Method for using circular spreading codes to achieve high bit densities in a direct-sequence spread spectrum communication system

A method for achieving high bit densities in a direct-sequence spread spectrum communication system by using circular spreading codes. The circular spreading code is a pseudo-noise spreading code that is shifted by n. This circular pseudo-noise spreading code is then used to spread an information signal by modulating the information signal with the circular pseudo-noise spreading code. The same circular pseudo-noise spreading code is also used to demodulate the signal. The value of n used is that which corresponds to the value of the signal to be spread.

FIELD OF THE INVENTION 
The present invention relates to the field of data communications. More 
particularly the invention describes a method of using circular spreading 
codes to achieve high bit densities in direct sequence spread spectrum 
communication systems. 
BACKGROUND OF THE INVENTION 
Direct Sequence Spread Spectrum (DSSS) techniques rely on the use of 
pseudo-noise carriers, also called spreading codes, spreading sequences, 
code sequences and chip sequences, and a transmission bandwidth which is 
much wider than the minimum required to transmit the information. The 
transmitter spreads the information by modulating the information with a 
pseudo-noise spreading sequence. At the receiver, the information is 
despread to recover the base information. This despreading is accomplished 
by correlating the received, spread-modulated, signal with the spreading 
sequence used for the transmission. DSSS is sometimes referred to by the 
shorthand name "direct spread." 
The modulating signal, such as a pseudo-random spreading code signal, 
possesses a chip rate (analogous to carrier frequency) which is much 
larger than the data rate of the information signal. This characteristic 
is required for efficient spreading. Each state of the pseudo-random 
spreading sequence is referred to as a chip. The spreading sequence (chip 
sequence) directly modulates each bit of the information signal, hence the 
name direct spread. Pseudo-randomness of the spreading signal is required 
in order to recover the original information signal. Since the spreading 
sequence is deterministic, it can be exactly duplicated at the receiver in 
order to extract the information signal. If it were truly random, 
extraction of the information signal via correlation receiver would not be 
possible. 
The spreading operation causes the signal power to be depleted uniformly 
across the spread bandwidth. Thus, the spread spectrum signal will appear 
buried in noise to any receiver without the despreading signal. 
Consequently, it is not only difficult to jam, but is also difficult to 
detect its presence in any bandwidth. Any undesired signal picked up 
during transmission is spread by the receiver in the same way that the 
transmitter spread the desired signal originally. In other words, the 
receiver spreads undesired signals picked up during transmission, while 
simultaneously despreading, or demodulating, the desired information 
signal. Processing gain is the term used to express this interference 
suppression in the overall transmit/receive operation. When viewed as a 
transmit/receive operation, the desired signal is spread-modulated twice, 
giving back the original signal, while in-band interference is 
spread-modulated once, and thereby depleted across the full spread 
bandwidth. 
SUMMARY OF THE INVENTION 
A method for achieving high bit densities in a direct-sequence spread 
spectrum communication system by using circular spreading codes. The 
circular spreading code is a pseudo-noise spreading code that is shifted 
by n. This circular pseudo-noise spreading code is then used to spread an 
information signal by modulating the information signal with the circular 
pseudo-noise spreading code. The same circular pseudo-noise spreading code 
is also used to demodulate the signal. The value of n used is that which 
corresponds to the value of the signal to be spread.

DETAILED DESCRIPTION OF THE INVENTION 
The disclosed technique utilizes a previously unexploited method of 
embedding multi-bit information content in the spreading code, via 
intra-symbol rotation. This circular rotation of the spreading code 
provides increased information transmission capacity through a mapping of 
user data to rotation angle. In this manner, the excess bandwidth inherent 
to spread spectrum modulation is exploited for an information capacity 
increase, without sacrificing benefits of spread spectrum techniques. This 
information capacity increase is achieved without any accompanying 
increase in the transmit power or bandwidth. 
By encoding the spreading code through circular rotation within the bounds 
of a symbol period, multi-bit information capacity is created. This is 
done through the rotation of the spreading code where the rotation angle 
is used to encode the user data information. The number of bits 
represented by a given rotation is equal to log-base-2 of the spreading 
code length. Implementation is relatively simple and the benefits of the 
spread spectrum technique are preserved. 
By storing a given spreading code in, for example, a circular shift 
register or ring buffer, and reading the register each symbol period, 
starting from a pointer location determined by the user data, additional 
information is embedded in the rotated spreading code. Each pointer 
increment corresponds to a rotation angle of 360 degrees divided by the 
log-base-2 of the length of the spreading code. The rotated spreading code 
at the ring buffer output is then used to modulate user data, as in a 
conventional direct-sequence spread spectrum modulator. 
FIG. 1(a) shows an example of what occurs to a signal when it is spread. 
Signal 100 is spread using a spreading sequence (not shown) into signal 
101. As can be seen, the amplitude of the signal is decreased, while its 
bandwidth is expanded. By reducing the amplitude, the signal will appear 
indistinguishable from noise, and can only be recovered by a receiver 
which processes the correct spreading sequence. FIG. 1(b) shows the spread 
signal 101 and an interference signal 102 which has been picked up during 
transmission. When the spread modulated signal 101 is demodulated by using 
the original spreading sequence (not shown), the original signal 100 is 
recovered and the interference signal 102 is spread into signal 103, 
thereby being reduced to noise. 
FIG. 2(a) is a diagram of an exemplary prior art method of spreading a 
signal. An information signal 210 is modulated, using known methods, by a 
pseudo-noise code 211. For each `1` in the information signal, the 
pseudo-noise code 211 is transmitted. Whereas for each `0` in the 
information signal, the inverse of the pseudo-noise code 211 is 
transmitted. Thus, through such modulation, the signal is spread out for 
transmission into the transmitted signal 212. For example, if the 
information signal 210 consists of the bits `101` and the pseudo-noise 
code 211 is `01011010` then the transmitted signal 212 is `01011010 
10100101 01011010.` This transmitted signal is created by `1` 
corresponding to the pseudo-noise code 211 (`010110101`) and `0` 
corresponding to the inverse of the pseudo-noise code (`10100101`). 
FIG. 2(b) is a diagram of an exemplary method of spreading a signal using a 
circular pseudo-noise spreading code. As described above, the information 
signal 210 is again modulated by a spreading signal to create a 
transmitted signal 214. However, in this case, instead of using a 
pseudo-noise code, a circular pseudo-noise spreading code is used. By 
using a circular pseudo-noise spreading code, multiple bits of information 
can be transmitted per each pseudo-noise code instead of a single bit, as 
described above. The circular pseudo-noise spreading code is a shifted 
pseudo-noise code. For example, if the circular spreading code was 
`01011010` then the code shifted by zero is still `01011010`. However, the 
code shifted by one is `10110100` where the second bit of the original 
spreading code is now the first bit, the third bit is now the second, 
etc., until the last bit is the first bit. As a trivial example, if two 
bits of information are to be sent per each pseudo-noise code, a four bit 
pseudo-noise code is required because two bits of information have a value 
ranging from zero to three. If the value of the information bits is 3 (the 
bits are `11`), then the pseudo noise code is shifted by three, thus the 
circular spreading code used begins with the fourth bit, or bit number 3 
where the bits are numbered zero through three. This scheme results in 
high bit densities of transmitted data while still containing high 
correlation. 
In FIG. 2(b), the same information signal 210 (`101`) and pseudo-noise code 
211 (`01011010`) of FIG. 2(a) is used. In this case, since a binary `101` 
equals a numeric 5, the circular pseudo-noise spreading code used is 
`01001011,` , where the circular pseudo-noise code is original 
pseudo-noise code shifted by five. Thus, the circular pseudo-noise code 
corresponds to `101` and the transmitted signal is therefore `01001011.` 
FIG. 3 shows the circular spreading code `01011010.` The rotation of the 
code corresponds to the multiple user bit value. Thus, if the code used is 
`01101001` then the original code was shifted by 2 and thus corresponds to 
a user value of `010.` Similarly, user values of zero to seven can be 
transmitted using different shifted versions of the circular spreading 
code. 
FIG. 4 shows the receipt and decoding of the transmitted signal. When the 
transmitted signal 214 from FIG. 2(b) is received, it is compared to the 
correlators for that circular pseudo-noise spreading code (shown in FIG. 
3) 418. Each correlator is the circular pseudo-noise spreading code 418 
shifted by zero to seven bits where the value of the number of bits 
shifted equals the value of the original information signal. The 
transmitted signal may be compared to the correlators simultaneously. When 
a match is found then the value corresponding to the correlator (which 
corresponds to the number of bits shifted) is read. This value is the 
value of the original signal. In this manner, the signal is demodulated, 
or despread. Using the example of the transmitted signal `01001011,` when 
it is compared with each correlator, it is found that it corresponds to 
correlator 415, where correlator 415 is circular pseudo-noise spreading 
code 418 shifted by five bits. Therefore the decoded signal 420 is equal 
to the numeric value `5` and in a binary signal is `101.` 
In the example described above, an eight bit pseudo-noise code was used to 
transmit three bits of information. Of course, other values could be used. 
For example, to transmit 2 bits of information at a time, a four bit 
pseudo-noise code is required. Similarly, to transmit 4 bits of 
information, a 16-bit pseudo-noise code is required, to transmit 5 bits of 
information, a 32-bit pseudo-noise code is required, to transmit 6 bits of 
information, a 64-bit pseudo-noise code is required, etc.