High Performance Receiver Design for RX Carrier Aggregation

Transcription

1 Aticle High Pefomance Receive Design fo RX Caie Aggegation Jusung Kim 1, Bon-Hyun Ku, Sanghun Lee 3, Sungchan Kim 1 and Keunkwan Ryu 1, 1 Depatment of Electonic Engineeing, Hanbat National Univesity, Daejeon , Koea; jusungkim@hanbat.ac.k (J.K.); sckim@hanbat.ac.k (S.K.) Qualcomm Technologies, San Diego, CA 911, USA; bonhyunku@gmail.com 3 Wavepia Copoation, Osan-si 18110, Koea; platune@wavepia.com * Coespondence: kkyu@hanbat.ac.k; Tel.: Academic Editos: Minsu Choi and Yong-Bin Kim Received: 7 Mach 017; Accepted: 7 Apil 017; Published: 1 May 017 Abstact: Caie aggegation is one of the key featues to incease the data ate given a scace bandwidth spectum. This pape descibes the design of a high pefomance eceive suitable fo caie aggegation in LTE-Advanced and futue 5 G standads. The poposed achitectue is vesatile to suppot legacy mode (single caie), inte-band caie aggegation, and inta-band caie aggegation. Pefomance with caie-aggegation suppot is as good as legacy eceives. Contadicting equiements of high lineaity and the low noise is satisfied with the single-gm eceive achitectue in addition to suppoting caie aggegation. The poposed cascode-shutoff low-noise tans-conductance amplifie (LNTA) achieves 57.1 db voltage gain, 1.76 db NF (noise figue), and 6.7 dbm IIP3 (Thid-ode intecept point) with the powe consumption of 1.3 mw in the inta-band caie aggegation scenaio. With legacy mode, the same eceive signal path achieves 56.6 db voltage gain, 1.33 db NF, and 6. dbm IIP3 with a low powe consumption of 7.4 mw. Keywods: low noise amplifie (LNA); caie aggegation; LTE-advanced; single-input multi-output (SIMO); multi-input multi-output (MIMO); multi-band 1. Intoduction The eve inceasing data cunch in wieless communication necessitates the means to incease the data ate with spectal efficiency. Thee ae thee main appoaches to inceasing the data ate in wieless communication: high-ode modulation schemes (e.g., 64 QAM (Quadatue Amplitude Modulation), 18 QAM, and 56 QAM), high caie fequency with lage factional bandwidth, and caie aggegation (CA). The high-ode modulation fomat is able to offe faste data ates and a highe level of spectal efficiency, but pactically, modulation fomat is dictated by the link-budget, signal-to-noise atio (SNR), and tansceive pefomance. High caie fequency (e.g, millimete-wave bands fom 30 GHz to 300 GHz) is beneficial because signal bandwidth is a cetain faction of caie fequency, and thus highe caie fequency leads to lage signal bandwidth. Howeve, hostile channel chaacteistics at millimete-wave bands educe sevice coveage and impai communication pefomance [1]. CA aises the bandwidth and capacity by combining two o moe adjacent o non-adjacent RF channels []. CA is also the technical solution to ovecome spectum fagmentation, and can be applied to a wide vaiety of spectum scenaios [3]. CA has been adopted by the Long-Tem Evolution (LTE) standad fo cellula systems, and is being extended to Licensed-Assisted Access (LAA) and LTE in Unlicensed spectum (LTE-U). In addition, CA will be one of the key featues to enable futue 5 G wieless systems. J. Low Powe Electon. Appl. 017, 7, 9; doi: /jlpea

2 J. Low Powe Electon. Appl. 017, 7, 9 of 10 A plethoa of wieless standads exist in the adio fequency egime, and the numbe of wieless devices is inceasing apidly. Reception of wieless signals is becoming moe challenging due to spectum cowding and educed channel capacity unde the hash wieless envionment. Low noise pefomance is a must to impove the SNR of the eception signal at the sensitivity level. On the othe hand, high lineaity of the eceive is the capacity to handle lage signal intefeences and jammes with the simultaneous eception of small desied signals. These consideations call fo new innovations on seveal fonts, fom the cicuit to the achitectual level. In this pape, we popose a high-pefomance eceive achitectue suppoting legacy single-caie opeation, inte-band caie aggegation, and inta-band caie aggegation. The poposed achitectue can be consideed a single-input, and multi-output (SIMO) eceive, and two contasting equiements noise and lineaity which set the dynamic ange pefomance of RFICs (Radio Fequency Integated Cicuits) ae satisfied simultaneously. We demonstate that the poposed achitectue does not sacifice pefomance in single-caie opeation, and no additional matching element is equied. Compaative pefomance in diffeent opeation modes ae elaboated. This pape is oganized as follows. The system desciption of the poposed eceive achitectue is discussed in Section. Seveal low noise amplifie (LNA) topologies suitable fo eceive (RX) caie aggegation ae pesented, and ae evaluated with the poposed eceive achitectue in Section 3. Concluding emaks ae given in Section 4.. Receive Achitectue Figue 1 shows the concuent tansceive achitectue suppoting N caies. When RF caies belong to the same band, it is called inta-band aggegation. If they come fom diffeent bands, then it is named inte-band aggegation. In the fome case, they ae defined as contiguous o non-contiguous when the channels ae adjacent to each othe o not, espectively. Suppot of N caies does not necessaily mean that N tansceives ae equied. Fo instance, ideal softwae defined adio envisioned by Mitola [4] can suppot concuent opeation of N caies with only a single tansceive. Howeve, existing tansceives specifically eceives in this pape ae limited in thei RF opeation fequency (RF bandwidth) as well as intemediate fequency (IF) (baseband bandwidth). Theefoe, we conside a geneic case whee N tansceives ae suppoting N caies simultaneously. Figue 1. Concuent tansceive achitectue suppoting N caies. CA allows fo the full utilization of fagmented spectum, and diffeent types of CA come fom the location of the fagmented spectum to be combined. The design of the digital baseband is almost invaiant to the diffeent CA types. On the othe hand, design complexity of adio-fequency

3 J. Low Powe Electon. Appl. 017, 7, 9 3 of 10 tansceive depends heavily on the CA types [3]. Without losing geneality, Figue shows two diffeent implementations of the eceive suppoting two CA caies. Figue a suppots two inte-band caie signals, and paallel signal paths ae optimized fo diffeent fequency bands. Fo instance, RF1 and RF denotes the caies at band ( MHz) and band 1 ( MHz), espectively. Due to the limited RF and IF bandwidth of the cicuity, a single signal path eceive that suppots both band and band 1 simultaneously is not easily feasible, and we thus have to adopt sepaate signal paths. Figue b is the embodiment of a two CA eceive with the suppot of inta-band CA. Due to the limited IF spectum coveage with the baseband cicuits (e.g., vaiable-gain amplifie, filte, etc.), N (= in Figue ) eceive signal paths ae equied to suppot both contiguous and non-contiguous inta-band CA. LNTA I 1 BBF_I I 1 BBF_I RF 1 Q 1 BBF_Q LNTA Q 1 BBF_Q LNTA I BBF_I RF 1 I BBF_I RF Q CA BBF_Q Q CA BBF_Q (a) (b) Figue. Caie aggegation (CA) eceive achitectues: (a) Inte-band CA eceive; (b) Inta-band CA eceive. The RF bandwidth that each signal path can suppot is mainly dictated by the input matching netwok (IMN) bandwidth. All the low-noise tans-conductance amplifies (LNTAs) discussed hee ae based on the inducto-degeneated common-souce stage. The matching netwok utilizing the C gs of the main tansisto is based on the seies RLC esonance cicuit, whee its quality (Q) facto is typically lage than 1. Passive voltage amplification due to the matching netwok is beneficial fo the eceive with impoved signal-to-noise atio. On the othe hand, the bandwidth is invesely popotional to the Q-facto (BW = ω o Q ). Fo instance, RF1 in Figue a has to suppot MHz to pactically eceive the signals fom band 1 ( MHz) to band 8 ( MHz) with the same eceive path. Then, maximum Q fo RF1 LNTA can be deived as follows. Q < ω o BW = = 3.6 (1) The limitation in the Q-facto applies to RF path as well. Pactically, IMN does not possess high Q-facto lage than 5, and this value is acceptable with vaious 3 GPP (3d Geneation Patneship Poject) bands suppot. The ealization in Figue a is no diffeent fom the conventional eceive. Recent advances in eceive achitectue opt fo the single-g m eceive implementation [5 8]. With single-gm appoach, low-noise active block intefacing the antenna is called a low-noise tans-conductance amplifie, denoted as LNTA in Figue. As the name indicates, LNTA is diving the subsequent stage with a small signal cuent. Inte-band CA implementation does not alte the chaacteistics of the LNTA, but the inta-band CA equies the LNTA to delive multiple outputs. Section 3 discusses the implementation issue of the LNTA in ode to suppot inta-band CA. Pio to that, in this section, we delve into the achitectual issues and concens to enable a high-pefomance eceive with lage dynamic ange.

4 J. Low Powe Electon. Appl. 017, 7, 9 4 of Single-g m Receive Achitectue Figue 3 shows the conventional zeo-if eceive achitectue with achitectual implications. Low-noise amplifie (LNA) is the cascade of V I (g m ) and I V (load) convesion, and Mixe equies sepaate tans-conductance (g m ) to dive the passive switching mixe. A low-pass filte with I V convesion is pefomed by the single block in eithe passive (R C) o active (tans impedance amplifie TIA) implementation. The eason fo having an LNA is best illustated with the Fiis equation [9] expessed in Equation (). Due to the lage gain and low noise figue of the LNA (G 1 and NF 1, espectively), system noise pefomance is impoved with the suppession of subsequent noise coming fom mixe and baseband amplifies. On the othe hand, lage gain which is desiable fo noise pefomance is detimental to the lineaity pefomance manifested by the cascade IIP 3 equation given in Equation (3) [10]. Lage gain due to the font-end amplifies the desied signal as well as lage intefeences in both in-band and out-of-band. Lage intefeences eithe satuate the baseband o wosen the lineaity pefomance of the baseband without pope filteing. Then, low input impedance and the jamme filteing in a boadband IF fequency is highly desiable, both of which ae not easily attainable with the cuent state of the at. On the othe hand, the lineaity of LNA itself is less of a concen compaed to subsequent active stages. Then, we do not adopt the lineaization technique, although thee have been many effots to impove the lineaity of the LNA [11 13]. NF = NF 1 + NF 1 G 1 + NF 3 1 G 1 G + () 1 A 1 IIP 3 A + A v,1 IIP 3,1 A + A v,1 A V, IIP 3, A + (3) IIP 3,3 Zin Vout AvVin LNA V-I f LPF I-V f BB Vin Figue 3. Conventional zeo-if (intemediate fequency) eceive achitectue. The eceive achitectue based on a single low-noise tans-conductance amplifie diving a cuent-mode passive mixe loaded by a low-impedance tans-impedance closed-loop amplifie has been widely adopted ecently due to its beneficial featues in tems of noise and lineaity [8]. Figue 4 shows the conceptual diagam of the single-g m eceive. I V convesions of the LNA ae eliminated, and thus the emaining V I convesion is pefoming as LNTA. Since the output of LNTA is in the cuent domain, the tans-conductance of the mixe can also be eliminated. Due to the single-g m pio to the baseband low-pass filte and I V convesion, the lage voltage swing pesent in taditional Gilbet-cell and voltage-mode mixes ae eliminated. The vitual gound of the TIA minimizes the distotion caused by lage in-band and out-of-band intefeences. Low-pass filteing pefomed inside TIA impoves the lineaity of the eceive system. Due to the low-pass filteing pio to the fist I V convesion in the baseband, the eceive s obustness to the blocke is impoved by the filteing effect. Assume fo simplicity that the single blocke is pesent at the mth hamonic fequency of LO (Local Oscillato) fequency and the ode of the low-pass filte is n. Then, enhancement in the blocke toleance (B 1dB ) is expessed as follows. ( ) fb m f B 1dB,enhancement n 0 log LO 10 + HR m (4) BW

5 J. Low Powe Electon. Appl. 017, 7, 9 5 of 10 whee f B is the blocke fequency and HR m is the hamonic ejection atio of the switching mixe. Due to the single-g m of the eceive, lage tans-conductance is essential to maintain low NF and low sensitivity. Inducto-degeneated LNA in common-souce configuation is ideal in that espect due to the passive voltage gain coming fom the impedance matching netwok. Moeove, simultaneous noise and powe match is achieved by shifting the optimum noise impedance Z opt to the desied value [14]. Then, CA-suppoting LNTA achitectues discussed in Section 3 ae consideed the extension of the inducto-degeneated LNTA with multi-output suppot. LPF f f V-I A LPF I-V B BB I-V V-I C Figue 4. Single-g m zeo-if eceive achitectue. 3. LNTA Topologies and Simulation Results In this section, seveal LNTA topologies ae investigated to suppot both inte-band and inta-band CA. LNTA needs to be in the fom of single input and multi-output (SIMO) in ode to povide sepaate signal paths fo diffeent caies. Although two CA caies ae assumed in the implementation, the achitectue can be extended to N caies. Figue 5 shows the simulated eceive font-end achitectue to evaluate diffeent LNTA topologies. LTE band 7 ( MHz) is chosen fo the implementation and simulation of the eceive. L-section off-chip matching lumped elements ae configued with RLC equivalent cicuit paametes adopted fom Muata Chip S-paametes & Impedance Libay. CA-capable LNTA is used as a single-g m of the eceive font-end. The main tansisto (g m ) is biased in the sub-theshold egion to impove the cuent efficiency (g m /I d ). The degadation in the unity-gain fequency( f t ) due to sub-theshold opeation is not a poblem pe se, since the esonance behavio of the impedance matching netwok (IMN) cancels out the paasitic effect of the main tansisto. The cascode-tansisto is half the size of the main tansisto, and its gate bias is supplied fom the egulated loop to make V DS of the main tansisto at the same potential of the cuent mio tansisto s V DS. A conventional single input, single output (SISO) LNTA is designed fo band 7, and is simulated with the test-bench without CA. The gain of the single-caie eceive font-end as a efeence is set at 56.6 db. NF and cuent consumption of the single-caie eceive is 1.33 db and 7.4 ma, espectively.

6 J. Low Powe Electon. Appl. 017, 7, 9 6 of 10 I 1 CA 1_BB I RF in LNTA XFMR_CA 1 XFMR_CA Q 1 I CA 1_BB Q CA _BB I Q CA _BB Q Figue 5. Receive font-end achitectue with two CA suppot. LNTA: low-noise tans-conductance amplifie Thee-Coil BALUN LNTA The thee-coil LNTA in Figue 6 enables CA opeation with magnetically-coupled inductos. LNTA tank impedance is loweed by a facto of two due to two inductive coupled loads. With N CA suppot, the numbe of magnetically-coupled coils inceases linealy. The gain of LNTA is educed and NF is inceased with this appoach. Moeove, isolation between diffeent CA paths is limited by the coupling coefficient (k k CA ). Two inta-band CA simulation with this appoach gives 51.6 db gain and 3. db NF. Compaed to the single-caie eceive case, 5 db gain eduction is obseved close to the expected 6 db loss. NF incease is due to the insufficient gain of the LNTA, and moe noise contibution fom the subsequent stages accodingly. Even with twice the cuent consumed in LNTA, modeate impovements ae achieved with 53 db and.67 db of gain and NF, espectively. CA V casc1 RF in Figue 6. Thee-coil BALUN LNTA.

7 J. Low Powe Electon. Appl. 017, 7, 9 7 of Two-Stage LNTA Two-stage LNA is shown in Figue 7. The second stage is sized smalle so as not to load the fist stage of the LNA. is a single-stage design, and its pefomance is compaable to the non-ca case. On the othe hand, the CA path has two-stage active blocks, and even with lage degeneation in nd stage, lineaity is poo due to the cascaded design appoach [10]. NF fo CA is pimaily set by that of fist stage design. Table 1 summaizes the simulation esults with the two-stage LNTA appoach. All of the pefomances ae compaable to the efeence (SISO), with the exception of CA path lineaity (IIP 3 ). CA V casc1 V casc RF in Figue 7. Two-stage BALUN LNTA. Table 1. Simulation esults with two-stage LNTA. SISO: single input, single output. Gain (db) NF (db) IIP 3 (dbm) I d (ma) Refeence (SISO) stage, non-ca stage, path stage, CA path Split-Cascode LNTA The split-cascode LNTA shown in Figue 8 ealizes multi-output opeation by signal splitting at the cascode node. Tans-conductance (g m ) is shaed between CA paths. Theefoe, gain is educed by 0 log 10 (N) (db) without cuent scaling. Even with cuent scaling by N x fo N CA, gain eduction of aound 10 log 10 (N) (db) is inevitable due to squae-oot dependence of gm with espect to the cuent consumption in CMOS devices (shot channel effects ae ignoed). Shaed g m dictates small vaiation in Z in fo diffeent opeation modes (non-ca and CA). NF is degaded due to significant noise contibution fom the cascode devices and lage contibution fom TIA due to smalle LNA gain. Significant noise contibution due to the cascode devices is coming fom the small impedance looking into the anothe cascode device in inta-band CA opeation. In non-ca mode, only the single cascode tansisto is enabled and diects the cuent to eithe o CA. Thus, cascode tansisto noise is highly degeneated, giving ise to a small noise contibution. In CA mode, on the othe hand, 1 both cascode tansistos ae enabled, and the small impedance ( g ) looking into the souce node m,cascode is influencing each othe. The noise cuent out of the cascode devices is expessed as Equations (5) and (6) fo non-ca and CA mode, espectively. Table summaizes the simulation esults with

8 J. Low Powe Electon. Appl. 017, 7, 9 8 of 10 split-cascode LNTA appoach. High NF due to the cascode device is obseved, and additional cuent consumption of the g m device gives limited impovement in its NF. i cascode,non-ca = i n 1 + g m,cascode o (5) i cascode,ca = 1 + g m,cascode i n ( ) i n 1 o g m,cascode (6) CA V casc1 V casc RF in Figue 8. Split-cascode LNTA. Table. Simulation esults with split-cascode LNTA. Gain (db) NF (db) IIP 3 (dbm) I d (ma) Refeence (SISO) Split-Cascode, non-ca Split-Cascode, CA (1 I d ) Split-Cascode, CA (3 I d ) Cascode Shut-off LNTA Figue 9 shows the cascode shut-off LNTA. g m is sepaated between diffeent CA paths. Noise contibution due to the cascode device is the same as a conventional LNTA without shaing g m as in the split-cascode LNTA. In non-ca opeation, gate bias of the second gm tansisto is popely ON. At the same time, the cascode bias of CA is shut-off to avoid thowing additional cuent. With this bias scheme, the second g m tansisto opeates in satuation and tiode egions in CA and non-ca, espectively. Impedance matching (S 11 ) is slightly distubed due to the change in C gs and g m fo diffeent CA modes. Input impedance (Z in ) of the cascode shut-off LNTA is expessed in Equations (7) and (8) fo non-ca and CA mode, espectively. Z in,non-ca = Z Z CA (7) Z in,non-ca = Z ( jωl deg + 1 ) jωc gs whee Z (= Z CA ) is the input impedance of path (= CA path), and it can be expessed as ω t L deg + jωl deg + 1 jωc gs. Two inductos fo souce degeneation puposes can be meged into the (8)

9 J. Low Powe Electon. Appl. 017, 7, 9 9 of 10 shaed degeneation without losing the popety of the cascode shut-off LNTA. Since two inductos ae meged in paallel, the size of the inducto deceases, and thus the size of the inducto is minimized accodingly. Table 3 summaizes the simulation esults with cascode shut-off LNTA appoach. CA V casc1 V casc M 3 M 4 RF in M M 1 Figue 9. Cascode shut-off LNTA. Table 3. Simulation esults with cascode shut-off LNTA. Gain (db) NF (db) IIP 3 (dbm) I d (ma) Refeence (SISO) Cascode Shut-off, non-ca Cascode Shut-off, CA Cascode Shut-off (shaed L d eg), CA Conclusions The cicuit was designed and simulated with TSMC 65 nm CMOS technology. Simulated pefomance of seveal LNA topologies within the context of a single-g m eceive font-end was investigated and compaed with a conventional SISO eceive. The single-g m eceive achitectue defes the voltage amplification to the TIA stage afte the signal goes though the low-pass filteing opeation. This popety gives bette lineaity, especially fo out-of-band blockes and jammes. Geneic eceive achitectues with CA suppot ae intoduced, and the poposed achitectues suppot vaious CA scenaios in both inte-band and inta-band CA. Table 4 summaizes the pefomance metic fo diffeent LNTAs that ae discussed in this aticle. Cascode shut-off topology shows pomising esults with the suppot of SIMO (CA) eceive. Simulation esults exhibit <0.5 db noise penalty in CA opeation. Lineaity pefomance (IIP 3 ) is compaable to the legacy opeation. Table 4. Pefomance compaison with diffeent LNTA achitectue. Gain (db) NF (db) IIP 3 (dbm) I d (ma) Refeence (SISO) Thee-Coil Balun, CA Two-Stage, CA Split-Cascode, CA (1 I d ) Split-Cascode, CA (optimized) Cascode Shut-off, CA ( I d ) Cascode Shut-off, CA (optimized) Acknowledgments: This wok was suppoted by the National Reseach Foundation of Koea (NRF) gant funded by the Koea Govenment (MSIP) (No. 016R1C1B10104).

10 J. Low Powe Electon. Appl. 017, 7, 9 10 of 10 Autho Contibutions: Jusung Kim is the the main autho and was esponsible fo witing the pape. Bon-Hyun Ku and Sanghun Lee contibuted to the theoetical analysis and the design of the cicuit. Sungchan Kim helped to eview the poposed cicuits and edit the pape. Keunkwan Ryu was esponsible fo evising and supevising the pape. Conflicts of Inteest: The authos declae no conflict of inteest. The founding sponsos had no ole in the design of the study; in the collection, analysis, o intepetation of data; in the witing of the manuscipt, and in the decision to publish the esults. Abbeviations The following abbeviations ae used in this manuscipt: LNA LNTA FE RX TIA SISO SIMO MIMO CA Low Noise Amplifie Low Noise Tans-conductance Amplifie Font End Receive Tans Impedance Amplifie Single Input, Single Output Single Input, Multi Output Multi Input, Multi Output Caie Aggegation Refeences 1. Kim, C.; Kim, T.; Seol, J.-Y. Multi-beam tansmission divesity with hybid beamfoming fo MIMO-OFDM systems. In Poceedings of the IEEE Globecom Wokshop, Atlanta, GA, USA, 9 13 Decembe 013; pp Hwu, S.-C.; Razavi, B. An RF eceive fo inta-band caie aggegation. IEEE J. Solid State Cicuits 015, 50, Pak, C.S.; Sundstöm, L.; Wallén, A.; Khayallah, A. Caie aggegation fo LTE-Advanced design challenges of teminals. IEEE Commun. Mag. 013, 51, Mitola, J. The softwae adio achitectue. IEEE Commun. Mag. 1995, 33, Valla, M.; Montagna, G.; Castello, R.; Toneitto, R.; Bietti, I. A 7-mW CMOS 80.11a diect convesion font-end with 3.5-dB NF and 00-kHz 1/f noise cone. IEEE J. Solid State Cicuits 005, 40, Kim, N.; Lason, L.E.; Apain, V. A highly linea SAW-less CMOS eceive using a mixe with embedded Tx filteing fo CDMA. IEEE J. Solid State Cicuits 009, 44, Ru, Z.; Moseley, A.; Klumpeink, E.A.M.; Nauta, B. Digitally enhanced softwae-defined adio eceive obust to out-of-band intefeence. IEEE J. Solid State Cicuits 009, 44, Kim, J.; Silva, J. Low-Powe, low-cost CMOS diect-convesion eceive font-end fo multistandad applications. IEEE J. Solid State Cicuits 013, 48, Fiis, H.T. Noise figue of adio eceives. Po. IRE 1944, 3, Razavi, B. RF Micoelectonics; Pentice-Hall: Englewood Cliffs, NJ, USA, 1997; pp Apain, V.; Lason, L. E. Modified deivative supeposition method fo lineaizing FET low-noise amplifies. IEEE Micow. Theoy Tech. 005, 53, Ding, Y.; Hajani, R. A +18 dbm IIP3 LNA in 0.35 µm CMOS s. In Poceedings of the 001 IEEE Intenational Solid-State Cicuits Confeence, San Fancisco, CA, USA, 8 10 Febuay 001; pp Geddada, H.M.; Pak, J.W.; Silva. J. Robust deivative supeposition method fo lineaizing boadband LNAs. IEE Electon. Lett. 009, 45, Kim, J.; Hoyos, S.; Silva, J. Wideband common-gate CMOS LNA employing dual negative feedback with simultaneous noise, gain, and bandwidth optimization. IEEE Micow. Theoy Tech. 010, 58, by the authos. Licensee MDPI, Basel, Switzeland. This aticle is an open access aticle distibuted unde the tems and conditions of the Ceative Commons Attibution (CC BY) license (