Abstract:
Portion-by-portion selection and/or combination of signals received from multiple base transceiver stations (BTSs) is used to improve the quality of reception in cellular communication systems. For any particular frame, bit, symbol, or chip, the highest-quality copy can be selected and concatenated onto the end of a sequence of data being generated by the system. In addition, the energies and/or voltages of multiple copies of bits or symbols received by multiple BTSs can be added and/or averaged in order to improve signal quality (e.g., increased signal-to-noise ratio (SNR) and/or signal-to-interference ratio (SIR)). In addition, a single communication system can utilize both selection and combination procedures. The resulting communication system reduces error rate and improves the quality of reception.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to wireless communication technology and more particularly to a wireless communication system with signal selection and combination. 
     BACKGROUND OF THE INVENTION 
     Typical cellular communication systems include base transceiver stations (BTSs) that engage in wireless communication with mobile devices such as cellular phones. An example of such a system is illustrated in  FIG. 1 . The BTSs  14  of the illustrated system connect to at least one base station controller (BSC)  20  through a local network  16 , and transmit and receive phone calls and other data using circuit-switched, time division multiplexed communications protocols, virtual circuit, asynchronous transfer mode (ATM) protocols, and/or other communications protocols. The term “local network” as used herein refers to a network served by a particular BSC  20 . The local network  16  is typically an Internet protocol (IP) network, and can generally be considered part of a wider communication network having other portions which can include, for example, other local cellular networks and/or other types of networks such as the Internet. The other network portions can be referred to, with respect to the local network  16 , as “outside” network portions. The local network  16  communicates to the outside network portions through a gateway  18 . 
       FIG. 2  illustrates an example of a gateway  18  for use in the cellular communication system of  FIG. 1 . The gateway  18  includes an interface  40  for communicating with outside network portions, an interface  42  for communicating with the local network  16 , a processor  44 , and a data storage device  46  which stores information for use by the other components of the gateway  18 . The stored information can include, for example, programs for execution by the processor  44 . 
     A mobile device—a/k/a a “mobile unit” (MU)  12 —engages in direct wireless communication with one or more of the BTSs  14  in order to ultimately communicate with another end-user device such as another MU or a hard-wired telephone (a/k/a a “land line”). The other end-user device can be within the geographic region served by the local network or can be elsewhere in the wider network—e.g., in an outside network portion. 
     A typical cellular network—which can include one or more local networks—covers a contiguous area that is divided into multiple cells. Each cell is served by a BTS  14  which provides a wireless link for at least one MU (e.g., a cellular phone) within the cell. The wireless link—which in many systems operates within the radio-frequency (RF) spectrum—is used to transmit electromagnetic data signals representing data being sent between the MU  12  and the BTS  14 . 
     Consider an MU  12  which is engaged in a communication session (e.g., a telephone call). As the MU  12  moves among the cells, the session (i.e., the call) is handed off among the BTSs  14  in order to provide continuous coverage. 
     Typically, a BSC  20  controls call set-up within the BTSs  14 , and inter-cell operations such as handoffs among the BTSs  14 . In addition, the BSC  20  in conventional systems generally collects information about the respective BTSs  14  and controls the wireless communication parameters of the BTSs  14 , such as transmission strength and modulation parameters. During call handoff, a local handoff controller  806  is used to control the allocation of resources among the other devices—e.g., the BSC  20  and the BTSs  14 —which are connected to the local network  16 . 
     For “uplink” communications—i.e., communications sent from a cellular phone or other MU  12 —it is common to utilize multiple BTSs  14  to receive data from the MU  12 . In conventional systems, the best-quality data signals from one or more of the BTSs  14  are selected by the BSC  20  in order to improve the quality of reception, as is well-known in the art. Typically, the stream of data transmitted from the MU  12  is broken into “frames” (i.e., portions of selected size). 
     For “downlink” communications—i.e., communications sent from one or more BTSs  14  to the MU  12 —multiple BTSs  14  can send signals to a single MU  12  in order to improve the quality of reception, as is well-known in the art. 
     The above-described functions of: (1) selecting uplink signals received by multiple BTSs  14 , and (2) distributing downlink signals through multiple BTSs  14  to a single MU  12 , are typically performed by a software and/or hardware system called a “selection and distribution unit” (SDU). The SDU controls various characteristics of the digital transmission of the data to and from each MU. Such characteristics typically include parameters such as frame size and allocation of digital capacity such as bit transmission and processing capacity. In conventional systems, the SDU function is performed by the BSC  20 . In addition, the allocation of wireless resources (e.g., wireless bandwidth) to an MU is also performed by the BSC  20 . In particular, the BSC  20  also includes a wireless resource allocation function which assigns wireless bandwidth, spreading codes (e.g., Walsh codes), and/or time slots to the respective MUs connected to the local network  16 . Moreover, digital transmission parameters such as digital capacity allocation are related to the quantity of wireless resources being used. For example, the digital capacity and the wireless capacity allocated to a particular MU must together increase with increasing data transmission rate. The BSC typically coordinates the SDU function and the wireless resource allocation function such that the allocation of wireless resources matches the allocation of digital resources. 
     The system of the claimed invention improves the capacity and/or coverage of a wireless communication system (e.g., Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), or Wideband Code Division Multiple Access (W-CDMA)) by reducing the required transmit power of the MU in simultaneous communications with multiple BSTs. 
     SUMMARY OF THE INVENTION 
     From the foregoing, it may be appreciated by those skilled in the art that a need has arisen for a communications system which can effectively utilize portions of signals from multiple BTSs, to thereby improve reception of signals from an MU. 
     It is therefore an object of the present invention to provide a communications system which can combine portions of signals from multiple BTSs to thereby construct an improved signal. 
     It is a further object of the present invention to provide a communication system which can select portions of signals from multiple BTSs to thereby construct an improved signal. 
     These and other objects are accomplished by a communication system comprising a first base transceiver station receiving from a mobile unit a first wireless signal, wherein the signal is comprised of a first signal portion having a first signal characteristic and a second signal portion. The system is further comprised of a second base transceiver station receiving from the mobile unit a second wireless signal, wherein the second wireless signal is comprised of a third signal portion having a second signal characteristic and a fourth signal portion. A fifth signal portion is generated by applying a processing operation to the first and third signal portions, independently from the second and fourth signal portions, wherein the processing operation comprises using the first and second signal characteristics to perform at least one of the steps of selecting one of the first and third signal portions and combining the first and third signal portions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like reference numbers represent like parts, in which: 
         FIG. 1  is a block diagram of a cellular communication system; 
         FIG. 2  is a block diagram of a gateway apparatus for use in the communication system of  FIG. 1 ; 
         FIG. 3  is a block diagram of a BTS for use in the communication system of  FIG. 1 ; 
         FIG. 4  is a block diagram of an additional cellular communication system; 
         FIG. 5  is a block diagram of a processor for use in the gateway of  FIG. 2  or the BTS of  FIG. 3 ; 
         FIG. 6  is a flow diagram of an algorithm for selecting a portion of a wireless signal in accordance with the present invention; 
         FIG. 7  is a block diagram of a data stream in accordance with the present invention; 
         FIG. 8  is a graph of signal portions which are received by a system in accordance with the present invention; 
         FIG. 9  is a block diagram of a data stream being processed in accordance with the present invention; 
         FIG. 10  is a block diagram of additional data streams being processed in accordance with the present invention; 
         FIG. 11  is a graph of signal portions, representing digital bits, which are received by a system in accordance with the present invention; 
         FIG. 12  is a chart illustrating a combination of signal portions in accordance with the present invention; 
         FIG. 13  is a block diagram illustrating processing of signal portions in accordance with the present invention; 
         FIG. 14  is a flow diagram illustrating an exemplary procedure for processing signal portions in accordance with the present invention; 
         FIG. 15  is a flow diagram illustrating an additional exemplary procedure for processing signal portions in accordance with the present invention; and 
         FIG. 16  is a block diagram illustrating an exemplary controller for use in a cellular communication system in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, data received through multiple BTSs can be combined and/or selected on a frame-by-frame basis.  FIG. 7  illustrates an exemplary wireless data stream divided into frames in accordance with the present invention. The frames are transmitted from one BTS  14  to another BTS  14  or from one BTS  14  to the BSC  20  across the local network  16 . The data stream includes a routing label  134 , a first wireless frame  132   a , a second wireless frame  132   b , and any number of additional frames as may be required to carry information during a communication session between a MU and the BTSs. The routing label  134  provides information which can be used to direct the frame to its ultimate destination. Each of the wireless frames  132   a  and  132   b  includes a synchronization bias field  140  which enables synchronization of different copies of the same wireless frame arriving through different routes (i.e., through different BTSs). Each wireless frame also includes a wireless frame header  136  which provides information about the type of data being transmitted within the frame. In addition, each of the wireless frames  132   a  and  132   b  includes a data payload  138  comprising one or more data bits  702 . 
     A wireless signal broadcast by an MU  12  can be received in the form of multiple copies, each copy being received by one of the BTSs  14 . Because the quality of reception can vary among the BTSs  14 , it is advantageous to employ a procedure which: (a) selects the copy or copies, or portions thereof, having the best quality; or (b) combines the copies, or portions thereof, to thereby obtain a signal having improved quality.  FIG. 10  is a block diagram illustrating an example of such a procedure for generating improved quality data in accordance with the present invention. A first set of signal portions  1004   a ,  1004   b , and  1004   c  is received by a first BTS. A second set of signal portions  1006   a ,  1006   b , and  1006   c  is received by a second BTS. Signal portions  1004   a  and  1006   a  are processed using a signal processing algorithm  1002  which either selects the best one of signal portions  1004   a  and  1006   a , or combines signal portions  1004   a  and  1006   a , to generate an improved quality output signal  1008   a . Similarly, signal portions  1004   b  and  1006   b  are processed using the algorithm  1002  to produce an additional improved quality signal portion  1008   b . Signal portions  1004   c  and  1006   c  are also processed using the signal processing algorithm  1002  to obtain yet another improved quality signal portion  1008   c . In fact, any number of pairs of signal portions can be similarly processed. 
     In accordance with the present invention, the processing can be done on a frame-by-frame basis—i.e., the signal portions  1004   a - 1004   c ,  1006   a - 1006   c , and  1007   a - 1008   c  can be frames. For example, the best copy of each frame can be selected, and the selected copies can be concatenated to form a more accurate version of the sequence of data frames transmitted by the MU  12 . Alternatively, or in addition, each copy of a frame can be combined to form a better quality output frame, and the set of output frames can be concatenated to form a better quality frame sequence. 
     Various types of signal quality parameters and/or characteristics can be utilized as indicators of signal quality. For example, there is a signal-to-noise ratio (SNR) and/or a signal-to-interference ratio (SIR) associated with each bit received by a BTS. High signal quality is generally associated with high SNR and/or high SIR. The mean (e.g., the arithmetic mean) of the respective SNRs or SIRs of a bit  702  within the payload  138  of a frame can be utilized as an indicator of the quality of reception of the frame, and based upon this criterion, the best frame can be selected from among the corresponding copies received by the respective BTSs  14 . 
     Other criteria can also be used to select a preferred frame from among the copies received by the various BTSs  14 . For example, an algorithm in accordance with the present invention can select the frame with the highest energy-per-bit (Eb), the lowest error-per-bit, or can select only those frames which pass an error-detection procedure such as, for example, the well-known Cyclic Redundancy Check (CRC). 
     In accordance with an additional aspect of the present invention, data from the various BTSs  14  can be selected on a bit-by-bit basis. In other words, the respective copies of each bit which are received by the respective BTSs  14  can be compared based upon SNR, SIR, energy, Eb, CRC, or another criterion indicating the quality of reception. The bit or bits having the best quality can be selected. 
     Each bit broadcast by the MU  12  is typically represented by a pair of analog “symbols” which are sinusoidal analog signals. Such signals are illustrated in  FIG. 8 . Whether a bit is a “logic 1” or a “logic 0” is determined by the relative phases of the symbols  802  and  804 . If a bit is considered a “portion” of a signal, a symbol can be considered a portion of the bit, or a “sub-portion” of the signal. Similarly to bits, associated with each symbol is an SNR, an SIR, an energy, and/or a voltage. An algorithm in accordance with the present invention can select the “best” copy of a symbol broadcast by the MU  12 , based upon which symbol has the highest SNR, the highest SIR, the highest energy, or the highest voltage. Each selected copy of symbol  802  can then be paired with a selected copy of the corresponding symbol  804  to form a bit having a low probability of error. The resulting bits can then be concatenated into a sequence which is delivered to the ultimate destination of the data. 
     As illustrated in  FIG. 9 , for applications requiring a heightened level of security, a group of bits  704  can be processed by a mathematical spreading process  904  to form “chips”  902  as is well-known in the art. An algorithm in accordance with the present invention can perform the above-described selection process on a chip-by-chip basis, similarly to the frame-frame procedure discussed above. 
       FIG. 6  illustrates an example of a data selection algorithm in accordance with the present invention. The algorithm can optionally be used to perform frame-by-frame, bit-by-bit, symbol-by-symbol, or chip-by-chip selection of portions of data streams. In the algorithm illustrated in  FIG. 6 , signals received by two BTSs are processed. However, such an algorithm can be used to process signals from any number of BTSs. 
     In the illustrated example, a signal having first and second signal portions is received through a first BTS, and a signal having third and fourth signal portions is received through a second BTS. The first and third signal portions can, for example, be derived from an earlier portion of a signal stream transmitted by an MU, and the second and fourth signal portions can be derived from a subsequent portion of the data stream transmitted by the MU. The algorithm first determines a first signal characteristic of the first signal portion, and a second signal characteristic of the third signal portion (step  616 ). As discussed above, a portion of a signal can be a frame, a bit, a symbol, a chip, or any other portion of a signal received through a BTS. As also discussed above, the signal characteristic of each of the portions can be an SNR, an SIR, an energy, a voltage, an Eb, the result of an error detection test such as a CRC, or any other parameter which can serve as an indication of signal quality. Preferably, an SNR or an SIR is used. 
     The first and second signal characteristics are compared (step  610 ). If, based upon the comparison of the first and second signal characteristics, the first signal portion received from the first BTS is preferable (i.e., is of better quality) (step  602 ), the first signal portion is selected (step  604 ). On the other hand, if the comparison of the first and second signal characteristics indicates that the third signal portion received from the second BTS is preferable (step  606 ), the third signal portion is selected (step  608 ). The selected portion is then concatenated onto any previously-received data in the data stream (step  612 ), and the algorithm is repeated for the next portion of the signal stream being transmitted by the MU  12  (step  614 ). For example, the algorithm can next process the second and fourth signal portions. 
     In accordance with another aspect of the present invention, signals received by multiple BTSs can be combined on a bit-by-bit or symbol-by-symbol basis by adding and/or averaging the energy and/or voltage associated with each copy of a bit or symbol received through a set of BTSs. For example, when the RF signal received by a BTS is converted into a digital data stream, there is an energy and/or voltage associated with each bit, as is illustrated in  FIG. 11 . The RF signal is converted to an analog waveform  1102  which represents a sequence of bits  1104 . One or more thresholds T 1  and T 2  are used to determine whether a particular portion of the waveform  1102  represents a 0 or a 1. For example, if the energy or voltage of a portion of the waveform exceeds T 1 , the corresponding bit is a 1. On the other hand, if the energy or voltage of the portion of the waveform is less than T 2 , then the portion of the waveform represents a 0. However, due to noise, interference, and/or other potential sources of error, there is associated with each bit a probability that the bit is represented incorrectly. In accordance with the present invention, the probability of an incorrect bit can be reduced by adding and/or averaging the waveforms from multiple BTSs. 
     The addition or averaging of bits received via multiple signal paths (i.e., through multiple BTSs) can be further understood with reference to  FIG. 12 . In the example illustrated in  FIG. 12 , M bits  1202  are being received through N signal paths  1204  (i.e., through N separate BTSs). For each of the bits  1202 , the N copies of the bits are added or averaged to form a sum or mean  1206 . The M sums or means are then ready to be transported to the recipient of the data. 
     A similar procedure, illustrated by the block diagram of  FIG. 13 , can be used to add or average the voltages of multiple copies of symbols received by the BTSs  14 . For example, as illustrated in  FIG. 8 , each symbol is represented by a waveform ( 802  or  804 ). If multiple BTSs  14  are being used to receive a first symbol waveform  802  transmitted by the MU  12 , multiple copies  1302  of the waveform  802  are available, and these copies  1302  of the waveform  802  can be added or averaged (block  1308 ) to form a resulting waveform  1306  which is likely to have a higher SNR and/or SIR than the constituent waveforms  1302 . Similarly, the corresponding symbol waveform  804  is also received, by multiple BTSs, as multiple copies  1314 . The copies  1314  are added or averaged to form a resulting waveform  1312  having improved SNR and/or SIR. Waveforms  1306  and  1312  form a pair of symbols which are then used to form a bit  1316  with a reduced probability of error. 
       FIG. 14  illustrates an example of an algorithm for combining signal sizes of multiple signal portions in accordance with the present invention. In the exemplary algorithm of  FIG. 14 , signal portions from only two BTSs are combined. However, such an algorithm can be used to combine signal portions from any number of BTSs. In the algorithm illustrated in  FIG. 14 , a signal received from a first BTS has two portions—referred to as first and second signal portions—and a signal received by a second BTS also has two signal portions—referred to as third and fourth signal portions. Each signal portion preferably is, or represents, a data bit. A first signal size associated with the first signal portion is determined (step  1402 ). A second signal size associated with the third signal portion is also determined (step  1404 ). The first and second signal sizes are combined to thereby generate a third signal size (step  1406 ). Because a data bit associated with a signal portion is typically represented by the size of the signal portion, the third signal size can be used to generate a fifth signal portion (step  1408 ). For example, if the respective sizes of the first and third signal portions represent data bits, and the third signal size is the sum or mean of voltages associated with the first and third signal portions, then the third signal size itself represents a data bit associated with the fifth signal portion. 
     Once the fifth signal portion is determined, this portion is concatenated onto the last portion of any prior data in the sequence being constructed (step  1410 ). The algorithm then proceeds to the next portion of the signal transmitted by the MU  12  (step  1412 ). For example, the algorithm can next process the second and fourth signal portions. The above-described procedure can be repeated any number of times, depending upon how many signals portions are being transmitted during a communications session. 
       FIG. 15  illustrates an example of an algorithm that can be used to combine symbols in accordance with the invention. In the algorithm illustrated in  FIG. 15 , symbol copies are received through two BTSs. However, the algorithm can be applied to symbol copies received through any number of BTSs. The signal received by the first BTS comprises first and second signal portions—in this case, first and second symbols. The signal received by the second BTS comprises third and fourth signal portions—in this case third and fourth symbols. After the first and third symbols are received (steps  1502  and  1504 ), the symbols are combined by adding and/or averaging the time-varying voltages of the waveforms representing the first and third symbols, thereby generating a fifth symbol (step  1506 ). In other words, the size of the changing voltage of each waveform is added to thereby generate a time-varying voltage which represents the fifth symbol. Preferably, the addition of the waveforms of the first and third symbols is performed using an analog voltage adder; such adders are well-known in the art. The fifth symbol is then paired with a set of symbols representing the same bit to thereby generate a data bit (step  1508 ). The data bit generated in step  1508  is concatenated onto the sequence of any previously-generated data bits (step  1510 ), and the procedure repeats itself for the next signal portion (i.e., the next symbol) (step  1512 ). For example, the algorithm can next process the second and fourth signal portions—i.e., the second and fourth symbols. 
     In accordance with the present invention, either the BSC or a BTS can be used to perform the SDU function. If the SDU function is to be performed at the BTS level, a particular BTS is generally selected to perform this function. The selected BTS can be referred to as the “primary” BTS (item  378  of the system illustrated in  FIG. 4 ). The other BTSs  382  can be referred to as “secondary” BTSs. Because there are generally no high-capacity links directly connecting the various BTSs, the data sent to and from the MU  12  typically travels to and from the primary BTS  378  through the local network  16  which performs the necessary communication with the secondary BTSs  382 . Specifically, wireless uplink data signals  502  are sent from the MU  12 , via wireless links  412 , to the secondary BTSs  382  which convert the wireless data signals  502  into digital data signals  510  representing the data being communicated. The digital data signals  510  are sent into the IP network  16  through high-capacity (typically “T1”) communication lines  404  capable of quickly transmitting large quantities of data. The data are then sent—in the form of digital data signals  516 —from the IP network  16  through an additional high-capacity communication line  406  to the primary BTS  378 . The primary BTS  378  also receives its own copies of the wireless data signals  502  directly, through a wireless link  414 , from the MU  12 . In accordance with the present invention, the SDU function includes selection of digital data signals corresponding to the best quality wireless data signal or signals received by the primary BTS  378  and the secondary BTSs  382 . As discussed above, the selection need not be based solely upon which signals, considered in their entirety, have the best quality. The selection can also be based which portions (e.g., frames) of the wireless signals have the best quality. The selection procedure is preferably performed on a frame-by-frame basis. Preferably, the SDU function includes combination of digital data signals corresponding to two or more of the wireless data signals  502  received by the primary BTS  378  and the secondary BTSs  382 . The combination procedure preferably includes adding and/or averaging of the respective amplitudes and/or power levels of the wireless data signals  502 , and is preferably performed on a frame-by-frame basis. 
     The primary BTS  378  uses a high-capacity line  410  to communicate the selected or combined data—in the form of digital data signals  514 —back into the IP network  16 , from which they are transmitted, through an additional high-capacity line  424 , to a gateway  18 . The gateway  18  is connected to an outside network portion  372 . The outside network portion  372  delivers the data to their ultimate destination. 
     In the downlink direction, data are received, from the outside network portion  372 , into the gateway  18  and are transmitted, through a high-capacity line  426 , into the IP network  16 . The data are then transmitted, in the form of digital data signals  516 , from the IP network  16 , through high-capacity line  406 , to the primary BTS  378 . The primary BTS  378  distributes the downlink data to the various secondary BTSs  382  by sending the data—in the form of digital data signals  514 —through high-capacity line  410  into the IP network  16 , from which the data are distributed—in the form of digital data signals  512 —to the secondary BTSs  382  through additional high-capacity lines  408 . The secondary BTSs  382  then communicate the data in the form of wireless data signals  504  to the MU  12  using wireless transmission—i.e., through wireless paths  412 . In addition, the primary BTS  378  sends data signals  504  directly to the MU  12  through wireless link  414 . 
     A communication system in accordance with the present invention can, for example, perform wireless communication within the widely used 800 MHz cellular band, the widely used 1.9 GHz PCS band, or the 2.4 GHz band which is currently unlicensed. However, the present invention can be used for wireless communication at any frequency over any wireless link, and the discussion herein is not meant to imply any limitation of the frequency range and/or electromagnetic spectrum within which the invention can be practiced. 
       FIG. 3  illustrates an example of a BTS for use in a cellular communication system in accordance with the invention. The BTS  300  includes an interface  50 , a wireless interface  52 , a processor  44 , and a data storage device  56 . The interface  50  couples the BTS  300  to the local network  16 , and the wireless interface  52 —which can be, e.g., an RF modem—couples the BTS  300  to one or more mobile units such as the mobile unit  12  illustrated in  FIGS. 1 and 4 . The data storage device  56  stores information for use by the other components of the BTS  300 . Such information can include computer code for execution by the processor  44 , and can also include information associating one or more MUs with multicast groups, time-slot assignments, frequency assignments, spreading code assignments, and/or other suitable information. The processor  44  manages and controls the operation of the various elements within the BTS  300 . 
       FIG. 16  illustrates an example of a controller  4000  for use in a cellular communication system in accordance with the present invention. The controller  4000  illustrated in  FIG. 16  can be, for example, a BSC  20  as illustrated in  FIGS. 1 and 4 . The controller  4000  includes an interface  4002 , a processor  44 , and a data storage device  4006 . The interface  4002  connects the controller  4000  to the local network  16 . The data storage device  4006  stores information for use by the other elements of the controller  4000 . Such information can include computer code for execution by the processor  44 . The processor  44  manages and controls the operation of the other components within the controller  4000 . 
       FIG. 5  is a functional block diagram illustrating an example of a processor  44  for use in the gateway  18  illustrated in  FIG. 2 , the BTS  300  illustrated in  FIG. 3 , or the controller  4000  illustrated in  FIG. 16 . The processor  44  generally includes a processing unit  510 , control logic  520 , and a memory unit  530 . Preferably, the processor  44  also includes a timer  550  and input/output ports  540 . The processor  44  can also include a co-processor  560 , depending on the microprocessor used in the processing unit  510 . The control logic  520  provides, in conjunction with the processing unit  510 , the control necessary to handle communications between the memory unit  530  and input/output ports  540 . The timer  550  provides a timing reference signal for the processing unit  510  and the control logic  520 . The co-processor  560  provides an enhanced ability to perform complex computations in real time. 
     The memory unit  530  can include different types of memory, such as volatile and non-volatile memory and read-only and programmable memory. For example, as shown in  FIG. 5 , the memory unit  530  can include read-only memory (ROM)  531 , electrically erasable programmable read-only memory (EEPROM)  532 , and random-access memory (RAM)  533 . Different processors, memory configurations, data structures, and the like can be used to practice the present invention, and the invention is not limited to a specific processor. 
     When included in a BSC such as the BSC  20  illustrated in  FIGS. 1 and 4 , or when used in a BTS such as the BTSs  14 ,  382 , and  378  illustrated in  FIGS. 1 and 4 , the processor  44  illustrated in  FIG. 5  can be used to perform an SDU function such as described above. 
     Although the present invention has been described in connection with specific exemplary embodiments, it should be understood that various changes, substitutions, and alterations can be made to the disclosed embodiments without departing from the spirit and scope of the invention as set forth in the appended claims.