Zero-forcing precoding: Difference between revisions

Revision as of 00:58, 20 June 2009

Revision as of 12:47, 24 June 2009

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In particular, '''Null-Steering''' is a method of [[beamforming]] for [[narrowband]] [[signal processing|signals]] where we want to have a simple way of compensating delays of receiving signals from a specific source at different elements of the antenna array. In general to make use of the antenna arrays, we better to sum and average the signals coming to different elements, but this is only possible when delays are equal. Otherwise we first need to compensate the delays and then to sum them up. To reach this goal, we may only add the weighted version of the signals with appropriate weight values. We do this in such a way that the frequency ___domain output of this weighted sum produces a zero result. This method is called null steering. The generated weights are of course related to each other and this relation is a function of delay and central working frequency of the source.

==Performance of Zero-forcing Precoding==

==Performance of Zero-forcing Precoding==

If the transmitter knows the downlink channel state information (CSI) perfectly, ZF-precoding can achieve almost the system capacity when the number of users is large. On the other hand, with limited channel state information at the transmitter (CSIT) the performance of ZF-precoding decreases depending on the accuracy of CSIT. ZF-precoding requires the significant feedback overhead with respect to signal-to-noise-ratio (SNR) so as to achieve the full multiplexing gain<ref name="Jindal_ZF">{{cite journal|author=N. Jindal|title=MIMO Broadcast Channels with Finite Rate Feedback|journal=IEEE Trans. Information Theory.|pages=5045–5059|volume=52|number=11|date=Nov. 2006|doi=10.1109/TIT.2006.883550}}</ref>. Inaccurate CSIT results in the significant throughput loss because of residual multiuser interferences. Multiuser interferences remain since they can not be nulled with beams generated by imperfect CSIT.

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where <math>s_i</math> is the transmitted signal of user <math>i</math>, <math>\mathbf{n}</math> is the <math>N_r \times 1</math> noise vector, <math>\mathbf{h}_k</math> is the <math>N_r \times 1</math> channel vector.

==See Alsoalso==

* [[Precoding]]

* [[MIMO]]

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== External links ==

*[http://www.antenna-theory.com/arrays/weights/zeros.php Schelkunoff Polynomial Method (Null-Steering)] www.antenna-theory.com

[[Category:IEEE 802]]

[[Category:Information theory]]

[[categoryCategory: radioRadio resource management]]

[[Category:Signal processing]]