Abstract:
A radio local loop system which uses an improved dynamic channel selection scheme whereby the fixed subscriber units can automatically select the available traffic channel that exhibits the best signal quality characteristics for establishing a connection. The radio system has fixed subscriber units which measure received signals from radio stations and determine a signal quality factor for each received signal. The fixed subscriber units will create and maintain a traffic channel register that stores the frequency, time slot, radio station number, and fixed subscriber unit integer scan angle for each signal. The fixed subscriber units are assigned an available radio channel and antenna scan angle based upon the signal quality.

Description:
This application claims priority under 35 U.S.C. §120 to provisional application Ser. No. 60/109,705, filed on Nov. 24, 1998. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to radio telecommunications systems, and more particularly, to radio local loop systems. 
     2. Brief Description of the Related Art 
     A radio local loop system (“RLL”) is a wireless telecommunications system, wherein fixed subscriber units or terminals communicate with the system over an air interface. Such radio systems are connected to private or public switched telephone networks and include a number of radio domains, each radio domain containing one or more radio stations (RS), or base stations. Each radio station controls the wireless communication links with any number of fixed subscriber units located in a corresponding geographical coverage area. A control unit for each radio domain stores and maintains a subscriber list containing the identification codes for each fixed subscriber unit assigned to that radio domain. 
     A fixed subscriber unit is typically either immobile or limited in its ability to be moved during operation (e.g., as is the case with a cordless telephone). All communication with the fixed subscriber unit is handled through a radio station servicing a corresponding coverage area in which the fixed subscriber unit is located. The fixed subscriber unit has a transceiver and an antenna for transmitting and receiving telecommunications data to and from the radio station via the air interface, over at least one pre-assigned radio channel, wherein a radio channel is defined by any number of different channel access schemes. 
     One such channel access scheme is known in the art as time division multiple access (TDMA). In a TDMA based system, such as a TDMA based RLL system, each of a number of frequency carriers is subdivided into a number of time slots. By subdividing each frequency carrier into multiple time slots, the traffic capacity of the system is substantially increased as each of a number of fixed subscriber units are able to communicate over a single frequency carrier by limiting the time during which each transmits or receives data and control information to one or more assigned time slots. 
     A TDMA based system may further be characterized as either a time division duplex (TDD) system or a frequency division duplex (FDD) system. In a TDMA/FDD system, each frequency carrier is subdivided into time slots as described above. However, certain frequency carriers are dedicated to carrying downlink traffic (i.e., data and/or control information being transmitted from a radio station to a fixed subscriber unit), while other frequency carriers are dedicated to carrying uplink traffic (i.e,. data and/or control information being transmitted from a fixed subscriber unit to a corresponding radio station). In contrast, each frequency carrier handles both uplink and downlink traffic in a TDMA/TDD based system, such that approximately half of the time slots associated with a given frequency carrier are predesignated for carrying downlink traffic, while the remaining time slots associated with that frequency carrier are predesignated for carrying uplink traffic. A RLL system that employs the well-known Digital Enhanced Cordless Telecommunications (DECT) standard is an example of a TDMA/TDD based system. 
     In recent years, the demand for wireless radio communication services, and in particular, fixed radio communication services, has increased at an extraordinary rate. This is problematic because radio network resources are generally limited, thereby limiting both the geographic area that a system is capable of covering and limiting the amount of traffic (i.e., the traffic load) that a system is capable of handling. Certainly, one way to address this problem would be to construct new networks and/or to expand existing networks; however, such a solution would be extremely expensive. 
     An alternative solution to these and other related problems has been to increase the maximum range (i.e., the maximum operating distance between a fixed subscriber unit and a radio station), thereby increasing coverage area, by increasing the gain factor G of the antenna associated with each of the fixed subscriber units, wherein range is generally determined by the following relationship:        RANGE   =     G     (     T   +   I     )                              
     and wherein T is the noise temperature at the receiver and I represents interference. The gain factor G can be increased in a number of different ways. First, the gain factor G can be increased by simply boosting transceiver power. Unfortunately, this is generally an unacceptable option because boosting power is likely to result in a corresponding increase in the interference level in the geographic coverage area, as well as neighboring geographic coverage areas. Moreover, in accordance with the relationship above, an increase in interference levels would actually have the effect of limiting range. 
     The gain factor G can also be increased through the use of directional antennas. Directional antennas achieve a greater gain factor G by producing a significantly more narrow antenna beam. The use of directional antennas to generally increase the coverage area of an RLL system is a more desirable option than boosting transceiver power because it does not typically lead to increased interference levels. However, there are other problems associated with the use of directional antennas. As the transmit and receive antenna beams are generally more narrow, the task of directing (i.e., steering) the antenna beams so that they are accurately pointing in the direction of the radio station is somewhat more difficult. 
     Presently, the use of directional antennas requires that highly trained personnel install or perform regular adjustments on fixed subscriber units to insure that the antennas are, in fact, accurately pointing toward the corresponding radio station. And, as one skilled in the art will readily appreciate, this is extremely expensive, particularly if the RLL system is constantly undergoing network reconfiguration and/or network expansion to include the addition of new radio stations. Accordingly, in a fixed RLL system, it would be desirable to have fixed subscriber units that employ directional antennas but without the need to perform complex installation and/or readjustment procedures, or the expense associated therewith. 
     SUMMARY 
     The present invention generally relates to a method and/or communication system that involves the dynamic selection of communication channels by fixed subscriber units in a RLL system, wherein the fixed subscriber units employ directional antennas whose scan angle (i.e., the angle representing the direction in which radio frequency energy is being transmitted and received through the directional antenna) can be automatically adjusted and then selected as part of the dynamic channel selection process, and wherein the communication channel over which a fixed subscriber unit communicates is defined not only by the radio station through which it communicates, the frequency carrier over which it communicates, and the one or more time slots during which it communicates, but also by the scan angle of the directional antenna. 
     In accordance with one aspect of the present invention, the communication system includes a radio station and a terminal. The terminal includes an antenna through which said terminal transmits and receives signals with a radio station while varying the scan angle of the antenna. The system also has means for determining a quality factor for each signal. Then, based on the quality factor at each scan angle, the system can select a communication channel over which the terminal can communicate with said radio station. 
     In accordance with an additional aspect of the present invention, the communication system includes a terminal and a plurality of radio stations. The terminal has an antenna whose scan angle can be automatically varied. The terminal measures received signals from the plurality of radio stations and determines a quality factor for each communication channel defined by a frequency carrier, time slot, antenna scan angle, and/or radio station combination. Based on the signal quality factor at each antenna scan angle, the terminal selects a communication channel over which said terminal can communicate with one or more radio stations. 
     In accordance with a further aspect of the invention, the improved method for performing dynamic channel selection includes the steps of transmitting and receiving signals from a radio station to a terminal while varying the scan angle of an antenna associated with the terminal, determining a quality factor for each communication channel while the transmit and receive antenna scan angle is varied, and then selecting a communication channel over which said terminal can communicate with said radio station based on the signal quality factor. 
     The present invention provides advantages over the prior art. First, this improved dynamic channel selection scheme allows the subscriber unit to automatically select an available traffic channel that exhibits the best signal quality characteristics for establishing a connection. Second, the improved dynamic channel selection scheme can be used in a communication system using narrow beam antennas. Thus, the cost efficiency from increased radio station range will not be lost since the installation or adjustment of antennas is not limited to highly trained personnel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described in greater detail with reference to the accompanying drawings, in which like elements bear like reference numerals, and wherein: 
     FIG. 1 illustrates a radio local loop system; 
     FIG. 2 is a block diagram of a fixed subscriber unit according to an exemplary embodiment of the present invention; 
     FIG. 3 illustrates a DECT frame structure; 
     FIG. 4 illustrates stored data in a fixed subscriber unit; 
     FIG. 5 illustrates stored data in a fixed subscriber unit, wherein the stored data includes antenna scan angle; 
     FIG. 6 illustrates “k” different scan angles for a directional antenna associated with a fixed subscriber unit; and 
     FIG. 7 is a flowchart showing the method of assigning a subscriber unit based on measured radio signals according to an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates the configuration of a typical radio local loop (RLL) system  10 . As shown in FIG. 1, the coverage area associated with the RLL system  10  is divided into smaller, adjoining geographical areas, herein referred to as radio domains  12 . FIG. 1 also illustrates that each radio domain  12  contains a control unit  34 , wherein the control unit connects the corresponding radio domain with a public switch telephone network  40 . In addition, the control unit  34  maintains, among other things, a subscriber list  36 , which identifies all of the fixed subscriber units (FSU) assigned to the corresponding radio domain  12 . Each radio domain  12  also includes one or more radio stations  14  which are linked to the control unit  34 , typically over a wireline connection, and linked to a plurality of FSU  26  over a wireless or air interface. Although the RLL system  10  is illustrated as including three radio domains  12  which, in turn, contain two radio stations  14 , it will be understood that the RLL system may include more than or fewer than three radio domains, while each radio domain  14  may contain more than two radio stations or as few as one radio station. 
     The FSUs  26 , as stated above, communicate with a corresponding radio station  14  over a wireless interface. Accordingly, each FSU  26  has a transmit and receive antenna  32  which, in accordance with conventional practice, have been manually installed and/or adjusted so that they effectively point in the general direction of a corresponding radio station  14 . In addition, each FSU  26  is associated with one or more communication devices, for example, cordless telephones  30 , which are connected to the FSU via a socket  28 . However, it will be understood by those skilled in the art that the communication devices may include devices other than cordless telephones, such as computer terminals, fax machines and the like. 
     FIG. 2 depicts the basic components in a typical FSU  26 . As shown in FIG. 2, a typical FSU includes, among other features, a transceiver  20 , a CPU  18 , a memory  22  and a channel selector  24 , the functions of which will be described in greater detail below. 
     In order for the FSU  26  and the various radio stations  14  to communicate with one another over a wireless interface, a channel access scheme is required, such as the exemplary TDMA/TDD channel access scheme illustrated in FIG.  3 . As illustrated in FIG. 3, the exemplary TDMA/TDD channel access scheme has ten frequency carriers, wherein each of the ten frequency carriers is divided into time frames, and wherein each time frame is further divided into a number of time slots, for example, 24 time slots. As the channel access scheme in FIG. 3 is a TDD based scheme, one skilled in the art will appreciate that half, or approximately half, of the time slots (e.g., 12 time slots) associated with each of the ten frequency carriers are set aside for downlink (i.e., from radio station to terminal) communication, while the remaining time slots associated with each of the ten frequency carriers are set aside for uplink (i.e., from terminal to radio station) communications. 
     In general, the ten frequency carriers are divided amongst the radio stations  14  in each radio domain  12 . For example, if the radio domain  12  has two radio stations  14 , the first radio station may be assigned frequency carriers  1 - 5  for use in communicating with a number of corresponding FSU, while the second radio station may be assigned frequency carriers  6 - 10 . Each of the various FSU then receives data and control information from a corresponding radio station  14  during an assigned downlink time slot and transmits data and control information to the corresponding radio station  14  during an assigned uplink time slot associated with one of the frequency carriers assigned to that radio station  14 . It will be understood, however, that if the traffic load is relatively low, a FSU may be permitted to communicate with its corresponding radio station  14  over more than one frequency carrier and/or more than one uplink and downlink time slot. Accordingly, the communication channel linking a particular FSU and a particular radio station  14  is defined by: i) an identifier code that uniquely identifies the radio station  14  through which the FSU is communicating, ii) a frequency carrier assigned to that radio station  14  over which the FSU is communicating, and iii) a downlink and an uplink time slot associated with that frequency carrier during which the FSU is communicating. The exemplary TDMA/TDD channel access scheme depicted in FIG. 3 is well known in the art. 
     When a connection is first established between a FSU  26  and the RLL, it is preferable that the channel or channels exhibiting the best possible signal quality characteristics be assigned to support the connection. Of course, the same is true for existing connections as well. To help ensure that the channel or channels exhibiting the best possible signal quality characteristics are assigned to support new or existing connections, the CPU  18  in each FSU  26  will be capable of continuously deriving a signal quality factor for each channel. The signal quality factor may, for example, be derived as a function of one or more link parameters such as carrier-to-interference ratio (C/I), bit error rate (BER), frame erasure rate (FER), radio signal strength indicator (RSSI), or a combination thereof, and the values associated with the one or more link parameters are measured by the FSU  26  during those periods of time where the FSU  26  is not transmitting or receiving data or control information. Once derived, the signal quality factors can be stored in the memory  22 , for example, in tabular form as illustrated in FIG.  4 . Then by continuously updating the signal quality factor values stored in the memory  22 , the channel selector  24  in the FSU  26  can dynamically select the channel or channels exhibiting the best signal quality characteristics when a connection is first established or during an existing connection, if the signal quality associated with the channel or channels supporting the existing connection degrade below an acceptable level. 
     The present invention concerns an improvement in the way RLL systems accomplish dynamic channel selection. More particularly, the present invention extends the principle of dynamic channel selection by taking into consideration the FSU antenna scan angle during the dynamic channel selection process, where scan angle is defined as the direction in which the peak radio frequency energy is being transmitted and received relative to a reference direction. While the present invention is primarily intended to be implemented in a fixed radio system, such as a DECT based system, the present invention is not limited thereto. 
     Because the present invention takes the scan angle of the antenna  32  into consideration during dynamic channel selection, the CPU  18  must continuously derive a signal quality factor for each channel, where a channel linking a particular FSU and a particular radio station  14  is now defined by: i) an identifier code that uniquely identifies the radio station  14  through which the FSU is communicating, ii) a frequency carrier assigned to that radio station  14  over which the FSU is communicating, iii) a downlink and an uplink time slot associated with that frequency carrier during which the FSU is communicating, and iv) the scan angle of the antenna  32 . Again, the signal quality factor values may be stored in a memory  22 , for example, in tabular form as illustrated in FIG.  5 . Appropriately, the table shown in FIG. 5 contains a signal quality factor value for each frequency carrier, time slot, radio station, and antenna scan angle combination. 
     In order to derive a signal quality factor for each channel, that is, each frequency carrier, time slot, radio station, and antenna scan angle combination, in accordance with a preferred embodiment of the present invention, the antenna  32  associated with a given FSU  26  is automatically swept through “k” different scan angles, as illustrated in FIG.  6 . As the antenna is swept through each of the “k” different scan angles, the FSU  26  measures the value of one or more link parameters such as BER, FER, C/I, RSSI or the like, and therefrom, derives a signal quality factor for each channel. The signal quality factor values are then stored in the memory  22  and repeatedly updated, for example, 500 times per second, thereby creating a more accurate, dynamic picture of the radio frequency environment surrounding the FSU  26 . In doing so, the FSU  26  can dynamically select and assign the one or more traffic channels that exhibit the best signal quality characteristics for a new connection or to an existing connection during call handover. 
     As mentioned, the antenna  32  is automatically swept through the “k” different scan angles. This may be accomplished by mechanically sweeping a rotatable antenna to each of the “k” different scan angles, by electronically sweeping a phased-array antenna to each of the “k” different scan angles, or by selecting each one of a number of fixed directional antennas, wherein the boresight associated with each directional antenna is coincident with each of the “k” different scan angles. However, regardless of whether the automatic redirection of antenna scan angle is accomplished mechanically, electronically, or through the selection of a number of directional antennas, it will be understood that the process of automatically sweeping through the “k” different scan angles, measuring the one or more link parameters, and deriving a signal quality factor for each channel can be controlled through a dynamic channel selection algorithm resident in, for example, the memory  22 . 
     FIG. 7 shows the steps of an exemplary technique for obtaining a signal quality factor for each channel in support of the dynamic channel selection process of the present invention. The block  46  indicates a first general step of a FSU resetting the value of the scan angle “k” in its communication channel signal quality table  44 . At blocks  48  and  50 , the FSU resets the values of the frequency carrier and time slot in its signal quality table  44 . Next at block  52 , the FSU measures one or more link parameters (i.e., link parameters  1  through “N”), such as C/I, BER, FER, and RSSI, for a communication channel corresponding to a first time slot, frequency carrier, scan angle, and radio station combination. The FSU at block  54  then determines a quality factor for that channel, and the FSU stores the quality factor in the signal quality table  44  in memory  22  at block  56 . 
     At block  58 , the FSU incrementally increases the time slot. The flow then moves to decision block  60  where it is determined whether or not the FSU reached the last time slot. If so, the flow moves to block  62  where the FSU resets the time slot and incrementally increases the frequency carrier. Otherwise, the flow loops back to block  52  where the FSU measures the link parameter(s) associated with a second or subsequent communication channel corresponding to time slot, frequency carrier, scan angle, and radio station combination. 
     Having increased the frequency carrier at the FSU, the flow then proceeds to decision block  64 . Here it is determined whether or not the FSU has reached the last frequency carrier. If so, the flow moves to block  66  where the FSU resets the time slot and the frequency carrier and then increases the scan angle. Otherwise, the flow loops back to block  52 , and the FSU continues to measure the link parameter(s) associated with the subsequent communication channel corresponding to time slot, frequency carrier, scan angle, and radio station combination. 
     At decision block  68 , it is determined whether or not the FSU reached the last scan angle. If so, the flow moves to the beginning at block  46 , and the FSU resets the value of the scan angle in its communication channel signal quality table  44 . If the answer to this determination is no, then the flow loops back to block  52  where the FSU measures the link parameter(s) of the subsequent communication channel corresponding to time slot, frequency carrier, scan angle, and radio station combination. A signal quality factor is then computed for the channel by the FSU. By continuously measuring and updating the signal quality factor for each communication channel, the FSU can, through this dynamic channel selection process, compare the signal quality factor values associated with the various communication channels and select the channel or channels that exhibits the best signal quality characteristics. Accordingly, the FSU retunes to the selected channel or channels. It might be preferable if the channel selection algorithm continuously compared the signal quality factor values and ranked them in order of signal quality. 
     In an alternative embodiment, the dynamic channel selection process selects the channel or channels for communication between the FSU and RLL system by determining whether the channel meets predefined quality criteria. The first channel which satisfies the quality criteria is allocated for the connection. 
     In a further embodiment, the order in which the FSU incrementally increases the time slot, frequency carrier, and scan angle can be varied such that the FSU incrementally increases the frequency carrier or scan angle before incrementally increasing the time slot. 
     The present invention concerns the improvement of a radio system using dynamic channel selection in such a way that the dynamic channel selection principle is extended to the antenna portion of the fixed terminal. This invention provides the advantages of automatic selection of terminal to radio station using a narrow beam antenna, resulting in increased range; significant reduction and probable elimination of interference from nearby terminals and terminal to radio station, thereby improving system capacity; and soft hand-over between several non-collocated radio stations, providing improved signal quality. 
     While the invention has been described in detail with reference to the preferred embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made and equivalents employed, without departing from the present invention.