For the signalling schemes in this class, the data have not been translated to a much higher carrier frequency prior to intensity modulation of the optical source. Thus, a significant portion of the signal power is restricted to the DC region. Baseband mod- ulation techniques are so called because the spectrum of the modulated data is in the vicinity of DC. Baseband schemes that include, among others, OOK and the family of pulse time modulation (PTM) techniques, are more tolerant to the effects of the multipath channel. OOK is the simplest technique, in which the intensity of an opti- cal source is directly modulated by the information sequence. In contrast, PTM tech- niques use the information sequence to vary some time-dependent property of a pulse train. Popular examples of such schemes include pulse width modulation (PWM), in which the width of the pulses convey the information, and PPM, in which the information is represented by the position of the pulses within fixed time frames [15,16]. In DPIM, the information is represented by the number of empty slots between pulses, potentially allowing higher data rates and improvements in power efficiency compared to OOK and PPM [17]. Dual-header pulse interval modulation (DH-PIM), which is a variation on PIM reduces the number of ‘empty’ slots, and therefore symbol length, by introducing a second pulse at the start of the information symbol. The technique offers a trade-off between the lower bandwidth requirement of the longer pulse and the subsequent higher average optical power requirement. At higher bit rates, the scheme is both bandwidth and power efficient compared to PPM [17]. Unlike the fixed symbol length of PPM, both DPIM and DH-PIM offer symbol synchronization due to the pulse always being at the start of the symbol. To reduce the performance degradation of pulse modulation schemes adopted in a highly dis- persive indoor environment with a large delay spread, maximum likelihood sequence detection (MLSD) as well as decision feedback equalizer can be employed, but at the cost of higher system complexity.
This high bandwidth requirement problem in PPM has led to the development of several alternative modulation schemes, including the differential PPM [18,19], dicode PPM [20], multiple PPM [21], edge position modulation (EPM) [4] and hybrid modulation. The latter includes the differential amplitude pulse position modulation (DAPPM), which offers additional advantages over PPM in terms of the peak-to-average power ratio [22]. In EPM, the time is divided into discrete slots greater than the rise time and jitter of a pulse; however, the pulse width can be wider than one time slot, thus allowing more information to be transmitted than for a comparable PPM scheme. Multilevel modulation schemes such as PAM, L-PAM and QAM offered improved bandwidth efficiency by transmitting more information per symbol through the channel at the cost of lower power efficiency. However, com- pared to the binary schemes, they are more sensitive to the channel nonlinearities as well as the noise.
đang được dịch, vui lòng đợi..