In the field of communication, multipath is a propagation phenomenon that results in radio signals reaching a receiver (receiving antenna) by two or more paths, thus, the same radio signals may arrive multiple times at the receiver at slightly different times, which may cause signal distortion. Multipath or multipath information may be caused by, for example, atmospheric ducting, ionospheric reflection and refraction, and/or reflection from terrestrial objects, such as mountains and buildings. Because there can be only one “direct” path, some process of reflection, refraction, or scattering must account for multipath information. Furthermore, multipath information may result in a form of echoing or one or more “ghost signals.”
Accordingly, equalizers are widely used to compensate for channel distortion, for example, multipath information, in signal transmitting and receiving systems used in the fields of communications, broadcasting, storage media, the military and the like. To effectively remove multipath information having a time delay with respect to a main signal, equalizers typically have a sufficient tap length, i.e., a sufficient number of taps, to handle multipath information. To remove multipath information from a signal, an equalizer typically includes an N-tap feed forward filter and an M-tap feed backward filter, which may reduce the ghost signals present before (pre-ghost signal) and after (post-ghost signal) the main signal.
Referring now to FIG. 1, a schematic block diagram illustrating conventional fixed tap length equalizers 100 will be discussed. As illustrated in FIG. 1, the fixed tap length equalizer 100 includes a feed forward filter 110 including a main tap 130, a feed backward filter 120, and an accumulator 140. The tap length (number of taps) in the feed forward filter 110 and the tap length in the feed backward filter 120 are fixed.
Based on the main tap 130, a pre-ghost signal is removed by the feed forward filter 110 and a post-ghost signal is removed by the feed backward filter 120. A ghost signal that exists in a short section behind the main tap 130 (or a main signal) and has a large magnitude (“strong short ghost signal”) may be removed by some of the taps of the feed forward filter 110, which are located behind the main tap 130 and/or some of the taps of the feed backward filter 120. Typically, a long pre-ghost signal and a long post-ghost signal, which have an enough magnitude to influence a main signal, do not exist simultaneously.
The accumulator 140 receives output signals of the feed forward filter 110 and output signals of the feed backward filter 120, sums the received output signals and outputs a summation result to a decision block (not shown).
Conventional fixed tap length equalizers 100 typically have enough tap length to cover most possible lengths of ghost signals and, therefore, typically use a lot of hardware. Conventional fixed tap length equalizer 100 may also be able to handle multipath information (or a ghost signal profile) that rapidly changes. However, when the amount of the change goes beyond the limit of conventional fixed tap length equalizers 100, coefficients of all taps included in the conventional fixed tap length equalizer 100 may be initialized.
Referring now to FIG. 2, a schematic block diagram of conventional main tap moving equalizers 200 will be discussed. As illustrated in FIG. 2, the conventional main tap moving equalizer 200 includes a feed forward filter 210 including a main tap 230, a feed backward filter 220, and an accumulator 240. The tap length of the feed forward filter 210 is the same as the tap length of the feed backward filter 220. The main tap 230 is moved based on multipath information so that a ghost signal may be removed.
For example, when a long pre-ghost signal exists in a channel, the conventional main tap moving equalizer 200 may be configured to move the main tap 230 to the right (R) to increase a pre-ghost signal handling range. Similarly, when a long post-ghost signal exists in a channel, the conventional main tap moving equalizer 200 may be configured to move the main tap 230 to the left (L) to increase a post-ghost signal handling range. Accordingly, the conventional main tap moving equalizer 200 typically has a very wide ghost signal handling range.
However, the conventional main tap moving equalizer 200 may not remove strong short ghost signals existing behind the main tap 230 effectively. Moreover, since a data bit width of a tap of the feed backward filter 220, which is smaller than a tap of the feed forward filter 210, is increased, the size of the hardware for the conventional main tap moving equalizer 200 may also be increased. Furthermore, when multipath information changes, the main tap 230 is moved, and therefore, coefficients of all taps included in the conventional main tap moving equalizer 200 may be initialized.