SWEPTNOTCH UWB TRANSMITTER

The present invention relates to radio signals, radio transmitters and radio receivers, particularly those using multicarrier or OFDM (Orthogonal Frequency Division Multiplex) technology.

A block diagram of a known ultra-wideband transmission system is shown in Figure 1. Input data is processed in turn by a scrambler 101, a convotutional encoder 103 followed by a puncturer 105, and by a bit interleaver 107. Constellation mapping is then performed (block 109.), followed by an IFFT operation 1 11 in which ptlotsrtones and CP (Cyclic Prefix) and GI (Guard Interval) information is inserted. The resulting digital samples are converted to an analog signal using a digital to analog converter and upcoπverted by a combination of a mixer 115 and an interleaving kernel 117. The interleaving kernel is essentially a very fast frequency hopping local oscillator generator. The resulting upconverted signal is transmitted via an antenna 119.

In the system of Figure 1, the mapping of OFDM sub-carriers shown in Figure,2 may be used.

UWB transmission systems have been discussed in the literature that have reduced emissions at certain frequencies to avoid impacting known victim services that may be operating in close proximity. Such schemes have been described as "Detect and Avoid" schemes since these ^re intended to detect a victim service and then avoid it. OFDM signalling is weli-suited to creation of such dynamic detection and avoidance since the appropriate hardware is already available for spectrum sculpting in the transmitter (using an IFFT) and spectrum analysis in the receiver (using an FFT). Furthermore, signal processing for active interference cancellation (AIC) using one or more pairs of nulling tones at the left and right edges of the desired nulling band has recently been described, resulting in deep notches of 30-4OdB.

The main disadvantage of the current art is that the frequency band of a local victim receiver cannot usually be known a priori. Thus, the UWB transmitter may increase the unwanted signal (interference + noise) floor of the local victim receiver, preventing it from correctly receiving a

distant, weak signal from a base station, Discovery of the local receiver may only be possible if the associated transmitter begins to transmit; but this event is unlikely to occur if the receiver remains blocked by the UWB transmission. Therefore, start-up of the local victim transmitter may be blocked until a silence period of sufficient duration occurs in the UWB transmission. It has been proposed to insert a regular silence period in the Medium Access .Controller (MAC) of the UWB systein to facilitate the start-up of victim services. However, initial estimates showed that the length and frequency of these silence periods would be highly onerous for the UWB device, consisting of approximately 50ms contiguous silence with a frequency of IHz or more. This manner of operation would virtually destroy any chance of maintaining an isochronous connection with sufficiently ]ow latency.

The current invention uses a frequency domain technique to ensure the appropriate silence periods for the victim service without directly aliocating any time-domain MAC resources. In an exemplary embodiment, a sub-set of the transmitted sub-carriers are nulled, and the location of the null is not static but is swept in a systematic way through a set of possible locations in the transmission band where a victim service may be located. Thus, from the point of view of the victim receiver, the interference associated with the UWB transmitter is removed for a regular, predicable duration corresponding to a regular, repeated interval when the notch is co-channel in the frequency domain with the victim service.

The present invention may be further understood from the following description in conjunction with the appended drawing. In the drawing:

Figure 1 is a block diagram of a known UWB transmission system.

Figure 2 is a diagram illustrating mapping of the OFDM sub-carriers in the system of Figure 1.

Figure 3 is a block diagram of a swept notch UWB transmission system.

Figure 4 is a diagram illustrating mapping of the OFDM sub-carriers at a particular time in the system of Figure 3,

Figure 5 is a diagram illustrating mapping of the OFDM sub-carriers over time in the system of Figure 3.

Figure 6 is a power spectral density plot of the transm itter of Figure 3 during a first time interval.

Figure 7 is a power spectral density plot of the transmitter of Figure 3 during a second time interval.

Figure 8 is a power spectral density plot of the transmitter of Figure 3 during a third time interval.

Figure 9 is a power spectral density plot of the transm itter of Figure 3 during a fourth time interval.

Figure 10 is a power spectral density plot of the transmitter of Figure 3 during a fifth time interval

Figure 11 is-a block diagram of the swept notch processing block of Figure 3.

Consider an OFDM UWB transmitter according to the block diagram schematic of Figure 3. The transmit chain of Figure 3 differs from that of Figure 1 in that a swept notch processing block 301 is coupled between the constellation mapping block 109 and the IFFT block H l . Operation of ^" the swept notch processing block 301 will be described presently. One realization of the swept notch processing block 301 is described in further detail below in connection with Figure ^] 1.

By way of an example embodiment, consider the case where deleting four consecutive sub- carriers create a null zone of 4x4, 125=16.5MHz as shown in Figure 4.

Initially, the four leftmost tones are removed by inserting null values in the corresponding IFFT register locations in the transmitter. This situation is maintained for a pre-determined period of time. In subsequent time-slots, the location of the notch is shifted b-y one sub-carrier location. This shifting is illustrated in Figure 5,

An exemplary time interval between shifts of the notch could be 20ms. This would allow a service that could be protected by the deletion of two sub-carriers to be protected for a period of 60ms. Assuming the cycle would be extended of 128 sub-carriers, the period between protection intervals would be 128 x 20ms = 2.56 seconds. This may be satisfactory from the point of view of a victim service starting up for the first time, since after power-on of a device, the user may accept several seconds to acquire service and establish a connection. Once uplink communication has commenced from the device receiving protection, a fixed notch may be substituted for the swept one in order to protect the known location of the victim service downlink signal. This protection may be based on a known duplex spacing, or a limited set of possible duplex spaciπgs known to be employed by the service receiving protection,

A second exemplary embodiment relates to the case where active interference cancellation is employed. (See Active interference cancellation technique for MB-OFDM cognitive radio Yaniaguchi, H.; Microwave Conference, 2004. 34th European Volume 2, 13 Oct.2004 Page(s): l 105 - 1108,)

In the above-cited work, it is shown that deep notches can be created by use of active interference cancelling tones, typically located at the edges of the region to be nulled. Without reproducing the entire mathematical basis, the essential notation is as follows:

The upsampied frequency domain vector, Y, can becalculated from the following matrix product :

Y[512,1] = P[512,128]- X[128,1]

A segment of Y to be nulled is defined as d , . d _ι [» _ϊ ,l] = P _ϊ K,128]- X ₁ [128,lJ, where X ₁ [HSJ] is the vector of transmitted OFDM tones with a contiguous segment of length n _mιl! - n _ak set to zero. ⁿ , _ι ~ ^( ⁿ i _m ii ~ ^ri _m c -" O ⁺ I' ⁿ aiQ ^l$ the number of AlC tones.

To minimized :|P, [»„, « _rf ] -hfn^ll+d, 0,,,l]f

=> h = -(P, ^T P, / P^d ₁ Moore - Penrose psuedo inverse = -W ₁ Ci ₁ = -W ₁ (P ₁ -X ₁ ) = ~W ₂ -X ₁

In the above equations, Pi and P ₂ are sub-matrices of the transform kernel, P, used to perform frequency-domain upsampling, h is a vector of active interference cancellation values to be inserted and X ₁ is the vector of information symbols to be transmitted but with the locations where active interference cancellation vector h will be inserted replaced by zeros. The value of matrix W ₂ is fixed for any given desired notch and can be pre-calculated.

It lias been noted that the "nulling matrix" Wa typically has only n _aic significant rows, where n _aic is the number of active interference tones used (typically 2). Taking account of this, a pair of vector dot products, one for each active interference cancellation tone employed, can replace the matrix multiplication h = -W ₂ X ₁ .

Mote that a given notch can be moved to any desired location by a corresponding cyclic shift of th& matrix Wj. In the context of the current invention, this is useful because it allows an active interference cancellation notch to be moved using low complexity operations, namely a cyclic shift by one or more locations. Thus, a regular, timed cyclic shift of the notch location can be accomplished very easily-

Referring to Figure 1 1. a block diagram is shown of the swept notch processing block of Figure 3 for producing a cyclically shifted AIC notch in the foregoing manner. A control circuit 1101 controls operation of the various blocks of Figure 1 1.

The size of the IFFT in the present example is assumed to be 128. In preparation for the IFFT, a [128, lj-sized array of information symbols is formed using a conversion circuit 1 103. A block 1 105 then performs an operation in which zeros are inserted in place of values for sub-carriers to be nulled, thereby forming a '"notched OFDM symbol" X ₁ . At the same time, a nulling matrix W2

selected in view of a desired notch width (assumed here to be five sub-carriers) is read out of a look-up table 1 107. The nulling matrix is cyclically rotated by a circuit 1 109 m accordance with a current desired position of the narrowband notch. The notched OFDM symbol and the rotated nulling matrix are multiplied by a multiplier 1 1 1 1 to obtain an interference cancellation vector Iu In a block 1113, the values of the interference cancellation vector h are substituted for the zero values of the notched OFDM symbol to produce an AIC OFDM symbol X' _t , The AIC OEDM symbol is then ready for the IFFT operation to follow.

Example MATLAB® code fora cyclically shifted AIC notch follows:

% Test λtC for OFDM showing cyclic shift of notch location

% 128 Random QPSK symbols are transmitted using QI-T)M

% A null region is created by writing zeros in the required IFFT locations

% λ MMSE solution is found to the set of corn pi ex tone values to replace

% the missing set for minimum energy in the desired null band.

% The resulting PSD lor 1000 such OFDM symbols is plotted. clear; close all;

nr=24:28; %null region range of sub-carriers unr=(nr-l)*4+l; % null region indices at 4x o\ersampling unrl=unr(2):unr(end-l); % this is the region for zero energy sub-carriers

P=zeros(length(uπrl),128);% transform kernel for 4κ ovcrsampling for k=0:]27 for l=unrl P(l+l-unrl(l),l<+l)=sum(expO*2*pi*(0:127) ^=i! (k-(l-l)/4)/128)); end end

fprintf(l,'doπe calculating P\n")

P!=P(:,nr); % smaller transform kernel

Wl=InV(Pl^Pl)*?! ¹ ; % pre-calctilation of Moore-Pen rose psuedo inverse

W2=W1 *P;

for shift=l :5 strearn=Q; for sim=l : 1000 a=(rand(l,128)>0.5)= ^!! 2-l+j*((rand(l, 128)>0.5)*2-l); % vector of 128 QPSK sub-carriers a(nr)=0; % this region is hereby nulled

h=-W2*a.'; % Calculation of required nulling tone set b=[a(l:nr(l)-l) h.' a(nr(end)+l :128)]; % insertion of tone set ofdm=ifft(b); % make an OFDM symbol in time domain stream=[stream ofdm zeros( 1,128}]; % calcuate OFDM symbol stream end iigure(shift) pwelch(stream,1024) % evaluate spectrum nr=nr+5; % shift notch location by 5 places

W2=[W2(:,128-5+l:end) W2(:,l:128-5)]; % corresponding cyclic shift of W2 end

Tiie last two lines of code preceding the final "end" statement are responsible for "shifting" the notch. The shift has been set to 5 IFFT bins for clarity in the graphical output. Without loss of generality, the shift can be set to a single IFFT bin for use in a real-world scenario in order to prolong the period of interference protection for the victim service.

Figures 5-10 are five figures resulting from executing the above code. They represent the average power spectral density with the notch in a sequence of five locations. Note that the notches are approximately 4OdB in depth and the quality of the notch is not impacted by the cyclic shift operation used to move it to a desired location.

The described transmitter may be used with a conventional receiver unaware of the swept- notch characteristics of the received signal. Alternatively, the receiver may be arranged such that energy below a threshold level for a given carrier or sequence of carriers is "zeroed out." In this manner, noise is not mistaken for transmitted information.

The present invention finds application, for example, in ultra-wide band radio transceivers employing detect and avoid methods to protect in-band victim sen' ices, and in cognitive radio solutions in general. Note that the application of this invention is highly detectable, due to the swept notch being a highly observable phenomenon.

It will be appreciated by those of ordinary skill in the art that the present invention may be embodied in other specific forms without departing from the spirit or essential character thereof. The foregoing description is therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, not the foregoing description, and all changes which come within the meaning and range of equivalents thereof are intended to be embraced therein.