output power dependency on repeater spacing was also pre- sented based on these results.

Acknowledgments: The authors would like to thank M. Aiki and S. Saito for several fruitful discussions and K. Hagimoto and T. Kataoka for providing the transmitter and receiver. We also thank T. Miki, H. Ishio, T. Ito and K. Nakagawa for their encouragement.

0 IEE 1993 18th June 1993 T. lmai, M. MpaLami and A. Naka (NTT Tranmission Systems Lmboraiories, 1-2356 Take, Yokowka-shi, Konagawa, Japan)

Rd- 1 r""y". M., KATAOKA, T., IbIAI, T., HAGIMOTU, K., and AuU, M.:

lOGbit/s, 6OOOkm transmispi on experiment using erbiumdopcd fibre in-line repeaters', Electron Lett, 1992.28, (24), pp. 22542255

w-vmn, H.: 'lOGbit/s, 9pOOkm trammission experiments using 274 Erdopcd fih repeater repeater'. OFC '93, PDl, 1993

3 mu, T., I(URIW(I, M., -A, Y., w M., and mu, T.: '2.5 Gbit/s, 10,073 km straight line transmission system experiment using 100 Erdoped 6bn a m p W , Eleciron. Lett., 1992,- (la), pp. 1- 1485

4 uucuse, D.: 'Singlachannd operation in very nonlinear fibers with optical repeaters at zero dispersion', J . Lighiwm Technol., 1991, LT4, pp. 356-361

5 NAKA, A., and m, s: 'Fibre transmission distana determined by eye opening degradation due to self-pbasc modulation and group velocity dispersion', Electron. Lett, 1992,28, pp. 222-2223

2 TAG& H., PDAGAWA, N., TAN- H., SuZurtI, M., YMW(M0, S., and

SUBOPTIMAL METHOD FOR TIME-SPACE SIGNAL PROCESSING IN AIRBORNE RADAR

X. G. Wang

Indexing tenns: Adaptiw sigd processing, Radar clutter, Adaptiw filters

A new suboptimal method for time-space signal proassing m airborne phased array radar is propod which can sign&- canUy reduce the ncaseary computation compared with other optimal or suboptimal methods. The numerical results obtained from simulation illustrate that this method pos- scss*l idcal performance and is a disable scheme for real- time proccaping

Introduction: The suppression of ground clutter received by airborne radar is a two-dimensional filtering problem because the clutter depends on both incident degree and platform velocity. Therefore sampling of the echo field in time and space is required, which in practice is fulfilled by a coherent pulse Doppler phased array radar.

Assume the radar receiving antenna is an Lsensor evenly spaced linear array and echo signal under processing is an M-pulse train, and the received signal by the nth sensor at time t is x&). We can then define an L x M-dimension time- space sampling vector under processing:

X~[x(l)',X(2)', ...,X(M)'] T = S + N + C (1)

where x(t) = [x,(t), x2(t) ,..., xL(t)]' (t = 1, 2, ..., M) is the space sampling vector, and S, N and C are target signal, noise and clutter contained in X. The optimal weight vector is given by the well known formula [l]

W = p R i 'S' (2)

where R, is the covariance matrix of X, S* is the conjugate vector of S, and p is any constant.

Although the optimal method is an effective measure for clutter suppression, the computation required restricts its application in practice. The Klemm auxiliary channel method

ELECTRONICS LETTERS 5th August 1993 Vol. 29 No.

[2, 31 cannot meet the needs of real-time signal procasing ather. The suboptimal method proposed in this Letter involves the smallest computation with almost the optihal performance.

System analysis: Consider the case of a platform moving &on- mntally at a constant velocity. We d e h e the phase'difference between two adjacent time samples as tirqe frequency o, and the phase difference between two adjacent samples as space frequency a,. For clutter arriving at azimuth 0 and'pitch cp:

-= w, 2 d . cos 0 cos cpf1 4x0 . cos e cos ( ~ I ~ ~ P R F

PRF . d 2 . 0

=-

= K

= constant (3)

with 1 being the radar wavelength, PRF the radar pulse repe- tition frequency, d the space between sensors and U the plat- form velocity. This equation explains that the two-dimensional power spectrum of clutter distributes only along a ridge in the o, - o, plane as shown in Fig. 1.

I

-40 -t1 -t frequency response of time filters

frequency response of time cancellers

A m r Y A A A 4

Fig. 1 Distribution ofclutter power spectrum

Suppose the main lobe of the antenna points at a target which represents the gene& case. We divided the clutter spec- trum into sections a, b and c, where section a indicates the main-lobe clutter, and section b and c indicate the side-lobe clutter. Only section c can be easily filtered out by time filters and space filters in cascade connection because it is located in the side-lobes of both of them. Their required frequency responses are shown in Fig. 1 along the co-ordinate axes, where o,,, are, respectively, the time and space frequencies of the target signal. However, sections a and b cannot be sulliciently filtered out by filters alone. This is because, con- trary to section c, sections a and b are located in the main lobe of either time or space filters, and in addition to this, the power spectrum amplitude of section a is originally much higher than section c.

The solution is to add extra time cancellers to cancel section a and extra space cancellers to cancel section b. Their required frequency responses are also plotted in Fig. 1, where q1 and a,, are, respectively, the cancellation frequencies of the time and space cancellers. Therefore the system consists of four parts as shown in Fig. 2, where FIT, DBF and MTI act as time filters, space. filters and time cancellers, respectively. Similar to time cancellers, the outputs of the space cancellers are the linear combination of several adjacent space samples.

From Fig. 1, we can find the relationship between (a,,, os,,) and (qlr as&

uti o d K o s 1 = at0 ' K (4)

It is important to determine wt0, o,, and K for canceller

16 1411

design. In fact, oso can be determined because we have assumed that the main lobe of the antenna points at the target. Although the target velocity is unknown, q0 can be

Fig. 2 System block diagram

obtained according to the channel number of the FFT. K is a constant depending on PRF, d and U, the first two of which are known and last one can be very exactly measured by other airborne equipment.

System performance: Generally, improvement factor I is used as the performance measure:

1 (5) -

SCNR o 1 SCNR, si . no. LM . C N R , + I .

where SCNR represents the target signal-to-clutter + noise ratio and CNR clutter-to-noise ratio, subscript i denotes input and o output, sJs, is target signal gain, nJn. is noise attenu- ation and I,, = LM . (CNR, + 1) is the performance limit. In our case, sJs, A 1, nJn, oc L x M . Later simulation will show that a CNR, of less than OdB is provided if the weight of the filters and cancellers are suitably selected; then

q will vary with the time and space frequencies of the target signal (wto, oxo) and the weight vectors of every part, and its value is several dB below zero, for example, typically - 3 dB. In this sense, the method proposed above is a kind of sub- optimal method.

Table 1 lists the computation of each party. Their sum is much less than the O [ ( L x M ) 3 ] required by the optimal method, and much less than the O [ ( L + M ) 3 ] required by the Klemm suboptimal method.

Table 1 COMPUTATION FORMULAS

Complex multidication Comdex addition

Time cancellers (L + k,)k, ( L + k,)k, Time filters Space cancellers LMk, LMk, Saace filters LM ( L - l ) M

( L + k,) log, M / 2 ( L + kJ log, M

System simulation: In simulation, assume that ground is a plane, and some main parameters are selected as follows: M + k, = L + k, = 16 + 2, PRF = 5 W H z , I = 0,2m, d = L/2, U = 250m/s, platform altitude H = lOkm, target azimuth 45”, target distance R = 100 km (flying close to ground), side lobe of FIT, DBF and transmitting antenna pattern: - 50, - 30, -25dB. respectively (Chebyshev weight), time and space can- celler weight vectors: W, = W , = [1/3.38, - 1.98/3.98, 1/3.98]’*. Simulation results show the CNR, < OdB except around target velocity = 0 provided C N R , < 70dB. The improvement factor curves for CNR, = 60dB are shown in Fig. 3. It is clear that the two curves, which represent the performance of this method and the performance limit, respectively, are quite close.

When CNRi > 70dB, to ensure the suboptimal per- formance or CNR, < OdB, lower side lobes for the filters and

cancellers are required. Nevertheless, system errors, for example phase and amplitude errors, among the sensors will reduce the system performance. However, the results of many simulations show that the system performance of this method is much less sensitive compared with other suboptimal methods.

1412 ELECTRONICS LETTERS 5th August 1993 Vol. 29 No. 16

20

-800-600-400-200 0 200 400 600 800

1110131 target velocity,m/s Fig. 3 System performance curves

Conclusion: This suboptimal method is proposed to detect moving targets in an airborne background. Besides the lowest number of computations and the suboptimal performance stated above, the performance of this method is much less sensitive to system errors than other suboptimal methods.

Acknowledgments: The author would like to thank Z. Z. Zhang and W. Q. Xu for constructive comments and helpful discussion of this work.

0 IEE 1993 2Ist May 1993 X. G. Wang (Department of Electronic Engineering, University of Elec- tronic Science and Technology of China, Chengu 610054, People’s Republic of China)

References

1 BRENNAN, L. E., MALLETT, I. D., and REED, I. s.: ‘Adaptive arrays in airborne MTI radar’, IEEE Truns., 1976, AP-7.4, (5). pp. 607-615

2 KL.F.MM, R.: ’Adaptive airborne MTI: an auxiliary channel approach’, I E E Proc. F, 1987,134, (3). pp. 269-276

3 KLEMM, R.: ‘Adaptive clutter suppression for airborne phased array radars’, I E E Proc. F, 1983,130, (l), pp. 125-132

4 MCWHIRTIX, I. G., and SHEPHERD, T. I.: ‘Adaptive algorithms in space and time domains’, IEE Proc. F, 1983,130, (1). pp. 17-21

5 ‘MTI system simulation and clutter output’. AD-A149 193, (14P, 1984)

LOW DIVERGENCE ELECTRICALLY PUMPED CIRCULAR-GRATING SURFACE-EMITTING DBR LASER ON AN InGaAsIGaAs STRUCTURE

M. Fallahi, M. Dion, F. Chatenoud, I. M. Templeton, K. A. McGreer, G. Champion and R. Barber

Indexinq terms: Smkonductor lasers, Circular gratings

The Letter reports the fabrication and low divergence oper- ation of an electrically pumped circular grating surface emit- ting DBR laser on an InGaAs/GaAs SQW structure. A threshold current below 85 mA, output power of more than U)mW and a divergence of less than 1” FWHM were obtained.

Introduction: Circular-grating surface emitting distributed feedback/distributed B r a g reflector (CG-SE DFB/DBR) lasers have the potential to produce lowdivergence high- power circularly symmetric surface emission. As a result they can be of great interest for fibre optic communications and free space optical interconnects. A number of theoretical