Efficient architecture for a multichannel array subbanding system with adaptive processing

AnEcientArchitectureforaMultichannelArraySubbandingSystemwithAdaptivePro cessingDaniel Rabinkin and HuyNguyenMITLincoln Lab oratory, 244 Wo o d Street,Lexington, MA02420e-mail:frabinkin,hnguyeng@l l.mit.eduABSTRACTAn architecture is presented for front-end pro cessing in a wideband array system which samples real signals.Suchasystemmaybeencounteredincellulartelephon,radar,orlowSNRdigitalcommunicationsreceivers.Thesubbanding of data enables system data rate reduction, and creates a narrowband condition for adaptive pro cessingwithin the subbands.The front-end p erforms passband ltering, equalization, subband decomp osition and adaptiveb eamforming.The subbanding op eration is eciently implemented using a prototyp e lowpass nite impulse resp onse(FIR) lter,decomp osedintop olyphaseform,combinedwithaFastourierTransform(FFT)blo ckandbankofmo dulatingp ostmultipliers.Ifthesystemacquiresrealinputs,asingleFFTmaybeusedtoop erateontwochannels, but a channel separation network is then required for recovery of individual channel data.A sequence ofsteps is describ ed based on data transformation techniques that enables a maximally ecient implementation of thepro cessing stages and eliminates the need for channel separation.Op eration count is reduced, and several layers ofpro cessing are eliminated.1.INTRODUCTIONAsensorarraymabeusedinconjunctionwithamulti-channelpro cessingreceiverarchitecturetoimplementadaptive b eamforming.Thisenables therecovery of asignal withanenhanced signal-to-noise ratio (SNR)inthepresenceofinterferenceandnoise.Mo dernsensorarraysystemsuseincreasinglywiderbandwidthsignals.Thistrendisdrivenbya desireforimproved p erformance andtheadvancingcapabilityofhardware.For instance,greater bandwidth is required to increase throughput in wireless communication systems, or improve resolution in aradar or sonar system.Narrowband array b eamformersaretypicallyimplementedwithaonetap/sensor architecture.ThisapproachisillustratedinFigure1.Systemsthatusewidebandsignalsrequiremo di cationthenarrowbandadaptiveb eamforming approach to counteract the disp ersion e ect.Disp ersion is prop ortional to the time-bandwidth pro ductBTar r ay, de ned as the bandwidth of the signal times the propagation time of the wavefront across the array.Thee ectis illustrated in Figure 2.The gure shows the b eam patternresp onse as a function of lo ok angle for a linearray of 41 identical sensors with=2 element spacing and one tap/sensor adaptive pro cessing and a time-bandwidthpro duct of 2.The array is steered towards a target lo cated at an angle=0.An interference is present at angle=10.At the system's center frequency!0, a single tap/sensor adaptive b eamformer intro duces a deep null in thedirection of interference.However, the lo cation of that null is shifted for frequency comp onents at the edge of thesystem's frequency range!0+B=2.As a result, a considerable p ortion of the energy from theinterference sourceleaksintotherecovered signal.More detailedcriteria fordeterminingwhendisp ersionb ecomes unacceptable canb e obtained by examining the eigenvalues of the correlation matrix formed from sampling the interference and noisereceived bythe array.9,7In order tomitigate thedisp ersion problem,oneof two pro cessing architectures istypically used:Thetapp eddelayline architecture, and the frequency channelization architecture.The tapp ed delay line metho d uses multipleadaptive taps in eachchannel, resulting in a frequency dep endent b eamformer phase resp onse.Prop er choice of tapweights allows for go o d b eam null depth to b e maintained across the op erating bandwidth.The frequency channelizermetho d decomp oses the received wideband signal intoanumb er of subband comp onents, each of which has reducedbandwidth.The disp ersion in each subband is therefore reduced, and indep endent one tap/sensor b eamformers areThis work was sp onsored byDARPA under Air Force Contract F19628-95-C-0002.Opinions, interpretations, conclusionsand recommendations are those of the authors and are not necessarily endorsed by the United States Air Force or DARPA.Ahybrid approach is also p ossible