Abstract:
A method ( 40 ) and device ( 42 ) for assigning transmission channels in a wireless communications network ( 38 ). The frequencies of the network ( 38 ) are discriminated to determine which transmission channels have a predetermined blocking threshold of interference. A transmission channel is preassigned with a channel quality less than the predetermined blocking threshold of interference to an incoming call. The channel quality and interference level of the preassigned transmissions channel are continually monitored, and another transmission channel is assigned to the call when the monitored levels of channel quality and interference levels change beyond a set of predetermined thresholds. Forward and backwards channel reassignments are performed depending on the channel quality and interference levels.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The following U.S. Patent Application is commonly assigned and is incorporated herein by reference: 
     
       
         
               
               
               
               
             
           
               
                   
               
               
                 Patent No. 
                 Serial No. 
                 Inventor 
                 Title 
               
               
                   
               
             
             
               
                 TBD 
                 09/224,069 
                 H&#39;mimy et al. 
                 System and Method for 
               
               
                   
                   
                   
                 Channel Assignment Based 
               
               
                   
                   
                   
                 Upon Interference and 
               
               
                   
                   
                   
                 Channel Quality 
               
               
                   
                   
                   
                 Measurements 
               
               
                   
               
             
          
         
       
     
    
    
     TECHNICAL FIELD 
     This invention relates generally to wireless communication systems, and more particularly to a method and device for reassigning a transmission channel based on interference level and channel quality measurements. 
     BACKGROUND OF THE INVENTION 
     Wireless communication technology has experienced unprecedented growth, fueled by advances in radio frequency, satellite, and microelectronic technologies and by the convenience of access to telephony and portable wireless devices. Several technologies have emerged for providing wireless communications, including analog systems such as Advanced Mobile Phone System (AMPS), and digital systems such as Global System for Mobile Communications (GSM) and Digital AMPS (D-AMPS). Cellular radio systems operate in an interference-limited environment and typically rely on frequency reuse plans and Frequency Division Multiple Access (FDMA), in analog systems, and Time Division Multiple Access (TDMA), in digital systems, for example, to maximize capacity. 
     Because bandwidth and radio spectrum are limited resources shared by all users in a wireless network, schemes to divide the available network bandwidth among as many subscribers as possible are often employed. One such scheme involves a combination Time and Frequency Division Multiple Access (TDMA/FDMA) method. For example, 25 MHZ bandwidth can be divided into 124 carrier frequencies spaced 200 kHz apart, with one or more carrier frequencies assigned to each Base Station in the network. Each of the carrier frequencies, in turn, can be divided in time using a TDMA scheme to define logical channels. Channels, in turn, may be divided into dedicated subchannels which are allocated to a mobile station with common channels used by mobile stations in the idle mode. 
     A Traffic Channel (TCH) is used to carry speech and data signals. Traffic channels are defined in GSM using a 26 multi-frame, or a group of 26 TDMA frames. With 26 frames, 24 can be used for traffic, one for a control channel, and one unused. In other systems, other methods are used, such as CDMA or FDMA. An Idle Channel Measurement (ICM) algorithm that uses dynamic channel assignment is usually utilized by the Base Station Controller (BSC) to select channels for transmission of speech and data signals. 
     In addition to dividing bandwidth among subscribers, transmission channels are assigned or allocated to incoming calls to the network. Two methods of channel assignment are typically used in the industry: fixed channel assignment and dynamic channel assignment. Fixed channel assignment involves assigning a predetermined set of channels or frequencies to the base station or BSC. If all the channels within the cell of the base station or BSC are occupied, either the call is blocked, meaning the mobile handset does not receive service, or borrowing strategies are used to borrow channels from neighboring cell. With dynamic channel assignment, channels are not allocated to a base station or BSC permanently. The Mobile Switching Center (MSC) assigns the BSC some frequencies, which remain fixed for a period of time. Periodically, some measurements are made regarding channel utilization, and adjustments are made to the channel assignments as needed. The frequencies or channels are assigned by the BSC each time a call request is made. 
     A problem with prior art channel assignment algorithms and devices is that channel assignments are made based on the best interference level available. This results in an unbalanced use of traffic channels, with all voice and data calls being assigned to the best quality transmission channels even when a lower quality channel is available to service a call. Continuous monitoring of transmission channel interference levels and improved assignment algorithms would provide a more balanced network which could be utilized by a greater number of subscribers. 
     SUMMARY OF THE INVENTION 
     The present invention achieves technical advantages by reassigning transmission channels in a wireless communications network based on interference levels and channel quality using forward and backward reassignment. Interference measurements and channel quality measurement are used to reassign a call to a higher or lower quality channel, freeing up the network for more calls. The use of medium level quality channels rather than high quality channels results in a better balanced network and improved transmission channels in the network. 
     According to the present invention, disclosed is a method of assigning transmission channels based on channel quality and interference level. The frequencies of the network are discriminated to determine which transmission channels have interference thresholds less than a predetermined blocking threshold of interference, and a transmission channel is preassigned with an interference level less than the predetermined blocking threshold to an incoming call. The channel quality and interference level of the preassigned transmission channel are continually monitored, and a transmission channel is reassigned to the incoming call when the monitored levels of channel quality and interference levels change beyond a set of predetermined thresholds. 
     Also disclosed is a method of assigning transmission channels in a wireless communications network based upon interference and channel quality measurements. The method includes the steps of discriminating the frequencies of the network to determine which transmission channels have a interference level less than a predetermined blocking threshold of interference. Next, the channel with the highest available interference level less than the blocking threshold is preassigned to an incoming call. The remaining transmission channels are categorized based on channel quality and interference levels in the network using a rule table that creates bands of interference and channel quality. Next, a decision is made whether to continue the call on the first channel, drop the call, reassign the call to another channel with more interference than the current channel, or reassign the call to a channel having interference less than the current channel, based upon the measured channel quality and the interference levels. A tradeoff is made between good quality on some channels and bad quality on other channels to achieve a more uniform quality distribution across the network. 
     Also disclosed is a device for assigning transmission channels in a wireless communications network. The device includes an algorithm comprising a discrimination function adapted to discriminate the frequencies of the network to determine which transmission channels are less than a predetermined blocking threshold of interference, and a preassignment function adapted to preassign a first transmission channel with an interference level within a band less than the predetermined blocking threshold of interference to an incoming call. The algorithm also includes a monitoring function adapted to continually monitor the channel quality and interference level of the preassigned transmission channel, and a reassignment function for reassigning a second transmission channel to the incoming call when the monitored levels of channel quality and interference levels change beyond a set of predetermined thresholds. A rule table is stored in the device and utilized to reassign transmission channels in a wireless network on channel quality and interference level measurements. 
     By providing for both forward and backward reassignment, the invention is particularly advantageous to fully utilized networks with heavy traffic conditions. A further advantage is improved balance in the use of transmission channels of the wireless network. When the overall channel quality becomes more uniform over the entire network, a higher capacity can be achieved. Another advantage includes the free-up of low interference channels. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above features of the present invention will be more clearly understood from consideration of the following detailed description in connection with accompanying drawings in which: 
     FIG. 1 illustrates a typical cellular network  10  of the prior art; 
     FIG. 2 shows a GSM network  11  in which the present invention may be employed; 
     FIG. 3 illustrates a GSM network  38  with the channel assignment algorithm  40  of the present invention; 
     FIG. 4 shows the interference bands  44  and interference levels  42  of the present invention; and 
     FIG. 5 is a flow chart of the algorithm of the present invention. 
     Corresponding numerals and symbols in the different figures refer to corresponding parts unless otherwise indicated. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a diagram of a wireless communications network  10  of the prior art. The network  10  preferably comprises a wireless telephony network including an originating mobile station (MS)  12 . The originating MS  12  is preferably a wireless communication device comprising a wireless cellular-type mobile station, such as a Personal Communications Service (PCS) or cellular phone, but may also include a computer, a Personal Digital Assistant (PDA), or other wireless terminal, for example. A base station  19  provides cellular coverage via an RF link to the MS  12  and other mobile stations within the cell  22  of the base station  19 . Several base stations  19  service a single MSC  30  to provide continuous coverage during mobility of the MS  12 . For illustrative purposes, the cell  22  surface area is shown as hexagonal, but the RF wave propagation is actually more circular in coverage area. As the MS  12  moves from one cell  22  to an adjacent cell  22 , the call or data transmission is handed over to another base station  19  by the Mobile Switching Center (MSC)  30 , which handles call switch-overs and mobility management. MSC  30  is in communication with fixed networks such as the Public Switched Telephone Network (PSTN)  24  or an optical network, for example. 
     FIG. 2 shows the layout of a GSM network  11  comprised of three main parts: a mobile station  12 , a Base Station Subsystem (BSS)  14 , and network subsystem  16 . Mobile station  12  is typically a cellular telephone that is carried by the subscriber, although other terminal devices such as laptops or fax machines may be used to communicate over the GSM network  11 . The network subsystem  16 , the main part of which is the Mobile Switching Center (MSC)  30 , performs the switching of calls between the mobile and other fixed or mobile network users, as well as mobility management. The mobile station  12  and the BSS  14  communicate with one another across the Um interface  18 , also known as the air interface or radio link, which is controlled by the BSS  14 . The BSS  14  communicates with the MSC  30  of network subsystem  16  across an A- interface  20 . MSC  30  communicates with fixed network  32  which may be the PSTN or other cellular networks, for example. 
     The mobile station  12  comprises a mobile transceiver of the type found in many cellular phonesets and a Subscriber Identity Module (SIM). The SIM includes a identity indicator, a secret key for authentication, and other relevant network/user information. The mobile transceiver is uniquely identified by the International Mobile Equipment Identity (IMEI). The identification features of the MS  12  are independent, thereby allowing mobility of the user about the service area of the GSM network  11 . 
     The Base Station Subsystem (BSS)  14  comprises two parts: the Base Transceiver Station (BTS)  26  and the Base Station Controller (BSC)  28 . BTS  26  communicates across the standardized Abis interface  30  with BSC  28 , allowing operation between components. BTS  26  houses radio transceivers that define a cell and handles the radio-link protocols with the mobile handset  12 . In a large urban area, there may be a large number of BTSs  26  deployed. BSC  28  manages the radio resources for one or more BTSs  26 , and, as such, there may be several BSCs  28  within a single BSS  14 . Among the functions of the BSC  28  are radio-channel setup, frequency hopping, and handovers. BSC  28  provides a connection mechanism between the mobile handset  12  and the MSC  30  of network subsystem  16  which acts as the interface to one or more fixed networks  32 . 
     The central component of the network subsystem  16  is the Mobile Switching Center (MSC)  30 , which acts like a normal switching node of the Public Switched Telephone Network (PSTN), and provides all of the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to roaming subscribers. These functions are provided in conjunction with several other network entities, which together form the network subsystem  16 . 
     The MSC  30  provides the connection mechanism to the fixed networks  32  which may include the PSTN or an Integrated Service Digital Network (ISDN), for example. The Home Location Register (HLR)  34  and Visitor Location Register (VLR)  36 , together with the MSC  30 , provide call routing and roaming capabilities for the GSM network  11 . In particular, the HLR  34  contains administrative information of the subscriber registered in the corresponding GSM network  11 , along with the current location of the mobile handset  12 . Likewise, the VLR  36  contains selected administrative information from HLR  34  necessary for call control and provisioning of the subscriber services for each mobile currently located in the geographical area controlled by the VLR  36 . Other registers are used for authentication and security functions within the network subsystem  16 . 
     Related patent application entitled “A System and Method for Channel Assignment Based on Uplink and Interference Level in Channel Quality Measurements” filed in December  1998  involves a forward reassignment, where channels are reassigned to channels with lower interference level if the quality measure satisfies certain conditions. Before the MS  12  is assigned to a channel upon a call request, the BSC  28  measures the interference level of all channels available, and then selects some of the channels and marks them as a preassignment channel which will be assigned to the MS  12  call request. Then when the MS  12  is reassigned to the specific channel, the quality is continually monitored. The quality measured may be the Bit Error Rate (BER) for digital networks, or carrier/interference (C/I) for analog networks, for example. If the quality measurement satisfies certain conditions, then the call is either reassigned to another channel having a better or lower interference level, keeps the channel the same as it is, or drops the call. This process of searching for a channel with a lower interference level and reassigning the call to a better channel is referred to as a forward reassignment. Other channel assignment algorithms of the prior art also focus on searching for a channel with a better quality, e.g., a lower interference level. 
     A problem with prior art assignment and reassignment algorithms is that a wireless network  10  or  11  may become saturated, especially in the channels having a low interference level, because all calls are being assigned to the channels with good quality. The wireless network becomes unbalanced, with many more calls being assigned to the lower interference level channel than for medium interference level channels or high interference channels. A method and algorithm is needed that provides a better balance to the channel assignment of the wireless network and prevents overloading of the low-level interference channels. 
     The present invention provides a more uniform distribution of transmission channels by using a reverse reassignment algorithm to redirect calls away from the low interference channels and reassign them to medium interference level channels. With the present invention, a device measures the interference level and channel quality of available channels and categorizes them into different grades or classifications. Rather than reassigning calls or data transmissions to low interference level and good quality transmission channels, the present algorithm focuses on balancing the network and performing reverse and forward reassignments as required. 
     Referring next to FIG. 3, an improved wireless communications network  38  according to the present invention is shown comprising mobile station  12  in communication with other networks such as fixed networks  32  through the various devices and systems as described in FIG. 2 of the prior art. BSC  42  of the present invention includes a channel assignment algorithm  40  which provides instructions to the BSC  42  for the set-up and iteration of the channel assignment. The Channel Assignment Algorithm (CAA)  40  of the present invention is shown located at the BSC  42 , but the CAA  40  may be located in other devices of the wireless communications network  38  depending on the type of network. For example, the CAA  40  may be located in a base station  19  or BSS  14 , or other similar devices. 
     The CAA  40  of the present invention includes a setup step in which the interference measurements are divided into bands and categories of interference and channel quality, respectively. The algorithm includes an iteration step where incoming requests for services (e.g., calls or data transmissions) are assigned to channels. First, the uplink interference is measured on channels in the system, and the measured interference is divided into “n” thresholds specified as interference levels. The actual value of “n” depends on the requirements of the system. The larger value of “n” usually imposes a greater amount of processing requirements to implement the algorithm CAA  40 . Given “n” interference levels, I 1 , I 2 , . . . I n , there are “n+1” interference bands I 1 , I 2 . . . I n+1 , as shown in FIG.  4 . The mapping between the interference levels and the interference bands is shown in Table 1, which is a definition of the interference band based interference levels in the system. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Condition 
                 Interference Band 
               
               
                   
                   
               
             
             
               
                   
                 I &gt; I n   
                 I n+1  (blocked interference band) 
               
               
                   
                 I n−1  &lt; I &lt; I n   
                 I n   
               
               
                   
                 . . . 
                 . . . 
               
               
                   
                 I 1  &lt; I &lt; I 2   
                 I 2   
               
               
                   
                 I &lt; I 1   
                 I 1   
               
               
                   
                   
               
             
          
         
       
     
     The highest interference band is always blocked, which implies that I n  is the “blocking threshold”. The blocking threshold depends on the system implementation. 
     The channel quality is divided into n+1 categories Q 1 , Q 2 , . . . Q n+1  where “n” is the number of interference thresholds. There is a correspondence between channel quality categories and the Channel Quality Measurements (CQM), carrier to interference ratio (C/I) for analog and BER measurements for digital. The channel quality categories according to one embodiment are shown in Table 2. 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Category 
                 Analog 
                 Digital 
               
               
                   
               
             
             
               
                 Q m+1   
                 C/I &gt; (C/I) m   
                 BER &lt; BER 1   
               
               
                 Q m   
                 (C/I) m−1  &lt; C/I &lt; (C/I) m   
                 BER 2  &gt; BER &gt; BER 1   
               
               
                 Q m−1   
                 (C/I) m−2  &lt; C/I &lt; (C/I) m−1   
                 BER 3 &gt; BER &gt; BER   2   
               
               
                 . . . 
                 . . . 
                 . . . 
               
               
                 Q 2   
                 (C/I) 1  &lt; C/I &lt; (C/I) 2   
                 BER m−2  &gt; BER &gt; BER m−1   
               
               
                 Q 1   
                 C/I &lt; (C/I) 1   
                 BER &gt; BER m   
               
               
                   
               
             
          
         
       
     
     For simplicity, the interference bands are divided coarsely into high, moderate and low interference subcategories. I H , I M , I L  and Q H , Q M , Q L  respectively. However, it is seen that more subcategories may be utilized, in the present invention. 
     The mapping between coarse CQM bands and CQM bands of Table 1 and 2 is shown in Table 3. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Coarse CQM Band 
                 CQM Band 
               
               
                   
                   
               
             
             
               
                   
                 Q H   
                 Q m+1 , . . ., Q j   
               
               
                   
                 Q M   
                 Q j+1 , . . ., Q k   
               
               
                   
                 Q L   
                 Q k+1 , . . ., Q l   
               
               
                   
                   
               
             
          
         
       
     
     Where m&gt;j&gt;k. Similarly, the coarse interference band mapping with interference bands in Table 1 is shown in Table 4, where n&gt;w&gt;u. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Coarse Interference Band 
                 Interference Band 
               
               
                   
                   
               
             
             
               
                   
                 I H   
                 I n+1 , . . ., I w   
               
               
                   
                 I M   
                 I w−1 , . . ., I u   
               
               
                   
                 I L   
                 I u−1 , . . ., I 1   
               
               
                   
                   
               
             
          
         
       
     
     The present invention utilizes a rule table that suggests forward and backward channel reassignment based on a fixed reassignment step. Table 5 is an example of a rule table with rules for reassignment based on the channel quality category and the interference band. FIG. 5 is a flow chart illustrating the re-assignment determinations of the rule table shown in Table 5. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Band/Category 
                 I H   
                 I M   
                 I L   
               
               
                   
                   
               
             
             
               
                   
                 Q H   
                 Continue 
                 R − &gt; I H   
                 R − &gt; I M   
               
               
                   
                 Q M   
                 Continue 
                 Continue 
                 Continue 
               
               
                   
                 Q L   
                 R − &gt; I M   
                 R − &gt; I L   
                 Continue 
               
               
                   
                   
               
             
          
         
       
     
     The CAA  40  requires measuring the interference level on each base station for all possible channels. Then the channels are discriminated based on their interference levels. The incoming service is pre-assigned to a relatively high interference channel depending on channel availability. Then the channel quality category “Q i ”, where {i=1, 2, . . . n+1} is determined based on the C/I or BER measurements, as shown in Table 2. 
     Depending on the channel quality category, the call or data transmission is either kept on the same channel, reassigned to a channel corresponding to a lower interference band in accordance with the reassignment step, or the call may be dropped/handed-off. A call is dropped if a hand-off candidate is not available. The reassignment to a channel in a lower interference band ensures an interference level on the reassigned channel below the one on the preassigned channel by a predetermined value, increasing channel quality by at least a predetermined value. The CAA  40  is triggered either by increased saturation in the cell and unavailability of channels with low interference, or by the service supplier on a regular basis. The two stages of the algorithm include a set-up stage, which comprises taking uplink interference measurements and channel discrimination based on interference levels; and an iteration stage, which includes preassignment and channel quality measurement, and action based upon on a rule table. 
     In accordance with the CAA  40 , the BSC  42  continuously measures the interference level on unused devices or traffic channels. Channels having interference levels (I) greater than I n  are blocked, meaning that they will not be assigned to incoming calls. The remaining unused channels are categorized in accordance with Table 1. Thus, a list of channels is compiled that categorizes them in accordance with the uplink interference level. 
     In the iteration stage of the present algorithm, each incoming request for service is pre-assigned a channel, followed by a channel quality measurement (CQM). Table 2 is utilized to ascertain the channel quality category Q j , {j=1, 2, . . . , m+1}. Then service is either continued, re-assigned or handed off based on rule Table 5 and the flow chart of FIG. 5 that utilizes both the interference band and the channel quality category as a basis. Incoming service or calls are pre-assigned to the channels with the highest available interference level (barring the blocked channels). The CQM (C/I for analog or BER for digital channels) is measured on the pre-assigned channels and then mapped to a channel quality category using Table 2. 
     The desired result of the rule Table 5 is to achieve uniform quality distribution and to achieve a homogeneous quality level of service throughout the network  38 . The algorithm trades good quality on some channels with bad quality on others to achieve a balance on the network  38  having a predominately medium level interference and channel quality. 
     The CAA  40  for implementing the rule table for the iteration shown in the flow chart  48  shown in FIG.  5 . The effect of reassigning channels based on the rule table is that if Q=Q H  and if I=l M , the reassignment to I H  results in Q=Q M . If Q=Q H  and if I=I L  then reassignment to I M  results in Q=Q M . Finally, if Q=Q L  and if I=I H , the reassignment to I M  results in Q=Q M ; and if I=I M , the reassignment to I L  results in Q=Q M . The reassignment actions of the present invention preferably result in obtaining a moderate or medium quality level. Service is continued if the call is in the medium quality band or if there is no further room to improve the service. 
     The coarse interference bands are optimized in a manner (for w, u) such that the objective of uniform quality is maximized. The values of w, u are preferably based on the statistical distribution of the interference level; however, the percentile for the moderate subcategory should be greater than the high or low subcategories, because all the reassignments fall into the moderate band. 
     The coarse quality bands are optimized in a manner (for k and j) such that the objective of moderate quality subcategory is maximized. This may be done on an interactive basis by first setting thresholds for the subcategories percentiles and calculating the new percentile after a determined time. If the percentile of the high and low subcategories exceeds the threshold, then the border band may be added/removed from the moderate subcategory until it meets the threshold. 
     The novel method and device for transmission channel assignment based on uplink interference level and channel quality measurements of the present invention achieves technical advantages with the use of a forward and backward reassignment step. Advantages include preventing or reducing the number of dropped connections where the wireless network  38  is fully utilized. A further advantage is improved balance in the use of transmission channels of the wireless network, with the emphasis on moderate quality channel reassignment. Another advantage includes preventing overloading of low interference channels. 
     While the invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications in combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. For example, while the invention is described herein for use in a GSM network  38 , the device and method may be used for channel allocation in other communications networks using FDMA or TDMA, for example. The channel quality measurements of the present invention are preferably based on either C/I (for analog) or BER (for digital) for the preassigned channels, but may be based on some other measure of channel quality. It is therefore intended that the appended claims encompass any such modifications or embodiments.