Self-Interference Cancellation via Beamforming in an Integrated Full Duplex Circulator-Receiver Phased Array

Transcription

1 Self-Interference Cancellaton va Beamformng n an Integrated Full Duplex Crculator-Recever Phased Array Mahmood Baraan Dastjerd, Tngjun Chen, Negar Reskarman, Gl Zussman, and Harsh Krshnaswamy Department of Electrcal Engneerng, Columba Unversty, New York, NY, 127, USA Abstract Ths paper descrbes how phased array beamformng can be exploted to acheve wdeband self-nterference cancellaton (SIC). Ths SIC s ganed wth no addtonal power consumpton whle mnmzng lnk budget (transmtter (TX) and recever (RX) array gan) penalty by repurposng spatal degrees of freedom. Unlke pror works that rely only on dgtal transmt beamformng, ths work takes advantage of analog/rf beamformng capablty that can be easly embedded wthn an ntegrated crculator-recever array. Ths enables () obtanng SIC through beamformng on both TX and RX sdes, thus ncreasng the number of degrees of freedom (DoF) that can be used to obtan SIC and form the desred beams, whle () sharng the antenna array between TX and RX. A 75MHz 65nm CMOS scalable 4-element full-duplex crculator-recever array s demonstrated n conjuncton wth a TX phased array mplemented usng dscrete components. A tled 8-element system shows () 5dB overall RF array SIC over 16.25MHz (WF-lke) bandwdth (BW) wth < 3.5/3dB degradaton n TX and RX array gans, respectvely, and () 1dB overall array SIC ncludng dgtal SIC, supportng +16.5dBm TX array power handlng. I. INTRODUCTION Over the past decade, the rapd growth of slcon-based phased array technology has started to make an mpact on commercal and mltary wreless applcatons. Over the next decade, phased array and Multple-Input-Multple-Output (MIMO) rados are expected to emerge as a key technology for next-generaton wreless networks. Phased arrays can provde coherent beamformng gan [1] and MIMO can sgnfcantly ncrease throughput [2]. Both technologes are a part of the upcomng standards for next-generaton Wreless Local Area Networks (WLANs) [3] and 5G [4]. Meanwhle, Full-Duplex (FD) wreless smultaneous transmsson and recepton on the same frequency channel has drawn sgnfcant research nterest, and s an emergng technology that can theoretcally double the channel capacty [5], ntegratng FD capablty wth phased array transcevers can provde hgher data rate and hgher spectrum effcency, whle substantally enhancng lnk range. Due to space constrants n both W-F Access Ponts (APs) and small cell Base Statons (BSs), there s a need for Integrated Crcut (IC) desgn and mplementaton of such transcevers. Although slcon-based mplementatons are plagued by low TX power handlng, partcularly n solutons that ntegrate the antenna nterface wth low TX-ANT TABLE I: Lnk budget calculatons. Metrc Calculaton Value Frequency (f) 73MHz # of ANT Elements (N) 8 TX Power per Element (P TX) 1dBm TX Array Gan (AT TX) N 2 3dB 15dB TX ANT Gan (G TX) 6dB Bandwdth (BW ) 16.26MHz RX Nose Fgure (NF ) 5dB RX Array Gan (AG RX) N 2 3dB 15dB RX ANT Gan (G RX) 6dB RX Array Nose Floor referred to the ANT Input (N floor ) kt BW NF N 88dBm Requred SNR 2dB RX Array Senstvty referred to the ANT Input (P sense) N floor SNR 68dBm Implementaton Losses (IL) 1dB λ 4π (PTXAGTXGTX Supported Range (R max) AG RXG RX 3.7km IL/P sense) 1/2 Requred Array SIC (SIC array) P TXAG TXAG RX/N floor 119dB loss [6] [9], phased arrays can substantally enhance range. Based on lnk budget calculatons shown n Table. I, an 8-element 73MHz array wth +1dBm TX power per element (P TX ), 6dB antenna gan, 15dB TX and RX array gans (AG TX and AG RX, 3dB degraded from the deal 18dB array gan), 16.25MHz bandwdth (BW), 5dB RX nose fgure (N F ), 2dB requred Sgnal-to-Nose Rato (SNR) and 1dB mplementaton losses can establsh an FD lnk over a dstance of 3.7km. Mult-antenna FD operaton s extremely challengng as not only the SI from each TX to ts own RX needs to be cancelled, but also the cross-talk SI (CTSI) needs to be suppressed between every TX-RX par (Fg. 1). Indeed, the total SI power level at the RX sde output of an N-element FD phased array wth shared antenna nterface can potentally be N 4 tmes hgher relatve to a sngle-element transcever, snce the SI and CTSI from every TX element can add up constructvely at an RX element and ncrease n power by N 2 (the TX array gan), and then ncrease by another N 2 across the RX elements (the RX array gan). Hence, the requred array SIC can be calculated as SIC array = P TX AG TX AG RX /N floor = 119dB, where N floor = kt BW NF N = 88dBm s the array recever nose floor referred to the antenna nput. In other words, although phased array beamformng provdes N 2 ncrease n array gan at both the TX and RX, t could also potentally ncrease the SI levels by a smlar amount. However, the actual combned SI level depends on the N N

2 SI Channel (db) -1 Antenna Index (2,4) (2,3) -15 (2,2) -2 (2,1) -25 (1,4) (1,3) -3 (1,2) -35 (1,1) (1,1)(1,2)(1,3)(1,4)(2,1)(2,2)(2,3)(2,4) -4 Antenna Index SI Channel Magntudes from ANT (1,1) Fg. 1: Full-duplex phased array transcever achevng SIC through TX and RX beamformng by sacrfcng a few spatal DoF H(1,1),(1,1) -3 H(1,2),(1,1) -4 H(2,1),(1,1) H(2,4),(1,1) -5 6 Magntude (db) Magntude (db) SI Channel Magntudes from ANT (1,2) H(1,1),(1,2) -3 H(1,2),(1,2) -4 H(1,3),(1,2) -5 6 H(2,2),(1,2) SI channel matrx as well as TX/RX beamformng weghts. In ths work, we manpulate beamformng weghts to acheve wdeband SIC whle mnmzng the penalty on the TX and RX array gans [1]. In [1], we manly dscussed the IC desgn of Crc.-RX array. In ths nvted paper, we dscuss () lnk budget calculaton, () desgn and mplementaton detals of the 8-element phased array TX made usng dscrete components, () slot loop antenna desgn, and (v) how Crc.-RX, TX and slot loop antenna are ntegrated to form FD phased array transcever. Combnng FD wth mult-antenna systems has been nvestgated at the system-level n [11], [12]. However, ths work explores the jont optmzaton of TX and RX beamformng weghts to acheve wdeband SIC for FD operaton. In addton, ths work consders a shared TX-RX antenna nterface through ntegrated crculators. Furthermore, the custom IC desgned enables performng analog beamformng to acheve SIC for the frst tme. Specfcally, a scalable 65nm CMOS FD crculator-recever array s ntroduced [1], where beamformng s performed n the baseband (BB) n a hgh-lnearty and low-nose manner wth lttle overhead by employng the mult-phase outputs avalable n an N -path-flter-based crculator-recever front-end [8]. the TX and RX can be repurposed so that total SI s suppressed after RX beamformng at the expense of some TX and RX beam characterstcs, such as a few nulls and/or some gan loss n the beam-pontng drecton(s). Fg. 2 depcts our mplementaton of a 2 4 rectangular array of slot loop antennas at 73MHz wth λ/2h spacng, SI whose SI channel matrx s denoted by HSI = H(m,n). As Fg. 2 shows, the antenna matchng s around 2dB at the center frequency of 73MHz, whle the magntude of the SI channel from the closest element can be as hgh as 1dB (e.g., H(2,1),(1,1) from element (1, 1) to element (2, 1)). Moreover, a vertcal par of elements have hgher SI channel magntude than a horzontal par of elements (e.g., H(2,1),(1,1) > H(1,2),(1,1) at 73MHz). Let x(t) be hthe transmt sgnal n the h tme doman, TX TX RX RX and w = w(m,n) and w = w(m,n) be the complex-valued TX and RX beamformng weght vectors, respectvely. Then, the phased array SI after RX beamformng, xsi (t), s gven by II. SIC THROUGH B EAMFORMING xsi (t) = (wrx )> HSI wtx x(t), An N -element phased array transcever wth a shared antenna nterface and control over phase and ampltude of each element on both transmt and receve features overall 2(N 1) complex-valued DoF on TX and RX sdes ((N 1) DoF on each sde). These DoF are a representaton of the complex-valued weghts (ampltudes and phases) of each element relatve to that of the frst element. Typcally, these DoF are employed to form the beams toward desred node locaton, and mnmze nterference to/from nearby rados by pontng nulls towards them or suppressng the sde-lobes of the radaton pattern. Alternatvely, a few beamformng DoF at where ( )> denotes the transpose of a vector. Consder a 3D coordnate system where the 2 4 rectangular array s located on the x-y plane. We denote the TX and RX beamformng drectons by the azmuth and elevaton angles (φ, θ) n a horzontal coordnate system. Then, the far-feld array TX/RX beamformng pattern s gven by Fg. 2: A element antenna array at 73MHz wth λ/2 spacng, the measured SI channel magntudes at 73MHz, and, examples of measured SI channel magntudes across frequency from antenna elements (1, 1) and (1, 2) to the adjacent elements, respectvely. E TX/RX (φ, θ) = (stx/rx (φ, θ))> wtx/rx = 2 X 4 X m=1 n=1 TX/RX (1) (2) w(m,n) ejπ[(m 1) cos φ cos θ+(n 1) sn φ cos θ)],

3 Syntheszed Array Pattern (x-z Plane) Syntheszed Array Pattern (y-z Plane) Array Gan (db) Conv. TX/RX SIC TX SIC RX Conv. TX/RX SIC TX SIC RX For <3dB TX/RX Array Gan Degradaton TX/RX Array Gan (db) Smulated Array SIC (db) 71 5 Array Gan (db) Nomnal TX/RX Array Gan TX/RX Array Gan for SIC BW = 2 MHz TX/RX Array Gan for SIC BW = 4 MHz Fg. 4: Block/crcut dagram and chp photo of the 65nm CMOS 73MHz 8-element FD crculator-recever (C-RX) phased array system employng scalable 4-element ICs. Desred SIC (db) Fg. 3:, Smulated TX/RX array patterns n the x-z and y-z planes whle achevng 6dB array SIC across 2MHz wth 3dB array gan degradaton n the TX/RX broadsde beam-pontng drectons, smulated array SIC where at least 6dB SIC s guaranteed to be acheved across 72-74MHz, and smulated TX/RX array gan for a desred array SIC based on solvng an optmzaton problem usng the measured SI channels depcted n Fg. 2. antenna couplng paths are cancellng each other, as opposed to havng an IC canceller duplcate the frequency characterstcs of an antenna couplng path, () the beamformng-fd trade-off can be dynamcally adapted n the feld, wth the number of DoF sacrfced dependent on the requred SI cancellaton, bandwdth, external nterferers that need to be nulled, etc., and (v) the trade-off between FD and beamformng wll become more favorable for larger arrays. where h TX/RX stx/rx (φ, θ) = s(m,n) (φ, θ) h = ejπ[(m 1) cos φ cos θ+(n 1) sn φ cos θ], denotes the TX/RX beam steerng vector. The goal s to acheve wdeband SIC n the near-feld (1) wth mnmal penalty of the TX/RX beamformng gans n the far-feld (2). We formulate and solve an optmzaton problem where the objectve s to maxmze the TX and RX array gans, subject to the constrant that a desred amount of array SIC s acheved after TX beamformng. We evaluated ths dea through smulatons usng the measured array SI channel (see Fg. 2) and the results are summarzed n Fg. 3. In the smulaton, the TX/RX array gans are maxmzed for broadsde beamformng (.e., θ = 9 ) subject to the constrant that at least 6dB array SIC s acheved between 72-74MHz. Fgs. 3 and 3 show that an array gan degradaton of only 3dB compared wth the maxmal array gan of N 2 = 18dB can be mantaned whle achevng 6dB array SIC across 2MHz. Moreover, Fg. 3 shows the tradeoff between the maxmum achevable TX/RX array gan for dfferent amounts of desred array SIC across 2MHz and 4MHz, respectvely. In general, four mportant features must be hghlghted: () SI suppresson s essentally acheved n the spatal doman through a trade-off between near-feld SI nullng and far-feld beamformng wthout any explct cancellers and assocated power consumpton, snce the RX/TX beamformers are repurposed, () the SI suppresson s wdeband snce dfferent III. I MPLEMENTATION A. Integrated Crculator-Recever Phased Array The N -path-flter-based combned-crculator-rx concept descrbed n [8] provdes the unque feature of havng 8-phase baseband nodes avalable n each crculator-rx (C-RX) whch can radcally smplfy the RX beamformng. Usng ths C-RX, an ntegrated scalable 73MHz 4-element C-RX phased array s fabrcated usng 65nm CMOS technology (Fg. 4). 7-bt programmable transconductor cells (gm ) are employed to convert each baseband (BB) node voltage n each C-RX nto a correspondng current. Then, the current sgnals across all the elements are summed n the current doman n low-nput mpedance IQ transmpedance amplfers (TIAs), mplemented usng two-stage op-amps. Therefore, the complex weghts (phase shft and gan) appled to each element and the summaton across all the elements are performed smultaneously whle mantanng low nose and hgh lnearty. Furthermore, the low nput mpedance combnng pont at the TIA nput eases scalablty across multple chps. By connectng a second chp s low mpedance node to the frst one, and turnng off the second chp s TIAs, the current from the gm cells of the second chp can be combned nto the TIAs of the frst chp n the current doman. Thanks to the the low mpedance provded by the TIA of the man chp, board trace capactance does not degrade the performance n terms of BW. B. A Custom-Desgned Transmtter Phased Array Fg. 5 shows the custom-desgned TX phased array, whch s mplemented usng off-the-shelf dscrete components.

4 Fg. 5: A custom-desgned 8-element transmtter phased array: Block dagram, and PCB mplementaton. The sgnal s frst amplfed and dvded nto 8 channels usng a Mn-crcuts JCPS-8-1+ spltter. Each channel contans a cascade of two 18 phase shfters (Mn-crcuts JSPHS-1+) to cover the full 36 range, n seres wth multple stages of a hghly-lnear low-nose amplfer (Mn-crcuts HXG-122+) and a programmable attenuator to obtan ampltude control whle mantanng good nose performance. The 7-bt attenuator (Skyworks SKY LF) provdes a maxmum attenuaton of 31.75dB wth.25db resoluton. System calculatons show that the desgn can acheve upto 3dB gan wth an output-referred IP3 as hgh as 31dBm whle the nose fgure (NF) s less than 3.3dB. C. Antenna Array A rectangular 2 4 array s mplemented usng a slot loop antenna structure. The dstance between adjacent elements s equal to half wavelength (λ/2). The antenna s fabrcated on an FR-4 PCB and can radate on both frontsde and backsde drectons. A metal sheet s used as a reflector at the back of the antenna array wth quarter wavelength dstance to redrect the radaton to the frontsde (Fg. 7). D. Complete 8-element Full-duplex Phased array Transcever Two C-RX phased array ICs are tled on a PCB to realze an 8-element FD C-RX phased array, whch s then mounted on top of the TX phased array to form an 8-element FD phased array TRX wth mnmum nterconnectons (Fg. 7). IV. M EASUREMENTS AND D EMONSTRATION The sngle-element performance of the C-RX array s measured by turnng ON only one element s gm cells and shuttng down all the other gm cells. The C-RX shows 1.7dB TX-ANT loss, and +28dBm TX-ANT IIP3. Although Fg. 6: Measured full-duplex phased array performance across 8-elements (tlng of 2 ICs): array SIC, mpact of optmzed weghts to acheve SIC on the TX/RX array gan, gan compresson of a small receved sgnal under the nfluence of TX power wth optmzed weghts wth and wthout the antenna tuner, and two-tone TX test trackng the TX total SI and ts IM3 products at the recever output wth addtonal dgtal SIC. the nomnal ANT-BB converson gan s 41dB, ths can go up to 5dB based on the programmable resstve feedback around the TIA. We also measured 31dBm/+22.5dBm n-band/out-of-band IIP3 and 5dB sngle-element NF n the ANT-BB path due to the low-nose and hgh-lnearty performance of the nverter-based gm cells. Array FD measurements are summarzed n Fg. 6. The solaton of each C-RX element s around 15dB. When the TX and RX arrays are confgured for nomnal broadsde beamformng, only an average of 23dB array SIC s acheved over 16.25MHz. Employng the optmzaton descrbed n Secton II, TX and RX DoF are repurposed to acheve SIC through beamformng whle allowng 3dB TX and RX array gan loss, whch leads to 4.7dB array SIC over 16.25MHz. The measured SIC profle s very wdeband, and very smlar to the smulated profle n Fg. 3. However, the acheved SIC s somewhat lower than smulaton as the crculator s nternal solaton and some second order effects such as quantzaton of beamformng weghts are neglected n the smulatons. Fnally, custom-desgned antenna tuners are ntegrated wth antenna array. Co-optmzng the tuners (confgured dentcally across all elements) wth TX/RX weghts leads to 5dB array SIC over 16.25MHz. The RX and TX array gan measurements verfy the 3dB array gan loss versus frequency for broadsde exctaton as expected from the syntheszed TX and RX array patterns for these weghts (Fg. 3). To evaluate array TX power handlng when confgured for SIC, a weak n-band sgnal s radated towards the array and montored whle the TX array power (PTX AGTX ) s swept. The 1dB desenstzaton of the C-RX array gan occurs at

5 TABLE II: Comparson wth state-of-the-art FD recevers wth an ntegrated shared antenna nterface. Fg. 7: Wreless FD demonstraton setup. VI. ACKNOWLEDGMENT Ths work was sponsored by Intel and the NSF EARS program (grant ECCS ). The authors would lke to thank Dr. Stefano Pellerano for useful comments. Fg. 8: Demo results: A 31dBm desred sgnal radated from 2ft away from a sngle antenna s recovered whle transmttng a 5MHz OFDM-lke sgnal wth +8.7dBm TX array power. a TX array power of +16.5dBm. Fnally, The effectve IIP3 referred to the TX array power s measured usng a two-tone TX test whle montorng the RX IF outputs wth weghts confgured for SIC. The effectve IIP3 referred to the TX array power s +17.5dBm. Nonlnear Volterra-seres-based dgtal SIC at +16.7dBm average TX array power s able to suppress the resdual total SI and ts assocated IM3 to below 84dBm, ndcatng 1dB total array SIC. However, an addtonal 19dB SIC s requred to suppress the SI to the array nose floor (on the effectve IIP3 graph, the nose floor would be at Nfloor /AGRX = 13dBm), whch can be potentally acheved wth addtonal analog SIC. We have successfully demonstrated around 8dB total self-nterference cancellaton for an OFDM-lke sgnal wth 1MHz RF bandwdth and a TX array average power of +8.7dBm, whle smultaneously recevng a 31dBm contnuous-wave desred sgnal radatng from a slot loop antenna placed 2ft away from the FD antenna array (Fgs. 7, 8). Ths shows that the dgtal cancellaton algorthm works effectvely even when the desred sgnal s present. V. C ONCLUSION In ths work, phased array functonalty s combned wth full-duplex operaton wth no addtonal power consumpton. Compared to the pror works, ths works acheves hgher total SI suppresson, hgher TX array power handlng, and far superor potental FD lnk range (Table. II). R EFERENCES [1] R. Hansen, Phased array antennas. John Wley & Sons, 29. [2] A. Goldsmth, S. A. Jafar, N. Jndal, and S. Vshwanath, Capacty lmts of MIMO channels, IEEE J. Sel. Areas Commun., vol. 21, no. 5, pp , 23. [3] B. Bellalta, IEEE 82.11ax: hgh-effcency WLANs, IEEE Wreless Commun., vol. 23, no. 1, pp , 216. [4] J. G. Andrews, S. Buzz, W. Cho, S. V. Hanly, A. Lozano, A. C. Soong, and J. C. Zhang, What wll 5G be? IEEE J. Sel. Areas Commun., vol. 32, no. 6, pp , 214. [5] J. Zhou, N. Reskarman, J. Dakonkolas, T. Dnc, T. Chen, G. Zussman, and H. Krshnaswamy, Integrated full duplex rados, IEEE Communcatons Magazne, vol. 55, no. 4, pp , 217. [6] D. Yang, H. Yuksel, and A. Molnar, A wdeband hghly ntegrated and wdely tunable transcever for n-band full-duplex communcaton, IEEE J. Sold-State Crcuts, vol. 5, no. 5, pp , 215. [7] N. Reskarman, J. Zhou, and H. Krshnaswamy, A CMOS passve LPTV nonmagnetc crculator and ts applcaton n a full-duplex recever, IEEE J. Sold-State Crcuts, vol. 52, no. 5, pp , 217. [8] N. Reskarman et al., Hghly-lnear ntegrated magnetc-free crculator-recever for full-duplex wreless, n Proc. IEEE ISSCC 17, 217. [9] S. Ramakrshnan, L. Caldern, A. Nknejad, and B. Nkol, An FD/FDD transcever wth RX band thermal, quantzaton, and phase nose rejecton and >64dB TX sgnal cancellaton, n Proc. IEEE RFIC 17, 217. [1] M. Dastjerd, N. Reskarman, T. Chen, G. Zussman, and H. Krshnaswamy, Full duplex crculator-recever phased array employng self-nterference cancellaton va beamformng, n Proc. IEEE RFIC 18 (to appear), 218. [11] E. Everett, C. Shepard, L. Zhong, and A. Sabharwal, Softnull: Many-antenna full-duplex wreless va dgtal beamformng, IEEE Trans. Wreless Commun., vol. 15, no. 12, pp , Dec 216. [12] J. P. Doane, K. E. Kolodzej, and B. T. Perry, Smultaneous transmt and receve wth dgtal phased arrays, n Proc. IEEE PAST 16, 216.