100gbps dmt asic for hybrid lte-5g mobile fronthaul networks · 2021. 1. 26. · le et al.: 100gbps...

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 39, NO. 3, FEBRUARY 1, 2021 801

100Gbps DMT ASIC for Hybrid LTE-5G MobileFronthaul Networks

Son Thai Le , Member, IEEE, Tomislav Drenski, Andrew Hills, Malcolm King, Kwangwoong Kim,Yasuhiro Matsui , and Theodore Sizer

(Post-Deadline Paper)

Abstract—Multiple access interfaces are required to provide thewireless coverage and capacity that operators need for consistentservice across their geographic footprint. As a consequence, futureradio access networks will be hybrid 4G/LTE-5G networks. Thistechnology evolution is creating many challenges in the mobilefronthaul (MFH) networks to support the co-existence of commonpublic radio interfaces (CPRI) and evolved common public radiointerfaces (eCPRI) traffic with diverse connectivity requirements interm of capacity and reach. To address these challenges, this paperintroduces a 100 Gb/s discrete multi-tone modulation (DMT) ASICwhich has been designed and fabricated in 16 nm CMOS process,specifically targeting MFH applications. This DMT ASIC can flex-ibly support various important data rates in hybrid 4G/LTE-5GMFH networks, including CPRI-10, 25 GbE, 50 GbE, 75 GbE,and 100 GbE (eCPRI rates) over the full industrial temperaturerange (−40 °C to +85 °C). We demonstrate the high performanceand reliability of this DMT ASIC in several real time transmissionexperiments, including: i) 200 Gb/s LAN-WDM2 transmission over20 km in O-band using two 25-GHz class directly modulated lasers(DMLs); and ii) 200 Gb/s, 300 Gb/s and 400 Gb/s WDM transmis-sions over up to 40 km in C-band using 4 wavelengths, each withdata rate of 50 Gb/s, 75 Gb/s, and 100 Gb/s, respectively. The successof these experiments clearly indicates the attractiveness of thepresented DMT ASIC for future MFH applications. In addition, inseveral transmission scenarios, the advantages of DMT format overconventional PAM4 format for MFH applications are highlighted.

Index Terms—5G, direct detection, Discrete Multi-Tone format,eCPRI, hybrid LTE-5G networks, LTE, mobile fronthaul.

I. INTRODUCTION

MANY operators around the world (e. g. from SouthKorea, China, and the United States) have already startedManuscript received July 30, 2020; revised September 30, 2020, Novem-

ber 16, 2020, and December 9, 2020; accepted December 10, 2020. Date ofpublication December 14, 2020; date of current version February 2, 2021.(Corresponding author: Son Thai Le.)

Son Thai Le, Kwangwoong Kim, and Theodore Sizer are with NokiaBell Labs, Holmdel NJ 07733 USA (e-mail: [email protected];[email protected]; [email protected]).

Tomislav Drenski, Andrew Hills, and Malcolm King are with the So-cionext Europe GmbH, Maidenhead SL6 4FJ, U.K. (e-mail: [email protected]; [email protected]; [email protected]).

Yasuhiro Matsui is with II-VI Incorporated, Fremont, CA 41762 USA (e-mail:[email protected]).

Color versions of one or more of the figures in this article are available athttps://doi.org/10.1109/JLT.2020.3044516.

Digital Object Identifier 10.1109/JLT.2020.3044516

the deployments of the Fifth Generation (5G) mobile communi-cation systems. In comparison to previous mobile generationsystems, e. g. 2G, 3G and 4G, the 5G distinguishes itselfby supporting versatile services including not only enhancedmobile broadband (eMBB), but also massive Machine TypeCommunication (mMTC) and Ultra-Reliable and Low Latency(URLLC) services [1]. To effectively support these services,5G networks promise 1000 times more cell capacity, 100 timeshigher peak rates, 10 times lower latency, and 10 times betterreliability than current 4G networks [2]. For eMBB services, thetarget maximum user throughput is as high as 20 Gbit/s at thefinal phase of 5G, which would be achieved by leveraging newfrequency bands, especially the mmWave band, and massiveMulti-input Multi-output (MIMO) technology.

Full deployment of 5G networks is a complex and expensiveprocess, which will take many years to complete. In addition,monolithic and homogeneous networks, including 5G networks,are no longer capable of addressing our growing and increasinglydiverse mobile device and connectivity needs in a satisfactoryway. In this aspect, legacy infrastructure, especially 4G net-works, will continue to have a crucial role in the hybrid wirelessaccess networks with multiple access interfaces to provide thecoverage and capacity that operators need for consistent serviceacross their footprint.

In fact, a big part of the 5G applications market is driven bythe evolution of 4G/LTE (Long Term Evolution). Technologyimprovements of 4G/LTE are ongoing, and it will continue tobe relied upon for many applications and services for a longtime. Accordingly, operators will continue to optimize LTEcapabilities such as improved uplink capacity to support videoand wireless communication to cloud-based applications andcoordinated multipoint (CoMP) transmission. LTE adoption willproceed at a fast pace with many innovations overlapping with5G operations. Thus, over the next decade, operators will keepon deploying, upgrading and improving their 4G networks inparallel with the deployment of 5G networks. This evolutionin hybrid LTE-5G networks is having a strong impact on everyaspect of the overall radio access network (RAN), including themobile fronthaul (MFH) and mobile backhaul (MBH) segments.

Cloud radio access network (C-RAN), sometime referred toas Centralized RAN, has been widely considered as the mainarchitecture for 4G/LTE and 5G networks [3]–[4] due to itssignificant benefit in capital and operation expenditure savings.

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802 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 39, NO. 3, FEBRUARY 1, 2021

In addition, C-RAN can also enhance the network’s capacity andperformance through load balancing and combined processingof radio signals from several closely located base stations [5]–[6]. In C-RAN, most of the signal processing functions on radiosignals are performed from remotely located baseband units(BBUs), which are connected to lightweight remote radio heads(RRHs) through MFH networks. For 4G/LTE MFH networks,Common Public Radio Interface (CPRI) has been considered asthe mainstream transport protocol [7]. The latest CPRI specifica-tion specifies CPRI line rates up to 24.33 Gbit/s (CPRI Rate 10)[7] which pumps more capacity to the LTE RRH for achievinghigher order MIMO and multi-carrier configuration. However,as the CPRI data rate scales with the number of antennas ratherthan the number of MIMO layers, the required CPRI data ratesin 5G systems with massive MIMO are in the order of severalhundreds of Gb/s [7]. This extremely high data rate makes itchallenging for implementing MFH networks in a cost-effectivemanner.

To address this problem, the 3rd generation partnershipproject (3GPP) has identified eight different functional split op-tions between the CU and RU for the 5G New Radio (NR) [8], outof which “Option 1” denotes the classical backhaul and “Option8” the physical layer (CPRI) approach. A new specification forMFH including these function splits called enhanced CPRI orevolved CPRI (eCPRI) has also been introduced [9]. Out of eightfunction split options specified by 3GPP, the most prominentoption appears to be the option 7 (intra PHY layer split) as it canreduce the fronthaul capacity by an order of magnitude whilestill effectively supporting features such as Carrier Aggregation,Network MIMO, Downlink CoMP and Uplink L1 joint process-ing [8]–[9]. In addition, with eCPRI protocol, fronthaul trafficnow can be encapsulated in Ethernet frames and subsequentlytransported via Ethernet and Ethernet/IP/UDP networks.

Unfortunately, eCPRI is not backward compatible with CPRI.To enable the co-existence of CPRI and eCPRI traffic in hy-brid 4G/LTE-5G fronthaul networks, eCPRI specification v2[10] has specified an Interworking function (IWF), acting asa CPRI/eCPRI bridge. More specifically, an IWF can be con-nected with both LTE Radio Equipment (RE) through a CPRIpoint-to-point link, or with an (enhanced) Radio EquipmentControl (eREC) via a packet-based transport network. In thisscenario, having optical transceivers which can support bothCPRI-10 and eCPRI Ethernet traffic is strongly desirable as theysimplify the network deployment and planning. In addition, suchoptical transceivers can be re-used when a RE is upgraded to aneRE to support 5G New Radio (NR) services. This representsa significant saving in capital expenditure. As a result, severaloptical module vendors have already started developing a newclass of optical transceivers with selectable retiming and dataclock recovery to support both 25GbE eCPRI and CPRI-10traffic [11]–[12]. To further reduce the cost and support futurebandwidth-hungry applications, it would be strongly desirableto increase the data rate of such optical transceivers to 100 Gb/s.

This paper is an extended version of our conference pa-per [13], presenting the industry-first 16 nm complemen-tary metal-oxide-semiconductor (CMOS) ASIC [14] using aDiscrete Multi-Tone (DMT) format that can support either

4× 25 Gb/s eCPRI or 4× 24.33 Gb/s CPRI-10 traffic on a singleoptical wavelength. In addition, this ASIC can be operated in1 × 25 Gb/s, 2 × 25 Gb/s and 3 × 25 Gb/s transmission modes,providing great flexibility for mobile network operators. Usingthis ASIC, we demonstrate real-time 200 Gb/s transmission inO-band and 400 Gb/s transmission in C-band over up to 40 km.

The paper is organized as follows: Section II discusses the co-existence of CPRI and eCPRI traffic in hybrid LTE-5G networks;Section III discusses the benefit of DMT over the conventionalPAM4 format in fronthaul applications; Section IV presentsthe architecture and characterization of 100 Gb/s DMT ASIC;Section V presents experimental setup and real-time trans-mission results for various practically relevant configurations;Section VI concludes the paper.

II. FRONTHAUL TRANSPORT IN HYBRID LTE-5G NETWORKS

As mentioned earlier, CPRI has been used as the main trans-port protocol for 4G/LTE MFH networks as shown in Fig. 1a,where the LTE RE is connected to LTE REC via a point-to-pointCPRI link. Compared to CPRI, eCPRI makes it possible todecrease the required data rate between eREC and eRE viaflexible functional decomposition while limiting the complexityof the eRE [9]. This is a crucial feature making eCPRI suitablefor 5G networks with massive MIMO. Alternatively, in recentyears, several spectral-efficient MFH transport schemes, suchas radio-over-fiber (RoF) [15]–[17] and intermediate frequencyover fiber (IFoF) [18]–[20] have been actively investigated.However, compared to these schemes, eCPRI provides an im-portant advantage that eCPRI data is encapsulated in Ethernetframes and can be transported via a packet-based transport net-work. This effectively positions Ethernet as convergence layerfor consolidating backhaul, fronthaul and possibly other packettraffic, providing great flexibility and efficiency.

On the other hand, as eCPRI is not backward compatiblewith CPRI, eCPRI specification v1 suggests that CPRI andeCPRI traffic are managed separately (Fig. 1b). In this case,mobile operators have to manage both REC and eREC, and inmany cases, in separate physical locations, which is very ineffi-cient. To address this issue, eCPRI specification v2.0 specifiesCPRI/eCPRI traffic mapping through an IWF, by which a REcan be connected to an eREC (IWF type 0) as shown in Fig. 1c.In this case, the REC can be integrated and managed togetherwith the eREC. Two example of implementation options (at thelogical data plane) are shown in Fig. 1d [10], namely i) – IWFis integrated with eREC and is connected to a LTE RE via apoint-to-point CPRI link (usually via a fiber optical cable) andii) – IWF is integrated with 5G eRE and connected with LTE REvia a short CPRI link (can be via either a fiber optical cable or anelectrical cable). These two implementation options are differentin terms of transceiver requirements. For example, in the firstimplementation scenario, one pair of optical CPRI transceiverand one pair of eCPRI transceivers providing similar reachare required. In the second implementation scenario, two pairsof long-reach eCPRI transceivers and one pair of short-reachCPRI transceiver are required if CPRI and eCPRI traffic areto be transmitted separately. In terms of hardware requirement,


LE et al.: 100GBPS DMT ASIC FOR HYBRID LTE-5G MOBILE FRONTHAUL NETWORKS 803

Fig. 1. Possible CPRI/eCPRI migration scenarios in hybrid 4G/LTE-5G MFH networks [10]; IWF – Interworking function.

Fig. 2. Block diagram of a conventional IM/DD transmission scheme; PD –photodiode.

it is clear that the first option is advantageous as it requiresone pair less of optical transceivers and it minimizes the powerconsumption and complexity of eRE. This option highlights theimportant roles of both CPRI and eCPRI optical transceiversin the hybrid 4G/LTE-5G MFH networks. In this case, a newclass of optical transceivers with selectable retiming and dataclock-recovery to support both CPRI (most likely CPRI-10)and eCPRI traffic would be of great interest. Such transceiversare available on the market, offering data rate of 25 Gb/s onone optical wavelength using NRZ format [11]–[12]. The nextcommon step would be to increase the data rate to 100 Gb/s tosupport future bandwidth-hungry applications. In term of reachrequirement, the majority of fronthaul connections in C-RANLTE/5G networks are within the range of 10 km – 40 km. Theupper limit on the fronthaul link distance is determined basedon the latency requirement of the RANs.

III. DMT FOR HYBRID LTE-5G MFH NETWORKS

A. Power Fading in IM/DD Transmissions

Due to the cost-sensitive nature of radio access networks,optical transceivers for MFH applications have to be low-cost.As a consequence, CPRI and eCPRI transceivers have beendesigned using the conventional intensity modulation – directdetection (IM/DD) transmission scheme where the detection isperformed by a single-ended photodiode (PD) [21] (Fig. 2).This transmission scheme is strongly dependent and affectedby the fiber chromatic dispersion (CD). For the sake of thecompleteness, in this section, we review some basic propertiesof the IM/DD channel and explain why DMT is a very attractivemodulation format for MFH networks.

In conventional IM/DD transmission systems, one can expressthe amplitude of the optical field after modulation as:

s (t)= A +m(t) . (1)

where m(t) is the real-valued modulated signal with zero meanand A represents the amplitude of the optical carrier which isbig enough such that the condition s(t) ≥ 0 is satisfied.

For simplicity of the analysis, it is assumed that the fiberchannel is characterized only by the CD. In this case, the timeimpulse response of the channel can be expressed as [22]:

h (t) =

√c

jDλ2exp

(jπc

Dλ2t

). (2)

where D (ps/nm) is the total chromatic dispersion of the link, λ(nm) is the optical wavelength and c (m/s) is the speed of light.

In the frequency domain, the fiber channel response can beobtained by taking the Fourier Transform of the Eq. (2), showinga characteristic of an all-pass filter:

H (ω) = exp

(−jDλ

2

4πcω2

). (3)

where ω = 2πf is the angular frequency.The received signal at the end of the fiber link can be simply

written as:

r (t) = s (t)⊗ h (t) = (A+m (t))⊗ h (t) . (4)where ⊗ stands for the convolution operation.

The detected photocurrent is the intensity of r(t):

I (t) = |A|2 + |m (t)⊗ h (t)|2 + 2A ·R (m (t)⊗ h (t)) .(5)

where � stands for the real part.Equation 3 shows that the detected current in an IM/DD

transmission system is the sum of three terms. The first termI1(t) = |A|2 is the DC part which does not have any im-pacts on the system performance. The second terms I2(t) =|m(t)⊗ h(t)|2 is the signal-signal self-beating term. The thirdterm I3(t) = 2A · �(m(t)⊗ h(t)) represents the linear detec-tion term impaired by the fiber CD, which is also the dominant



Fig. 3. (a) Electrical spectra of 112 Gb/s 4-level pulse-amplitude modulation(PAM4) signal with raised-cosine (RC) pulse shaping at the transmitter and afterPD over 20 km of standard single mode fiber (SSMF) in C-band; (b) – amplituderesponse of the linear detection term impaired by the CD.

term in the detected photocurrent. Taking the Fourier transformof this term, we obtain:

I3 (ω)=M (ω) · cos(Dλ2

4πcω2

)=M (ω) · cos

(πDλ2

cf2

).

(6)where M(ω) is the spectrum of the modulated signal m(t).

From Eq. (6), considering only the linear detection term, thechannel response of the DD channel can be expressed as:

R (f) = cos

(πDλ2

cf2

). (7)

It is evident that this channel response has notches (R(f) =0) at the following frequencies:

fn =

√(2n+ 1) c

2λ2D, n = 0, 1, 2. (8)

This result indicates that at several frequencies, the trans-mitted signal is completely faded after propagating through anIM/DD channel. In addition, frequency components around thenotches are attenuated causing a significant loss in the signalpower. This phenomenon is usually called CD-induced powerfading effect [24], which is a major effect limiting the perfor-mance of IM/DD transmission systems.

Figure 3(a) shows the electrical spectrum of 112 Gb/s 4-levelpulse-amplitude modulation (PAM4) signal with raised-cosine(RC) pulse shaping over 20 km of standard single mode fiber(SSMF) in C-band (D = 340 ps/nm), where several notchesfalling into the signal’s bandwidth can be observed. An RC pulseshaping was chosen with a small roll-off factor of 0.05 so that thesignal-signal self-beating term can be clearly observed outside

Fig. 4. Frequency of the first notch as function of distance for IM/DD trans-mission system in C-band.

the signal’s bandwidth. The amplitude response of the lineardetection term is shown in Fig. 3(b).

A major issue associated with the power-fading effect is thatthe faded frequency components cannot be recovered throughconventional receiver equalizers. This will limit the usable band-width of the IM/DD channel from DC to the first notch whenthe signal is modulated using conventional modulation formatssuch as non-return to zero (NRZ) and PAM4. Such limitation inthe usable bandwidth subsequently reduces the achievable datarate of the IM/DD transmission system.

Figure 4 shows the first frequency notch as function of dis-tance for IM/DD systems in C-band. One can note that at 20 kmthe usable system bandwidth is up to ∼13 GHz, which mightrestrict the achievable data rate of the system to be below 50 Gb/swhen PAM4 format is used. For 100 Gb/s PAM4 IM/DD sys-tems, the required electrical bandwidth is around 25 GHz, whichmight impose a limitation in the reach of ∼6 km, according toFig. 4.

In summary, the estimated reach of 50 Gb/s and 100 Gb/sPAM4 IM/DD systems based on the location of the first notchare below 20 km and 6 km respectively, which is not sufficientfor supporting MFH links in hybrid 4G/LTE-5G networks. In-creasing further the reach of IM/DD systems in C-band requiresmitigating the impact of the power fading effect. Alternatively,IM/DD systems for fronthaul applications can be implementedin O-band where the chromatic dispersion is minimal. However,due to the lack of cost-effective O-band amplifiers and tunableO-band laser sources, only a limited number of wavelengthsin O-band (up to 8 wavelengths in LAN-WDM8 scheme) canbe used for transmissions [24] (Grey optic). In this case, Greyoptic can deliver a total capacity per fiber up to 400 Gb/s. Inthe context of LTE/5G networks, this capacity (up to 400 Gb/s)might not be sufficient. Operators will either need to rent morefibers or to leverage a more expensive transmission technologysuch as coherent detection. This situation is undesirable from acost point of view.

B. Discrete Multi-Tone Modulation

The power fading effect just described in IM/DD transmissionsystems is similar to the multi-path fading effect in wirelesscommunication [25]. To maximize the transmission capacity



Fig. 5. General block diagram IM/DD transmission with DMT format; P/S and S/P– serial to parallel and parallel to serial conversions; CP – cyclic prefix; BL/PL– bit and power loading; Conj – conjugation operation.

of fading channels, wireless communication systems (e. g. 5Gsystems) use a multicarrier modulation format called orthogonalfrequency division multiplexing (OFDM) [26]. OFDM encodesdigital data on multiple carrier frequencies and optimizes thedata rate on each carrier frequency based on the fading profileof the channel. Due to its superior performance, OFDM formathas been chosen as the standard signaling technique for 5GNR. Based on the success of OFDM, for IM/DD transmissionsystems, a special version of OFDM called DMT has beenintensively investigated [27]–[30]. A general block diagram ofan IM/DD transmission scheme with DMT format is shown inFig. 5. DMT divides the frequency spectrum into orthogonalsubcarriers but it employs the properties of Hermitian symmetryand the inverse discrete Fourier transform (IDFT) to create areal-valued signal which is required for IM/DD systems [27].Each subcarrier can be modulated with a quadrature amplitudemodulation (QAM) format and the power of each subcarrier canbe allocated using a water filling algorithm [31] to maximizethe system’s capacity in the presence of power-fading effects.This process is known as bit and power loading (BL, PL).BL/PL effectively minimize the impact of notches due to thepower fading effect and extend the usable bandwidth in a DMTIM/DD system beyond the first notch’s frequency. In addition,this technique also mitigates the impact of bandwidth limitationsof opto-electronic components in the system. At the receiver, thetotal channel response can be compensated effectively using asimple 1-tap equalizer.

The benefit of DMT format over the conventional PAM4format for IM/DD systems is shown in Fig. 6, where the requiredoptical power as function of total dispersion of the link for112 Gb/s DMT and 112 Gb/s PAM4 systems are comparedthrough simulation. For the 112 Gb/s DMT system the IDFTsize was 512 and the number of modulated subcarriers was 250.The transmitter’s bandwidth limitation was modelled using thefirst order Gaussian filter with a 3-dB bandwidth of 16 GHz.The fiber channel includes only CD, the receiver includes bothshot noise and thermal noise from a TIA with resistor of 20kΩ and a noise figure of 5 dB. For 112 Gb/s PAM4 system afeed-forward equalizer (FFE) with 127 T/2-space taps was usedfor signal equalization. A long filter had to be used to makesure that the best possible performance of PAM4 system wasachieved. For the DMT system, a single-tap equalization wasused. It is evident that when the CD is small (up to ∼40 ps/nm)both systems offer very similar sensitivities. However, when the

Fig. 6. Sensitivities at BER of 0.004 for 112 Gb/s DMT and 112 Gb/s PAM4systems as functions of total dispersion of the link.

CD is increased further, the performance of the PAM4 systemis quickly degraded and the system breaks down when the firstnotch appears within the signal bandwidth. Due to the abilityof optimizing BL/PL based on the channel response (changingwith the link distance), DMT can tolerate a much higher CD,up to 240 ps/nm when accepting a ∼2 dB penalty in sensitivity.This result clearly shows a significant benefit of DMT formatover PAM4 for IM/DD transmission systems. On the other hand,the DMT format has higher requirements on the dynamic rangeand on the linearity of the opto-electronic components due toits higher peak-to-average power ratio (PAPR) compared tothose of PAM4 format. This is the main reason that practicalimplementations of the DMT format in IM/DD systems for datacenter and other short-reach applications are limited.

IV. 100 GB/S DMT ASIC

There are practical applications in hybrid 4G-LTE/5G net-works that can highly benefit from the DMT solution. In thefollowing investigations, a 100 Gb/s DMT ASIC has been usedthat is designed and fabricated in 16 nm complementary metal-oxide-semiconductor (CMOS) technology [14]. Some prelim-inary transmission measurements using this DMT ASIC havebeen also reported in [32]. The block diagram of the DMT ASICis depicted in Fig. 7(a). It can support various client interfaces,including CPRI-10, 100 GbE (IEEE CAUI-4 compliant) andOTL4.4. The ASIC can be operated at ¾, ½ and ¼ client rates(75 Gb/s, 50 Gb/s and 25 Gb/s) providing a great flexibility



Fig. 7. (a) General block diagram of the 16nm CMOS ASIC; (b) – Picture of the ASIC evaluation board

for 5G MFH applications. As a 25 Gb/s rate would be enoughto support eCPRI traffic of a typical 5G radio unit employingthe splitting option 7.3 with a 100 MHz carrier, 8 spatial layersand a 64 transmit/receiver antenna [10]. A typical small cell-siteconsists of 3 sectors and can be supported by a single 75 Gb/sDMT system. In a macro cell, more sophisticated traffic aggre-gation schemes can be used and 100 Gb/s per wavelength systemcan provide significant cost saving. All of these scenarios can beeffectively supported by this DMT ASIC, making it an attractiveand viable solution for hybrid 4G/LTE-5G networks.

The ASIC incorporates a complete single channel DMT trans-mission PHY for up to 100 Gb/s data rates over short reachoptical fiber. It includes an 8-bit Digital-to-Analog Converter(DAC) and Analog-to-Digital Converter (ADC) with samplingrates up to 71 GS/s, a DMT core engine, an on-chip digitalRX timing recovery, a low-jitter RX clock generation and highcoding gain FECs. The DMT core engine at the transmitterside uses 512pt iFFT to encode data onto 256 subcarriers withadaptive QAM formats with 0-8 bits per subcarrier. For achiev-ing the best transmission performance, the QAM modulation isdetermined by measuring the signal-to-noise ratio (SNR) of thechannel at each subcarrier frequency. A water-filling algorithmthen computes the bit and power loading for each subcarrierand an adaptive background equalization guarantees optimumadaptation to any variations in the channel. The DMT ASIC alsoincludes configurable cyclic prefix for total bitrate optimizationand frequency domain equalization. The receive path accepts aDMT modulated electrical signal which is sampled and digitizedby the ADC. The DMT data stream is then demodulated, backto the original time domain-based data before running throughthe FEC decoder to ensure error free data is passed to the clientinterface.

To meet the strict requirements of LTE/5G fronthaul networks,the DMT ASIC was designed to achieve low latency variationand stable performance over the full industrial temperature range(−40 °C to +85 °C). All DSP operations of the DMT chip(including DMT framing/deframing, bit mapping/demappingand FEC coding/decoding) were designed to operate in a fixednumber of clock cycles and contribute no latency variation;additionally the transparent nature of the DMT transmissionmeans that latency is unaffected by the format or framing of thecarried data and all data bits experience the same ‘bit-latency’.The remaining source of latency variation is due to on-chipFIFO’s whose delay is stable once initialized but can change

between initializations. For this reason, the latency variation wasmeasured across multiple initializations; a long-sequence PRBSdata stream input to the transmitter of one DMT chip and theoutput PRBS data stream from a connected receiver DMT chipwere both captured in the time-domain and the resulting delaybetween them was accurately measured. For any one particularinitialization the latency variation is small, with typically 130ps change measured when subjected to a temperature changefrom 25 °C to 100 °C, demonstrating an excellent stability. Fig-ure 8(a) then shows the latency variation observed across 200initializations of the DMT ASIC, the resulting variation is lessthan ±3 ns (Fig. 8a). This value is well below 65 ns, which isthe maximum allowed time alignment error between antennaports in 5G networks [33], confirming that the latency variationcontribution of the DMT ASIC is small relative to the totalallowed latency variation budget of the system.

Figure. 8(b) shows the variation in performance in terms ofmeasured chip temperature and pre-FEC BER and post-FECBER of the DMT ASIC in the electrical back to back configura-tion when the temperature is swept from−40 °C to+85 °C (eachtime point is 10 s). One can note that the pre-FEC BER slightlyincreases at high temperature, but the rise is insignificant as itis always below 10−9, and the slight variation does not haveany impact on the post-FEC BER. This result confirms thatthe ASIC performs stably over the full industrial temperaturerange (−40 °C to+85 °C), therefore qualifying for fronthaulapplications in uncooled outdoor enclosures.

V. EXPERIMENTAL DEMONSTRATIONS

A. O-Band Transmission

The first experiment was to evaluate the performance ofthe DMT ASIC in O-band (Grey optic) applications. For thistransmission scenario, a directly modulated laser (DML) is thepreferred optical modulator due to several reasons: i) – DMLis a low cost solution since the optical signal is modulatedby directly modulating the injected current of the laser and noexternal modulator is required; ii) – DML offers higher opticaloutput power than external modulated lasers (EMLs), whichsubsequently increases the system reach in the O-band.

For the experiment in O-band, a 200 Gb/s LAN-WDM2 realtime transmission over 20 km of SSMF was tested using two25-GHz-class DMLs at 1305 nm and 1310 nm with + 8 dBmof optical output power (when the DML is bias at 70 mA).



Fig. 8. (a) Latency variation measurement of the DMT ASIC; (b) – Electrical mode temperature cycling pre-FEC and post-FEC BER measurement versustemperature.

Fig. 9. Experimental setup of real time O-band, amplifier-less, LAN-WDM2 200 Gb/s transmission supporting 8 × 25 GbE Ethernet or 8 × 24.33 Gb/s CPRI-10traffic over 20 km.

This system can support 8 eCPRI channels at 25 Gb/s or 8CPRI-10 channels. The experimental setup is shown in Fig. 9having two DMT ASICs operating at full rate of 4 × 25 Gb/s(supporting eCPRI traffic) and 4 × 24.33 Gb/s (supportingCPRI-10 traffic). For each channel, the appropriate CPRI-10traffic at 4 × 24.33 Gb/s and eCPRI Ethernet traffic at 4 ×25.78125 Gb/s (IEEE CAUI-4 compliant) was emulated, whichwere then mapped onto 256 subcarriers (after FEC-encoding) bythe DMT core engine through bit and power mapping based onthe channel condition measured during the initialization stage.During operation, the channel condition was automaticallytracked and updated by the DMT core engine after a predefined(adjustable) time interval. The modulated DMT signal of theASIC was amplified using a 25-GHz RF driver to a Vpp of ∼4.5V before bias-adding for directly driving a DML. The TX 3-dBbandwidth for each channel was ∼16 GHz (Fig. 10).

After optical modulation, the two optical channels were com-bined using a LAN-WDM MUX and then fed into 20 km ofstandard single mode fiber (SSMF). At the receiver, two DMTchannels were demultiplexed using a LAN-WDM DeMUX.Each channel was detected using a 25-GHz PIN-TIA receiverand then fed back into the ASIC for real-time processing anddecoding. The total insertion loss of the link, including 20 kmof SSMF and LAN MUX/DEMUX in the O-band was ∼10 dB,providing a maximum received optical power for each channelof ∼−2 dBm.

The overall experimental results for the O-band transmissionare depicted in Fig. 11. Herein the pre-FEC BER was used asthe performance metric. Important to note is that the FEC wasapplied in all transmission cases and all reported pre-FEC BER

Fig. 10. Opto-electronic responses of 1310 nm and 1305 nm channels.

below 0.04 (CI-BCH FEC limit) corresponded to the post-FECBER value below 10−15. Fig. 11(a) and Fig. 11(b) depict the backto back (B2B) measurements for 1310 nm and 1305 nm chan-nels, showing excellent sensitivities of ∼−6 dBm and −7 dBmfor 4 × 25 Gb/s (eCPRI traffic) and 4 × 24.33 Gb/s (CPRI-10traffic), respectively. For the 4 × 25 Gb/s transmission scenario,the firmware provided an option of varying the DAC samplingrate from 64 GS/s to 71 GS/s. Fig. 11(a)–Fig. 11(b) show thatincreasing the DAC sampling rate significantly increases thesystem’s performance. This is due to the fact that the signal band-width is expanded when the DAC sampling rate is increased. InDMT transmissions, given a fixed data rate, increasing the usablebandwidth improves the overall transmission performance. Thisis a major advantage in performance and flexibility of DMT com-pared to the PAM4 format where increasing the bandwidth does



Fig. 11. Overall experimental results for 200 Gb/s transmission in O-band including (a)–(b) – B2B sensitivities of 1310 nm and 1305 nm channels, respectively;(c)–(d) SNR and allocated bits per subcarrier for the 1310 nm channel at 0 dBm of Rx power;

not necessarily increase the system’s performance because of thepower fading effect. For the 4 × 24.33 Gb/s transmission mode,the DAC sampling rate was fixed at 64 GS/s. This limitation wasdue to the firmware used at the time of experiment. However, dueto the slightaly smaller data rate, the 4× 24.33 Gb/s transmissionmode with a DAC sampling rate of 64 GS/s still performs betterthan 4 × 25 Gb/s mode with a DAC sampling rate of 71 GS/s.

Figure 11c shows the typical SNR profile of the IM/DD chan-nel for 4 × 24.33 Gb/s transmission mode (DAC sampling rateof 64 GS/s) and 4 × 25 Gb/s transmission mode (DAC samplingrate of 71 GS/s). Due to the different DAC sampling rates used,the subcarrier spacings in these cases was also different (thesubcarrier spacing is proportional to the DAC sampling rate).Based on this SNR profile the DMT engine optimizes the BL foreach subcarrier for minimizing the pre-FEC BER. The resultingBL profiles for 4 × 24.33 Gb/s and 4 × 25 Gb/s transmissionmodes are shown in Fig. 11d. These BL profiles show that mostof the first 64 subcarriers are transmitted with 64 QAM format.Subcarriers from 90 to 160 are loaded mostly with 16 QAMformat following by 8 QAM and QPSK.

Figure 12 depicts the overall performances of 4 × 24.33 Gb/sand 4 × 25 Gb/s transmission modes over 20 km for both 1310nm and 1305 nm channels. Compared to the B2B cases, thesensitivity is reduced by ∼1 dB, to – 6 dBm for all consid-ered transmission scenarios. Given the fact that the maximumreceived optical power was around −2 dBm, there is still 4 dBpower margin for practical implementations. This high-powermargin confirms the reliability and usability of the DMT ASICfor real transmission systems.

B. C-Band Transmissions With Double Sideband DMT

As discussed in Section II, many MFH links in 4G/LTE-5Gnetworks will require extremely high transmission capacity (be-yond 1 Tb/s). In this case, transmission in the C-band becomesnecessary. To demonstrate the potential performance of the DMT

Fig. 12. Performance over 20 km of 200 Gb/s LAN-WDM2 transmission.

ASIC for C-band transmission applications, a 4-channel WDMtransmission as shown in Fig. 13 was set up. The ASIC was usedto modulate two interleaved optical carriers (either 1545 nm with1557 nm or 1549 nm with 1561 nm) using a Mach-ZehnderModulator (MZM). After multiplexing, the WDM signal wasamplified and fed into a single span of SSMF with span lengthfrom 10 km to 40 km. Various net data rate settings of the DMTASIC, namely 50 Gb/s (2× 25 Gb/s), 75 Gb/s (3× 25 Gb/s) and100 Gb/s (4 × 25 Gb/s and 4 × 24.33 Gb/s) were considered.When the data rate of the DMT ASIC was 75 Gb/s or below,no DCF was used. At the receiver, each channel was filteredout using a 0.7 nm tunable optical filter where the filter’s centerfrequency was tuned to the carrier frequency of the DMT signal.In this case, the full DMT signal passed through optical filterbefore being detected using a PIN-TIA photo-receiver. With thisfilter setting, the system is usually referred to as double sideband(DSB) DMT.

For the data rate of 50 Gb/s a comparison of DMT format(with real time processing) versus 50 Gb/s PAM4 and 50 Gb/s



Fig. 13. Experimental setup of real time C-band, WDM transmissions with up to 400 Gb/s supporting 16 × 25 GbE Ethernet or 16 × 24.33 Gb/s CPRI-10 traffic.The transmission distance is from 10 km to 40 km, DCF is optional.

Fig. 14. (a) Performance comparison between 50 Gb/s DMT (real time) over20 km and 40 km, 50 Gb/s PAM4 and NRZ (offline signal processing) for 1549nm channel over 20 km (without DCF); (b) – Bit loadings for 50 Gb/s DMTsignals over 20km (with – 1 dBm of Rx power) (upper) and 40 km (lower).

NRZ was performed and the results are shown in Fig. 14(a) forthe 1549 nm channel. For the DMT format, similar performanceswere observed also for other wavelengths (not shown here). For50 Gb/s PAM 4 and NRZ systems offline signal processing wasused. For both cases, FFEs with an optimum length of 127T/2-space taps were used. It is evident in Fig. 14(a) that the50 Gb/s systems with either PAM4 or NRZ could not reach20 km without dispersion compensation because of the powerfading effect (the first notch appears at ∼13 GHz). As a result,for 20 km MFH applications either DCF or advanced codingare required if the conventional PAM4 format or NRZ are tobe used. Both options are not desirable as they increase the

Fig. 15. (a) – Performance comparison of 75 Gb/s DMT (real time) over 10 kmand 20 km and 75 Gb/s PAM4 (offline signal processing) for 1549 nm channelover 10 km; (b) – Bit loadings for 75 Gb/s DMT signals (with – 1 dBm of Rxpower) over 10 km (upper) and 20 km (lower).

complexity and cost of the systems. On the other hand, withDMT format, appropriate bit loading can be used to extendthe usable bandwidth beyond the first notch. The bit loadingsfor the cases of 20 km and 40 km transmissions are depictedin Fig. 14(b), where subcarriers around notches are not usedwhile subcarriers beyond the first notch can still be used fordata transmission. This result confirms that DMT can maximizethe usable bandwidth of the IM/DD channel and thus deliver amuch better performance in the C-band compared to PAM4. Thisimportant feature of DMT enables 50 Gb/s transmission over



Fig. 16. Overall experimental results for 400 Gb/s transmission in C-band including (a) – B2B sensitivities of 4 WDM channels; (b) – performance over 20 km;(c)–(d) SNR and allocated bits per subcarrier for the 1549 nm channel at 0 dBm of Rx power in the B2B case.

Fig. 17. (a) Performances of 1557 nm channel versus distance when the DCFis fixed at −338 ps/nm, the Rx signal power was −1 dBm; (b) – Bit loadings for100 GbE DMT signals (at -1 dBm of Rx power) over 18km (upper) and 23 km(lower).

40 km without DCF, showing an attractive and viable solutionfor MFH links where massive capacity is required.

Similar comparison between DMT and PAM4 for 75 Gb/s isshown in Fig. 15(a). Again, it is evident that the PAM4 systemwith offline signal processing could not reach 10 km in the

Fig. 18. Optical spectra for DSB and VSB 100 GbE DMT signals. The centralfrequency is at 1557 nm.

C-band without dispersion compensation. However, when usingthe DMT format, 75 Gb/s could be transmitted successfully over20 km. The bit loadings for 75 Gb/s DMT signal over 10 kmand 20 km are shown in Fig. 15(b). This is an important resultbecause 75 Gb/s is required to support a typical 5G cell with 3sectors, each with 25 Gb/s of eCPRI traffic.

Next considerations were for the full 100Gb/s data rate ofthe DMT ASIC. In this case, for the DSB-MDT signals, thereach of at least 10 km could not be achieved without DCF(target was set to 20 km). Therefore, to compensate the CD over20 km a DCF with dispersion of −338 ps/nm was used. Theoverall performance of 400 Gb/s WDM system in C-band isshown in Fig. 16. In the B2B case (where the optical filter isalso included), all four channels showed a similar sensitivity of∼−6 dBm and a BER floor was reached at∼0 dBm of Rx power.In this transmission scenario the limitation is set by the CD ratherthan the fiber loss and RX noise due to the usage of EDFA.Over 20 km of transmission distance, the sensitivity droppedby ∼1 dB (to ∼−5 dBm). Overall, both transmission modesdelivered pre-FEC BER significantly below the FEC limit, whichconfirmed the usability and reliability of the experiment. For the1549 nm channel in B2B case, the channel’s SNR profile and BLper subcarriers are depicted in Fig. 16(c) and Fig. 16(d), clearlyindicating that high SNR and high order QAM were critical for



Fig. 19. Overall experimental results for 400 Gb/s VSB-DMT transmission in C-band without using DCF including. (a) B2B sensitivities of DSB and VSBsignals for 1557 nm channel; (b) – performance versus distance for all 4 100 GbE VSB-DMT channels, the Rx power was −1 dBm; (c) – performance versusdistance for all 4 CPRI-10 VSB-DMT channels, the Rx power was −1 dBm.

achieving such a high data rate given the bandwidth limitationof the system (∼16 GHz of 3-dB bandwidth).

In practice, tight dispersion management is challenging. Forstudying the dispersion tolerance of DMT signals at 100 GbE,we fixed the DCF at−338 ps/nm and varied the distance for 1557nm channel from 18 km to 23 km. The transmission result in thisscenario is shown in Fig. 17(a). We observed that the optimumperformance was achieved at 21 km where the estimated residualdispersion is ∼17 ps/nm. We suspect the reason behind thisrelates to the non-ideal (linear) phase response of the opticalfilter. Overall, performance below the FEC limit was achievedfor the distance range of 18 km to 23 km. This indicates that the100 GbE DSB-DMT signals can tolerate ± 40 ps/nm of residualdispersion.

C. C-Band Transmissions with Vestigial Sideband DMT

To achieve 20 km of reach without using DCF for 100GbE transmission mode, appropriate optical filter setting canbe chosen to generate vestigial side band (VSB) DMT signalprior to the photodetector [34]–[35]. Alternatively, the opticalfiltering operation can also be applied at the transmitter side.In our experiment, to generate VSB DMT signals, we tuned thefilter center frequency∼0.32 nm away from the center frequencyof the modulated DMT signal. The filter bandwidth was keptat 0.7 nm. For practical implementations, a VSB DMT signalcan be generated by assigning the transmitter laser frequencyto the edge of the MUX/DEMUX filters response. For WDMDD transmission systems, MUX/DEMUX are mandatory so theimplementation effort of a WDM VSB DMT system would besimilar to those of a WDM DSB DMT system. In addition,for both abovementioned systems, the requirement for opticalamplification should be also similar.

For the 1557 nm channel a comparison of optical signalspectra for DSB and VSB DMT signals is shown in Fig. 18,where one can note that one sideband of the DMT signal has

been effectively removed. In this case, the power-fading issueis strongly suppressed. In the ideal case (sharp filter roll-offand ideal photodetector), the dominant impairment is the signal-signal self-beating term as shown in Eq. 5.

In the B2B scenario, we compare the sensitives of DSB DMTand VSB DMT schemes at 100 GbE in Fig. 19(a) for 1557nm channel. One can note that VSB DMT scheme shows aslightly better performance than DSB DMT scheme for both100 GbE and 4 × 24.33 Gb/s CPR-10 transmission modes,especially at high received signal powers. This indicates thatVSB DMT scheme also mitigates the non-ideal phase responseof the particular optical filter used in the setup. One shouldnote that optical filter was also required in the DSB WDMtransmission case as a DD receiver cannot tolerate inter-channelinterference. On the other hand, VSB DMT scheme does notshow significant benefit at the low power regime, where thereceiver noise is the dominant performance limitation.

The transmission performances of all 4 VSB-DMT channelswith the received power per channel of −1 dBm are depictedin Fig. 19(b) and Fig. 19(c), showing that pre-FEC BER valuesbelow the FEC limit were achieved over 40 km for all 4 channels.Similar to the case of DSB DMT schemes, 4 × 24.33 Gb/sCPR-10 transmission mode provides better performance than the100 GbE mode. One should note here that no DCF was used inthe case of VSB DMT transmissions. This result is encouragingas it shows that the VSB DMT scheme can cover all fronthaulnetwork at 100 Gb/s without the need for optical dispersioncompensation.

VI. CONCLUSION

The first 100 Gb/s DMT ASIC fabricated in 16 nm CMOStechnology for applications like future hybrid 4G/LTE-5G MFHnetworks was presented. To address the diverse connectivity andcapacity needs of hybrid 4G/LTE-5G MFH networks, the DMTASIC has been implemented with great flexibility supporting



various data rates (from 25 Gb/s to 100 Gb/s) of both CPRI andeCPRI traffic. Using this DMT ASIC, 200 Gb/s real-time trans-missions in O-band have been demonstrated using two DMLs.In addition, C-band real time transmissions up to 400 Gb/s havealso been demonstrated successfully, showing several significantbenefits of the DMT formats over the conventional PAM4 for-mats for MFH applications. The successful and reliable resultsof these real time demonstrations clearly show the high potentialof a DMT solution for high capacity and flexible MFH networksin hybrid 4G/LTE-5G environments.

ACKNOWLEDGEMENT

We would like to thank Peter Winzer for valuable discussions.

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