Abstract:
A system for TDMA-SS signal prioritization and collision mitigation is provided. The system includes a Hub transceiver having plurality of parallel PN correlation branches, wherein each of the plurality of parallel PN correlation branches is prioritized with respect to each of the other plurality of parallel PN correlation branches. The system also includes a first Spoke transmitter adapted to transmit a first prioritized PN encoded signal corresponding to a first one of the first plurality of prioritized parallel PN correlation branches. In addition, a second Spoke transmitter is adapted to transmit a second prioritized PN encoded signal corresponding to a second one of the first plurality of prioritized parallel PN correlation branches.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   U.S. patent application Ser. No. 10/619,864, entitled “System And Method For Priority Communication Management In A Hybrid TDMA-SS System” filed Jul. 14, 2003. The disclosure of this Non-provisional Patent Application is incorporated by reference herein in its entirety to the extent it does not conflict with the teachings presented herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to communication systems using spread spectrum Pseudo-Noise (PN) coding techniques, and pertains more specifically to methods and systems for PN encoded signal prioritization and collision mitigation. 
   2. Prior Art 
   A variety of multiple access communication systems has been developed for transferring information among a large number of system users. Techniques employed by such multiple access communication systems include time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation schemes, such as amplitude companded single sideband (ACSSB), the basics of which are well known in the art. 
   In Spread Spectrum (SS) or TDMA-SS transmission systems, a succession of short-duration bursts emanating from a number of different stations are presented to a demodulator. Each burst may contain data frames from one or more data channels. Each data frame generally contains a synchronization or sync word and a data payload area. 
   The TDMA structure is composed of a stream of frames with a number of fixed-time slots per frame. Each time slot may be of an assigned type: entry and registration, routine maintenance, priority messages, mass data transfer, and interrupt. The composition of slot types in a frame may be reassigned from frame to frame. A time slot in a frame may be assigned to one specific user; or a time slot may be a free-for-all slot; any number of users may attempt to use it on a first-come, first-serve basis. 
   Frequently, a class of users may need to communicate messages on an ad-hoc basis during a time slot that is reserved for ad-hoc messages, e.g., an interrupt time slot. The interrupt time slot is not assigned a priori to any specific user, but is available to all users on a free-for-all basis. For the case when multiple users occasionally transmit a message during the same interrupt time slot, the possibility exists that the different user transmissions will arrive nearly simultaneously at the receiver, thus “colliding” and interfering with each other. Prior art approaches design the spread spectrum correlation receiver to demodulate the received signal that arrives first in time and to reject other signals that are outside the correlation window of the correlation receiver. One disadvantage of this method is that higher priority messages from one user may be rejected in favor of lesser priority message from another user. Another disadvantage is that all messages received in the interrupt time slot may be rejected if multiple received spread spectrum signals arrive within the receiver&#39;s correlation window (e.g., within 2 PN code chips) of each other. 
   Therefore, a signaling method is desired that will enable interrupt message priority to be assigned to different users (i.e., Spokes) of the system and for the receiver (i.e., Hub) to automatically “sort” the messages in the correlation receiver to automatically select the highest priority message. Messages having lesser priority are thus automatically rejected in favor of a higher priority message. 
   SUMMARY OF THE INVENTION 
   The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings. 
   In accordance with one embodiment of the present invention a TDMA-SS signal prioritization system is provided. The system includes a Hub transceiver having a plurality of parallel PN correlation branches, wherein each of the plurality of parallel PN correlation branches is prioritized with respect to each of the other plurality of parallel PN correlation branches. The system further includes a first Spoke transmitter adapted to transmit a first prioritized PN encoded signal corresponding to a first one of the plurality of prioritized parallel PN correlation branches. The system also includes a second Spoke transmitter adapted to transmit a second prioritized PN encoded signal corresponding to a second one of the plurality of prioritized parallel PN correlation branches. 
   The invention is also directed towards a method for selecting a prioritized TDMA-SS signal. The method includes in at least one Spoke transmitter, Spoke prioritizing a TDMA-SS signal; and in a Hub receiver accumulating the prioritized TDMA-SS signal. Accumulating the prioritized TDMA-SS signal includes delaying the prioritized TDMA-SS signal by a PN chip or a specified offset in chips; despreading the PN chip delayed prioritized TDMA-SS signal; and determining a detection threshold. The method also compares the accumulated prioritized TDMA-SS signal to the detection threshold and determines a priority of the accumulated prioritized TDMA-SS signal in accordance with a result of the comparison. 
   In accordance with another embodiment of the present invention a program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform method steps for generating TDMA-SS signal prioritization and collision mitigation is provided. The method includes in at least one Spoke transmitter, Spoke prioritizing a TDMA-SS signal; and in a Hub receiver accumulating the prioritized TDMA-SS signal. Accumulating the prioritized TDMA-SS signal includes delaying the prioritized TDMA-SS signal by a PN chip or a specified offset in chips; despreading the PN chip delayed prioritized TDMA-SS signal; and determining a detection threshold. The method also compares the accumulated prioritized TDMA-SS signal to the detection threshold and determines a priority of the accumulated prioritized TDMA-SS signal in accordance with a result of the comparison. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other aspects of these teachings are made more evident in the following Detailed Description of the Preferred Embodiments, when read in conjunction with the attached Drawing Figures, wherein: 
       FIG. 1  is a pictorial diagram of a Hub-Spoke system architecture incorporating features of the present invention; 
       FIG. 2  is a block diagram of a hybrid TDMA-SS communication system incorporating features of the present invention shown in  FIG. 1 ; 
       FIG. 3  is a block diagram showing parallel correlators (rake receiver) in accordance with fast acquisition features of the present invention shown in  FIG. 2 ; 
       FIG. 4  is a block diagram detailing PN code tap delay element in accordance with spoke prioritization features of the present invention shown in  FIG. 1 ; and 
       FIG. 5  is a method flow chart showing steps for one method of implementing priority selection features of the present invention shown in  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , there is shown a pictorial diagram of a HUB  12 -SPOKE  14  telecommunications system incorporating features of the present invention. Although the present invention will be described with reference to the embodiment shown in the drawings, it should be understood that the present invention might be embodied in many alternate forms of embodiments, e.g., point-to-point duplex links or point-to-multipoint links. In addition, it should be understood that the teachings herein may apply to any group or assembly of hybrid TDMA-SS receivers, including those that are fixed in place; vehicle mounted; and/or hand carried; as illustrated by  14   z.    
   The Hub  12  transmits a continuous “broadcast” transmission waveform that is composed of a wideband direct sequence spread spectrum system composed of component PN codes (XYZ) described in copending patent application Ser. No. 10/352,295 entitled “Method and System for Rapid automatic Data Rate Discovery for PN Codes” filed Jan. 27, 2003 and incorporated herein in its entirety. The waveform is received, de-spread, and the underlying data is demodulated as taught by copending application Ser. No. 10/352,295 by each Spoke  14  in the system. 
   The Spoke(s)  14  derive the TDMA timing and slot structure from the Hub&#39;s  12  broadcast spread spectrum waveform PN code and from time slot definition tables transmitted from the Hub  12  to the Spokes  14  in the broadcast data ( FIG. 2 , FL). In a preferred embodiment of the present invention, the time of arrival and PN code phase of the transmitted TDMA spread spectrum signal is controlled to arrive at, referring also to  FIG. 3 , the Hub correlation receiver, item  30 , within ±1 PN chip uncertainty. However, it will be appreciated that any suitable chip uncertainty may be used. 
   Still referring to  FIG. 1 , in a preferred embodiment, the Hub  12  assigns priorities to the Spokes  14 . The priority for each Spoke is preferably unique such that collisions can be managed. For example, in a system  10  composed of M number of Spokes  14 . Each Spoke  14  preferably has a unique pre-assigned priority “m=[1, 2 . . . M]”, where “1” is the highest priority. The priority for each Spoke is transmitted from the Hub to the Spokes in the broadcast data and is contained in the Spoke&#39;s  14  respective time slot definition table. 
   Referring also to  FIG. 2 , there is shown a block diagram of a hybrid TDMA-SS communication system incorporating features of the present invention shown in  FIG. 1 .  FIG. 2  shows a full-duplex system  10  that is suitable for practicing this invention. Specifically, the system  10  employs direct sequence spread spectrum based techniques over an air link to provide data transfer between HUB  12  and SPOKE  14 . The forward link (FL) from HUB  12  to SPOKE  14  contains a spread spectrum waveform that is constructed in the manner described herein, with the PN code being composed of even-length and/or maximal length codes. In a similar manner, the return link (RL) from SPOKE  14  to HUB  12  contains a spread spectrum waveform that is similar, or identical, to that of the FL. 
   Still referring to  FIG. 2 , HUB  12  includes a Spread Spectrum Modulator (SSM)  12   b ; the SSM  12   b  generates a desired spread spectrum waveform at a desired RF frequency. The SSM  12   b  also provides a Tx clock  12   d  that is used to clock the Tx Data  12   e  into the SSM  12   b . The SSM  12   b  then combines the Tx data  12   e  with a spread spectrum PN code to produce the desired spread spectrum waveform. HUB  12  also includes an antenna  12   a , which may transmit at any suitable RF frequency. 
   The signal generated by HUB  12  and transmitted by antenna  12   a  via the FL is received by SPOKE  14  via antenna  14   a . Spoke  14  includes a spread spectrum correlator  14   c   1 , PN generator  14   c   2 , clock generator  14   c   3 , and spread spectrum demodulator (SSD)  14   c   4 . The received signal is then demodulated by SSD  14   c   4 . Once the signal is acquired and the SPOKE  14  is tracking the received signal, the Rx Clock  14   g  and Rx Data  14   f  are output to the intended receiver circuitry. It will be appreciated that the clocks  14   g  and  12   d  are synchronous and may be commanded to change frequency to correspond with PN code epochs as will be described herein; thus advantageously providing means to vary the data rate without interruption; and without the need for conventional bit synchronizers with associated synchronization time. 
   Similarly, SPOKE  14  generates a Tx Clock  14   d  and Tx Data  14   e  using the Spread Spectrum Modulator  14   b  in a similar fashion described earlier for HUB. Likewise, HUB  12  may receive the RL signal via antenna  12   a , and demodulate and track the signal as described earlier with receiver  12   c  to provide Rx Data  12   f  and Rx Clock  12   g  to the intended user. Referring to  FIG. 4  there is shown a block diagram detailing PN code tap delay elements in accordance with spoke prioritization features of the present invention shown in  FIG. 1 . Referring to  FIG. 2 , during an interrupt time slot(s) (not shown), a Spoke  14  has a message to transmit, the Spoke  14  selects a transmitted PN code phase to correspond to the pre-assigned priority. The Spoke PN Generator  42  is shown with inputs consisting of a master clock  41 , generally operating at the TDMA chipping rate, and timing commands  411  from the Broadcast Receiver (not shown). The timing commands include the information necessary to control the frequency and phase of the Spoke  14  transmitted PN code to arrive at the Hub  12  within an accuracy of ±1 chip in a preferred embodiment. The timing commands  411  also adjust for Doppler frequency between the Hub  12  and Spoke  14 , and for master clock  41  drift. 
   The Spoke Transmitter PN Generator  42  receives as inputs a signal from Master Clock  41  and timing commands from Spoke Broadcast Receiver  411 . The Spoke Transmitter PN Generator  42  may generate suitable PN codes such as taught in co-pending application Ser. No. 10/352,295 filed Jan. 27, 2003. The PN code from PN Generator  42  then passes to M−1 3-chip delay elements  43 – 46 , where M is the number of Spokes in the HUB/SPOKE system ( FIG. 1 , item  10 ). The delay elements shown in  FIG. 4  are preferably set at delays of 3 chips each to account for an uncertainty of ±1 chip and to allocate a 1 chip guard band. However, any suitable delay may be used. The PN code having passed through up to M−1 3-chip delays is selected by priority selector  47  based on input priority m  47   a . The selected PN code with priority m is then modulated by modulator  48  and transmitted by transmitter  49 , depicted in  FIG. 4 , to Hub  12  depicted in  FIG. 1 . 
   The delaying of the PN code may be expressed as:
 
Delay=( P− 1)*(CD)  (Eq. 1)
 
   In equation 1, delaying of the PN code (Delay) is expressed as an integer variable representing priority (P) minus one, times an integer variable representing chip delay units (CD). 
   It will be appreciated that the number of delay elements ( 43 – 46 ) selected by each Spoke  14  is selected by selector  47  in accordance with priority assignment “m” from the Hub  12 . In alternate embodiments, the Spoke  14  priority may be fixed, thus allowing the Spoke priority circuitry to be hardwired. For example, a Spoke  14  may be permanently assigned priority 2, thus obviating the need for chip delay units  44 – 46 . 
   Referring now to  FIG. 3  there is shown a block diagram of a correlation rake receiver  30  showing multiple correlator branches  31   a  in a parallel rake receiver architecture in accordance with the teachings of the present invention. Each correlator branch  31   a  includes a chip-delayer  31   b , a PN despreader  31   c , an accumulator  31   d , a symbol rate timing selector switch  32 , and a sample weighting controller  31   e.    
   In a preferred embodiment the number of correlator branches  31   a  is 3 times M (M equaling the number of Spokes); in other words, three correlator branches  31   a  per Spoke. The TDMA-SS signal received from the Spoke(s) ( FIG. 1 , items  14 ) is chip delayed from 0 to 3M−1 chip delays by chip-delayers  31   b   0 -to- 31   b   3M−1 , respectively. During interrupt time slots, the symbol rate timing selector switches  32   0 – 32   3M−1  send despread accumulated signals from accumulators  31   d   0 – 31   d   3M−1  to the Weight Update Algorithm Controller  31   f . Accumulator  31   d   0 – 31   d   3M−1  outputs that do not exceed the detection threshold are discarded because there is insufficient signal correlation to be of interest. After the priority accumulator set is identified (see  FIG. 5  and description below), the rake receiver  30  sample-weighting-taps for all other delay paths  31   e  are set to zero to discard other received signals having lesser priority. Sample weighted signals not set to zero are summed via summer  31   g  and output to a data demodulator. Thus, it will be appreciated that this arrangement advantageously complements the Spoke prioritization arrangement shown in  FIG. 4 . For example, in a two or more Spoke system, there is, according to  FIG. 4 , at least one 3-chip delay element ( FIG. 4 , item  43 ) in at least one Spoke. Thus, the transmitted signal may not be delayed at all, i.e., m=priority  1 , and therefore, when received by the Hub  12  rake receiver  30 , the within tolerance signal that is despread by one of the despreaders  31   c   0 ,  31   c   1 , or  31   c   2  and accumulated by its corresponding accumulator  31   d   0 ,  31   d   1 , or  31   d   2 , will correspond to the signal received through chip delayers  31   b   0 ,  31   b   1 , or  31   b   2 . Likewise, continuing the example, a m=priority 2 message is chip delayed by 3 chips ( FIG. 4 , item  43 ). Thus, when received by the Hub  12  rake receiver  30 , the within tolerance signal that is despread by one of the despreaders  31   c   3 ,  31   c   4 , or  31   c   5  and accumulated by its corresponding accumulator  31   d   3 ,  31   d   4 , or  31   d   5 , will correspond to the signal received through chip delayers  31   b3 ,  31   b4 , or  31   b5 . 
   The following numeric example further illustrates the prioritization and collision avoidance features of the present invention. Consider two transmitting Spokes  14 , in a three Spoke system, having transmitting interrupt messages having priorities  2  and  3 , respectively. Both transmitting Spokes  14  transmit such that their transmitted signals arrive at the Hub  12  with a PN code phase error that does not, in a preferred embodiment, exceed ±1 chip. The transmitted PN code phase for the priority 2 message is (2−1)×3=3 PN chip delays; hence, this signal will arrive at the Hub rake receiver  30  with a delay of 3±1 chips. In a similar manner for the priority 3 message, the transmitted PN code phase for the priority 3 message is (3−1)×3=6 PN chip delays; hence, this signal will arrive at the Hub receiver with a delay of 6±1 chip. The Hub correlation receiver  30  accumulators  31   d   0 – 31   d   8  will get simultaneous detections at one of the accumulator sets  31   d   3 – 31   d   5  and  31   d   6 – 31   d   8 . The accumulator set having the higher priority, in this example, accumulator set  31   d   3 – 31   d   5 , is selected by weight update controller  31   f  positively weighting the higher priority-2 signal (delayed by 3 chips) and rejecting, or zero weighting, the lower priority-3 signal (delayed by 6 chips) through sample weighting controllers  31   e   6 – 31   e   8 . 
   Thus, the signals from Spokes  2  and  3  do NOT collide in-phase, thus enabling reception of the higher priority signal from the higher priority Spoke. Thus, the Hub receiver advantageously avoids fatal message collisions such that neither priority message is received. 
   It will be appreciated that the number of correlation branches per set  31   a  per priority may be any suitable number corresponding to the system tolerance. For example, a communication system having a PN chip tolerance of plus or minus 4 chips would have 5 correlation branches per set  31   a  per desired priority. 
   Referring also to  FIG. 5  there is shown a method flow chart showing steps for one method of implementing the priority selection features of the present invention shown in  FIG. 3 . The first step  51  accumulates PN phase information over one symbol period. The next step  52  compares each accumulator  31   d   0 – 31   d   3M−1  output to a detection threshold. A check, step  53  determines whether any accumulated output exceeds a predetermined minimum threshold. If the result of step  53  is negative then the process starts over at step  51 ; if the result of step  53  is positive then the 3-set correlation branch ( FIG. 4 , item  31   a ), in this example, corresponding to the desired priority signal is selected  54 . The three taps exceeding the predetermined minimum threshold are optimized for performance, step  56 . All other Rake Receiver Taps from weight update controller  31   f  are set to zero, step  55 , thereby rejecting lower or non-desired priority signals. Finally, step  57  determines whether the end of the interrupt time slot has been reached. 
   In addition, in alternate embodiments features of the present invention may be implemented in a programmable device such as an integrated circuit (IC). It will be further appreciated that the IC may be a field programmable gate array (FPGA), an application specific IC (ASIC), or a function of Modulator, Demodulator, Controller (MDC) firmware. The operation of the ICs or firmware may be defined by a suitable programming language such as a Very High Speed Integrated Circuit (VHSIC) Hardware Description (VHDL) Language file. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.