Monthly Archives: March 2012
Valence Band:The highest energy band,in a solid, that is completely filled or occupied by electrons at 0K is known as the valence band.
Conduction Band: The higher level of valence band that is partially occupied by electrons or vacant is known as the conduction band.
Energy gap is the gap between these two energy bands and an indication of several electrical or optical properties of solid.In semiconductor the forbidden gap or energy gap is 0.25eV-2.5eV
An atomic system is characterized by discrete energy levels,and the constituent atoms or molecules can exist in one of the allowed levels orstates
- Spontaneous Emission
- Stimulated Emission
- The source wavelength should correspond to the low-loss windows of silica viz.around 1.30 micro.m and 1.50 micro.m because the low fiber loss yield the larger repeater spacings.
- The spectral linewidth should be as small as possible typically <1nm
- It is possible to modulate the source at speed in excess of several 1Gb/s.To meet this requirement, one can either choose a suitable source that can be directly modulated or use an external modulator in tandem with a source that gives steady power output.
1.Makes efficient use of the spectrum by allowing overlap.
2.By dividing the channel into narrowband flat fading subchannels, OFDM is more resistant
to frequency selective fading than single carrier systems are.
3.Eliminates ISI and IFI through use of a cyclic prefix.
4.Using adequate channel coding and interleaving one can recover symbols lost due to the
frequency selectivity of the channel.
5.Channel equalization becomes simpler than by using adaptive equalization techniques with
single carrier systems.
6.It is possible to use maximum likelihood decoding with reasonable complexity.
7.OFDM is computationally efficient by using FFT techniques to implement the modulation
and demodulation functions.
8.Is less sensitive to sample timing offsets than single carrier systems are.
9.Provides good protection against cochannel interference and impulsive parasitic noise.
1.The OFDM signal has a noise like amplitude with a very large dynamic range, therefore it
requires RF power amplifiers with a high peak to average power ratio.
2.It is more sensitive to carrier frequency offset and drift than single carrier systems are due
to leakage of the DFT.