Common Public Radio Interface Research Articles

Fronthaul Network Modeling and Dimensioning Meeting Ultra-Low Latency Requirements for 5G

Enabling the transport of fronthaul traffic in next-generation cellular networks [fifth-generation (5G)] following the cloud radio access network (C-RAN) architecture requires a redesign of the fronthaul network featuring high capacity and ultra-low latency. With the aim of leveraging statistical multiplexing gains, infrastructure reuse, and, ultimately, cost reduction, the research community is focusing on Ethernet-based packet-switch networks. To this end, we propose using the high queuing delay percen-tiles of the G/G/1 queuing model as the key metric in front-haul network dimensioning. Simulations reveal that Kingman's exponential law of congestion provides accurate estimates on such delays for the particular case of aggregating a number of evolved Common Public Radio Interface fronthaul flows, namely functional splits and II D . We conclude that conventional 10 G, 40 G, and 100 G transponders can cope with multiple legacy 10–20 MHz radio channels with worst-case delay guarantees. Conversely, scaling to 40 and 100 MHz channels will require the introduction of 200G, 400G, and even 1T high-speed transponders.

Digital Code-Division Multiplexing Channel Aggregation for Mobile Fronthaul Architecture With Low Complexity

A low-complexity mobile fronthaul architecture via digital code-division multiplexing (CDM) is proposed to enable channel aggregation of 4G-LTE signals. In comparison with traditional frequency division multiplexing based aggregation scheme, the fast Fourier transformation/inverse fast Fourier transformation operations are replaced by simple sign selection and addition, leading to the significant reduction of computational complexity. Moreover, synchronous transmission of both the I/Q waveforms of wireless signals and the control words (CWs) used for the purpose of control and management using the CDM approach is also presented to be compliant with the common public radio interface (CPRI). In a proof-of-concept experiment, we demonstrate the transmission of 48 × 20 MHz LTE signals with CPRI equivalent data rate of 59 Gb/s, achieving an average error vector magnitude (EVM) of ∼3.6% and ∼4.3% after 5 and 20 km transmission over standard single-mode fiber (SSMF), respectively. Furthermore, we successfully demonstrate the transmission of 32 × 20 MHz LTE signals together with CPRI-compliant CWs, corresponding to CPRI-equivalent data rate of 39.32 Gb/s, only using single optical wavelength channel with analog bandwidth of ∼1.96 GHz. After transmission over 5 km SSMF, CWs can be error-free recovered while the LTE signals are recovered with an EVM of ∼3.6%.

Spectrally efficient digitized radio-over-fiber system with k-means clustering-based multidimensional quantization.

We propose a spectrally efficient digitized radio-over-fiber (D-RoF) system by grouping highly correlated neighboring samples of the analog signals into multidimensional vectors, where the k-means clustering algorithm is adopted for adaptive quantization. A 30 Gbit/s D-RoF system is experimentally demonstrated to validate the proposed scheme, reporting a carrier aggregation of up to 40 100MHz orthogonal frequency division multiplexing (OFDM) channels with quadrate amplitude modulation (QAM) order of 4 and an aggregation of 10 100MHz OFDM channels with a QAM order of 16384. The equivalent common public radio interface rates from 37 to 150 Gbit/s are supported. Besides, the error vector magnitude (EVM) of 8% is achieved with the number of quantization bits of 4, and the EVM can be further reduced to 1% by increasing the number of quantization bits to 7. Compared with conventional pulse coding modulation-based D-RoF systems, the proposed D-RoF system improves the signal-to-noise-ratio up to ∼9 dB and greatly reduces the EVM, given the same number of quantization bits.

Broadband IF-Over-Fiber Transmission With Parallel IM/PM Transmitter Overcoming Dispersion-Induced RF Power Fading for High-Capacity Mobile Fronthaul Links

We demonstrate a broadband and long-distance intermediate frequency-over-fiber (IFoF) transmission scheme employing a transmitter composed of parallel intensity/phase (IM/PM) modulators with appropriate bandwidth allocations to IM and PM. Due to the proposed scheme, we can eliminate all the null frequencies caused by dispersion-induced RF power fading, which, in turn, enables us to significantly increase the available bandwidth. In addition, our system does not require any synchronization between IM and PM, which reduces complexity compared to conventional parallel transmitter architecture. We successfully transmitted 20 × 360 MHz filtered orthogonal-frequency-division-multiplexed signals corresponding to a common public radio interface equivalent data rate of 524.28 Gbps over a 30- and 40-km single-mode fiber satisfying the 8% threshold for the error-vector magnitude values for all the subcarriers. These results show that our proposed IFoF transmission scheme is scalable to long-distance mobile fronthaul links for 5G and beyond.

A Transport Scheme for Reducing Delays and Jitter in Ethernet-Based 5G Fronthaul Networks

Fronthaul networks are leveraging communication links between the remote radio heads and baseband units in cloud radio access networks, and common public radio interface (CPRI) is an interface to carry time-sensitive traffic over the fronthaul. For the fronthaul networks which impose stringent bandwidth, latency, and jitter requirements, Ethernet can be a cost-effective solution to carry the CPRI traffic. However, Ethernet networks have limitations to transport the multiple CPRI streams simultaneously and satisfy the strict service requirements due to the accumulation of unavoidable intrinsic delays at switches. To address these issues, this paper evaluates the impact of intrinsic delay on the performance of CPRI over Ethernet (CoE) networks with experimental results and proposes a virtual local area network-based CoE network architecture along with two transport algorithms to carry the multiple CPRI streams over Ethernet despite the intrinsic delays. We confirmed with computer simulations that with the proposed techniques, the end-to-end delays and jitter for a number of CPRI streams within the acceptable range of fronthaul requirements can be achieved.

Toward an Efficient C-RAN Optical Fronthaul for the Future Networks: A Tutorial on Technologies, Requirements, Challenges, and Solutions

The exponential traffic growth, demand for high speed wireless data communications, as well as incessant deployment of innovative wireless technologies, services, and applications, have put considerable pressure on the mobile network operators (MNOs). Consequently, cellular access network performance in terms of capacity, quality of service, and network coverage needs further considerations. In order to address the challenges, MNOs, as well as equipment vendors, have given significant attention to the small-cell schemes based on cloud radio access network (C-RAN). This is due to its beneficial features in terms of performance optimization, cost-effectiveness, easier infrastructure deployment, and network management. Nevertheless, the C-RAN architecture imposes stringent requirements on the fronthaul link for seamless connectivity. Digital radio over fiber-based common public radio interface (CPRI) is the fundamental means of distributing baseband samples in the C-RAN fronthaul. However, optical links which are based on CPRI have bandwidth and flexibility limitations. Therefore, these limitations might constrain or make them impractical for the next generation mobile systems which are envisaged not only to support carrier aggregation and multi-band but also envisioned to integrate technologies like millimeter-wave (mm-wave) and massive multiple-input multiple-output antennas into the base stations. In this paper, we present comprehensive tutorial on technologies, requirements, architectures, challenges, and proffer potential solutions on means of achieving an efficient C-RAN optical fronthaul for the next-generation network such as the fifth generation network and beyond. A number of viable fronthauling technologies such as mm-wave and wireless fidelity are considered and this paper mainly focuses on optical technologies such as optical fiber and free-space optical. We also present feasible means of reducing the system complexity, cost, bandwidth requirement, and latency in the fronthaul. Furthermore, means of achieving the goal of green communication networks through reduction in the power consumption by the system are considered.

1.032-Tb/s CPRI-Equivalent Rate IF-Over-Fiber Transmission Using a Parallel IM/PM Transmitter for High-Capacity Mobile Fronthaul Links

We report high-capacity intermediate-frequency-over-fiber (IFoF) transmission using parallel intensity/phase modulators (IM/PM). Thanks to the parallel IM/PM transmitter, we can avoid all the null frequencies caused by dispersion-induced RF power fading. Compared to other techniques to avoid RF power fading, the parallel IM/PM transmitter offers a simple solution because it does not rely on any sophisticated digital signal processing. We transmitted 14 $\times$ 1.2-GHz orthogonal-frequency-division-multiplexed signals over a 20-km single-mode fiber using the parallel IM/PM transmitter and achieved a Common Public Radio Interface equivalent data rate of 1.032 Tb/s. Thus, an IFoF system with the parallel IM/PM transmitter shows great potential to be applied to high-capacity mobile fronthaul links in the upcoming fifth-generation mobile system and beyond.

Efficient polling cycle adaptive passive optical network for low-latency 5G fronthaul

Under the large and ubiquitous bandwidth requirements of the upcoming 5G era, new common public radio interface (CPRI) solutions (such as eCPRI) characterized by scale flexible, packet-based bandwidth reduction are receiving extensive attention. In this context, applying a passive optical network (PON) into the mobile fronthaul (MFH) has good prospects due to the features of low cost and wide coverage. However, the millisecond-level latency of the dynamic bandwidth allocation (DBA) procedure exceeds the harsh latency constraints of 5G MFH, and the uneven distribution of traffic caused by user mobility brings waste or congestion of MFH bandwidth resources. To meet the latency constraint and simultaneously improve bandwidth utilization, we propose a flexible time- and wavelength-division multiplexed passive optical network (TWDM-PON)-based MFH architecture, in which optical network units (ONUs) can be dynamically grouped to support efficient use of the bandwidth and adaptive polling cycle. Based on this architecture, the influence of the propagation time on latency is analyzed, and a joint bandwidth and latency constraint (JBL) grouping algorithm is proposed to obtain the appropriate ONU group for different network slice scenarios. The simulation results indicate that, in the case satisfying the latency constraint, the bandwidth utilization reaches 94% and above (maximum 99.5%). Simultaneously, within different slicing scenarios, the polling cycle can be increased by 98% on average (maximum 122%) or decreased by a maximum of 36μs. Compared with the non-grouping situation, the adaptive polling cycle lowers the performance requirement of the hardware device (such as a burst mode transmitter), expands the MFH coverage, and improves the channel utilization.

Digital mobile fronthaul employing differential pulse code modulation with suppressed quantization noise.

A differential pulse code modulation (DPCM) based digital mobile fronthaul architecture is proposed and experimentally demonstrated. By using a linear predictor in the DPCM encoding process, the quantization noise can be effectively suppressed and a prediction gain of 7~8 dB can be obtained. Experimental validation is carried out with a 20 km 15-Gbaud/λ 4-level pulse amplitude modulation (PAM4) intensity modulation and direct detection system. The results verify the feasibility of supporting 163, 122, 98, 81 20-MHz 4, 16, 64, 256 QAM based antenna-carrier (AxC) containers with only 3, 4, 5, 6 quantization bits at a sampling rate of 30.72MSa/s in LTE-A environment. Further increasing the number of quantization bits to 8 and 9, 1024 quadrature amplitude modulation (1024 QAM) and 4096 QAM transmission can be realized with error vector magnitude (EVM) lower than 1% and 0.5%, respectively. The supported number of AxCs in the proposed DPCM-based fronthaul is increased and the EVM is greatly reduced compared to the common public radio interface (CPRI) based fronthaul that uses pulse code modulation. Besides, the DPCM-based fronthaul is also experimentally demonstrated to support universal filtered multicarrier signal that is one candidate waveform for the 5th generation mobile systems.

Adaptive Digitization and Variable Channel Coding for Enhancement of Compressed Digital Mobile Fronthaul in PAM-4 Optical Links

The standardization and development of LTE-A and 5G introduced advanced wireless technologies including multiple-input multiple-output and carrier aggregation, which require multiple wireless carriers to be delivered to and from each remote radio head. The common public radio interface (CPRI) as the mainstream standard in mobile fronthaul (MFH) with on-off-keying-based optical links cannot fulfill the capacity and efficiency requirement. Instead, using compressed CPRI in a high-speed pulse-amplitude-modulation-4 (PAM-4) link is actively researched and demonstrated. In this paper, we propose and demonstrate adaptive digitization and channel coding based on these compression and capacity boosting technologies. Depending on the optical link condition, the digitization bits and channel coding rates can be adaptively and dynamically changed to achieve the lowest error vector magnitude (EVM) of wireless carriers. By separating digitization bits into high bits and low bits, the coding overhead can be different between groups, while still keeping the same bit rate per wireless carrier. Based on the existing digital MFH infrastructure, the proposed scheme can significantly improve the capacity and sensitivity in the PAM-4-based compressed digital MFH. Capacity gains from 30% to 68%, sensitivity improvement of 2–9 dB, and significant EVM improvements are demonstrated experimentally, comparing with other compressed CPRI MFH solutions.

Time- Versus Size-Based CPRI in Ethernet Encapsulation for Next Generation Reconfigurable Fronthaul

The utilization of Ethernet in the cellular terrestrial cloud radio access network (C-RAN) fronthaul will potentially improve C-RAN reconfigurability and cost efficiency in terms of both capital and operational expenditures. Moreover, Common Public Radio Interface (CPRI) line bit rate dynamic reconfiguration may spare further capital expenditures by avoiding peak traffic capacity allocation. A possible solution for introducing Ethernet in the fronthaul is the encapsulation of CPRI in Ethernet. This solution is now part of the radio over Ethernet interface specified by the IEEE P1914 working group. However, it must be assured that fronthaul strict requirements on delay and jitter are still met. In this study, the combined impact of encapsulating CPRI in Ethernet and of the dynamic CPRI line bit rate reconfiguration on delay and jitter of the communication between radio equipment and a radio equipment controller is evaluated. The evaluation is both analytical and based on the presynthesis emulation of dynamic CPRI line bit rate reconfiguration. Both a size-based and a time-based encapsulation are considered. Results show that dynamic CPRI line bit rate reconfiguration can be achieved within about 1 ms. However, if a size-based encapsulation of CPRI in Ethernet is utilized, dynamic CPRI line bit rate reconfiguration might cause delay variations (i.e., jitter) up to few microseconds. Thus, to avoid jitter upon CPRI line bit rate reconfiguration, time-based encapsulation is preferable.

Cascaded Phase Modulation for AMCC Superimposition Toward MFH Employing CPRI

A cascaded phase modulation is proposed for auxiliary management and control channel (AMCC) superimposition toward mobile fronthaul employing common public radio interface (CPRI). Different from other AMCC implementations, like the baseband intensity overmodulation scheme and the radio frequency pilot-tone scheme, the proposed cascaded phase overmodulation shows negligible impact on CPRI performance. Differential binary phase-shift keying is employed to encode the AMCC message, which is then overmodulated on CPRI via a cascaded phase modulator. Demodulation of AMCC message is achieved by a delay line interferometer, which converts the phase modulation to amplitude modulation. In the proof-of-concept demonstration, a 500-kb/s AMCC data are superimposed on a 10.1376-Gb/s nonreturn to zero signal with CPRI option 8, which is transmitted over 20-km standard-single-mode fiber. A -30-dBm receiver sensitivity is achieved for CPRI, with negligible penalty compared to the AMCC-free transmission of CPRI data.

A Novel and Efficient Vector Quantization Based CPRI Compression Algorithm

The future wireless network, such as the Centralized Radio Access Network (C-RAN), will need to deliver data rate about 100–1000 times the current fourth-generation (4G) technology. For the C-RAN-based network architecture, there is a pressing need for tremendous enhancement of the effective data rate of the common public radio interface (CPRI). Compression of CPRI data is one of the potential enhancements. In this paper, we introduce a vector quantization based compression algorithm for CPRI links, utilizing the Lloyd algorithm. Methods to vectorize the I/Q samples and enhanced initialization of the Lloyd algorithm for codebook training are investigated for improved performance. Multistage vector quantization and unequally protected multigroup quantization are considered to reduce codebook search complexity and codebook size. Simulation results show that our solution can achieve compression of four times for uplink and 4.5 times for downlink, within $2\%$ error vector magnitude (EVM) distortion. Remarkably, vector quantization codebook proves to be quite robust against data modulation mismatch, fading, signal-to-noise ratio (SNR), and Doppler spread.

Performance Evaluation of Ethernet Network for Mobile Fronthual Networks

<p>Increasing mobile data traffic due to the rise of both smartphones and tablets has led to high-capacity demand of mobile data network. To meet the ever-growing capacity demand and reduce the cost of mobile network components, Cloud Radio Access Network (C-RAN) has emerged as a promising solution. In such network, the mobile operator’s Remote Radio Head (RRH) and Base Band Unit (BBU) are often separated and the connection between them has very tight timing and latency requirements imposed by Common Public Radio Interface (CPRI) and 3rd Generation Partnership Project (3GPP). This fronthaul connection is not yet provided by packet based network. To employ packet-based network for C-RAN fronthaul, the carried fronthaul traffic are needed to achieve the requirements of fronthaul streams. For this reason, the aim of this paper is focused on investigating and evaluating the feasibility of Ethernet networks for mobile fronthaul. The fronthaul requirements used to evaluate and investigate this network are maximum End to End (E2E) latency, Packet Loss Ratio (PLR) and Packet Delay Variation (PDV). The simulated results and numerical analysis confirm that the PDV and PLR of High Priority (HP) traffic in Ethernet network meet the requirements of mobile fronthaul using CPRI. However, the PDV of HP traffic meets the fronthaul network when the number of nodes in the Ethernet network is at most four. For Ethernet network, the number of nodes in the network limits the maximum separation distance between BBU and RRH (link length); for increasing the number of nodes, the link length decreases. Consequently, Radio over Ethernet (RoE) traffic should receive the priority and Quality of Service (QoS) HP can provide. On the other hand, Low Priority (LP) classes are not sensitive to QoS metrics and should be used for transporting time insensitive applications and services.</p>

Key Technologies for Next-Generation Digital RoF Mobile Fronthaul With Statistical Data Compression and Multiband Modulation

As a counterpart of analog radio-over-fiber technology, digital radio-over-fiber (D-RoF) system, such as common public radio interface (CPRI), is a matured and robust solution to support RF signal delivery in traditional mobile fronthaul networks. In view of recent progresses in delta-sigma modulation, data compression, and advanced error correcting coding, the efficiency of D-RoF is significantly improved, which motivates researchers to re-evaluate the role of D-RoF in future mobile fronthaul networks to support 5G and beyond wireless communications. In this paper, we demonstrate two critical technologies to improve the transmission efficiency and flexibility of D-RoF systems. A fast-statistical-estimation based data compression algorithm is proposed to reduce the number of quantization digits in a D-RoF-based mobile fronthaul with low complexity and high quality. Combined with resampling and advanced modulation formats, data-transmission efficiency of a 25-Gbit/s D-RoF testbed is improved by around five times compared with uncompressed CPRI systems. On the other hand, we also experimentally demonstrate a point-to-multi-point (PTMP) D-RoF system with multiband modulation, which exhibits higher flexibility and better compatibility with multiple services and different radio-access technologies compared to existing schemes based on time interleaving. An experiment of 13.3-Gbit/s 4-band PTMP bidirectional D-RoF MFH is demonstrated. Combined with data compression, error free delivery of 6.4-Gbit/s 1024-QAM 5G-New-Radio-like signals is realized.

Delay analysis of mixed fronthaul and backhaul traffic under strict priority queueing discipline in a 5G packet transport network

AbstractVirtualization of the base station for the purpose of centralization is being actively studied and researched as an implementation option for 5G mobile networks. Proposed as Cloud radio access network, the technology is expected to facilitate easier operation and maintenance than regular radio access networks. However, the base stations traffic has stringent delay requirements. In this paper, we explore the possibility of multiplexing fronthaul traffic and traditional backhaul traffic as it traverses over the metropolitan network while keeping the average fronthaul queueing delay and jitter under control. We analyze and simulate the cases of a single fronthaul flow and multiple fronthaul flows arriving at the packet switch assuming strict priority for the fronthaul queue. We propose a fronthaul frame aggregation strategy to improve the packet transmission efficiency while keeping the average fronthaul queueing delay and jitter constant regardless of the percentage of fronthaul traffic. While the criteria for aggregation is different for the 2 cases, we show that the optimal number of basic frames to aggregate is between 3‐10 frames assuming the Common Public Radio Interface protocol.

Real-Time Demonstration of CPRI-Compatible Efficient Mobile Fronthaul Using FPGA

Mobile fronthaul is regarded as an important network segment for supporting cloud radio access network and massive multiple-input multiple-output in next-generation wireless networks. To achieve bandwidth-efficient mobile fronthaul (EMF), transmission of multiple wireless signals in a single wavelength channel has been demonstrated by digital signal processing (DSP)-assisted channel aggregation. The DSP-assisted channel aggregation has been realized via frequency-division multiple access. In addition, the control word (CW) information, as specified in the common public radio interface (CPRI) specification, has been transmitted with the wireless I/Q data synchronously. In this paper, we present CPRI-compatible efficient mobile fronthaul scheme for synchronous transmission of CWs and IQ data via equalized time-division multiple access (TDMA) We further experimentally demonstrate the real-time implementation of a TDMA-EMF transceiver based on field-programmable gate array. Long-term stable operation has been achieved showing the feasibility of using this EMF technology to meet the fronthaul demands of future 4.5 G and 5 G wireless systems in terms of bandwidth efficiency, power efficiency, and cost efficiency.

5G Fronthaul–Latency and Jitter Studies of CPRI Over Ethernet

Common Public Radio Interface (CPRI) is a successful industry cooperation defining the publicly available specification for the key internal interface of radio base stations between the radio equipment control (REC) and the radio equipment (RE) in the fronthaul of mobile networks. However, CPRI is expensive to deploy, consumes large bandwidth, and currently is statically configured. On the other hand, an Ethernet-based mobile fronthaul will be cost-efficient and more easily reconfigurable. Encapsulating CPRI over Ethernet (CoE) is an attractive solution, but stringent CPRI requirements such as delay and jitter are major challenges that need to be met to make CoE a reality. This study investigates whether CoE can meet delay and jitter requirements by performing FPGA-based Verilog experiments and simulations. Verilog experiments show that CoE encapsulation with fixed Ethernet frame size requires about tens of microseconds. Numerical experiments show that the proposed scheduling policy of CoE flows on Ethernet can reduce jitter when redundant Ethernet capacity is provided. The reduction in jitter can be as large as 1 μs, hence making Ethernet-based mobile fronthaul a credible technology.

Digital Mobile Fronthaul Based on Delta–Sigma Modulation for 32 LTE Carrier Aggregation and FBMC Signals

We first experimentally demonstrated a digital mobile fronthaul (MFH) architecture using delta-sigma modulation as the new digitization interface to replace the conventional common public radio interface (CPRI). Both one-bit and two-bit delta-sigma modulations were demonstrated, and the digitized signals were transmitted over on-off keying (OOK) or 4-level pulse-amplitude-modulation (PAM4) optical intensity modulation-direct detection (IM-DD) links. 32 LTE component carriers (CCs) were digitized and transmitted over a 25-km single-λ 10-Gbaud OOK/PAM4 link, so that 32 LTE carrier aggregation (CA) specified by 3GPP release 13 can be supported. Compared with the conventional digitization interface based on CPRI, the fronthaul capacity is increased by four times. Error vector magnitude (EVM) less than 5% or 2.1% for all LTE CCs was obtained using one-bit and two-bit delta-sigma modulators, so that high-order modulations (256QAM/1024QAM) can be supported. As a waveform-agnostic digitization interface, delta-sigma modulation can digitize not only 4G-LTE but also 5G waveforms, and its 5G compatibility was verified by filter-bank-multicarrier (FBMC) signals. The tolerance to bit error ratio (BER) of the proposed delta-sigma modulation-based digital MFH was evaluated, and no significant EVM degradation was observed for BER up to 3 χ 10−5. A comparison with analog MFH reveals that the proposed digital MFH based on delta-sigma modulation can offer improved resilience against noise and nonlinear impairments, and it increases the fronthaul capacity by four times compared with the conventional CPRI-based digital MFH without significant EVM penalty.

Bidirectional Fiber-Wireless Access Technology for 5G Mobile Spectral Aggregation and Cell Densification

Empowered by spectral aggregation and cell densification, future 5G mobile data networks pose a huge challenge to building next-generation mobile fronthaul systems with higher capacity, scalability, and energy efficiency. Under this circumstance, traditional solutions based on the Common Public Radio Interface or channel aggregation with digital signal processing (DSP) are not sufficient to support heterogeneous ubiquitous wireless access. In this paper, we demonstrate a point-to-multipoint bidirectional mobile fronthaul system. Wavelength division multiplexing plus frequency division multiplexing is applied to support independent asynchronous small cells. Intensity-modulation and direct-detection downlink (DL) as well as field-modulation and heterodyne-detection uplink (UL) are proposed. Combined with efficient virtual tone-based DSP for phase recovery and carrier-frequency-offset estimation, signal degradations from beating among incoherent asynchronous UL signals are mitigated. Proof-of-concept experiments are demonstrated where 20 and 16 80-MHz component carriers are transmitted over 25-km standard single-mode fiber for DL and UL, respectively. Less than 6% error vector magnitudes with 64-, 16-, and 4-ary quadrature amplitude modulation are obtained.