8.6. CODE-DIVISION MULTIPLEXING 391
Figure 8.33: Frequency hopping in CDM lightwave systems. Filled square show frequencies for
different time slots. A specific frequency-hop sequence (3, 2, 0, 5, 1, 4) is shown.
Fig. 8.33. The matrix rows correspond to assigned frequencies and the columns corre-
spond to time slots. The matrix elementmijequals 1 if and only if the frequencyωiis
transmitted in the intervaltj. Different users are assigned different frequency-hop pat-
terns (or codes) to ensure that two users do not transmit at the same frequency during
the same time slot. The code sequences that satisfy this property are said to be orthog-
onal codes. In the case of asynchronous transmission, complete orthogonality cannot
be ensured. Such systems make use of pseudo-orthogonal codes with maximum auto-
correlation and minimum cross-correlation to ensure the BER as low as possible. In
general, the BER of such CDMA systems is relatively high (typically> 10 −^6 ) but can
be improved using a forward-error correction scheme.
Spectrally encoding of CDM lightwave systems requires a rapid change in the car-
rier frequency. It is difficult to make tunable semiconductor lasers whose wavelength
can be changed over a wide range in a subnanosecond time scale. One possibility
consists of hopping the frequency of a microwave subcarrier and then use the SCM
technique for transmitting the CDM signal. This approach has the advantage that cod-
ing and decoding is done in the electrical domain, where the existing commercial mi-
crowave components can be used.
Several all-optical techniques have been developed for spectral encoding. They can
be classified as coherent or incoherent depending on the the type of optical source used
for the CDMA system. In the case of incoherent CDM, a broadband optical source such
as an LED (or spontaneous emission from a fiber amplifier) is used in combination with
a multipeak optical filter (such as an AWG) to create multiwavelength output [256].
Optical switches are then used to select different wavelengths for different chip slots.
This technique can also be used to make CDM add–drop multiplexers [234]. An ar-
ray of fiber gratings having different Bragg wavelengths can also be used for spectral