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9.8 References 449the length of the segment and may not provide adequate sample support. This creates atrade-off between cancellation performance loss due to estimation errors, which motivatesmaking the segments larger, and performance loss due to the jammer nonstationarity, whichmotivates making the segments smaller. A second drawback is that changing the weightsperiodically in the receive window can impose small modulations on the target signal thatcreate increased range sidelobes after pulse compression.9.6 SUMMARYDBF architectures provide significant functionality enhancements for phased array radar.At the current state of digital technology, element-level DBF is often impractical for largearrays operating at high frequencies, so subarray level implementations are commonly usedto reduce the digital receiver count. The choice of subarray architecture heavily impactsperformance due to grating lobes and grating nulls that result from under sampling thearray. Adaptive algorithms provide the ability to cancel unwanted jamming and can uselow gain auxiliaries, subarrays, and high gain beams as spatial degrees of freedom; eachof these work best against jamming located in specific regions of the antenna pattern. Theadaptive filters can also be augmented for wideband operation by optimize over both thespatial and frequency domains.9.7 FURTHER READINGIntroductory material on adaptive filtering is available in [1,8].9.8 REFERENCES[1] Johnson, D.H. and Dudgeon, D.E., Array Signal Processing: Concepts and Techniques,Prentice Hall, Englewood Cliffs, NJ, 1993.[2] Haykin, S. and Steinhardt, A., Ed., Adaptive Radar Detection and Estimation, Wiley, NewYork, 1992.[3] Hudson, J.E., Adaptive Array Principles, Peter Peregrinus Ltd., London, 1989.[4] Monzingo, R.A. and Miller, T.W., Introduction to Adaptive Arrays, Wiley, New York, 1980.[5] Compton, R.T., Adaptive Antennas: Concepts and Performance, Prentice Hall, EnglewoodCliffs, NJ, 1988.[6] Widrow, B. and Stearns, S., Adaptive Signal Processing, Prentice Hall, Englewood Cliffs, NJ,1985.[7] Van Trees, H.L., Optimum Array Processing (Detection, Estimation, and Modulation Theory,Part IV), Wiley, New York, 2002.[8] Sayed, A.H., Fundamentals of Adaptive Filtering, IEEE Press, John Wiley & Sons, 2003.[9] Mailloux, R.J., Phased Array Antenna Handbook, 2d ed., Artech House, Boston, MA, 2005.[10] Haykin, S., Adaptive Filter Theory, 4th ed., Prentice-Hall, Englewood Cliffs, NJ, 2001.[11] Laakso, T.I., Valimaki, V., Karjalainen, M., and Laine, U.K., “Splitting the Unit Delay,” IEEESignal Processing Magazine, pp. 30–60, January 1996.