 bandwidth used to transfer information. To expand the bandwidth here, we add the information to the spreading spectrum code before modulation. This approach ap- plies for example to the FDD mode, which uses a code sequence to determine RF bandwidth. The FDD mode has robustness to interference due to higher system process- ing gain³ Gp. The latter quantifies the degree of interference rejection and can be de- fined as the ratio of RF bandwidth to the information rate: (2.3) From Ref. [23] in a spread-spectrum system, thermal noise and interference determine the noise level. Hence, for a given user, the interference is processed as noise. Then, the input and output SIN ratios can relate as: (2.4) Relating the SIN ratio to the Eb/No ratio4, where Eb is the energy per bit and No is the noise power spectral density, we get: (2.5) From the preceding equations we can express Eb/No in terms of the SIN input and output ratios as follows: Reference processing gains for spread spectrum systems have been established between 20 and 50 dBs. 4 Unless otherwise specified, here we assume that No includes thermal and interference noise. System Analysis Fundamentals (2.6) 2.1.2.2 Modulation and Spreading Principles In wide-band spread-spectrum systems like the FDD mode, the entire bandwidth of the system remains available to each user. To such systems, the following principles apply: first, the spreading signal has a bandwidth much larger than the minimum bandwidth required to transfer desired information or base-band data. Second, data spreading OC- curs by means of a code spreading signal, where the code signal is independent of the data and is of a much higher rate than the data signal. Lastly, at the receiver, de- spreading takes place by the cross-correlation of the received spread signal with a syn- chronized replica of the same signal used to spread the data [23]. 2.1.2.2.1 Modulation If we view Quadrature Phase-Shift Keying (QPSK) as two independent Binary Phase- Shift Keying (BPSK) modulations, then we can assume the net 