turning now to the figures , fig1 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 fig2 , 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 fig1 , 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 fig2 , fig2 is a more detailed block diagram of the receive baseband processor 120 shown in fig1 . 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 ( fig1 ). still referring to fig2 , 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 ( fig1 ) 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 fig2 , 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 fig8 ) 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 fig6 , the control unit 820 functions as a finite state machine capable of switching between two states 610 , 620 . referring to fig6 , the control unit 820 ( fig8 ) of the energy threshold detection unit 235 ( fig2 ) 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 fig7 . 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 fig2 , 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 fig7 , 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 fig7 . 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 fig2 , 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 fig2 , 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 fig2 , 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 ( fig9 ) 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  ) ) , 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 ( fig9 ) 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 : 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 ( fig9 ). as shown in fig5 , 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 fig5 , 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 fig5 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 fig2 , the ed and cs validation signals are sent to the cca unit 250 of the embodiment shown in fig2 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 fig1 , the cca unit includes a control unit 1010 configured as a finite state machine ( as depicted in fig4 ) 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 . 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 . 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 . 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 . 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 fig1 ) initiated at the beginning of the frame detection processing sequence times out or the end of the recognized frame is reached . 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 fig3 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 .