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12 - Transmit beamforming

from Part III - Transmit beamforming

Published online by Cambridge University Press:  04 December 2009

Eldad Perahia  and
Robert Stacey
Eldad Perahia
Affiliation:
Intel Corporation, Hillsboro, Oregon
Robert Stacey
Affiliation:
Intel Corporation, Hillsboro, Oregon
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Summary

The capability to perform adaptive transmit beamforming is provided in the 802.11n standard. With transmit beamforming (TxBF), we apply weights to the transmitted signal to improve reception. The weights are adapted from knowledge of the propagation environment or channel state information (CSI). Since by definition transmit beamforming weights are derived from channel information, spatial expansion as defined in Section 6.2 is not considered transmit beamforming.

The key advantage with transmit beamforming is the ability to significantly improve link performance to a low cost, low complexity device. This advantage is illustrated in Figure 12.1, which depicts a beamforming device with four antennas. Such a device could be an AP or a home media gateway. The device at the other end of the link has only two antennas, typical of a small client device. Such a system would benefit from 4 × 2 transmit beamforming gain from device A to device B. However, when transmitting from device B to device A, the system gain would be matched with 2 × 4 SDM with MRC as described in Section 6.1. Therefore link performance would be balanced in both directions.

In Figure 12.2, the generic MIMO system is modified to illustrate the application of beamformer weights to the transmitted signal. To simplify notation, the system description is given in terms of the frequency domain for a single subcarrier. Transmit beamforming as described is applied to each subcarrier in the frequency band.

Type
Chapter
Information
Next Generation Wireless LANs
Throughput, Robustness, and Reliability in 802.11n
 , pp. 307 - 367
Publisher: Cambridge University Press
Print publication year: 2008

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References

Anderson, E., Bai, Z., Bischof, C., et al. (1999). LAPACK User's Guide, 3rd edn. Philadelphia: SIAM, available at: www.netlib.org/lapack/lug/lapack_lug.html.CrossRefGoogle Scholar
,Institute of Electrical and Electronic Engineers (2007). IEEE P802.11nTM/D3.00, Draft Amendment to STANDARD for Information Technology – Telecommunications and Information Exchange Between Systems – Local and Metropolitan Networks – Specific Requirements. Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY). Amendment 4: Enhancements for Higher Throughput.
Lebrun, G., Ying, T., and Faulkner, M. (2002). MIMO transmission over a time-varying channel using SVD. IEEE GLOBECOM'02, 1, 414–18.Google Scholar
Nanda, S., Walton, R., Ketchum, J., et al. (2005). A high-performance MIMO OFDM wireless LAN. IEEE Communications Magazine, February, 101–9.CrossRef
Sadowsky, J. S., Yamaura, T., and Ketchum, J. (2005). WWiSE Preambles and MIMO Beamforming?, Institute of Electrical and Electronic Engineers 802.11–05/1635r1.Google Scholar
Smith, G. S. (2004). A direct derivation of a single-antenna reciprocity relation for the time domain. IEEE Transactions on Antennas and Propagation, 52(6), 1568–77.CrossRefGoogle Scholar

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