinator Using Third-Party UWB Devices”, filed Dec. 22, 2000, which is incorporated herein by reference in its entirety. 
     In steps  740  and  745  the projected bit error rate (PBER) algorithms are performed. In steps  750  and  755  each of the base stations  10  transmits an associated rating to the mobile device  70 . In step  760  the mobile device  70  calculates the data integrity of each base station  10  and establishes a link with a base station  10  having the highest data integrity. In step  770  the mobile device  70  transmits a link curtailment to the first base station  10 . 
     This process is accomplished in a fashion that is transparent to the mobile user.  FIG. 8  illustrates a mobile ultra-wideband device  70  that is within transmission range of two ultra-wideband towers  810  and  820  and is in a location that would benefit from a hand-off. In this diagram, the transmission distance to base station  810  is a 1 , and the transmission distance to base station  820  is b 1 . The mobile ultra-wideband device  70  will require less power (e.g., RF power) to transmit to base station  820  at distance b 1  than to base station  810  at distance a 1 . 
     The present invention additionally manages system bandwidth by restricting power levels, such as RF power levels, in mobile ultra-wideband devices  70  to the smallest amount necessary to maintain a data link with an acceptable level of service. In  FIG. 8 , the limited RF power output would keep the mobile unit  70  primarily within the broadcast range of base station  820 . Since the mobile ultra-wideband device&#39;s transmission is limited to base station  820 , only bandwidth from base station  820  is utilized and no adjoining base station&#39;s bandwidth is encumbered. As the mobile ultra-wideband  70  moves away from the base station  10 , the bit error rate will increase and the signal strength will drop to the pre-determined point where data is unacceptable and a soft hand-off will be initiated. Power level can be further minimized by combining the present invention with the technology described in U.S. patent application Ser. No. 09/677,082, entitled “Communication System”, filed Sep. 29, 2000, which is incorporated herein by reference in its entirety. 
     D. Scenario #4: Mobile Unit to Mobile Unit to Base Station 
       FIG. 9  illustrates the process when a mobile unit  70  detects an increase in bit error rate and/or a reduction in RF signal strength. The mobile unit  70  also has been unable to contact another base station  10  directly and the currently linked base station  10  (base station two ( 820 ) in this case) is unable to initiate a hand-off routine to another adjoining base station  10 . In this case the mobile unit  70  (mobile unit two ( 920 )) initiates the “mobile unit to mobile unit link hand-off request” to any mobile unit  70  that will respond. The mobile unit  70  that responds to this request (mobile unit one ( 910 )) will then perform as a temporary repeater to pass mobile unit  2  ( 920 ) geo-location data to base station one ( 810 ). This may be useful, for example, for emergency  911  linking. 
     III. Hand-off Procedure Cycle 
       FIG. 10  shows the procedural flow  1000  for conducting a soft hand-off. This shows the four scenarios and the situations in which they would be conducted. When the bit error rate or the signal strength has reached the level where the quality of service is no longer acceptable, the base station  10  or the mobile unit  70  will initiate the procedures for a hand-off to an adjoining base station  10 . Due to various field conditions, both the base station  10  and the mobile unit  70  have the individual capability to “request” a hand-off to another base station  10  to maintain quality of service. In the event that a mobile unit  70  cannot contact a base station  10 , including the previous linked base station  10 , the mobile unit  70  will conduct a “Power Range Linking” procedure. This procedure will locate and link with a neighboring mobile unit  70  for the purpose of using the contacted mobile unit  70  as a temporary repeater and as an emergency “911” link back to a base station  10 . 
     The overall scheme  1000  begins in step  1010  when the mobile unit  70  link to base station  10  degrades, either due to an increased bit error rate or a signal decrease. In step  1020  it is determined whether the base station  10  detects the degraded link. If yes, then the hand-off procedures are initiated in step  1040 . If no, then in step  1030  the mobile unit  70  detects the degraded link. 
     Then, in step  1050 , it is determined whether the mobile unit  70  has successfully conducted a soft hand-off through the linked base station  10 . If yes, then the hand-off procedures are initiated in step  1040 . If no, then in step  1060 , the mobile unit  70  initiates power range linking. 
     In step  1070  it is determined whether the mobile unit  70  has conducted a soft hand-off through power range linking. If yes, then the hand-off procedures are initiated in step  1040 . If no, then the mobile unit  70  initiates power range linking with the nearest mobile unit  70  for an emergency repeater link to a base station  10 . 
     IV. Adaptive Link Controller 
     An adaptive link controller (ALC) is structured to provide at least one of the following acts at a mobile unit, a base station or some combination thereof: (a) constantly monitoring the Bit Error Rate (BER); (b) after reaching a predetermined threshold, searching active cells for a link with a greater signal strength based on a minimum acceptable level; (c) monitoring signal strengths of other signals within the cell; (d) maintaining a two-way link between base stations and mobile units in hand-off process; (e) performing hand-off request and hand-off actions; (f) perfoming emergency link mangagement; (g) creating emergency message set; (h) performing an overall data link coordination; and/or (i) performing vector manipulation beam tracking. 
     Thus, for example, the ALC may provide the hand-off coordination and execution. As indicated in steps  1110  and  1120 , the adaptive link controller monitors the bit error rate and the overall “quality of service” in both the mobile ultra-wideband unit  70  and the base stations  10 . When the channel quality has dropped below acceptable level and the quality of service is diminished, the adaptive link controller in either the base station  10  or the mobile unit  70  performs the steps necessary to conduct a hand-off. 
       FIG. 11  illustrates how the adaptive link controller monitors the channel quality with the assigned mobile units  70  and the adjoining base stations  10 . As a mobile unit  70  traverses through the coverage area  20  of a base station  10 , the mobile unit&#39;s adaptive link controller maintains the status of the link with the base station  10 . Additionally, in step  1130  the base station  10  adaptive link controller monitors the link status of the mobile units  70  in its coverage area  20 , as well as the “local” adjoining base stations  10  that would be used for hand-off. The adaptive link controller provides and maintains a dual link (i.e., the mobile unit  70  is in communications with two base stations  10  on two separate channels) during the hand-off process. In step  1140  the adaptive link controller initiates a link curtailment after the hand-off has effectively transferred control to the receiving base station  10 . 
     While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.