Abstract:
Techniques for and apparatus capable of implementing packet detection and signal recognition in wireless communications systems are disclosed. In particular, the disclosed techniques and apparatus incorporate at least one of relative energy detection operable on assessment of a relative energy threshold for an inbound signal borne across an RF channel, carrier sense operable upon on assessment of at least one of a peak-to-sidelobe ratio and peak-to-peak distance defined by the inbound signal, and comparison operable upon demodulated data corresponding to the inbound signal as compared to predetermined preamble data. Clear channel assessment is performed based on determinations undertaken by one or more of the aforementioned relative energy detection, carrier sense and comparison operations.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/268,156, filed Oct. 9, 2002. The disclosure of the above application is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to wireless communications, and is specifically concerned with clear channel assessment techniques to determine the presence or absence of a valid inbound signal. 
     BACKGROUND OF THE INVENTION 
     The past few years has witnessed the ever-increasing availability of relatively cheap, low power wireless data communication services, networks and devices, promising near wire speed transmission and reliability. One technology in particular, described in the IEEE Standard 802.11b-1999 Supplement to the ANSI/IEEE Standard 802.11, 1999 edition, collectively incorporated herein fully by reference, and more commonly referred to as “802.11b” or “WiFi”, has become the darling of the information technology industry and computer enthusiasts alike as a wired LAN/WAN alternative because of its potential 11 Mbps effective throughput, ease of installation and use, and transceiver component costs make it a real and convenient alternative to wired 10 BaseT Ethernet and other cabled data networking alternatives. With 802.11b, workgroup-sized networks can now be deployed in a building in minutes, a campus in days instead of weeks since the demanding task of pulling cable and wiring existing structures is eliminated. Moreover, 802.11b compliant wireless networking equipment is backwards compatible with the earlier 802.11 1 M/2 Mbps throughput standard, thereby further reducing deployment costs in legacy wireless systems. 
     802.11b achieves relatively high payload data transmission rates or effective throughput via the use of orthogonal class modulation in general, and, more particularly, 8-chip complementary code keying (“CCK”) and a 11 MHz chipping rate to bear the payload. As such, previously whitened or scrambled bitstream data of interest is mapped into nearly orthogonal sequences (or CCK code symbols) to be transmitted, where each chip of the CCK code symbol is quaternary phase modulated using QPSK (“quadrature phase shift keying”) modulation techniques. Meanwhile the common phase of each CCK symbol is jointly determined by the current and previous symbols using differential QPSK or DQPSK modulation scheme. Subsequent conversion into the analog domain prepares these CCK symbols for delivery over a wireless medium RF modulated on a carrier frequency within the internationally recognized 2.4 GHz ISM band to form the payload or PLCP Service Data Unit of an 802.11b compliant Physical Layer Convergence Procedure (“PLCP”) frame, a type of packet. The high-rate physical layer PLCP preamble and header portions forming the frame overhead are still modulated using the 802.11 compliant Barker spreading sequence at an 11 MHz chipping rate. In particular, the preamble (long format—144 bits, short format—72 bits) is universally modulated using DBPSK (“differential binary phase shift keying”) modulation resulting in a 1 Mbps effective throughput, while the header portion may be modulated using either DBPSK (long preamble format) or DQPSK (short preamble format) to achieve a 2 Mbps effective throughput. 
     An IEEE 802.11b compliant receiver receives and downconverts an incident inbound RF signal to recover an analog baseband signal bearing the PLCP frame, and then digitizes and despreads this signal to recover the constituent PLCP preamble, header and payload portions in sequence. The preamble and header portions are Barker correlated and then either DBPSK or DQPSK demodulated based on the preamble format used to recover synchronization data and definitional information concerning the received PLCP frame, including the data rate (Signal field in the PLCP header) and octet length (Length field in the PLCP header) of the variable-length payload or PSDU portion. The CCK encoded symbols forming the PLCP payload portion are each correlated against 64 candidate waveforms in received per symbol sequence in combination with DQPSK demodulation to verify and reverse map each into the underlying bitstream data of interest, at either 4 bits per symbol (5.5 Mbps) or 8 bits per symbol (11 Mbps) increments. 
     It should be appreciated that 802.11 and 802.11b signals operate in the 2.4 GHz ISM band and must therefore coexist with quite an array of dissimilar signals operating in the same frequency, including microwave ovens and digital phones. By definition, there are no licensure restrictions within the available RF channels of the ISM band, so 802.11 and 802.11b compliant transceivers must employ clear channel assessment techniques to determine if it is safe to transmit. In particular, there is an expected amount of ambient noise that the 802.11/802.11b transceivers must tolerate but still be able to transmit, but should not attempt to transmit while another in-range 802.11/802.11b transceiver is transmitting so as to maximize channel use and system throughput. In other words, it is desirable for 802.11 and 802.11b transceivers to know when the operating channel is occupied with valid traffic, and thus enter receive mode without attempting to transmit over such traffic. Likewise, it is desirable that these transceivers should be free to transmit on the operating channel while that channel is free of 802.11/802.11b traffic, even in the presence of a tolerable amount of noise or interference. 
     To this end, the 802.11 and 802.11b standards specify clear channel assessment (CCA) guidelines which are used to determine if a tuned RF channel contains valid PLCP frame traffic. Inbound signals in the tuned or operating RF channel which do not meet these CCA guidelines are considered to bear either corrupted frames, or represent interference or noise in the channel. The 802.11/802.11b CCA guidelines are organized in modes as follows:
         CCA Mode 1: Energy above threshold. CCA shall report a busy medium upon detecting any received energy above the ED threshold.   CCA Mode 2: Carrier Sense only. CCA shall report a busy medium only upon detection of a DSSS signal. This signal may be above or below the ED threshold.   CCA Mode 3: Carrier Sense with energy above threshold. CCA shall report a busy medium upon detection of a DSSS signal with energy above the ED threshold.   CCA Mode 4 (802.11b): Carrier sense with timer. CCA shall start a timer whose duration is 3.65 ms and report a busy medium upon the detection of a High Rate PHY signal. CCA shall report an IDLE medium after the timer expires and no High Rate PHY signal is detected. The 3.65 ms timeout is the duration of the longest possible 5.5 Mbps PSDU.   CCA Mode 5 (802.11b): A combination of carrier sense and energy above threshold. CCA shall report busy at least while a High Rate PPDU with energy above the ED threshold is being received at the antenna.
 
The 802.11 DSSS PHY receiver must perform CCA according to at least one of modes 1-3, and the 802.11b High Rate PHY must perform CCA according to modes 1, 4 or 5.
       

     Three of the five conventional CCA modes require thresholding inbound signal energy, and so this guideline is believed important. However, conventional transceivers simply compare inbound signal energy against the specified threshold, and report an energy threshold validation signal whenever the threshold is exceeded. Thus, the presence of strong interference in the operating channel, will cause (in the case of a CCA mode 1 implementation) or potentially may (in the case of a CCA mode 3 or 5) cause a false busy to be reported, and thus prevent the transceiver from transmitting, which may in turn cause transmission delay and lower effective data throughput. 
     Moreover, to implement CCA modes 2-5, conventional CCA carrier sense techniques are used to determine if a DSSS or High Rate PHY inbound signal is present, typically by thresholding a measure of the perceived Barker code lock. However, known techniques are relatively complex and are thus power inefficient and expensive to implement. Both cost and power consumption reduction are critical design goals in 802.11/802.11b transceiver implementation, it would be advantageous if simpler carrier sense techniques could be incorporated without materially affecting carrier sense sensitivity or recognition performance. 
     Further, while conventional CCA techniques look for valid PLCP header information (via CRC validation), there is no post-demodulation confirmation during receipt of the preamble. Checking for valid preamble receipt would be advantageous, especially where the inbound signal fades potentially below the inbound signal energy threshold, but the receiver is still able to successfully recover recognizable preamble information from the signal. 
     Finally, while the defined 802.11/802.11b CCA modes account accommodate a range of operational environments, they are not appropriate for every environment and channel condition. Therefore, it would advantageous to provide a transceiver capable of handling further CCA modes other than those defined by the 802.11/802.11b standards, preferably while retaining backwards compatibility with such standards. 
     SUMMARY OF THE INVENTION 
     To address these and other perceived shortcomings and desires, the present invention is directed in part to a packet detection unit and signal recognition method that includes at least one of relative energy detection operable on assessment of a relative energy threshold for an inbound signal borne across an RF channel, carrier sense operable upon on assessment of at least one of a peak-to-sidelobe ratio and peak-to-peak distance defined by the inbound signal, and comparison operable upon demodulated data corresponding to the inbound signal as compared to predetermined preamble data. Clear channel assessment is performed based on determinations undertaken by one or more of the aforementioned relative energy detection, carrier sense and comparison operations. 
     Further aspects of the present invention include a tranceiver, network interface apparatus, and information processor incorporating this packet detection unit, as well as a computer program product including computer readable program code capable of causing an information processor to perform one or more of these signal recognition aspects. 
     Additional aspects and advantages of this invention will be apparent from the following detailed description of certain embodiments thereof, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a high level block diagram of a wireless transceiver in accordance with an embodiment of the invention. 
         FIG. 2  is a more detailed block diagram of a receive baseband processing unit shown in  FIG. 1 . 
         FIG. 3  is a detailed block diagram of a packet detection unit according to an alternative embodiment of the invention. 
         FIG. 4  is a state transition diagram for the CCA unit shown in  FIG. 2 . 
         FIG. 5  is a state transition diagram for the CS unit shown in  FIG. 2 . 
         FIG. 6  is a state transition diagram for the ED unit shown in  FIG. 2 . 
         FIG. 7  is a sample plot of gain perceived by the ED unit over time. 
         FIG. 8  is a block diagram for the ED unit shown in  FIG. 2 . 
         FIG. 9  is a block diagram of the CS unit shown in  FIG. 2 . 
         FIG. 10  is a block diagram of the CCA unit shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Turning now to the figures,  FIG. 1  illustrates a wireless communications transceiver  100  according to an embodiment of the invention. In this embodiment, inbound RF signals potentially conveying an 802.11 or 802.11b compliant PLCP frame are picked up by the duplex antenna  110  and routed to the RF receiver unit  115  of a receiver  150  arranged in a manner consistent with the present invention. The RF receiver unit  115  performs routine downconversion and automatic gain control of the inbound RF signals, and presents an analog baseband signal containing the aforementioned 802.11b PLCP frame to the receive baseband processor  120 . The functions of the receive baseband processor  120  will be detailed below with reference to  FIG. 2 , including packet detection and channel busy consistent with the present invention, along with conventional symbol correlation and/or demodulation of the preamble, header and payload portions of each inbound 802.11b PLCP frame to recover bitstream data for receiver synchronization (preamble), frame or packet definition (header), or the actual inbound data of interest (payload). 
     Once recovered by the receive baseband processor  120 , the inbound data contained in the PSDU of each received 802.11b PLCP frame is delivered to a network interface such as the MAC layer interface  125  and then on to higher layer applications and devices being serviced by the transceiver  100 . 
     Outbound data intended for wireless transmission originating from the device(s) or application(s) being serviced by the transceiver  100  are delivered to the transmit baseband processor  135  of the transmitter  160  from the MAC interface  125 . Directives from the PMD sublayer (not shown) forming part of the MAC interface  125  and expressing the desired transmission mode, including the 802.11b 1, 2, 5.5 and 11 Mbps effective throughput modes are transferred to the transmit baseband processor as well for each PLCP frame/packet. The transmit baseband processor  135  formulates appropriate 802.11b PLCP frame, and symbol encodes the outbound data as specified by the PMD sublayer to generate a complete outbound 802.11b PLCP frame. As the frame or packet is being developed, it is converted into analog form suitable for upconversion and RF transmission by the RF transmitter unit  140  consistent with 802.11b physical layer requirements. 
     Though not shown in  FIG. 1 , the transceiver  100  may form an operational part of a network interface apparatus such as a PC card or network interface card capable of interfacing with the CPU or information processor of an information processing apparatus such as a desktop or laptop computer, and may be integrated within and constitute a part of such information processing apparatus. This network interface apparatus may alternatively form an operational component of a wireless communications access point such as a base station as will be appreciated by these ordinarily skilled in the art. 
     Turning now to  FIG. 2 ,  FIG. 2  is a more detailed block diagram of the receive baseband processor  120  shown in  FIG. 1 . So as not to obfuscate the teachings of the present invention, several 802.11 and 802.11b compliant directives and signals are not shown. As such, an inbound analog baseband signal potentially conveying an inbound 802.11/802.11b PLCP frame recovered by the RF receiver unit  110  of the receiver  150  is fed to the input of the automatic gain control unit (AGC)  210 . With the assistance of a feedback loop tied to the input of the FIR filter  315 , the AGC  210  adjusts the gain of the inbound baseband signal to maximize the dynamic range and performance of the analog to digital converter  310 , as is known in the art, assuming the inbound signal is valid. The AGC  210  also reports the gain adjusted signal to the RSSI unit  220  of the frame detection unit  200  for channel busy, as will be described in greater detail below. 
     The gain adjusted inbound analog signal generated by the AGC  210  is then sent to the digital converter  310  to convert it into digital form. With the aid of the 44 MHz clock, the ADC produces a corresponding digital signal sampled at 44 MHz. Next, this digital signal passes through the digital FIR LPF  315  to reject out-of-band interference, and the down sampler  320  to provide a 22 Mhz digital signal potentially bearing a PLCP frame of interest. 
     This 22 MHz signal next encounters two parallel baseband demodulation pathways. The first demodulation pathway, including the Barker correlator  330 , down sampler  335 , Rake  340  and the down sampler  345  is used to recover a despread 1 MHz signal representing the preamble and header portions of the inbound frame for symbol demodulation by the combination DBPSK/DQPSK demodulator  375 . This first demodulation pathway-demodulator combination  375  is also used for symbol decoding a base 802.11 PLCP frame payload in 1 Mbps/2 Mbps modes. The second demodulation pathway is used to symbol demodulate a high rate 802.11b payload portion of the inbound frame, and includes a 22 MHz to 11 MHz down sampler  350  following by a decision feedback equalizer  355 . To begin the CCK symbol decode process for 802.11b compliant payloads at 11 Mbps or 5.5 Mbps transmission modes, a CCK correlator  360  is provided. 
     An 802.11b compliant receive state machine (not shown) issues the HI_RATE_PSDU semaphore to control the modulation pathway selection mux  370  based on which portion of the inbound frame is being demodulated, as well whether 5.5 Mbps+ payload modulation modes are specified. The combination DBPSK/DQPSK demodulator  375  is used to recover the symbol encoded inbound data presented in the preamble, header and payload portions. The DBPSK/DQPSK demodulator is clocked at the symbol rate; i.e., 1 MHz for 1 Mb and 2 Mb modes, and 1.375 MHz for 5.5 Mb and 11 Mb modes. After symbol demodulation, the recovered inbound data is descrambled by the descrambler  380  in a known fashion, and delivered to the MAC interface  125  ( FIG. 1 ). 
     Still referring to  FIG. 2 , the frame detection unit  200  is provided to determine if the inbound baseband signal recovered from an RF channel tuned by the RF receiver  115  ( FIG. 1 ) is a valid signal at least capable of bearing an 802.11b compliant PLCP frame, and if so, keep the transceiver  100  in receive mode for the duration of the frame. 
     To implement relative energy threshold detection consistent with the invention, in the frame detection unit  200  of the embodiment shown in  FIG. 2 , an RSSI unit  220  is provided to receive and down sample the digital adjusted gain signal for the inbound baseband signal generated by the AGC  210  to a 2 MHz gain signal E, with E k  representing the signal E at the kth 2 MHz sample in time. E is sent directly to one input of the relative energy detection unit  235 , and is concurrently sent to a noise floor unit  230 . The noise floor unit  230  here constitutes an IIR low pass filter to generate a 0.5 μsec delayed signal n which tracks the original signal E using the following relationship: n k =αE k +(1−α)n k-1    
     The relative energy threshold detection unit  235  receives both E and n, and calculates the difference between the gain values each represents (via the gain differential unit  810  shown in  FIG. 8 ) as a gain change over time. Through the control unit  820 , the relative energy threshold detection unit  235  compares the gain change over time against one of two preferably programmable energy detection thresholds, based on the current state of the control unit  820 . Though not required, the relative energy threshold detection unit  235  of the present embodiment operates on a 1 MHz synchronized clock, and so E and n are effectively downsampled at a 1 MHz rate before their difference is compared against one of these thresholds. As shown in  FIG. 6 , the control unit  820  functions as a finite state machine capable of switching between two states  610 ,  620 . 
     Referring to  FIG. 6 , the control unit  820  ( FIG. 8 ) of the energy threshold detection unit  235  ( FIG. 2 ) is initialized to state  610  at the beginning of each frame detection processing sequence (such as when a new inbound signal is first received on the operating RF channel). As such, the energy threshold validation signal (ED) is set to false (e.g. logic level 0). While in state  610 , the control unit  820  monitors the gain change over time generated by the gain differential unit  810 . The control unit  820  remains in state  610  while the gain remains relatively stable (transition or trans. 3). If the gain change over time exceeds a first energy detection threshold, meaning that the gain is changing rapidly with the AGC  210  in a gain unlock condition and attempting to transition to receive mode from transmit protect mode in response to an inbound signal having relatively significant energy, the control unit  820  transitions to state  820  (trans. 2) and the energy threshold validation signal transitions to true (logic level 1). In turn, assertion of true energy threshold validation signal may cause assertion of the channel busy signal (CCA) true by the CCA unit  250 , depending on the CCA mode being implemented. 
     The control unit  820  remains at state  620  (trans. 4) while the gain change over time continues to exceed a second energy detection threshold to hold energy threshold validation signal true. However, once the gain change over time settles and the gain stabilizes, the control unit  820  transitions back to state  610  (2), and the energy threshold validation signal transitions back to logic level 0 or false. 
     Note that in this embodiment the second energy detection threshold is less than the first energy detection threshold to lengthen the window in which the energy threshold validation signal is asserted high by the control unit  820 . However, in alternative embodiments, the first and second thresholds may be equal or even reversed depending on particular CCA implementation goals. 
     Due to this “relative energy thresholding”, certain gain signal transitioning rather than the receive/transmit state of the AGC  210  is used to toggle the energy threshold validation signal. This difference is subtle, yet important in handling relatively strong, persistent interference in the operating channel. In conventional absolute RSSI thresholding, the energy threshold validation signal would be held as long as the strong interference is perceived by the AGC  210  as the inbound signal, thus at least potentially causing a conventional CCA unit implementing legacy CCA modes to consider the channel to be busy for the duration of the interference and erroneously holding the transceiver in receive mode. Take, for example, the gain curves shown in  FIG. 7 . The left curve  710  illustrates the RSSI output E, and the conventional absolute threshold is shown as horizontal line  730 . As E transitions from a relatively high value to a relatively low value over time (indicating that the AGC has unlocked and is transitioning from the high gain transmit protect state to a low gain receive state) a conventional energy threshold validation signal would transition true once E crosses the absolute threshold  730  and would remain so until the inbound signal diminishes significantly or disappears, thereby permitting E to drift upward towards high gain or receive mode. 
     However, in “relative energy thresholding” according to the present embodiment, the energy threshold validation signal is only held high between the point from where the difference between the n  720  and E  710  curves exceed the first energy detection threshold T RED1  to where it no longer exceeds the second energy detection threshold T RED2 . This helps the CCA unit discriminate between interference and a valid signal and respond more quickly with an idle channel determination in the presence of such interference, especially where additional validating criteria based on the content of the inbound signal such as preamble, carrier sense and header validation is assessed. 
     In the embodiment shown in  FIG. 2 , the noise floor unit  230  is only operational (and so tracks the RSSI output E) while the channel busy signal (CCA) is false and the AGC is not in the aforementioned gain unlock state (as indicated by the GAIN_UNLOCK signal). When not operational the output of the noise floor unit  230 , n is frozen to the last tracked value. For example, if the inbound energy is a valid 802.11/802.11b signal, the noise floor computation will stop upon gain unlock or CCA assertion. Thus, in this case and though not shown in  FIG. 7 , curve  720  representing n would remain constantly high while the inbound energy is detected. Thus, the energy threshold validation signal would hold true for the duration of the 802.11/802.11b packet. 
     If, however, strong noise, is instead detected, the noise floor unit  230  is initially held high due to the AGC  210  unlocking (GAIN_UNLOCK is asserted true). However, if the channel busy signal (CCA) remains idle for a few microseconds while the AGC is still in the unlock state, the AGC  210  of the present embodiment resets the GAIN_UNLOCK signal to false and the noise floor unit  230  begins to track (n curve  720 ) the falling gain signal E  710  as shown in  FIG. 7 . Before the noise floor unit  230  completely tracks the noise level, the energy threshold validation signal is temporarily asserted true—from where the difference between the n  720  and E  710  curves exceed the first energy detection threshold T RED1  to where it no longer exceeds the second energy detection threshold T RED2 , as previously discussed. Thus, CCA can still temporarily report a false busy depending on the CCA mode being utilized. 
     In particular, if 802.11/802.11b CCA mode 1 is supported (ED only) the CCA will report a false busy once the difference between n and E exceed the first energy detection threshold T RED1 . In the configuration shown in  FIG. 2 , this will cause the noise floor unit  230  to suspend tracking of the gain signal E. To counteract this, the CCA unit  250 , and the comparison unit  1020  in particular, can be used verify that the inbound signal is 802.11/802.11b compliant through preamble or header verification, such as through looking for the presence of the Start Frame Delimiter (“SFD”) situated at the end of a proper PLCP frame preamble. If the selected field is found in the correct sequence, e.g. the SFD field is confirmed immediately after the end of the sync symbols, approximately 128 μsec (long preamble format) or 28 μsec (short preamble format) after the signal begins, it may be assumed that the inbound signal is valid, and that CCA unit  250  will report a busy channel for the remaining duration of the packet. If, however, the SFD or other selected field not found at its predesignated place, the inbound signal is presumed invalid and the CCA unit will override the ED mode logic and transition CCA false, indicating that the channel is free and the transceiver  100  may transmit. 
     Alternatively, if the selected CCA mode also includes carrier sense thresholding (e.g. 802.11/802.11b modes 3 or 5), it is less likely, though still possible, for the CCA unit to still report a false busy, particularly where the carrier sense threshold levels are kept low. Again, SFD or other predefined field verification can be used to clear a false busy a number of microseconds after perception of the inbound signal by the AGC  210  begins. 
     Though not shown in  FIG. 2 , an absolute energy thresholding unit may be provided in addition to or as an alternative to the relative energy detection unit  235  to perform routine absolute energy thresholding of the inbound signal. 
     Referring back to  FIG. 2 , the frame detection unit  200  also includes the capability of providing carrier sense feedback to the CCA unit  250  through the carrier sense unit  240 . This carrier sense unit  240  takes the results of Barker correlation to the inbound signal to verify the presence of a valid DSSS signal. In particular, the Barker correlator  330 , in addition to feeding the div2 downconverter  335  and RAKE filter  340 , presents the correlation result of the digital form of the inbound baseband signal against the 802.11 Barker PN code to the input of the carrier sense unit  240 . 
     A peak-to-sidelobe ratio determination unit  910  ( FIG. 9 ) examines the real (I) and complex (Q) components of this correlation result to help determine that, in fact, the inbound signal presents a valid Barker modulated preamble or header. In particular, the determination unit  910  calculates a peak-to-sidelobe average ratio as follows: 
                 SQ   ⁢           ⁢   1     =         max   i     ⁢     (            I   i     ⁢        +        ⁢     Q   i     ⁢        )                 1   16     ⁢     (         ∑   i     ⁢          I   i            +          Q   i          -     2   ×       max   i     ⁢     (            I   i          +          Q   i            )           )           ,         
where i=half-chip index=0 . . . 21. Or, alternatively:
 
                 SQ   ⁢           ⁢   1     =         max   i     ⁢     (            I   i     ⁢        +        ⁢     Q   i     ⁢        )                 1   16     ⁢     (         ∑   i     ⁢     (            I   i          +          Q   i            )       -     2   ×       max   i     ⁢     (            I   i          +          Q   i            )         -       ∑   k     ⁢     (            I   k          +          Q   k            )         )           ,         
where i, k are half-chip indexes each ranging from 0 . . . 21, where k is the index of four sidelobes relatively distant from a local peak in the received signal. In the latter case, near-peak sidelobes are excluded from the SQ1 computation to counteract potential multipath interference and more effectively validate the inbound signal as being 802.11/802.11b compliant.
 
     In order to better validate the inbound signal as bearing valid Barker modulated information, the carrier sense unit  240  also includes a peak-to-peak detection unit  920  ( FIG. 9 ) to calculate the distance between consecutive peaks in the Barker correlation results provided by the Barker correlator  330 . In particular, the consecutive peak-to-peak distance is calculated as follows:
 
 pp   n =|max —   ind   n −max —   ind   n-1 |
 
     After the peak-to-sidelobe average ratio and the peak-to-peak distance are calculated, their results are thresholded against preferably programmable signal quality thresholds, again based on the current state of the control unit  930  ( FIG. 9 ). As shown in  FIG. 5 , the control unit  930  of the carrier sense unit  240  functions as a finite state machine capable of switching between two states  510 ,  520 . 
     Referring to  FIG. 5 , the control unit  930  of the carrier sense unit  240  is initialized to state  510  at the beginning of each frame detection processing sequence. At state  510 , the control unit  930  issues the carrier sense validation signal (CS) as false. While in state  510 , the control unit  930  monitors the peak-to-sidelobe average ratio (SQ1) generated by the peak-to-sidelobe determination unit  910  along with the consecutive peak-to-peak distance (pp n ) calculated by the peak-to-peak distance determination unit  920  (trans. 3). In this embodiment, if the SQ1 signal meets or exceeds a first signal quality threshold at least 3 out of 4 preceding SQ1 calculation iterations undertaken by the determination unit  910  and the pp n  is less than a maximum acceptable distance in at least 3 out of the 4 preceding pp n  calculation iterations undertaken by the determination unit  920 , the control unit  930  transitions to state  520  (trans. 1) and the carrier sense validation signal transitions to logic level 1 or true, thereby indicating that an 802.11/802.11b DSSS signal (e.g. PLCP preamble/header) has been perceived. Note, that in alternative embodiments consistent with the teachings of the present invention, satisfaction of either these SQ1 or pp n  conditions alone and/or setting different threshold criteria for the SQ1 or pp n  may be used to trigger the transition from state  510  to state  520 . 
     Returning to the embodiment shown in  FIGS. 5 and 9 , the control unit  930  maintains this state (trans. 4) and continues monitor SQ1 and pp n  while asserting the carrier sense validation signal true. If, at state  520 , the SQ1 signal meets or exceeds a second signal quality threshold no more than once out of the preceding four calculation iterations undertaken by the determination unit  910 , and/or (depending on the desired carrier sense tolerance) the pp n  meets or exceeds the maximum acceptable distance more than once during the preceding  4  calculation iterations undertaken by the determination unit  920 , the control unit  930  will transition back to state  510  and the carrier sense validation signal will transition back to false. This can occur in the case of a corrupted PLCP frame preamble or header, or in the case of a valid PLCP frame that has transitioned to the High Rate PPDU. The latter case is expected, and consistent with 802.11 and 802.11b CCA guidelines, the CCA unit  250  will nevertheless report the channel busy until the end of the packet (as indicated by the packet length in the received PLCP frame header) has been reached, and CS is ignored. 
     Note here that the number of calculation iterations used to assess the SQ1 and pp n  signals in this embodiment is a matter of design choice, and, consistent with the present invention any number of calculation iterations and thresholding may be used as long as a valid symbol-modulated signal can be recognized within relevant CCA tolerances such as those specified in the 802.11/802.11b standards. 
     Returning to  FIG. 2 , the ED and CS validation signals are sent to the CCA unit  250  of the embodiment shown in  FIG. 2  in order to at least assist in determining whether the transceiver  100  should consider the inbound signal as valid. This CCA unit  250  is configured to operate and support at least a subset of the conventional 802.11/802.11b CCA modes 1-5 described above as well as support additional signal validation modes A-E as detailed below. As shown in  FIG. 10 , the CCA unit includes a control unit  1010  configured as a finite state machine (as depicted in  FIG. 4 ) operating in one of two states  410 ,  420  based on either the ED and/or CS validation signal semaphores in isolation, or in combination with a PLCP preamble validation to be described below, depending on the desired signal validation mode A-E. In this embodiment, though not required, the CCA control unit  1010  assesses the ED, CS and/or preamble validation signals every microsecond. 
     The operation of the CCA unit  250 , including the CCA control unit  1010  will now be described with reference to the following signal validation modes. State  410  is in the initial state at the beginning of each frame detection processing sequence. 
     Signal Validation Mode A: ED Only 
     The ED validation signal is initially assumed to be false. While in state  410  (trans. 3), the CCA control unit  1010  monitors or polls ED, preferably every microsecond. The channel busy signal is held false, indicating that the operating RF channel is free of traffic and is idle. If the ED validation signal transitions to true (logic level 1), The CCA control unit  1010  transitions from state  410  to state  420  (trans. 1), and the channel busy signal transitions to true, indicating that the operating RF channel is considered to be busy with valid traffic. In this embodiment, State  420  is maintained (trans. 4) and the channel busy signal is held true until ED transitions false after the earlier of either antenna selection in a diversity implementation is complete or the CCA timer (default of 12 μsec) expires, the SFD field in the PLCP preamble is not found and the comparison unit  1020  of the CCA unit  250  determines that valid PLCP frame preamble symbols are not being received. The CCA comparison unit  1020  determines this in this embodiment by comparing the output of the descrambler  380  against predetermined preamble data including strings of 8 consecutive 1&#39;s or 0&#39;s identifiable with the 802.11/802.11b long preamble format or strings of 16 consecutive 1&#39;s or 0&#39;s identifiable with 802.11b&#39;s short preamble format. Consistent with the 802.11/802.11b standards, if the PLCP header is found to be corrupted or the SFD field is omitted, the CCA control unit  1010  will consider the inbound signal as invalid noise and consider the channel to be idle, thus transitioning the control unit  1010  back to step  410 . Further, if frame transmission is deemed complete, transition to state  410  will occur. 
     Though not required, the CCA control unit  1010  can be configured to delay a transition back to state  410  if, for example a recent antenna switch was performed in a receiver selection diversity implementation so as to allow e.g. the AGC  210  to retrain and settle. 
     Signal Validation Mode B: ED &amp; CS→Busy, ED &amp; CS→Idle 
     In this mode, the CCA control unit  1010  will transition from idle state  410  to busy state  420  if both the ED and CS validation signals are concurrently asserted true. In this embodiment, state  420  will be held until both the ED and CS validation signals transition false, the SFD field in the PLCP header is not found and the comparison unit  1020  of the CCA unit  250  determines that valid PLCP frame preamble data is not being received. Again, a recent antenna selection in a diversity implementation can forestall, at least temporarily, the transition back to state  410 . 
     Signal Validation Mode C: ED&amp; CS→Busy, ED∥CS→Idle 
     The operation of the CCA control unit  1010  is similar to that as described for validation mode B previously discussed, but for the 420 to 410 transition requires only that one of the ED and CS validation signals to transition false. 
     Signal Validation Mode D: ED with Timer 
     The transition from idle state  410  to busy state  420  occurs similarly to that described previously for signal validation mode A. However, the CCA control unit  1010  remains in busy state  420  with the input validation signal held true unit either the first of an 802.11/802.11b standards compliant 3.65 millisecond timer (e.g. timer  1030  shown in  FIG. 10 ) initiated at the beginning of the frame detection processing sequence times out or the end of the recognized frame is reached. 
     Signal Validation Mode E: ED &amp; CS with Timer 
     The transition from idle state  410  to busy state  420  occurs similarly to that described previously for signal validation mode B, and the transition back from busy state  420  to idle state  410  occurs when the first of either the 3.65 millisecond timer expires or the end of the recognized frame is reached. 
     The above embodiments were described with reference to an 802.11/802.11b PLCP frame format. However, several aspects and features of the present invention are not limited to the particular wireless frame format chosen. For example, since relative energy thresholding according to the present invention is not dependent on the content of the inbound signal, it could be applied to a range of wireless communications which could benefit from clear channel assessment, including, but not limited to, those compliant with one or more of IEEE 802.11a, IEEE 802.16a, and the forthcoming IEEE 802.11g High Rate PHY supplement to the 802.11 standards. 
     Moreover, carrier sense determination according to the present invention, though dependent on the type of modulation used, can accommodate a wide range of orthogonal class modulated communications, including, but not limited to, the aforementioned Barker modulated communications, CCK modulated communications consistent with 802.11b, and OFDM modulated communications consistent with IEEE 802.11a, IEEE 802.16a, and the forthcoming IEEE 802.11g High Rate PHY supplement to the 802.11 standard. 
     In the case where the inbound signal may be modulated in one of several ways at the outset, such as proposed in draft IEEE standard 802.11g, a frame detection unit such as that shown in  FIG. 3  as frame detection unit  390  may be used which includes plural carrier sense units  240  and  380 , to determine the inbound signal bears either Barker or OFDM modulated information. In such case, the CCA unit  385  here may include appropriate processing logic to handle plural carrier sense validation signals (which in this case would have an XOR relationship). 
     Moreover, while the above described embodiments describe certain componentry in terms of function and functional relationships, it should be realized that such functions and relationships can be conveniently implemented using a wide variety of discrete circuitry and logic, in combination with or alternatively through one or more general purpose or specific purpose information processors such as a microprocessor or digital signal processor programmed in accordance with these functions and relationships, as will be appreciated by those of ordinary skill in the art. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.