Abstract:
A multi-channel spread spectrum communications system utilizes the combination of spreading codes to represent and transfer information. On the transmitter side, every block of data bits is divided into two sub-blocks of data bits. One sub-block is mapped to a number of spreading codes, while another sub-block is separated further into several smaller sub-blocks with each of them mapped into an M-ary phase. Each selected spreading code is modulated by the corresponding M-ary phase. On the receiver side, first determine the most likely transmitted spreading codes and their respective phases, then reverse map the set of estimated transmitted spreading codes into a block of bits and each of the phases into another block of data bits, and finally combine all these blocks to a single block of data bits.

Description:
FEDERALLY SPONSORED RESEARCH 
     Not Applicable 
     SEQUENCE LISTING OR PROGRAM 
     Not Applicable 
     FIELD OF THE INVENTION 
     The invention relates generally to multi-channel direct sequence spread spectrum communications system, and more particularly to using the elements of the power set of the set of spreading sequences to represent and transmit information in order to achieve a higher data rate without requiring more precision on A/D converter and more linearity on linear power amplifier. 
     BACKGROUND 
     Spread spectrum is a method of modulation that needs a transmission bandwidth usually substantially higher than data rate. In a direct sequence spread spectrum communications system, the transmitter modulates a data signal with a pseudo random chip sequence to generate spread spectrum signal. Usually the chip rate of the pseudo random sequence is much higher than the data rate and therefore the direct sequence spread spectrum communications system will take substantially wider bandwidth than the bandwidth needed by data signal itself. The spread spectrum signal is then transmitted over a communications media as a radio wave to a receiver. The receiver despreads the spread spectrum signal to recover the information contained in the received spread spectrum signal. 
     Having many advantages over other communications systems, direct sequence spread spectrum communications system is one of the major communications systems widely used in today&#39;s society. However, spread spectrum communications system has some disadvantages. One of the major disadvantages is low spectrum efficiency. Nowadays more and more applications require higher and higher data rate but the available bandwidth is both very expensive and limited. The low spectrum efficiency inherited in a spread spectrum communications system will greatly restrict its opportunity to be used in many high data rate applications. 
     Multi-channel direct sequence spread spectrum communications system is one of the attempts to increase the data rate within a given bandwidth and therefore to improve the spectrum efficiency of direct sequence spread spectrum communications system. However, in a regular multi-channel direct sequence spread spectrum communications system, the more channels, the more different signal levels. In order to have enough resolution, more linear power amplifier and higher precision A/D converter will be needed. Both of these factors will make cost greatly increased especially when the working clock is very high. 
     U.S. Pat. No. 6,324,209 to Don Li et al, herein incorporated by reference, disclosed a method of applying multi-channel technology in direct sequence spread spectrum communications system. Different from regular multi-channel direct sequence spread spectrum communication system in which each sub-channel transmits information simultaneously, U.S. Pat. No. 6,324,209 separated spreading codes into several subgroups and at any time, transmitted only one spreading code from each subgroup. In this way, it limited the possible number of signal levels and therefore the requirement on both A/D and linear power amplifier was under control. However, the data rate was still not high enough. In some case, it can not meet the requirements such as reasonable cost and limited transmission bandwidth set up by more and more high data rate applications. 
     Therefore, there is a need to further increase the spectrum efficiency of a multi-channel direct sequence spread spectrum communications system without increasing the requirement on linear power amplifier and A/D converter. 
     In a regular communications system, the information is represented and transmitted directly by each transmittable signal. Let S be the set of all these transmittable signals. The power set of the set S is a set denoted by 2 S , whose elements are the subsets of S. In other words, 2 S ={X|X ⊂ S}. If there are L components in S, then there will be 2 L  elements in the power set 2 S . 
     Suppose in a regular communications system, each element of S can carry 1 bit information. By transmitting all of the elements of S, one can send L bits information. 
     One can also send L bits information by alternative. Due to the fact there are 2 L  elements in the power set 2 S , one can correspond each element of the power set 2 S  to a different L bits binary number. Since there are C L   i  elements in 2 S  with each element consisting of i components from set S, the element of 2 S  on average consists of 
                 (       ∑     i   =   0     L     ⁢     i   ·     C   L   i         )     /     2   L       =     L   /   2           
components from set S. In this alternative way, instead of transmitting L signals all the time, one can, on average, transmit only half of L signals.
 
     Furthermore, instead of using all the elements of a power set to transmit information, one can use a particular subset of the power set to transmit information. 
     SUMMARY OF THE INVENTION 
     This invention is based on the fact that the number of the elements in the power set of a set is more than the number of the components in the set. 
     The primary objective of the invention is to achieve a multi-channel direct sequence spread spectrum communications system with a higher data rate without further requirement on linear power amplifier and A/D converter. 
     Another objective of the invention is to represent and transmit information by the elements of the power set of the set of spreading sequences instead of spreading sequences themselves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawing figures depict preferred embodiments of the present invention by way of example, not by way of limitations. In the figures, like reference numerals refer to the same or similar elements. 
         FIG. 1  is a block diagram of a communications system implementing the concept of power set. 
         FIG. 2  is a block diagram of the transmitter of a multi-channel direct sequence spread spectrum communications system implementing concepts of the present invention. 
         FIG. 3  is a block diagram of the receiver of a multi-channel direct sequence spread spectrum communications system implementing concepts of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Detailed description of the preferred embodiment is provided herein. The embodiment illustrates a multi-channel direct sequence spread spectrum communications system. However, it is to be understood that the present invention may be embodied in many different ways. For those skilled in the art, it may be easy to modify the embodiment. For example, instead of using phase mapping device, one can use quadrature amplitude modulation (QAM) or pulse amplitude modulation (PAM). Therefore, specific details disclosed are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one to employ the present invention in virtually any appropriately detailed system, structure or manner. 
     The principles described here can be easily deployed in other multi-channel communication systems. For instances, one skilled in the art can apply the principles in a communication system with the capability to transmit signals at multiple frequencies simultaneously or in a communication system with the capability to transmit signals at multiple time slots. One skilled in the art can further apply the principles in a communication system involved with time domain, frequency domain, and code domain simultaneously. For example, one can define a mapping scheme between a block of data bits and a subset of the power set based on available frequency points and time slots. 
       FIG. 1  illustrates the general structure of a communications system transmitting information by the elements of a power set of the set of transmittable signals. For a set S having L components, the power set of the set S will have 2 L  elements. Each element in the power set is a subset of the set S, consisting of 0 to L components from the set S. The huge difference between the number of components in a set and the number of elements in the power set of the set makes it possible on average to transmit more information under same or similar transmission conditions. 
     Suppose that there are N bits b 1 , b 2 , . . . , b N  to be transmitted and suppose that there are L codes in the set S. Let&#39;s arrange these L codes into order and call them code 1 , code 2 , . . . , code L . The power set 2 S  of the set S is the set of all subset of the set S. The elements of the power set of the set S consist of none, some or all of the codes from code 1 , code 2 , . . . , code L . 
     The mapping device  110  maps a different block of N data bits b 1 , b 2 , . . . , b N  into a different element of the power set 2 S . Let the mapping device  110  map a particular block of N data bits b 1 , b 2 , . . . , b N  into ch 1 , ch 2 , . . . , ch l , an element of the power set 2 S , where ch i ≠ch j  if i≠j, ch i =code i′ , for 1≦i≦l≦L and 1≦i′L, and i′&lt;j′ for ch i =codes i′ , ch j =code j′  and i&lt;j. The mapping is always possible as long as N≦L. 
     ch 1 , ch 2 , . . . , ch l  are transmitted through the transmission mechanism  120 , which may include RF modulation, power amplifier and transmitting antenna at transmitter side and receiving antenna, low noise amplifier, automatic gain control and frequency down converted device at receiver side. 
     The output of transmission is connected to a matched filter bank  130 . There are L matched filters in the bank with each one matching to one of signals represented by code 1 , code 2 , . . . , code L . 
     The maximum likelihood detection  140  determines which codes among code 1 , code 2 , . . . , code L  are most likely transmitted at the transmitter side. In other words, to determine ch′ l , . . . , ch′ l′ . This detection could be carried out under some conditions such as false alarm probability and detection probability as well as some precondition on ch 1 , ch 2 , . . . , ch l . 
     The reverse mapping device  150  is used to find the likely transmitted bits b′ 1 , . . . , b′ N  once ch′ 1 , . . . , ch′ l′  have been determined. If there is no mistake, b′ 1 , . . . , b′ N  should be identical to b 1 , b 2 , . . . , b N . 
     By properly arranging a mapping scheme between messages, which correspond to different blocks of data bits, and elements of the power set, one can make more efficient use of the scheme depicted in  FIG. 1 . For example, if every active component has a same power, by making a message having higher probability represented by an element of the power set with less components then on average, one can save power. If the total power for every element of the power set has a same power, by making an important message associated with an element of the power set with less components, then on average, one can enhance the probability to detect correctly the message. 
       FIG. 2  illustrates the transmitter of a multi-channel direct sequence spread spectrum communications system implementing the concept of power set. 
     In a multi-channel direct sequence spread spectrum communications system such as the one described in U.S. Pat. No. 6,324,209, each of the transmitted spreading codes may carry information by itself. Suppose in the system described by in U.S. Pat. No. 6,324,209, there are 64 codes separated into 8 groups with 8 codes in each group. Further suppose each code can have 16 different phases. Then during each symbol period, 8·(3+4)=56 bits could be transmitted. Using the concept of power set, it is possible to transmit more bits with only 8 codes transmitted at any time. 
     The total combinations to choose 8 codes from 64 codes are C 64   8 =4,426,165,368. Since C 64   8 &gt;2 32 =4,294,967,296, from a given 8 codes, 32 bits can be obtained from reverse mapping. Also 4 bits can be obtained from the phase associated with each code of the 8 codes. Therefore a total of 32+8*4=64 bits can be transmitted. 
     There are 2 64 −C 64   8  extra elements in the power set 2 S  which have not been used yet. These extra elements could be used for signaling, such as using one extra element to indicate no symbol repetition, another extra element to indicate 1 symbol repetition, a third extra element to indicate 2 symbol repetitions. A transmitter transmits an extra element corresponding to the desired symbol repetition. A receiver, after detecting the extra element, is able to find the intended symbol repetition set up by the transmitter. 
     Among these extra elements, there are 4,426,165,368−4,294,967,296=131,198,072 elements with 8 codes in each element and there are 
                 ∑     i   =   1     7     ⁢     C   64   i       =     704   ,   494   ,   192   ⁢           ⁢   elements           
with 7 or less codes in each element. If only the extra elements with exactly 8 codes in each element are used for signaling, the energy associated with each code of the element for signaling will be equal to the energy associated with each code of the element used for information transmission. If the extra elements with less than 8 codes in each element are used for signaling, the energy contained in each code for signaling will be larger than the energy in each code for information transmission. Therefore, more reliable signaling can be provided when the extra elements with fewer codes are used for signaling.
 
     Suppose on the data bus  205 , there are 64 bits needed to be transmitted simultaneously. The bit separation device  210  separates bus  205  into two data buses  215  and  220  with each bus having 32 bits. The mapping device  225  maps the 32 bits on data bus  215  into 64 bits with 8 bits set to 1 and the other bits set to 0. 
     There are many way of mapping 32 bits into 64 bits with 8 bits set to 1 and the other bits set to 0. The simplest mapping algorithm for the mapping device  225  could be done as following. First, list the entire binary table for 64 bits in ascending order. Then delete all the items which do not include exactly 8&#39;s 1 and 56&#39;s 0. Next, start from beginning, map each of the 64 bits of the remaining first 2 32 =4,294,967,296 items to a 32 bits sequence 0 . . . 000, 0 . . . 001, 0 . . . 010, 0 . . . 011, . . . , 1 . . . 111 correspondingly. 
     Set  235  consists of 64 codes denoted C 1 , . . . , C 64 . The code selection device  240  select codes according to corresponding bit on the data bus  230 . If a bit on the data bus  230  is 1, the corresponding code will be selected. Exactly 8 codes will be selected and they are denoted as C′ 1 , C′ 2 , . . . , C′ 8 . 
     Data bus  220  is connected to bit separation circuit  250 , where the 32 bits are further divided into 8 groups with each group having 4 bits on each data bus  255 . The phase mapping device  260  maps each of the 4 bits on data bus  255  into a complex number which corresponds to one of the 16 possible phase of a 16PSK signal. Each of these 8 complex numbers is multiplied with C′ 1 , C′ 2 , . . . , C′ 8  by one of the multipliers  245  respectively. The complex signal combiner  265  adds all the 8 complex products of the output of multipliers  245  together and then separates the real signal and image signal as I and Q. The I signal is multiplied by PN code  270 , modulated by cos(ωt); Q signal is multiplied by PN code  270 , modulated by sin(ωt). Both I and Q are coupled together, then sent to power amplifier  305  for amplification, and then to antenna  310  for transmission. 
       FIG. 3  illustrates the receiver of a multi-channel direct sequence spread spectrum communications system implementing the concept of power set. 
     The signal enters the receiver through the antenna  405  and goes through the voltage controlled amplifier (AGC)  410 . The output of AGC  410 , is multiplied by cos(ωt)  415  at multiplier  420  and by sin(ωt)  425  at multiplier  430 . The phase rotator  435  is used to correct the phase error. The PV code from the PN generator  440  and all the spreading codes C 1 , . . . , C 64  are fed to the matched filter bank  445 , which is used to find that how closely the input signal is matched with each of spreading codes C 1 , . . . , C 64  scrambled by PN code. The maximum likelihood detection device  450  will determine 8 most likely transmitted codes. Device  450  has 8 groups of output with each group of output consisting of an I component, a Q component and the index of corresponding code. Index i&lt;Index j for i&lt;j. 
     The 8 indexes with one from each group are fed to the reverse mapping device  455  to obtain 32 data bits, which should be same as the one on data bus  215  in  FIG. 2  if no error is made. 
     The I and Q signals from each group are fed to one of the 16PSK slicers as denoted by  460   a  through  460   h , which selects the most likely transmitted phases among the 16 possible phases. The outputs of these phase slicers are sent to the reverse phase mapping devices  465   a  to  465   h  to obtain the remaining 4 bits from each. The bit combination device  470  combines the 4 bits from each of the 8 reverse phase mapping devices to form demodulated 32 data bits, which should be identical to the bits on the data bus  220  of  FIG. 2 . 
     The combiner  475  combines the 32 bits form the bit combination  470  and the 32 bits from the reverse mapping device  455  to form a total of 64 demodulated bits. If no error occurs during transmission, the demodulated bits should be identical to the 64 bits on the data bus  205  in  FIG. 2 . 
       FIG. 2  and  FIG. 3  show a transmitter and a receiver respectively according to a subset of a power set whose elements in the subset have same number of spreading codes. One skilled in the art can easily extend the basic ideas shown in  FIG. 2  and  FIG. 3  to more general situation with a subset including any selected elements of the power set. Since two elements in the power set could include different number of spreading codes, in order to make sure each spreading code modulated by a same M-ary PSK scheme or any other same modulation scheme, the blocksize of data bits transmitted and received each time could change. To handle this case properly, one could define a mapping scheme that maps a block of data bits into an element of the power set with every block of data bits having a same blocksize. Depending on the number of components in an element and a particular modulation scheme used, an extra block of data bits with more or less data bits will be taken for conducting the modulations on all the components of that element. The mapping scheme could further define an associating relationship between every component of that element and a corresponding portion of data bits in that extra block of data bits. The corresponding portion of data bits will be used to perform a modulation, such as an M-ary PSK, on that particular component. The size of each block of data bits for determining the selection of an element of power set is fixed while the size of extra block of data bits is variable. In this way, each element associates with two types of blocks. The first type of block has a fixed content and is for determining which element will be selected. All first blocks have a same blocksize. The second type of block is for conducting modulation on all components of an element. Since the blocksize of a second block depends on the number of the components in an element and the modulation scheme, two different second blocks may have different blocksizes. A second type block may change its content but its blocksize will not change. To distinguish the blocks taken from the data stream from the blocks used as templates in a mapping scheme, one can call a first block in the mapping scheme as a first block template and a second block in a mapping scheme as a second block template.