Chapter 3
PHY Layer Issues — System Imperfections
In this chapter, we investigate signal processing solutions for a few performance-limiting system imperfections in OFDM communications. We first identify a set of key problems whose solutions will enable OFDM radio networks to deliver on its potential; the rest thus balances fundamental investigations with system design and implementation aspects.
To effectively modulate and demodulate OFDM signals, a number of pre-and post-modem tasks must be performed. Signal processing plays an essential role in these tasks. The most important ones among others include
- frequency synchronization
- channel estimation
- phase noise compensation
- peak-to-average power radio (PAPR) reduction
- I/Q imbalance compensation
3.1 Frequency synchronization
The inherent immunity of OFDM to multipath comes at the price of increased sensitivity to inaccurate frequency reference [4] - contrasting to its single-carrier counterparts which are robust to frequency offset but critical to timing inaccuracy. A carrier offset at the OFDM receiver can cause losses in subcarrier orthogonality, and thus introduces interchannel interference (ICI) and severely degrades the system performance [4]. High accuracy carrier offset estimation and compensation is of paramount importance in OFDM communications.
Similar to other communication systems, carrier synchronization in OFDM is usually carried out in two phases, namely, acquisition and tracking. While the acquisition range is the ...
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