The main objective in the communication systems is to transmit as much as possible information in as low as possible bandwidth and cost. Different modulation formats such as M-ary ASK, QPSK/DQPSK, QAM and DB are proposed to improve WDM channel capacity. M-ary ASK improves the channel capacity by transmitting more than one bit per signal element utilizing different signal levels or multilevel signals. QPSK/DQPSK utilizes multiple phases for transmit more than one bit per signal element. QAM improve channel capacity by transmitting more than one bit per signal element utilizing different phases and different amplitudes. DB use either three amplitude levels or two amplitudes (like binary signals) and one redundant phase element instead of the third level. All the advanced modulation schemes are limited to double or triple the channel utilization but all of them improve the channel capacity or the spectral efficiency. On the other hand, different multiplexing techniques such as TDM, WDM, OFDM, PDM and DCDM are proposed to improve the channel utilization. In TDM, different users share the same WDM channel by allocating each user different time slot. Using WDM, the available optical spectrum can be to utilize to support multiple numbers of users. In this technique, each user is assigned a wavelength as the carrier signal. In OFDM, utilizing IDFT many different RF signals can be assigned as the carrier for many low bit rate users. PDM allows two users to be carried over two different polarizations, vertical and horizontal polarizations. In DCDM, different users signed with different RZ duty cycles to share the same WDM channel. Except PDM, all other multiplexing techniques can support multiple users. Eventhough multiplexing techniques improve the channel utilization, but all of them except PDM, failed to improve the spectral efficiency of the link. PDM act like modulation schemes that can double the channel capacity or the spectral efficiency. Amongst all these multiplexing techniques and modulation schemes, PDM and QPSK/DQPSK or combinations of them have obtained the most attention in the communication systems. Finally, DCDM can reduce the clock recovery frequency significantly.
The main objective in the communication systems is to transmit as much as possible information in as low as possible bandwidth and cost.
