The Code division multiple access (CDMA) enhanced security based on the advanced encryption standard (AES) operation and it used 128, 192, and 256 bits AES-CDMA PN code
sizes to raise the security level against eavesdropping .
However, only the signal call matching its own PN code
will be recovered.
Starzyk, "Hardware Implementation for Fast Convolution with a PN Code Using Field Programmable Gate Array," Proceedings of the 33rd Southeastern Symposium on System Theory, Athens, OH, March 2001, pp.
The used PN codes have an auto-correlation property only with a synchronized copy of itself.
In order to be able to detect a PN modulated measurement signal, it is necessary to lock on the PN code for demodulation.
These two replica of the PN code are 2[delta] apart in time, where the rime shift [delta] must be smaller than the duration [T.
2] causes a tracking error because the error signal that controls the VCO and the PN code timing is non-zero at the proper time shift t.
The PN code
acquisition is instantaneous and burst data rate is configurable.
From research in PCS sytems, a PN code length of 127 is the suggested minimum spreading in the unlicensed bands for applications with multiple cells, low power and short range transmission at low data rates (9600 bps).
In theory, a PN code length of 2047 offers better than 30 dB process gain and jam margins of around 25 dB.
TABLE 1 REQUIRED BANDWIDTH FOR DATA RATE AND PN CODE LENGTH COMBINATIONS Chip Rate Channels Data PN (Data Rate X Process in 83 MHz Rate Code Code Length) Gain (dB) with BPSK 9600 511 4,905,600 27 16 9600 2047 9,820,800 30 8 100,000 31 3,100,000 15 26 100,000 127 12,700,000 21 6 100,000 255 25,500,000 14 3