Important OFDM characters for Wimax
Here I list out some important OFDM characters for Wimax. Let’s check it in detail.
OFDM Pros and Cons:
OFDM enjoys several advantages over other solutions for high-speed transmission.
Reduced computational complexity:
OFDM can be easily implemented using FFT/IFFT and the processing requirements grow only slightly faster than linearly with data rate or bandwidth The computational complexity of OFDM can be shown to be, where B is the bandwidth and Tm is the delay spread. This complexity is much lower than that of a standard equalizer-based system, which has a complexity
Graceful degradation of performance under excess delay:
The performance of an OFDM system degrades gracefully as the delay spread exceeds the value designed for greater coding and low constellation sizes can be used to provide fallback rates that are significantly more robust against delay spread.
In other words, OFDM is well suited for adaptive modulation and coding, which allows the system to make the best of the available channel conditions. This contrasts with the abrupt degradation owing to error propagation that single-carrier systems experience as the delay spread exceeds the value for which the equalizer is designed.
Exploitation of frequency diversity:
OFDM facilitates coding and interleaving across subcarriers in the frequency domain, which can provide robustness against burst errors caused by portions of the transmitted spectrum undergoing deep fades. In fact, WiMAX defines subcarrier permutations that allow systems to exploit this.
Use as a multiaccess scheme:
OFDM can be used as a multiaccess scheme, where different tones are partitioned among multiple users. This scheme is referred to as OFDMA and is exploited in mobile WiMAX. This scheme also offers the ability to provide fine granularity in channel allocation. In relatively slow time-varying channels, it is possible to significantly enhance the capacity by adapting the data rate per subscriber according to the signal-to-noise ratio of that particular subcarrier.
Robust against narrowband interference:
OFDM is relatively robust against narrowband interference, since such interference affects only a fraction of the subcarriers.
Suitable for coherent demodulation:
It is relatively easy to do pilot-based channel estimation in OFDM systems, which renders them suitable for coherent demodulation schemes that are more power efficient. Despite these advantages, OFDM techniques also face several challenges.
First, there is the problem associated with OFDM signals having a high peak-to-average ratio that causes nonlinearities and clipping distortion. This can lead to power inefficiencies that need to be countered. Second, OFDM signals are very susceptible to phase noise and frequency dispersion, and the design must mitigate these imperfections. This also makes it critical to have accurate frequency synchronization.