Abstract:
A first node initiating communications with a second node already in a secure network sends a discovery burst having a preamble portion and a payload portion. The preamble portion is sent at a varying frequency between high and low thresholds that are reflective of Doppler uncertainty between the nodes. The second node continuously listens at a frequency, termed an acquisition frequency. A data sequence in the preamble portion, known to the second node, is received and used to determine the receive instant in the preamble portion, and thereby compare against the known frequency ramp to determine the frequency at which the payload portion will be received. Preferably, the first node varies the preamble portion between thresholds more than once within the time span of a single preamble portion, and the preamble and payload portions are spread with different spreading codes. The preamble portion may also be disguised with noise generated by the first node.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to co-owned U.S. patent application Ser. Nos. 11/136,943 11/136,782 and 11/136,789, filed the same date as this application. Those related applications are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to signal acquisition, specifically, Doppler searches between a transmitter and receiver initiating contact with one another, which usually occur in a preamble of a transmission burst. It is particularly advantageous for spread spectrum communication systems. 
     BACKGROUND 
     In digital spread spectrum (DSS) communication, a wide band carrier signal is modulated by a narrow band message signal. The wide-band carrier is typically generated by modulating a single frequency carrier using a pseudo-random noise (P/N) code sequence. The data rate at which a message is communicated is usually much lower than the P/N code symbol or “chip” rate. The ability of DSS to suppress interference is proportional to a ratio of the chip rate to data rate. In many applications, there are thousands of code chips per data bit. 
     At the receiver, a carrier replica is generated by reducing the DSS signal to baseband and multiplying it with a locally generated replica of the original narrow-band carrier using a local oscillator. If the frequency and phase of the carrier replica is the same as that of the received original narrow-band carrier, then the multiplier output signal will be the product of the bipolar P/N code and intended message. The P/N code is removed by multiplying the wide-band data stream with the locally generated replica of the P/N code that is time aligned with the received P/N code. This is the de-spreading process. 
     Generating the carrier replica with proper carrier frequency and phase and generating the P/N code replica at the proper rate and time offset is a complex problem. In many DSS communication systems, the necessary carrier frequency, carrier phase, and P/N code offset are not known a priori at the receiver, which tries different values until a large signal is observed at the data-filter output. This is termed the search or acquisition process, and a DSS signal is said to be acquired when the proper frequency, phase, and code offset have been determined. A receiver selects and detects a particular transmitted signal by choosing the appropriate P/N code and performing the acquisition search. In some cases the acquisition search must include examination of different PIN codes from a known list when the transmitting node is not known, as is the likely scenario in  FIG. 1 . When many different codes, code offsets and carrier frequencies must be examined and the SNR is low, the acquisition task can be both time and energy consuming. 
     The above constraints are more pronounced in a secure environment such as that depicted in  FIG. 1  (detailed below), where a new node termed a hailing node  34  seeks to join an existing network while maintaining security for the joining node and those nodes already on the network. In addition, an established network requires a method of discovering the existence of another separate network that may have migrated into communication range, so that a cross-link can be established between the networks in order to form a larger network. This process of nodes “discovering” each other is termed herein node discovery, and is where DSS signal acquisition occurs. Typically, node discovery is done on channels separate from the primary data communication channels. Limited data exchange on the ‘discovery channel’ is preferable for network optimization. As a result, the discovery waveform must be flexible in the messages it carries and not be constrained to one specific message type or size. 
     The air interface should consist of a flexible and symmetric full-duplex or half-duplex link. The transmitting node or hailing node is that node that sends a discovery burst, essentially a message inquiring as to the presence of receiving nodes. Receiving nodes are the nodes that listen for that discovery burst. The receiving nodes are therefore target nodes, which may already have formed a network. These receiving nodes may become transmitting nodes when they send an acknowledgement back to the initiating new node. In this way, a new node that flies into range of an established network will transmit burst discovery messages on that transmitting node&#39;s transmit link. When a receiving node in the established network hears the discovery message on its receive link, it will respond via its transmit link which is the hailing node&#39;s receiving link. Subsequent handshaking can then be performed via the two node&#39;s transmit and receive links to bring the initiating new node into the network. The transmitting and receiving links may occupy separate time slots in a time division duplex (TDD) system, or may be separate frequency bands in a frequency division duplex (FDD) system. 
     An exemplary but non-limiting environment in which node discovery may be important is illustrated in perspective view at  FIG. 1 , a prior art arrangement of disparate nodes operating in a traffic data network and one hailing node seeking to join the traffic network. The nodes may be airborne as in aircraft; terrestrial as in autos, trucks, and trains; or waterborne as in ships and other surface watercraft. They may be stationary or mobile, fast or slow moving, as for example, communications between nodes in a building, an aircraft, and an auto. For additional flexibility, it is assumed that a hailing node  34  may not have a clock signal synchronized with the network prior to joining. The range  22  of the traffic data network is centered on a command node  24 , absent relays by other nodes within the network. Where the network links members via a satellite link, the line-of-sight range  22  is not particularly relevant. The range  22  is included to show further advantages of the invention that may be exploited when network communications are geographically limited. 
     The command node is representative of the node that receives the discovery burst, and may be a true command node that controls access to the secure network (in that no other nodes receive and acknowledge discovery bursts) or it may represent any node already established within the network that receives the discovery burst (such as where all established nodes listen for discovery bursts). In  FIG. 1 , all nodes depicted as within the traffic network range  22  communicate on the traffic network, either through the command node  24  or directly with one another once granted network entry. The traffic network typically operates by directional antennas  24   a , at least at the command node  24 , to maximize the network range  22 . This is because directional antennas typically enable a higher antenna gain and a higher tolerable path loss as compared to omni-directional antennas. Therefore, a range (not shown) of a discovery network that operates using omni-directional antennas  24   b  is somewhat less, at least in the prior art. The command node  24  maintains communication with stationary nodes  26 ,  28 . When two nodes are aircraft, they may be closing or separating from one another at very high rates, rendering Doppler effects significant. When a hailing node  34  sends a discovery burst to locate and request entry into the traffic network, its signal is typically not received at the command node  24  until the hailing node is within the traffic network range  22 . Since the hailing node  34  is not yet identified as authorized, this potentially puts communications within the network at risk, or alternatively unduly delays granting the hailing node  34  access to the network. Because access to the traffic network is obtained through the discovery protocol, that protocol must exhibit security features to prevent compromise of the traffic network. 
     Considering the issues apparent in light of  FIG. 1 , a good node discovery scheme for a highly secure communications network would therefore exhibit (a) high speed and reliability; (b) long range; (c) low probability of intercept (LPI) and low probability of detection (LPD) by unauthorized parties; (d) universal discovery and recognition among the various nodes; (e) asynchronous discovery; and (f) reliability for both stationary and fast-moving nodes. Each of these aspects are detailed further at co-owned and co-pending U.S. patent application Ser. No. 10/915,777 (filed on Aug. 10, 2004), herein incorporated by reference in its entirety. 
     Transmission bursts are normally divided into preamble and payload sections, payload carrying the substantive data. In a discovery burst of the prior art, the preamble and payload sections were at the same frequency and the receiving node would search among the possible frequency bins until it acquired the burst preamble. This prior art approach has been described as the receiver spinning its frequency search. In Doppler environments where transmitter and receiver may move relative to one another at a rate unknown prior to acquisition, as with the hailing and command nodes of  FIG. 1 , the frequency at which a discovery burst reaches a receiver is unknown to the extent of Doppler uncertainty. Ensuring the prior art receiver locks onto a discovery burst payload within the very short time of that burst preamble (e.g., on the order of milliseconds) with a high degree of probability requires a large hardware commitment. The present invention uses a different discovery burst regimen to reduce the hardware requirement in the receiver while simultaneously decreasing acquisition time in a highly secure communication environment. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect, the present invention is a method for establishing communications between a first and a second communication node. The method includes transmitting a waveform from the first node towards the second node. The waveform has a preamble portion and a payload portion, and the preamble portion is transmitted with a carrier frequency that varies in a range of carrier frequencies. The preamble portion further has a data sequence known to a second node. Further in the method, the preamble portion is received at a reception instant at the second node and at an acquisition frequency that lies within the range of carrier frequencies. A value of the data sequence at the reception instant is correlated at least with an expected value of the carrier frequency during reception of the payload portion. For example, any Doppler uncertainty may be resolved by the second node determining where along a varying carrier frequency ramp the reception instant occurred. Where the frequency ramp and the carrier frequency of the payload portion are known to the second node, the second node may correct its reception frequency for the payload portion by applying a Doppler correction calculated from the payload portion. 
     In accordance with another aspect of the invention, a transmitter includes a first and second switch, a controller, a modulator, and at least one transmit antenna. The first switch operates to switch between a variable frequency generator and a substantially constant frequency generator by alternately coupling to one or the other of them. The second switch operates to switch between a first PN code input and a second PN code input by alternatively coupling to one or the other of them. The controller operates the first and second switches simultaneously. The modulator has an input coupled to an output of each of the first and second switches, and has an output coupled to an input of the at least one transmit antenna. Preferably, the variable frequency generator provides a carrier frequency that varies non-linearly in a range of carrier frequencies, and the varying carrier frequency crosses a particular frequency within the range at least twice while the first switch remains continuously coupled to the variable frequency generator. That the carrier frequency varies does not imply is must vary continuously; it may be stepped in discrete frequencies that vary from one another in a manner that approximates a linear or a specific non-linear continuous frequency ramp. 
     In accordance with another aspect, the invention is a transmitter for sending a discovery burst. The transmitter includes means for impressing a data sequence in a burst preamble, wherein each and every point of the data sequence is indicative of a unique position within the preamble. These means may be a digital counter or any number of number generators known in the art, or may be drawn from a memory of the transmitter. The data sequence may be sequential (as in a known sequence, not necessarily increasing or decreasing by one count with each data point), or each data point may be matched to a specific frequency or limited range of frequencies. The transmitter further has means for applying a variable frequency profile to the burst preamble and means for applying a constant frequency to a burst payload. These may be, respectively, variable and constant frequency generators as known in the art. The transmitter further has means for combining the burst preamble at the variable frequency and the burst payload at the constant frequency within a single discovery burst. These means may include a gate, a switch, or any of numerous apparatus known for combining signals from different inputs in a seriatim manner. The transmitter further has means for transmitting the discovery burst, which is preferably one or more transmit antennas. 
     These and other features, aspects, and advantages of embodiments of the present invention will become apparent with reference to the following description in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is described below more particularly with reference to the following drawing figures, which are not to scale except where stipulated. 
         FIG. 1  is a schematic diagram of a particularly challenging prior art communication system in which a hailing node seeks entry into a communication network, and is an apt environment for the present invention. 
         FIG. 2A  is a profile of a frequency ramp over which the hailing node of  FIG. 1  transmits the burst preamble according to an alternative embodiment of the present invention. 
         FIG. 2B  is a profile of a frequency ramp over which the hailing node of  FIG. 1  transmits the burst preamble according to the preferred embodiment of the present invention. 
         FIG. 2C  is a profile of the frequency ramp of  FIG. 2B , compressed in time within a search interval that is about one-fourth the duration of the burst preamble, with consecutive search intervals alternating the direction of the frequency ramps. 
         FIG. 3A  is an overview of discovery search intervals by a command node of  FIG. 1  plotted against a discovery burst by a hailing node of  FIG. 1 . 
         FIG. 3B  is a detailed view of  FIG. 3A  showing response of the command node during the preamble of a discovery burst when the hailing node is not synchronized, in accordance with the preferred embodiment of the present invention. 
         FIG. 4  is a high-level block diagram showing a receiver having a controller directing various burst receivers to investigate and lock onto an individual candidate discovery burst that is reported by the search engine of  FIG. 3 . 
         FIG. 5  is a block diagram of a transmitter according to the preferred embodiment of the present invention. 
         FIG. 6  is a graph of transmission power for the burst preamble and payload, and depicting an approach to improve security for the burst preamble. 
     
    
    
     DETAILED DESCRIPTION 
     Consider again  FIG. 1 . A hailing node  34  seeking entry into the network is unaware of the location of the command  24  or other nodes already communicating on the network. Communication on the traffic channels may be done with a very long P/N code for high security, but discovery of new nodes such as the hailing node  34  generally operate with less complex PN codes apart from traffic P/N codes to ensure security for the longer traffic codes. The present invention is particularly described in the context of a discovery protocol for a hailing node  34  to join the secure communications network of  FIG. 1 . 
     As is typical, a discovery burst  36  waveform is divided by time into a burst preamble  38  portion and a consecutive burst payload  40  portion (see  FIGS. 3A-3B ). Security in the network of  FIG. 1  is enhanced when a PN code used in a burst preamble  38  differs from that used in the burst payload  40 . As compared to the prior art, the present invention shifts some complexity from the receiver of the command node  24  to the transmitter of the hailing node  34  by having the hailing node  34  transmit discovery bursts  36  at various frequencies, between a high threshold (maximum Doppler) and a low threshold (minimum Doppler) within the burst preamble  38 , while the receiver of the command node  24  listens at an acquisition frequency between those thresholds. Since the complexity of spinning a signal at the transmitter in a regular way is potentially lower than performing bin searches at the receiver, overall system complexity is reduced. The command node  24  essentially ‘parks’ its receiver at an acquisition frequency awaits the discovery burst, which it detects at a reception instant. A known data sequence in the preamble  38  discloses to the command node  24  exactly where in the discovery burst preamble  38  the reception instant lays. From this, the command node  24  may calculate when the payload begins and set up one of several receivers to receive the payload  40  in time. 
       FIG. 2A  is a graph showing a simple constant-ramp  48   a  frequency profile over which the hailing node  34  transmits the discovery burst  36 . In general, the hailing node  34  transmits a burst preamble within each search interval  42  according to a variable frequency profile that varies between a high  52  and a low  50  threshold. The command node  24  may simultaneously listen among several frequencies in parallel receivers as detailed below to minimize instances of no receiver lock. Arbitrarily, the frequency profile is depicted as beginning at a minimum Doppler frequency  50  and continuing to a highest Doppler frequency  52 . Given some knowledge of the maximum likely or absolute maximum speeds of platforms (e.g., aircraft, ship, etc.) that may define each of the various nodes in a network, the Doppler limits  50 ,  52  (the Doppler uncertainty) between unaware nodes are readily determined. An acquisition frequency  54  is that frequency between the minimum  50  and maximum  52  Doppler frequencies at which the command node  34  receiver detects and locks onto the burst preamble  38 . 
     While the constant ramp  48   a  frequency spin of  FIG. 2A  provides for a simple description, the preferred embodiment uses a continuously variable rate ramp  48   b  such as that depicted in  FIG. 2B . This takes advantage of the statistical probability that for two nodes  24 ,  34  unaware of the other&#39;s relative position or velocity, the probability curve that any particular frequency is the acquisition frequency describes a bell curve centered exactly between the min  50  and max  52  thresholds. The continuously variable rate ramp  48   b  of  FIG. 2B  approximately reflects that acquisition frequency probability curve, spinning frequency at a rate more slowly in the areas of highest probability (i.e., nearer the median frequency). Specifically, for a single burst preamble  38 , the transmitter of the hailing node  34  begins transmitting at a threshold frequency and spins toward a median frequency at a decreasing rate. Upon reaching the median frequency, it spins toward the other threshold at an increasing rate. This ensures more time transmitting the burst preamble  38  near the median frequency, where statistically the acquisition frequency is more likely to be, and less time near the threshold frequencies where it is less likely. It is understood that the frequency ramp need not be analog and continuous, but may be digitally stepped and frequency-discontinuous to approximate an analog ramp. 
       FIG. 2C  illustrates that the frequency ramp of  FIG. 2B  is compressed within a search interval  42  that is less in time than the entire burst preamble  38 . The frequency ramp is reversed in direction for each consecutive search interval within the burst preamble, allowing multiple crossings of the acquisition frequency  52  (which is unknown to the transmitting node) and more opportunities for the potential receiver to detect the discovery preamble  38  in time to set up and receive the burst payload  40 . 
     The content of the burst preamble is now described. The data pattern sent during the preamble  38  is preferably a simple countdown, or it may be a pseudo-random pattern known in advance to the command node  24 . In either case, each data bit or symbol denotes a position within the preamble  38  (either in time or along a known frequency ramp) that makes it possible for the command node  24  to resolve its Doppler frequency uncertainty and location in the preamble as soon as lock has occurred. For example, assume the frequency varies among four frequency steps: 900 MHz, 1100 MHz, 1300 MHz, and 1500 MHz. One possible data sequence embedded within the burst preamble  38  would include four unique symbols, for example, 00 representing 900 MHz, 01 representing 1100 MHz, 10 representing 1300 MHz, and 11 representing 1500 MHz. As the data sequence is already known to the command node  24  (as well as the profile of the frequency ramp), it then knows where along the frequency ramp the preamble  38  was when received, and may then set-up and tune a receiver to meet the frequency of the preamble  38  anywhere else along the frequency ramp. Where the preamble  38  uses a different PN code than the payload  40 , the preamble preferably also contains some information to inform the receiver of the command node  24  what the PN code of the payload may be (e.g., symbols indicating which constituent sub-codes the payload PN code is made from and/or in what order they are combined, and perhaps an indication of phase, a time that the payload PN information becomes valid, and any necessary encryption. Where the command  24  and hailing  34  nodes are asynchronous, a synch word  59  ( FIG. 3A ) is preferably disposed at the end of the preamble  38  to denote exactly where the payload  40  begins. 
       FIG. 3A  is an overview of discovery search intervals  42  plotted against a discovery burst  36  that is transmitted by a hailing node  34  of  FIG. 1 . Preferably, the search intervals  42  are seriatim and of equal periods that repeat within the span of one burst preamble  38 , allowing the command node&#39;s receivers several ‘looks’ for the preamble  38  within the time span of a single preamble burst  38 , further ensuring enough time for the command node to set up one of the receivers to receive the payload  40 . 
     The terminus of each search interval  42  is marked in  FIG. 3A  as either “no-detect”,  42 , wherein the command node  24  does not detect a burst preamble  38  (e.g., there is no reception instant), or “detect”  46 , wherein the command node  24  senses the presence of a burst preamble  38 . Because more than one hailing node  34  may seek entry into the network even within the span of one discovery burst  36 , the command node  24  continues to search for additional hailing nodes  34  even when it detects  46  the presence of one hailing node  34 . Because the burst preamble  38  and payload  40  preferably use different PN codes, detect  46  as used in this detailed description relates only to the burst preamble  38 , not to the burst payload  40 . As such, the command node  24  continues to listen even after one hailing node  34  is detected, though not all operable burst receivers may be available if one is locked onto a payload of another hailing node  34  or setting up to investigate another potential burst preamble  38 . Preferably and as detailed below, the period of the search interval  42  is less than half of the time period of the burst preamble  38  to allow at least two detect opportunities for a single burst preamble  38 . Most preferably, the search interval  42  allow at three detect opportunities in a single burst preamble  38 , necessitating that the period of the burst preamble  38  be greater than three times the search interval  42  in order to allow for receiver lock, setup, and carrier phase adjustment. 
       FIG. 3B , which is not to scale with  FIGS. 2A-2B , depicts further detail of timing within the burst preamble  38  where the command  24  and hailing  34  nodes are not synchronized to a common clock. Where a common clock is available, the synchronization features described below may be eliminated. Preferably, the discovery burst  36  runs at a symbol rate of 500 symbols per second with QPSK modulation and direct sequence spreading. The burst preamble  38  is preferably not encoded with forward error correction. The burst preamble  38  preferably is very short and of fixed length, preferably no more than several hundred msec. 
     Assume for  FIG. 3B  that a receiver detecting a burst preamble will require N dwell  symbols to detect and lock onto the acquisition frequency  54 . T RXsetup , reference number  55 , represents the number of symbols (e.g., sixteen) designated for receiver setup, which permits a link control processor (LCP) of the command node  24  to be notified by a search engine that a potential burst has been located. The LCP will respond by assigning a burst/traffic receiver out of a pool of available receivers to further investigate the potential burst. Several symbols (e.g., seven) are allotted for the newly assigned burst receiver to begin running at the correct frequency and phase (T phase , reference number  57 ) as determined by the hailing node&#39;s discovery burst  36 . A known symbol pattern in the burst preamble  38  may be used to permit the receiver in the command node  34  to identify its correct phase (e.g., four symbols). In an asynchronous discovery protocol, the burst preamble  38  terminates with a sync sequence or synch word  59  that permits the receiver to unambiguously identify when the burst preamble  38  ends and the burst payload  40  begins. For purposes of description, assume the synch word  59  is a Barker sequence spanning T synch  symbols (e.g., thirteen symbols). The payload begins immediately after the sync word  59 . When the payload section  40  of the burst  36  begins, the PN code will change to a more secure long-code and the payload data is preferably encoded with forward error control coding. 
     Preferably, the discovery burst preamble  38  is spun so as to allow command node  24  at least three opportunities or search intervals  42  to detect a single discovery burst  36  with sufficient time to prepare to receive the payload  40  of that same discovery burst. The worst possible timing between search interval  42  and burst preamble  38  has the search interval  42  beginning one dwell time prior to the start of the burst preamble  38  and resulting in a ‘no detect’  44 . Term this an incomplete search, terminating at reference number  56  of  FIG. 3B , and occupying (N dwell −1) symbols. The next three search intervals each occupy N dwell  symbols and terminate in a first  58 , second  60 , and third  62  attempt to detect. Assume detect  46  occurs on the third try  62 . The remaining time within the burst preamble  38  must be at least T RXsetup +T phase +T synch  symbols. The total burst preamble  38  size must therefore be at least (N dwell −1)+3N dwell +T RXsetup +T phase +T synch  symbols to ensure in all cases three complete detect attempts. To accomplish this within the several hundred msec limit for the burst preamble size noted above (e.g., 87 symbols) leaves the command node  24  about 32 msec (at 500 symbols/sec) for each detect attempt. 
     To minimize security risk, the preamble  38  carries no substantive data, allows detection while minimizing intercept, and informs as to the payload&#39;s presence and perhaps some information regarding its spreading code (where preamble  38  and payload  40  use different spreading codes). Even though every potential hailing node  34  may use the same PN code for the side channels at any given time, the probability of two users “colliding” and destroying each other&#39;s burst preamble is low. Due to the autocorrelation properties of properly doped composite codes (explained below), two separate discovery bursts  36  offset in time by more than a chip from one another should both be received by the command node  24  simultaneously without errors. 
     Longer code sequences create implementation challenges in that they require more memory in both the transmitter and receiver and more computation time and power in the receiver during the detection (autocorrelation) process. To enable a very high probability of successfully detecting any single arbitrary discovery burst preamble  38  of such a short duration, preferably the burst preamble uses a composite code constructed from one or more shorter sub-codes, and preferably doped in a manner that destroys autocorrelation at periodic intervals within the composite code that would otherwise be defined by the manner in which the sub-codes are combined. Further details may be obtained at co-owned U.S. patent application Ser. Nos. 10/915,776, and 10/915,777, each filed on Aug. 10, 2004. Each of those applications are incorporated herein by reference. Constraining the PN code used for the burst preamble  38  to repeat, for example every symbol, significantly reduces the search space to be scanned during those N dwell  symbols. Assume such a composite code is of length 100,000 and is doped so that autocorrelation is suppressed everywhere except symbol boundary epochs where the code repeats. 
     Since offset versions of a properly doped composite PN code do not correlate highly with the matched filter  66 , it is possible for any two discovery bursts  36  that are offset in time from one another by more than a chip to be simultaneously received. The search engine of  FIG. 4  reports to the LCP processor  78  the chip phase, carrier frequency, and chip frequency of each candidate discovery burst  36 . The LCP processor  78  responds by assigning burst/traffic receivers  80  from a pool  81  of available receivers to perform a further investigation of each discovery burst  36 . As long as the pool  81  of available receivers  80  is not depleted, there will be an available receiver to further investigate every received discovery burst  36  and extract the data from that burst&#39;s payload  40 . The number of receivers  80  in the pool  81  may be readily scaled for the estimated extent of the system. 
       FIG. 5  is a block diagram of a transmitter  66  according to a preferred embodiment of the present invention. A frequency spinner  67  varies a carrier frequency along a frequency ramp as described above, and a constant frequency multiplier  68  provides a constant carrier frequency. Each of these  67 ,  68  are alternatively coupled to a first switch  69 . A first PN code  70  such as the composite PN code made from two or more constituent sub-codes as noted above, and a second PN code such as one that does not repeat in 100 years, are each alternatively coupled to a second switch  72 . The first PN code  70  further includes the data sequence described above that informs the receiver as to a reception instant&#39;s position within the burst preamble portion  38 . The second PN code  71  further includes substantive data carried in the payload portion  40  of the discovery burst  36 . Each of the first and second switches  69 ,  72  are actuated in tandem by a common control  73 , which may be a single actuator or a common instruction from a processor that directs individual actuators associated with each switch  69 ,  72  to operate their respective switch. Each of the switches are coupled to a modulator  74 . An output of the modulator  74  is amplified at an amplifier  75  and transmitted by one or more transmit antennas  76 . 
     When the switches are in the position shown in  FIG. 5 , the burst preamble portion is processed and transmitted as follows. At the modulator, the first PN code and data sequence  70  are modulated onto a carrier frequency that varies according to a ramp (continuous or stepped) as detailed above and with reference to  FIGS. 2A-2C . At the end of the preamble portion  38 , which may be designated by a synch word added separately (not shown) from the first PN code and sequence  70 , the controller  73  flips both switches  69 ,  72 . Now the second PN code with data  71  is coupled to the modulator, where it is modulated onto a carrier wave at a constant carrier frequency. The output of the modulator  74  is amplified  75  and otherwise processed as known in the art to be transmitted via the transmit antenna  76 . 
     Considering that the secure environment described may possibly be compromised by an unwanted party receiving the burst preamble  38 , and that the burst preamble  38  has the least secure spreading code (e.g., shortest, higher autocorrelation than that of the burst payload  40 ), following is a method to increase its security. Using extra transmit power that must be reserved in the link budget in any system design (as detailed below), the hailing node  34  may ‘self-jam’ the discovery burst preamble  38  as shown graphically in  FIG. 6 . Assuming a large processing gain in the overall communication system, the hailing node can self-generate electronic ‘noise’  90  that is preferably stronger than the burst preamble  38 , preferably at least 5 or 10 dB stronger. While this would negligibly impact the ability of the command node  24  to detect and receive the burst preamble  38  as described above, it increased LPI by masking the very existence of the burst preamble  38  to eavesdroppers. It is possible to self-jam using self-generated noise that is at a power level less than the transmit power of the discovery burst preamble, or at a power level substantially the same as the discovery burst. Masking is more effective in minimizing probability of intercept when self-generated noise at least exceeds the transmission power of the substantive burst, and the more it exceeds the more effective the jamming masks. 
     Clearly, there is a tradeoff of masking the burst preamble  38  in noise  90 ; such an excess of noise greatly increases the probability of detection even while reducing probability of intercept. However, an eavesdropper in actual possession of a stolen transmitter will in many cases be unable to separate the burst preamble  38  sent by that transmitter from the overpowering noise  90  it also transmits (assuming the noise is at a higher transmit power). Because this self-jamming aspect is expensive in terms of link budget, and because the burst payload  40  preferably uses a much more secure PN code, self-jamming preferably occurs on the burst preamble  38  but not on the burst payload  40 . 
     If self-jamming is used, the level of the jamming should be at least a few decibels above the level of the true signal, unless the hailing node  34  is operating near its power amplifier saturation level. If this is the case, then it will be desirable to dedicate more of the hailing node&#39;s power to the true signal and less to the jamming. The true signal should, in general, be transmitted with an amount of power necessary to achieve the “power control set point” at the receiver of the command node  24 . The power control set point is defined as the energy level required by the command node, and is known in the art. Note that this is defined in terms of energy rather than power because hailing nodes  34  may send signals at different data rates and the power they must use to satisfy the command node  24  will vary with data rate. The ideal is that every signal will arrive at the command node  24  with the same energy per bit, regardless of rate. This implies that one hailing node transmitting at ten times the rate of another hailing node will need to have his signal arrive at the command node  24  at a 10 dB higher power level. Since it is anticipated that the discovery protocol operate on side channels (apart from traffic channels) at reduced data rates (e.g., on the order of 500 bps), it will generally be the case that the transmitters of hailing nodes will be backed off considerably from their peak level when sending discovery bursts  36 . Thus, it is normally the case that the hailing node transmitters will have ample available power to self-jam. 
     It is noted that the drawings and description presented herein are illustrative of the invention and not exhaustive. While there has been illustrated and described what is at present considered to be preferred and alternative embodiments of the claimed invention, it will be appreciated that numerous changes and modifications are likely to occur to those skilled in the art. It is intended in the appended claims to cover all those changes and modifications that fall within the spirit and scope of the claimed invention.