Patent Publication Number: US-6215982-B1

Title: Wireless communication method and device with auxiliary receiver for selecting different channels

Description:
TECHNICAL FIELD 
     The present invention relates generally to wireless communication systems, and more particularly to a device used therein including an auxiliary receiver for selecting different channels. 
     BACKGROUND OF THE INVENTION 
     In recent years, the use of cellular communication systems having mobile terminals which communicate with a hardwired network, such as a local area network (LAN) and a wide area network (WAN), has become widespread. Retail stores and warehouses, for example, may use cellular communications systems to track inventory and replenish stock. The transportation industry may use such systems at large outdoor storage facilities to keep an accurate account of incoming and outgoing shipments. In manufacturing facilities, such systems are useful for tracking parts, completed products and defects. 
     A typical cellular communication system includes a number of fixed base stations or access points interconnected by a system backbone. Also included in many cellular communication systems are intermediate base stations which are not directly connected to the system backbone. Intermediate base stations, often referred to as wireless base stations or repeaters, increase the area within which base stations connected to the system backbone can communicate with mobile terminals. 
     Associated with each base station is a geographic cell. A cell is a geographic area in which a base station has sufficient signal strength to transmit and receive data from a mobile terminal or other device with an acceptable error rate. Typically, base stations will be positioned along the backbone such that the combined cell area coverage from each base station provides full coverage of a building or site. Further, it is also typical to have the cell area of coverage from two or more base stations to overlap or be co-located. 
     Wireless communication systems such as those described above require that a base station and a mobile terminal communicate on the same frequency channel in order to exchange information. Often times, the noise level on a particular channel may become excessive and therefore a base station, for example, will initiate a move to a different frequency channel where better system performance can be achieved. Prior to changing to a different frequency channel, however, the base station must go off-line from its current channel to search for other channel candidates and to evaluate the current noise conditions of those channels. Unfortunately, by going off-line the base station can no longer communicate with other devices on what had been the current channel utilized by the base station. As a result, communications between the base station and any mobile terminals registered thereto are suspended so as to reduce overall system performance. 
     As an example, in a known frequency agile direct sequence spread spectrum (DSSS) system a base station is able to select among a plurality (e.g., five) available channels on which to communicate. On occasion, the base station may determine that the noise conditions on the current channel are too high for reliable communications and therefore decide to move to a new channel among the available channels. In order to determine which channel to move to, the base station broadcasts a message to be received by all mobile terminals registered to the base station indicating that the base station will be going temporarily off-line. This avoids the possibility of a mobile terminal transmitting information to the base station during such a time when the base station is not configured to receive such information. The base station then utilizes its transceiver to scan communication conditions (e.g., noise conditions) on all other available channels. 
     Based on such analysis, the base station then determines whether it is desirable to change to a new channel which may offer improved communication conditions (e.g., lower noise conditions). If more favorable conditions are available on another channel, the base station then returns to the original channel and attempts to inform all mobile terminals to jump to the newly selected channel. Otherwise, the base station simply remains on the original channel and informs the mobile terminals that the base station is back on line. 
     There are, however, a number of drawbacks associated with such an approach for determining to which channel the base station should change, if at all. The requirement that the base station go off-line in order to search for other channels significantly reduces overall system performance. Additionally, the base station typically assesses the noise conditions on each of the other channels over a short period of time and often leads to skewed results when, for instance, noise conditions are high or low on a particular channel due to conditions which are only temporary. Similar difficulties also exist for mobile terminals which evaluate the noise conditions in order to initiate channel switching. 
     In view of the aforementioned shortcomings associated with conventional communication systems involving different channels on which the base stations and mobile terminals may communicate, there is a strong need in the art for a system and method for minimizing loss in system performance associated with devices initiating a change in communication channels. Further, there is a strong need in the art for a system and method of changing channels utilizing information which accounts for temporary fluctuations in the communication conditions on other channels. 
     SUMMARY OF THE INVENTION 
     The wireless communication device and method according to the present invention minimizes the aforementioned problems associated with searching for a new channel. Specifically, the present invention introduces an auxiliary receiver or transceiver which is included in a base station or mobile terminal in addition to a transceiver used to communicate between devices. The auxiliary receiver or transceiver serves to monitor substantially continuously the noise conditions on available communication channels other than the channel currently being utilized for communication between devices. Whenever the noise conditions on the current channel goes above a preset threshold level, for example, the base station or mobile terminal is informed by virtue of the operation of the auxiliary receiver or transceiver as to the best alternative channel to which to change. Alternatively, even if the noise conditions on the present channel are not above a predetermined threshold, the base station or mobile terminal still may change to another channel if there is significantly less noise on another channel. Further, since the auxiliary receiver or transceiver can substantially continuously monitor all other channels, an average noise condition can be determined so that temporary noise conditions on a given channel do not skew the selection process. 
     According to one particular aspect of the invention, a wireless communication device is provided, including: a transceiver including a transmitter and a receiver for transmitting and receiving wireless communications selectively on any of a plurality of channels; and an auxiliary receiver for evaluating communication conditions on at least one of the plurality of channels while the transceiver communicates on another of the plurality of channels, and for providing information based on the communication conditions to the transceiver. 
     According to another aspect of the invention, a method is provided in relation to a wireless communication device including a transceiver having a transmitter and a receiver for transmitting and receiving wireless communications on any of a plurality of channels, and an auxiliary receiver. The method includes the steps of: the transceiver communicating on a channel selected from the plurality of channels; the auxiliary receiver evaluating communication conditions on each of the plurality of channels while the transceiver communicates on the selected channel, and selecting another channel for the transceiver to communicate on from among the plurality of channels based on the information provided by the auxiliary receiver. 
     In accordance with still another aspect of the invention, a cellular communication system is provided. The system includes a backbone; a plurality of base stations each coupled to the backbone; at least one mobile terminal, each of the at least one mobile terminal communicating with the backbone via a selected one of the plurality of base stations, and each of the base stations including: a transceiver for communicating with the at least one mobile terminal and including a transmitter and a receiver for transmitting and receiving wireless communications selectively on any of a plurality of channels; and an auxiliary receiver for evaluating communication conditions on at least one of the plurality of channels while the transceiver communicates with the at least one mobile terminal on another of the plurality of channels, and for providing information based on the communication conditions to the transceiver. 
     To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a wireless cellular communication system in accordance with the present invention; 
     FIG. 2 is a block diagram of a base station including an auxiliary receiver in accordance with the present invention; 
     FIG. 3 is a table containing noise condition information which is maintained in memory based on information provided by the auxiliary receiver in accordance with the present invention; 
     FIG. 4 is a flowchart suitable for programming the base station of FIG. 2 to select a new channel based on noise conditions in accordance with the present invention; 
     FIG. 5 is a block diagram of a base station including an auxiliary receiver and transmitter according to another embodiment of the present invention; 
     FIG. 6 is a flowchart suitable for programming the base station of FIG. 5 to select a new channel based on noise conditions in accordance with the present invention; 
     FIG. 7 is a block diagram of a mobile terminal including an auxiliary receiver or an auxiliary receiver and transmitter in accordance with the present invention; and 
     FIG. 8 is a block diagram of a wireless base station including an auxiliary receiver or an auxiliary receiver and transmitter in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. 
     Referring initially to FIG. 1, a wireless network in the form of a cellular communication system  100  is shown in accordance with the exemplary embodiment of the present invention. The cellular communication system  100  includes a local area network  105  having a system backbone  110  and a plurality of base stations  115  coupled thereto. The backbone  110  is shown to be a hardwired data communication path made of twisted pair cable, shielded coaxial cable or fiber optic cable, for example. Alternatively, the backbone  110  could be wireless in nature so as to provide an added dimension of flexibility. As is conventional, each base station  115  serves as an access point through which wireless communications may occur between devices coupled to the system backbone  110  and one or more mobile terminals  117  included in the system  100 . 
     In the exemplary embodiment, the system  100  is a direct sequence, spread spectrum (DSSS) system in which each of the base stations  115  is capable of communicating on any one of a plurality of channels at different respective frequencies. Thus, for example, if the noise level for a particular channel on which a base station  115  is operating becomes excessive, the base station  115  will determine whether to initiate a change to another channel found to have less noise. In this sense the system  100  is considered to be frequency agile whereby each base station  115  can selectively choose among a plurality of channels on which to operate. In the event a base station  115  initiates a change to another channel, the base station  115  communicates such information to any mobile terminals  117  which are registered to the base station  115 . Each mobile terminal  117  is configured to adjust its own parameters accordingly so as to operate on the newly selected channel. 
     By way of example, each base station  115  together with the other devices in the system are designed to operate in either the 902-928 MHz or 2.4-2.48 GHz bands. Such bands represent unlicensed bands provided by the FCC for low power communication devices in the U.S., although operation in other bands is certainly within the scope of the invention. Within each band there are five predefined channels A through E at different respective frequencies on which DSSS communications can be carried out. Each base station  115  is capable of selecting any one of the channels A through E on which to communicate as is discussed more fully below. 
     According to conventional DSSS techniques, communications involving the base stations  115  and mobile terminals  117  involve utilizing a predetermined spreading code known as a pseudo noise (PN) sequence to spread the data being transmitted. This involves spreading each data bit which is transmitted into a plurality of sub-bits, commonly referred to as chips, using the PN sequence. Data which is received is despread according to the same PN sequence. 
     In order to expand the effective communication range of the base stations  115 , one or more wireless base stations  120  also may be included in the cellular communication system  100 . As is conventional, each wireless base station  120  associates itself, typically by registration, with another base station, whether hardwired or wireless, such that a communication link is formed between itself and other devices situated on the system backbone  110 . 
     Each base station  115 ,  120  is capable of wirelessly communicating with other devices in the system  100  via a respective antenna  125 . A geographic cell  127  associated with each base station  115 ,  120  defines a region, or area of coverage, in which successful wireless communication may occur. Depending on the type of antenna  125  and the output power of the respective base station, the cell  127  may take one of several different forms and sizes. For example, in the event the antenna  125  is an omni-directional antenna, a generally spherical cell area coverage is obtained. However, a directed yagi-type antenna or other form of antenna could also be used as will be readily appreciated. 
     According to the exemplary embodiment of the present invention, each base station  115  also includes an auxiliary antenna  129 . The auxiliary antenna  129  preferably is of the same type as described above with reference to the antenna  125  and provides equivalent cell coverage  127 . As is discussed more fully below, the auxiliary antenna  129  is coupled to an auxiliary receiver or transceiver included in the base station  115 . The auxiliary receiver or transceiver is configured to operate substantially independently of a main transceiver also included in the base station  115  and which is used for carrying out conventional cellular communications. Specifically, the auxiliary receiver or transceiver in combination with the antenna  129  allows the base station  115  to scan substantially continuously the noise conditions of any or all of the available communication channels A through E to determine if, and when, the base station  115  may wish to switch from one channel to another. For example, if a base station  115  is currently operating on channel A and the auxiliary receiver determines that noise conditions on channel C are much lower, the base station  115  may initiate a change from channel A to channel C. Such configuration has advantages over prior art systems having only one transceiver and one antenna, in that the base station  115  is able to maintain continuous, uninterrupted communications with the mobile terminals  117  in its cell  127  area even during the scanning process. 
     As previously mentioned, the cellular communication system  100  includes one or more mobile terminals  117 . Each mobile terminal  117  communicates with devices on the system backbone  110  via a selected base station  115 ,  120 . Upon roaming from one cell  127  to another, the mobile terminal  117  is configured to associate itself with a new base station  115 ,  120 . 
     FIG. 2 represents a block diagram of a given base station  115  within the system  100 . The base station includes a main transceiver  200  and an auxiliary receiver  201  which are each controlled by a microprocessor  202 . The main transceiver  200  includes a transmitter  204  and a receiver  206  for respectively transmitting and receiving conventional cellular communications via the antenna  125 . The transceiver  200  is conventional in design and is capable of operating on any one of five different channels A through E as controlled by the microprocessor  202 . Control signals and data are exchanged between the microprocessor  202 , the transmitter  204  and the receiver  206  by way of a control/data bus  208  connected therebetween. Since the particular design of the main transceiver  200  is generally conventional and is not necessarily germane to the present invention, further detail is omitted. 
     The auxiliary receiver  201  as shown in FIG. 2 is provided in the base station  115  to scan substantially continuously the channels A through E on which the main transceiver  200  is not currently operating on in order to evaluate noise conditions. Thus, the provision of the auxiliary receiver  201  avoids the necessity of the receiver  206  in the main transceiver  200  having to go “off-line” with respect to the mobile terminals  117  in order to evaluate noise conditions on the other available channels as in conventional devices. Consequently, overall system performance is greatly enhanced as will be appreciated. The auxiliary receiver  201  includes a radio frequency (RF) section  222  and a modulation section  224 . As is described more fully below, the receiver  201  is controlled by the microprocessor  202  with respect to the operation of the RF section  222  and the modulation section  224 . A memory  228  such as a RAM or the like is also included in the receiver  201  and can serve as data storage. In addition, the memory  228  includes a non-volatile portion for storing appropriate operating code to be executed by the microprocessor  202  for carrying out the functions described herein. The manner in which the microprocessor  202  can be programmed to carry out such functions will be readily apparent to those having ordinary skill in the art based on the description provided herein. 
     Signals which are received by the auxiliary antenna  129  are provided to an RF downconverter circuit  234  included in the RF section  222 . The RF downconverter circuit  234  is driven by a frequency synthesizer  236  which produces an output frequency on line  238  which is input to the RF downconverter  234 . The RF downconverter circuit  234  includes a mixer (not shown) which mixes the incoming signals from the antenna  129  down to a corresponding base band signal provided on line  240 . The frequency synthesizer  236  is controllable by the microprocessor  202  via line  242  in order to control the specific channel on which the receiver  201  receives a signal. Specifically, the microprocessor  202  provides control information to the frequency synthesizer  236  which causes the synthesizer to selectively produce an output frequency on line  238  corresponding to the carrier frequency of any of channels A through E. Accordingly, by adjusting the output frequency of the frequency synthesizer  236  the microprocessor  202  can determine whether the receiver  201  receives signals on channel A, B, C, D or E. 
     The RF section  222  also includes a conventional received signal strength indicator (RSSI) circuit  242  which produces an output on line  244  indicative of the RSSI level of any signals received on a particular channel at a given time. The output of the RSSI circuit  242  is provided to the microprocessor  202  which samples the output as described below in order to evaluate the noise conditions of each particular channel A through E. 
     The base band signal provided on line  240  from the RF downconverter circuit  240  is input to a PN decoder circuit  250  included in the modulation section  224 . The PN decoder circuit  250  is designed to despread the signal from the RF downconverter circuit  240  according to the particular PN sequence utilized in the system  100 . The actual PN sequence is not critical to the invention, although the PN decoder circuit  250  is designed to include an output on line  252  which is indicative of a degree of correlation between any data in the received signal and the PN sequence. As will be appreciated, as the receiver  201  receives signals on any given channel (i.e., channels A through E), the receiver  201  receives any noise which may be existent on the particular channel at that time. In addition, the receiver  201  may receive intelligible signals which are being transmitted from the base stations  115 ,  120  or mobile terminals within the system  100  and which happen to be operating on that particular channel. Thus, if the output on line  252  indicates a strong degree of correlation (correlated) it can be assumed that the received signal represents an intelligible signal and that a device within the system  100  currently is using that particular channel. If the output on line  252  indicates a low degree of correlation (non-correlated), this indicates that the received signal represents unintelligible noise. The output on line  252  is provided to the microprocessor  202  which utilizes such information in the manner described below to determine which, if any, channel to change to during operation. Although the preferred embodiment utilizes both the RSSI signal and the degree of correlation to determine which channel is best to be operating on, it should be appreciated that alternative embodiments may only use one or the other but not both. 
     The despread data which is output from the PN decoder  250  is provided to the input of demodulator circuit  251  which is designed to carry out on the received signal whatever particular type of demodulation utilized in the system  100 . For example, if the system  100  is configured to utilize binary phase shift keying (BPSK) modulation the demodulator circuit  251  is designed to demodulate the received signal according to BPSK techniques. Of course, other types of modulation could also be used without departing from the scope of the invention. 
     The output of the demodulator  251  is provided to a symbol to data converter  253  which performs symbol to data conversion in accordance with the communication parameters of the system  100 . Thus, the output of the symbol-to-data converter  253  on line  254  represents any system data which may have been included in the received signal. Such data is coupled to the microprocessor  202  where it is evaluated for content if desired. 
     The base station  115  in this embodiment includes a single microprocessor  202  which operates at a sufficiently high rate to control both the main transceiver  200  and the auxiliary receiver  201  substantially simultaneously (e.g., via multitasking). However, it will be appreciated that the transceiver  200  and receiver  201  could each have their own dedicated microprocessor for providing control. Information between the dedicated microprocessors can then be exchanged in order to control which particular channel the respective devices operate on. 
     The base station  115  is connected to the system backbone  110  via a connector  256 . The connector  256  is connected to the backbone  110  at one end and to a network adapter transceiver  258  included in the base station  115  at the other end. The network adapter transceiver  258  is configured according to conventional network adapter transceiver techniques to allow the base station  115  to communicate over the network. The network adapter transceiver is coupled to the microprocessor  202  via line  260 . Information received via the backbone  110  can be transmitted via the main transceiver  200  and vice versa. 
     FIG. 3 represents a table  264  maintained by the processor  202  in the memory  228 . The table  264  contains noise condition information for each of the available channels A through E in the system  100 . As is described below in connection with FIG. 4, the microprocessor  202  is programmed to cause the auxiliary receiver  201  to scan continuously through each of the channels A through E sampling the noise conditions on each of the channels on which the transceiver  200  is not operating. By simply adjusting the output of the frequency synthesizer  236  the auxiliary receiver  201  is controlled so as to receive incoming signals according to the sequence B, C, D, E, B, C, . . . , etc. assuming, for example, the transceiver  200  is operating on channel A. For each channel in the sequence, the microprocessor  202  is programmed to sample the RSSI signal on line  244  and the correlation output on line  252  for a sample time period of Tsample. The period Tsample is preselected so as to be at least as long as the maximum time interval between any beacons which are transmitted by a given base station  115 ,  120  within the system. As is conventional, base stations  115 ,  120  in a passive scanning system broadcast beacons periodically which enable mobile terminals  117  to lock on to a base station and register therewith. In the exemplary embodiment the period Tsample is one second, although other time periods are certainly possible. 
     The information obtained via the auxiliary receiver  201  from the RSSI level and correlation output samples from each channel is then maintained in the table  264 . Further, the main transceiver  200  under the control of the microprocessor  202  periodically samples the RSSI level and correlation degree of the current channel on which the main transceiver  200  is operating. Such information is obtained by the receiver  206  in the same manner as done by the auxiliary receiver  201  with respect to the channels which the main transceiver  200  is not currently working on. This avoids the auxiliary receiver  201  having to spend time sampling noise conditions on the channel the main transceiver  200  is currently operating on; and since the main transceiver  200  already is on the current channel, there is no need for the main transceiver  200  to go “off-line” and change to another channel. Nevertheless, in another embodiment the auxiliary receiver  201  is responsible for obtaining the noise condition information with respect to all of the available channels. 
     The noise condition information obtained by the auxiliary receiver  201  for the other available channels and by the main receiver  206  for the current channel is provided to the microprocessor  202  which processes and stores the information in the table  264 . More specifically, the table  264  includes five different rows corresponding to channels A through E, respectively. For each channel the table  264  includes information relating to the noise level thereof as determined by the RSSI level and correlation output from the PN decoder. Specifically, in column  266  the table includes the average RSSI level (e.g., in decibels) for the corresponding channel during the most recent sample period Tsample. However, the average RSSI level as stored in column  266  is calculated by the microprocessor  202  only for such times during the time period Tsample that the correlation output on line  252  does not exceed a predetermined threshold (i.e., is considered non-correlated). Column  268  includes the average RSSI level as calculated by the microprocessor  202  during the most recent sample period Tsample, but in this case only for such times during the sample that the correlation output on line  252  is equal to or exceeds the predetermined threshold (i.e., is considered correlated). Such existence of a correlated signal is considered herein to be indicative of a use condition of a channel whereby the channel is actually in use by a device. 
     In order to detect the occurrence of temporary noise conditions on a particular channel, the table  264  also includes an average of the RSSI levels for both non-correlated and correlated signals over a predetermined number of most recent samples. For example, column  270  in the table  264  contains the average RSSI level over the five most recent samples of the respective channel as computed by the microprocessor  202  during such times when the correlation output on line  252  indicates a non-correlated signal. Similarly, column  272  contains the average RSSI level over the five most recent samples of the respective channel as computed by the microprocessor  202  during such times when the correlation output on line  252  indicates a correlated signal. 
     The table  264  also includes a column  274  which includes a flag for indicating whether a beacon was received during the most recent sample of the respective channel. As discussed above in relation to FIG. 2, in the event an intelligible signal is received by the auxiliary receiver  201 , such signal is demodulated, decoded and converted in the modulation section  224 . The information in the received signal is then provided to the microprocessor  202  via line  254 . The microprocessor  202  is programmed to analyze the information received on line  254  to determine whether the received signal includes a beacon which was sent from another base station  115 ,  120  in the system  100 . Such determination is based on conventional techniques including analyzing the appropriate data fields contained in the information provided on line  254 . If a beacon is received on a particular channel during a time Tsample, this indicates that a base station  115 ,  120  within the system  100  is currently operating on the particular channel and is within the cell coverage of the present base station  115 . Since it is desirable to avoid two or more base stations using the same channel within range of each other, a flag is set in column  274  to indicate whether a beacon has been received in the most recent sample. It is noted that such information is maintained only for the most recent sample since there is the possibility that the base station from which a previous beacon originated may itself have changed to another channel. 
     FIG. 4 is a flowchart representing the operation of the base station  115  with respect to monitoring the noise conditions of the available channels via the auxiliary receiver  201  and changing channels based thereon. Step  300  represents startup of the base station  115  in response to being powered up via an on/off switch or the like. During step  300  the base station  115  under the control of the microprocessor  202  carries out a self-initialization routine as is conventional. In addition, during step  300  the main transceiver  200  selects a default channel (e.g., channel A) on which to operate. Of course, in an alternative embodiment the main transceiver  200  may not select a default channel but rather may immediately perform a scan as discussed below to select on which channel to begin operations. 
     Following step  300 , the microprocessor  202  proceeds to step  302  in which the auxiliary receiver  201  is utilized to obtain a sample of the RSSI level on line  244  and the correlation output on line  252  for each channel. Specifically, if the main transceiver  200  is currently operating on channel A the microprocessor  202  controls the output frequency of the frequency synthesizer  236  such that a sample can be obtained by the auxiliary receiver  201  for each of channels B, C, D and E. The microprocessor  202  computes the average RSSI level for each sample for both the non-correlated and correlated signals and stores such information for each channel in the table  264  (FIG. 3) in columns  266  and  268 , respectively. In addition, the microprocessor  202  in step  302  computes the average RSSI levels for the non-correlated and correlated signals over past five samples for each channel and stores such information in columns  270  and  272 , respectively. As mentioned above, identical information for channel A on which the transceiver  200  is currently operating on is obtained by the main receiver  206  and is also stored in the table  264 . In the event the microprocessor  202  in step  302  has not yet obtained five samples for each channel, the average RSSI levels are computed based on the actual RSSI samples and a remaining number of phantom samples set to equal a total of five. Each phantom sample has an average RSSI level of zero, for example. 
     Furthermore, the microprocessor  202  in step  302  determines if a beacon was received on any of the channels during the samples taken in step  302 . If yes, the microprocessor  202  sets a flag in column  274  indicating the same. If no, the microprocessor  202  resets a flag in column  274  so as to indicate such fact. 
     The microprocessor  202  then proceeds from step  302  to step  304  in which the microprocessor  202  determines whether at least five RSSI samples have been acquired for each channel. If no, the microprocessor  202  proceeds to step  306  in which the base station  115  pauses before returning to step  302  to obtain another RSSI sample from each channel A through E. The duration of the pause in step  306  can be anywhere from zero to several seconds, for example, depending on how often it is desired that the auxiliary receiver  201  update the noise condition information. In the exemplary embodiment, the microprocessor  202  pauses for five seconds in step  306  before returning to step  302 . 
     If in step  304  the microprocessor  202  determines that at least five samples have been obtained for each of the channels A through E, the microprocessor  202  proceeds to step  308 . In step  308 , the microprocessor  202  is programmed to determine whether the noise conditions for the channel on which the main transceiver  200  is currently operating exceed a predetermined threshold level. More specifically, in step  308  the microprocessor  202  looks to the information in column  270  of table  264  to determine if the average RSSI level for non-correlated signals over the last five samples exceeds a predetermined threshold indicating noisy conditions. If the main transceiver  200  is currently operating on channel A, the microprocessor  202  will look to column  270  in relation to channel A. Similarly, if the main transceiver  200  is currently operating on one of the other channels, the microprocessor  202  compares the information for that particular channel with the predetermined threshold. If the noise conditions are above the threshold then it is assumed that the current channel is too noisy to maintain reliable communications and therefore the microprocessor  202  should attempt to identify another channel on which the base station  115  can operate. 
     It is noted that in step  308  the microprocessor  202  does not consider the information in column  272  pertaining to the RSSI signal level for correlated signals. Since the main transceiver  201  is operating simultaneously with the steps shown in FIG. 4, the main transceiver  201  and/or mobile terminals registered thereto is likely to be actively involved in communications on the current channel at such times when the RSSI levels are sampled for that particular channel. Hence, the RSSI levels during the correlated signals are not likely to represent noise or activities of another base station, but rather are likely to be indicative of operations of the base station itself. 
     If in step  308  the microprocessor  202  determines that the average RSSI level in column  270  does not exceed the predetermined threshold so as to indicate a low-noise condition, the microprocessor  202  proceeds to step  310 . In step  310 , the microprocessor  202  determines whether any of the other available channels exhibit even better noise conditions (i.e., less noise). More specifically, the microprocessor  202  looks to the average RSSI levels over the last five samples as reflected in column  270  of table  264  for each of the channels other than the channel on which the main transceiver  200  is currently operating on. The microprocessor  202  determines whether the average RSSI level in column  270  is more than a predetermined amount less than that of the current channel. For example, the microprocessor  202  determines if any of the other channels have an average RSSI level in column  270  which is more than 10 db less than that of the current channel. If no, it is assumed that the main transceiver  200  is currently on the optimum channel and the microprocessor  202  proceeds to step  312 . In step  312  the base station  115  pauses in the same manner described above with respect to step  306  prior to returning to step  302  in order to reevaluate the noise conditions on each channel. 
     In the event the microprocessor  202  in step  310  determines that the noise conditions on another channel are significantly better, the microprocessor  202  proceeds to step  314  in which a new channel is selected according to a predefined selection criteria. Similarly, if in step  308  the microprocessor  202  determines that the noise conditions on the current operating channel of the main transceiver  200  exceed the predetermined threshold, the microprocessor  202  proceeds to step  314 . In step  314  the microprocessor  202  carries out the following predefined selection criteria for selecting a new channel on which the main transceiver  200  is to operate: 
     a) with the exception of the current channel being utilized by the main transceiver  200 , exclude from consideration any channels on which a beacon was received in the most recent sample as determined from the information in column  274 ; 
     b) among the remaining channels and again with the exception of the current channel, determine which channels (if any) have an average RSSI level for correlated signals with respect to the last five samples (as reflected in column  272 ) which is zero or near zero; then, to the extent any such channels exist, compare the channel having the lowest average RSSI level for non-correlated signals (column  270 ) with the current channel, and select as a new channel whichever has the lowest average RSSI level for non-correlated signals as represented in column  270 ; 
     c) to the extent that none of the remaining channels have an average RSSI level for correlated signals which is zero or near zero as determined in b), compare from among the remaining channels the channel having the lowest average RSSI level for non-correlated signals (column  270 ) with the current channel of the main transceiver  200 , and select as a new channel whichever has the lowest average RSSI level for non-correlated signals as represented in column  270 . 
     Thus, the predetermined selection criteria in step  314  is utilized by the microprocessor  202  to select a new channel on which the main transceiver  200  can operate. The particular criteria applied in step  314  can be modified to include different parameters, weighting of various criteria, etc., and the criteria explained above is intended merely to be exemplary. Furthermore, it should be appreciated that it is possible that the same channel on which the main transceiver  200  is currently operating on may be selected in step  314  as the new channel in the event the current channel continues to exhibit the best operating conditions as far as the channels available. 
     Following step  314 , the microprocessor  202  proceeds to step  316 . In step  316 , the microprocessor  202  provides control information on line  208  which causes the main transceiver  200  to broadcast information on the current channel informing each of the mobile terminals  117  and wireless base stations  120  registered to the base station  115  of an upcoming channel change. Specifically, the main transceiver  200  broadcasts an information packet indicating to the mobile terminals  117  and wireless base stations which particular channel the base station  115  will be changing to and at what time such change will occur. A protocol for implementing such a channel change routine may be based on the type described in U.S. Pat. No. 5,142,550, although other protocols are certainly within the intended scope of the invention. Such information allows the mobile terminals  117  and wireless base stations  120  to reconfigure themselves in order to change together with the base station  115  to the newly selected channel. Alternatively, such devices may independently change after a time-out period in the event the mobile terminal or wireless base station did not receive the channel change notice. Following step  316 , the microprocessor  202  proceeds to step  318  in which it causes the main transceiver  200  to switch to the newly selected channel at the time noted in the information broadcast in step  316 . Thereafter, the microprocessor  202  proceeds to step  312  and then back to step  302  as the main transceiver  200  continues to operate on the new channel. The above procedure is then repeated. 
     In the event that the newly selected channel in step  314  ends up being the same channel as which the main transceiver  200  was currently operating on, there is no need for the microprocessor  202  to carry out steps  316  and  318 . As a result, in the preferred embodiment the microprocessor  202  is programmed to detect in step  314  whether the new channel is the same as the current channel. If so, the microprocessor  202  bypasses steps  316  and  318  and proceeds directly to step  312  (not shown). 
     As will be appreciated, the process for evaluating the noise conditions of the various channels and instructing the main transceiver  200  to change channels as represented in FIG. 4 is carried out while the main transceiver  200  carries out normal operations. In other words, it is not necessary for the main transceiver  200  to go “off-line” as in the past so as to evaluate the noise conditions on other channels. Consequently, system performance is increased in the present invention. 
     Referring now to FIG. 5, a different embodiment of the base station  115  is shown. In this embodiment, an auxiliary transmitter  350  is added to the base station  115  in addition to the auxiliary receiver  201  so as to form an auxiliary transceiver  352 . The construction and operation of the main transceiver  200  and the auxiliary receiver  201  are substantially identical to the embodiment of FIG.  2 . Consequently, only the relevant differences will be discussed herein. The auxiliary transmitter  350  provides the base station  115  with the ability to continue to communicate with any mobile terminals  117  or wireless base stations  120  which may have missed a broadcast message to change channels. Although the mobile terminals  117  and wireless base stations typically are designed to start searching for another base station and/or channel upon realizing that the base station to which they were previously registered is no longer available (e.g., has changed to a different channel unbeknownst to the mobile terminal), the auxiliary transmitter  350  is used to actively seek out such mobile terminals and/or wireless base stations. Thus, after the main transceiver  200  has switched to a new channel the auxiliary transceiver  350  is used to rebroadcast information to any remaining mobile terminals and/or wireless base stations informing them to change channels as is discussed more fully below. 
     As is shown in FIG. 5, the auxiliary transmitter  350  includes a data-to-symbol converter  354  which receives binary information to be transmitted from the microprocessor  202  via line  356 . The data-to-symbol converter  354  converts the data from the microprocessor  202  into symbol data by performing a conversion inverse to that performed by the symbol-to-data converter  253  in the auxiliary receiver  201 . The symbol data from the data-to-symbol converter  354  is then modulated by a modulator  356  which modulates the data based on the same modulation technique employed by the demodulator  251  in the auxiliary receiver  201 . The modulated data is then encoded or spread by a PN encoder  358  using the same PN sequence employed by the PN decoder  250  discussed above. 
     The output of the PN encoder  358  is provided to an RF upconverter circuit  360  which mixes the modulated signal onto an RF carrier as determined by the microprocessor  202 . More specifically, the RF upconverter circuit  360  is driven by the frequency synthesizer  236  in the same manner as the RF downconverter circuit  234  in order to transmit the modulated information selectively on either channel A, B, C, D or E. By providing a control signal on line  242 , the microprocessor  202  causes the transceiver  352  to transmit and receive signals on a selected channel. The RF upconverter circuit  360  includes a mixer (not shown) which mixes the modulated signal from the modulator  358  onto a carrier frequency provided by the frequency synthesizer  236 . The carrier frequency is the same frequency provided to the RF downconverter  234  which allows signals to be received on the same channel. 
     The modulated RF signal which is output from the RF upconverter circuit  360  is coupled to the transmit terminal of an antenna switch  362 . The receive terminal of the antenna switch  362  is coupled to the input of the RF downconverter  234 . The antenna  129  is coupled to the antenna terminal of the antenna switch  362 . The position of the antenna switch  362  is controlled by the microprocessor  202  via a line  364 . When the antenna switch  362  is in a receive position, the antenna  129  is coupled to the input of the RF downconverter circuit  234  so that signals may be received via the antenna  129  and the auxiliary receiver  201 . When the antenna switch  362  is in a transmit position, the antenna  129  is coupled to the output of the RF upconverter circuit  360  so that signals may be transmitted via the antenna  129  and the auxiliary transmitter. 
     FIG. 6 is a flowchart representing the operation of the base station  115  according to the embodiment of FIG. 5 with respect to monitoring the noise conditions on all of the available channels. Steps  300  through  318  in FIG. 6 are identical to those described above in relation to the flowchart of FIG.  4 . Such steps are carried out in the embodiment of FIG. 5 while the microprocessor  202  causes the antenna switch  362  to be in the receive position so that the noise conditions of the various channels can be evaluated. Following step  318 , however, the microprocessor  202  in the embodiment of FIG. 5 proceeds to step  370 . In step  370 , the microprocessor  202  initially switches the antenna switch  362  to the transmit position. Next, the microprocessor  202  causes the auxiliary transmitter  350  to broadcast a probe on the same channel which the main transceiver  200  had been communicating on immediately prior to changing channels in step  318 . As is conventional, the probe contains information which prompts any mobile terminals  117  and/or wireless base stations  120  which may be searching for a base station  115  with which to register to respond by transmitting a response to the probe on the same channel. Then, still in step  370 , the microprocessor  202  switches the antenna switch  326  to the receive position and begins “listening” for whether a response to the probe or any other activity is received via the receiver  201  to determine if any devices remain on the channel. 
     Following step  370  the microprocessor  202  determines in step  372  if a probe response or any other activity is received via the auxiliary receiver  201 . A probe response in particular can be detected by the microprocessor  202  based on the content of any received signal as provided on line  254 . Additionally, however, other activity on the channel can be detected simply by way of the correlation output on line  252 . If the output on line  252  exhibits a degree of correlation which exceeds a predetermined threshold (e.g., is considered correlated), the received signal is considered to represent activity on the channel as opposed to random noise. If no probe response nor any other activity is detected in step  372 , the microprocessor  202  proceeds to step  374  in which it determines if a predetermined amount of time has passed since the main transceiver  200  first switched to a new channel in step  318 . Such predetermined amount of time is selected to be of sufficient duration to assume that any devices not responding to a probe during such period have now changed over to the new channel selected in step  314 . For example, the predetermined amount of time in step  374  may be equal to the maximum amount of time required for a mobile terminal or wireless base station within the system to begin scanning on its own and identify a new base station and/or channel with which to register. The mobile terminal or wireless base station typically will begin to scan for such new base station and/or channel upon realizing that the transmissions of the mobile terminal or wireless base station are no longer being acknowledged by the previous base station. 
     If in step  374  the predetermined amount of time has not expired, the microprocessor  202  returns to step  370  in which the auxiliary transceiver  352  again broadcasts a probe and listens for a response or any other activity on the previous channel. If in step  374  the predetermined amount of time has passed, the microprocessor  202  assumes that no devices remain unregistered following the channel change. Consequently, the microprocessor  202  returns to step  302 . 
     In the event a probe response is received or activity is otherwise detected as determined in step  372 , this indicates that mobile terminals and/or wireless bases stations might exist which did not receive the channel change broadcast of step  316 . Consequently, in such case the microprocessor  202  proceeds to step  376  in which the microprocessor  202  causes the auxiliary transceiver  350  to rebroadcast the channel change information on the previous channel. Specifically, the microprocessor  202  switches the antenna switch  362  to the transmit position and causes the auxiliary transmitter  350  to broadcast again the information alerting any mobile terminals and wireless base stations of the change in channel of the main transceiver  200 . Thereafter, the microprocessor  202  in step  376  returns the antenna switch  362  to the receive position and begins again to listen for a response or any other activity via the auxiliary receiver  201 . Following step  376 , the microprocessor  202  returns to step  374 . 
     Accordingly, the embodiment of FIG. 5 provides for the auxiliary transceiver  352  to monitor noise conditions while the main transceiver  200  carries out normal communications. The auxiliary receiver  201  evaluates the noise conditions of the available channels, and the auxiliary transmitter  350  is utilized to contact any devices which may have inadvertently missed a channel change. As a result, channel changes may be carried out smoothly without the necessity of the main transceiver  200  having to go off-line temporarily. By utilizing an auxiliary transceiver  352  to access operating conditions, further enhancements may be added wherein the auxiliary transmitter  350  transmits test packets to base stations which respond with an indication of how well the base station was able to receive the transmitted packet. With this information the mobile device may better analyze operating conditions on a particular channel from both the mobile&#39;s position and the base station&#39;s position. 
     The present invention has been described above as it relates to the provision of an auxiliary receiver or transceiver in a base station. However, the invention can be implemented in a mobile terminal or a wireless base station as well. For example, it may be the case that a noise condition assessment is done by the auxiliary receiver or transceiver in a mobile terminal  117  or wireless base station  120 . The information regarding whether a less noisy channel exists is transmitted from the mobile terminal  117  or wireless base station  120  to the base station  115  which is notified by the mobile terminal or wireless base station when and to which channel to switch. The base station  115  then broadcasts such information to any other mobile terminals and wireless base stations registered thereto so that all of the devices know when to change channels. 
     FIG. 7 is a block diagram of a mobile terminal  117  in accordance with the invention. The mobile terminal  117 , like either of the base station embodiments in FIGS. 2 and 5, includes an auxiliary receiver  201  or transceiver  352 , a main transceiver  200 , a microprocessor  202  and a memory  228 . Instead of a hardwired connection to a system backbone, however, the mobile terminal  117  includes a display  400  driven by a display driver  402  which is coupled to the microprocessor  202 . The display  400  is utilized by the mobile terminal  117  to display data which is received by or input into the mobile terminal  117  as is conventional. The mobile terminal  117  also includes a user input device  404  such as a keypad or the like for allowing a user to input data and/or commands. In addition, the mobile terminal  117  includes a bar code reader  406  which allows the user to input information via a bar code. 
     The operation of the mobile terminal  117  with respect to evaluating noise conditions is substantially identical to the procedures described above in relation to FIG. 4 or  6 . However, with respect to the operation as described in FIGS. 4 and 6, step  316  instead involves the mobile terminal  117  transmitting the channel change information and timing to the base station  115  to which it is registered. In the meantime, the main transceiver  200  is free to carry on conventional mobile terminal communications within the system. 
     FIG. 8 shows an embodiment of a wireless base station  120  incorporating an auxiliary receiver  201  or transceiver  352 . The construction of the wireless base station  120  is substantially identical to that of the base station  115  with the exception that the wireless base station  120  is not coupled to the system backbone  110 . Instead, the wireless base station  120  serves as an intermediary with respect to communications between the mobile terminals  117  and the base stations  115 . The operation of the wireless base station  120  with respect to evaluating noise conditions is substantially identical to the procedures described above in relation to FIG. 4 or  6 . However, with respect to the operation as described in FIGS. 4 and 6, step  316  would be eliminated and upon deciding to change channels, the wireless base station  120  would simply register with a new base station which would in turn inform all other base stations of the new association, as is conventional. 
     In any of the above embodiments, a benefit is gained in that the communication link between the base stations and other devices need not be broken or interrupted so that an assessment of other potential channels to switch to can take place. 
     Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. For example, the auxiliary receiver/transceiver and the main transceiver are described above as utilizing separate antennas. However, in another embodiment the same antenna may be shared by both. Furthermore, additional steps may be taken when evaluating the noise conditions. For example, immediately prior to the main transceiver  200  broadcasting the channel change notice in step  316  as shown in FIG. 4, the auxiliary receiver  201  may sample the noise conditions on the new channel one last time to ensure that conditions have not changed since last sampling the channel and determining that the channel provides the optimum conditions. Further, in order to better avoid problems associated with intermodulation, the auxiliary receiver/transceiver  201 / 352  may be configured to only take RSSI samples when the main transceiver  200  is not transmitting information. 
     According to another variation, during such time as the microprocessor  202  samples the noise conditions of each of the available channels, the microprocessor  202  is programmed to ascertain the data rate of the received signal based on the correlation output on line  252 . As is conventional, the data rate between mobile terminals and base stations is increased in some systems in the event the mobile terminals and base stations are in close physical proximity and share a strong signal. This data rate information is stored for each respective channel in the table  264  and is used as part of the predetermined selection criteria performed in step  314  in order to select a new channel. Specifically, if the data rate exceeds a predetermined threshold thereby indicating that the source is relatively close to the base station seeking a new channel, the microprocessor  202  is programmed to give preference to other channels not exhibiting such a high data rate. This helps to avoid two base stations operating on the same channel in relatively close physical proximity. 
     In addition, it is noted that the present invention is not limited to a frequency agile DSSS system. The invention is useful in any system wherein a base station and a mobile terminal can communicate on two or more different communication channels. Also, although the analysis is performed as described above primarily with respect to noise level evaluation, changes between channels can also be based primarily on the amount of use of the other channels. For example, the auxiliary receiver determines which channel involves the least use as determined by the amount of time during each sample that a correlated signal is received as determined by the correlated output on line  252 . 
     The present invention includes all such equivalents and modifications, and is limited only by the scope of the following claims.