The recent explosion in wireless technology opens a new dimension to wireless communications without regard to mobility or location. Personal communication networks need to support high quality data including voice, facsimile, still pictures and streaming video. All of which require high transmission rates of several Mega bits per second (Mbps). The radio channels in mobile radio systems are usually multipath fading channels which cause inter-symbol interference (ISI) in the received signal. To remove ISI from the signal, there is a need for a strong equalizer which requires knowledge of the channel impulse response (CIR). That knowledge is provided thru use of a separate channel estimator.

Usually the channel estimation is based on the known sequence of bits, which is unique for a certain transmitter and which is repeated in every transmission burst. Thus, the channel estimator is able to estimate CIR for each burst separately by exploiting the known transmitted bits and the corresponding received samples.

For mobile or wireless applications, the channel is often described as a set of independent multipath components. Among the most important parameters when choosing the modulation scheme are the delay and the expected received power for different delays. Typically channel estimation is performed using one of three methods and/or a combination of these methods. Trade-offs between complexity and performance dictate the choice of algorithm.

-         Data Aided Channel estimation, known pilot symbols are transmitted, at the receiver end the channel estimation algorithm operates on the received signal along with its stored symbols to generate an estimate of the transmission channel.

-         Decision-Directed Channel Estimation, a rough estimate of the channel is obtained using a suitable estimation method. This estimate is used to make symbol decisions as pilot symbols.

-         Blind Channel Estimation, characteristics of the modulated signal are used to differentiate among estimated channels.

A popular candidate in eliminating ISI is Orthogonal Frequency Division Multiplexing (OFDM). In OFDM signals, bandwidth is divided into many narrow sub-channels which are transmitted in parallel. Each sub-channel is typically chosen narrow enough to eliminate the effect of delay spread.

Pilot-based approaches, where pilot means the reference signal used by both transmitters and receivers, are widely used to estimate the channel properties and correct the received signal. There are two types of pilot arrangement; block-type and comb-type. Block-type is sent periodically in time-domain and suitable for slow-fading radio channels thus relatively insensitive to frequency selectivity. Comb-types are uniformly distributed within each OFDM block and have higher re-transmission rates. With better resistance to fast-fading channels, comb-type pilot arrangement is sensitive to frequency selectivity when compared to block-type. Because the error rate of comb-type pilot schemes is higher than the lowest error rate than can be achieved by block-type, data with high importance/priority are optimal.

The data rate and spectrum efficiency of wireless mobile communications have been significantly improved over the last decade or so. Can we expect similar advances in the coming years? Determining a channel estimation technique that successfully solves the problems of ISI and CSR are necessary before the systems can successfully handle the coming volume of data.

http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing

http://jp.fujitsu.com

http://ethesis.nitrkl.ac.in

http://cache.freescale.com/files/dsp/doc/app-note/AN2253.pdf?fsrch=1&sr=2

http://www.comlab.hut.fi/opetus/260/chan_est.pdf