5G small-cell networks leveraging optical technologies with mm-wave massive MIMO and MT-MAC protocols
S. Papaioannou
1, G. Kalfas
1, C. Vagionas
1, C. Mitsolidou
1, P. Maniotis
1, A. Miliou
1and N. Pleros
11
Department of Informatics, Aristotle University of Thessaloniki, Greece
ABSTRACT
Analog optical fronthaul for 5G network architectures is currently being promoted as a bandwidth- and energy-efficient technology that can sustain the data-rate, latency and energy requirements of the emerging 5G era. This paper deals with a new optical fronthaul architecture that can effectively synergize optical transceiver, optical add/drop multiplexer and optical beamforming integrated photonics towards a DSP-assisted analog fronthaul for seamless and medium-transparent 5G small-cell networks. Its main application targets include dense and Hot-Spot Area networks, promoting the deployment of mmWave massive MIMO Remote Radio Heads (RRHs) that can offer wireless data-rates ranging from 25Gbps up to 400Gbps depending on the fronthaul technology employed. Small-cell access and resource allocation is ensured via a Medium-Transparent (MT-) MAC protocol that enables the transparent communication between the Central Office and the wireless end-users or the lamp-posts via roof-top-located V-band massive MIMO RRHs. The MT- MAC is analysed in detail with simulation and analytical theoretical results being in good agreement and confirming its credentials to satisfy 5G network latency requirements by guaranteeing latency values lower than 1 ms for small- to mid- load conditions. Its extension towards supporting optical beamforming capabilities and mmWave massive MIMO antennas is discussed, while its performance is analysed for different fiber fronthaul link lengths and different optical channel capacities. Finally, different physical layer network architectures supporting the MT-MAC scheme are presented and adapted to different 5G use case scenarios, starting from PON-overlaid fronthaul solutions and gradually moving through Spatial Division Multiplexing up to Wavelength Division Multiplexing transport as the user density increases.
Keywords: 5G networks, MT-MAC protocols, Analog-optical transmission, Optical Beamformers, ROADM
1. INTRODUCTION
The ever-increasing global demand for high bandwidth connectivity alongside the vast explosion of smart handheld devices has driven an immense data traffic growth, pushing Mobile Network Operators (MNOs) on a quest to satisfy the increasing demands. Today the 4G mobile networks already target to deliver 300Mb/s Downlink and 50 Mb/s Uplink connectivity per user in urban environments, and 25Mb/s and 50Mb/s respectively in densely crowded areas, as e.g.
concert halls, stadiums etc. [1]. These data-rates result in very high aggregate bandwidth demands, scaling up to the order of 1 Tb/s/km2 for the DL communication of 200-2500 devices in a km2 urban area, or even 7.5 Tb/s/ km2 for the UL for 30,000 devices in case of a stadium, pushing tremendous load on the shoulders of the 4G infrastructure, that is not expected to suffice in order to meet the future demands. This need has stimulated intense research for further radical technological advances and architectural innovations, with millimeter wave (mmWave) antennas and massive Multiple Input and Output (mMIMO) systems being heralded as the key enabling technologies.
To this end, Cloud-RAN (C-RAN) architecture has been suggested as the possible means to reduce cost and complexity by decoupling the Remote Radio Head (RRH) of the antenna from the Baseband Unit (BBU). The C-RAN architecture pushes all higher-layer functionalities at the BBU module in a centralized pool that is shared between multiple Remote Radio Head (RRH) units, which can then be released from complex network functionalities, facilitating reduced costs. In order to handle the interfacing between the BBU Box and the distant RRH, the most widely adopted specification is the Common Public Radio Interface (CPRI). However, CPRI requires transmitting the digitized In-phase (I) and Quadrature (Q) waveform of a wireless signal in a binary sequence in the time domain using On-Off keying modulation format, turning it into a bandwidth inefficient transmission standard. Indicatively, utilizing a single 100MHz LTE signal sampled with 16bits/sample resolution and employing two antennas per cell and 30% compression would allocate 20.644 Gb/s CPRI Fronthaul (FH) traffic [2], while considering an 8x8 MIMO system with three directional sector antennas would necessitate 147.5 Gb/s optical transmission [3]. Furthermore, when extending to higher, massive MIMO systems, the FH
transmission capacities would scale to Tb/s range per cell, questioning the adequacy of the current FH infrastructure to satisfy the requirements for the Next Generation Fronthaul Interface (NGFI). The critical key issues are:
1) NGFI will need to deploy multi-Gb/s mmWave wireless links to provide FH connectivity in areas where fiber installation is not reachable.
2) mmWave antennas will need to exploit low hardware complexity, reduced cost-expenses and energy-efficiency to facilitate feasible Capital (CAPEX) and Operational Expenditure (OPEX).
3) the current digital CPRI standard is rather bandwidth-hungry, requiring dense oversampling of the I/Q-wireless traffic to transport digital data streams towards the BBU, yielding intolerable linerates for the transceivers.
On the way towards addressing the above challenges, a complete analog mobile FH infrastructure is proposed in this communication, leveraging a Physical Layer (PHY) split (split-PHY), pushing all higher functionalities and complex expensive equipment at the centralized unit, while the RRH units include only the analog optical transceiver, mMIMO RF-IC antennas with Optical Beamformer.
2. CONVERGED FIWI NETWORK ARCHITECTURE
Towards the forthcoming 5G era, we have identified two prevailing network scenarios, where increased densification will certainly be required, i.e. a typical metropolitan/urban area network and a hot-spot scenario. Each network scenario addresses different operational needs and characteristics, as illustrated in Figure 1 and discussed below. The envisioned architecture assumes centralized BBU boxes connected through fiber with the RRH units, which are in turn equipped with mmWave mMIMO antennas, while the fiber-optical transceivers utilize advanced modulation formats and channel aggregation for enhanced spectral efficiency.
Figure 1. Proposed analog optical-wireless FH in Urban and hot-spot areas.
In the urban network scenario and in compliance with the incubating 25Gb/s and 4x25Gb/s PON infrastructures [4], the BBU Box is capable of handling optical wireless signals at aggregate traffic up to 25Gb/s utilizing 10GHz optics (Modulator/Photodiode). The RRH is equipped both with mmWave and sub-6GHz antennas for simultaneous connectivity with the Pico Cell (PC) and Small Cell (SC) access antennas respectively, which in turn feature two network interfaces, one for the static mmWave link back to the RRH unit and one for providing network access to end- point wireless users. This configuration allows establishing a mmWave FH link from the RRH to the access antennas, replacing the respective fiber-based FH implementation and overcoming the need for large brown-field fiber installation.
Scalability of the aggregated optical communication capacities can then be achieved either spatially by multiplexing
multiple FH links over a bundle of fibers each with a dedicated wavelength or optically through Wavelength Division Multiplexed (WDM) of separate light-streams in a single fiber.
Especially in the case of hot-spot scenarios with thousands of users highly confined in a confined geographical region, the RRH units and their antennas can be directed towards the wireless-terminals to speed up the access processes. In this case, the multiplexed fiber FH comprises a fiber bus architecture, starting from the BBU box and connecting a cascade of multiple RRH units that are scattered around the hot-spot area. The MIMO antennas can then direct the mmWave beam towards several users, while the increased number of RRH may eventually exceed the number of available wavelengths in the network. In this case, the RRH unit will need to be equipped with a Reconfigurable Add/Drop Multiplexer (ROADM), that will facilitate dynamic wavelength reconfiguration of the fiber bus architecture, so as to effectively handle the statistical and time varying distribution of the wirelessly connected users, by introducing wavelength selectivity and re-configurability scheme. This in turns requires the development of a suitable Medium Transparent MAC (MT-MAC) protocol to manage and allocate the shared network resources, i.e. wavelength, time and wireless frequency.
The above architecture utilizes the proposed analog RoF (aRoF) split-PHY architecture, as displayed in Figure 2(a), combining all the MAC-, DSP- and Software Defined Network (SDN)-related functionalities in a centralized BBU Box, leaving only the minimum necessary physical layer tasks and components at the RRH. The BBU Box would then utilize Digital-to-Analog Converters (DAC) inserted before the optical transmitter for the downlink communication and an Analog-to-Digital (ADC) converter after its photoreceiver for the uplink communication, undertaking the signal-format translation from the analog format to a digital data signal for backhauling to the rest of the metro network. In addition, the BBU-Box will also have to perform the Digital Signal Processing (DSP) functionalities, including channel estimation and signal predistortion, to mitigate the signal impairments introduced by both the wireless and fiber propagation. The proposed configuration can then enable squeezing the data around a subcarrier Intermediate Frequency (IF) for wireless transmission in a spectrally efficient manner, as recently introduced in [5], requiring only RF-to-IF and IF-to-RF up- and down-conversions, while the use of a few-GHz IF can overcome the dispersion-related signal impairments associated with mmWave RoF transmission [6].
Figure 2. (a) Split-PHY FH architecture (b) Schematic representation of the DSP-assisted layouts of a RoF FH architecture equipped with RF- beamformer and mmWave feeder network (upper inset) and of the proposed analog RoF FH architecture based on an optical beamformer network (lower inset).
One possible way to implement the above network architectures is to combine a mmWave mMIMO system with RF- beamformer and mmWave feeder network placed at the RRH antenna, as schematically illustrated in Figure 2(a) and thoroughly described in [5]. However, at the RRH units, the complex signal gets opto-electronically converted and then up-converted back to the RF before being fed to the mmWave RF-chain and the electronic beamformer, MmWave beamforming can be performed by analog, digital or hybrid schemes with DSP signal processing at the RRH unit, increasing the complexity-, cost- and energy- requirements [5]. Another alternative approach that can jointly address all the aforementioned FH challenges comes from the optical-beamforming domain, where photonic integrated chips can introduce the necessary time-delay at different signal constituents of the RoF signal directly in the optical domain, as schematically illustrated at the bottom inset of Figure 2(b), a solution that pushes optics further closer to the antennas.
3. KEY OPTICAL COMPONENTS FOR ANALOG ROF FRONTHAUL
Towards enabling the proposed aRoF FH, a set of main optical devices is needed: a) linear optical modulators for transferring the mmWave signals on an optical carrier, b) low-loss optical beamformers for seamlessly interfacing mmWave mMIMO antennas with the optical FH, c) ROADMs for adding WDM capabilities in the network.
The proposed aRoF approach exploits advanced modulation formats imprinted on microwave photonic links at Intermediate Frequencies (IF) for high data transferring between the BBU-Box and RRH modules. This brings to the foreground the increased demands on the linearity of optical transceivers. Extensive research studies have been carried out on modulator linearity, especially in Mach-Zehnder interferometric (MZI) arrangements, have so far focused on limiting the non-linear signal distortion for improved performance and thereby leading to various configurations, e.g.
Dual Signal, Cascaded or Resonator-Assisted MZI modulators [7], that may indeed achieve to minimize the nonlinearities, at the cost of increased layout complexity. Alternative approaches may utilize simpler solution, e.g.
driving a MZI modulator at the Quadrature point with small driving voltage amplitude variation in order to remain in the linearity region, or even deploying Externally Modulated Lasers (EMLs) combined with high-linear Electro-Absorption Modulators (EAM), in order to imprint mmWave wireless signals on optical IF sub-carriers in a cost-effective way.
The introduction of mmWave and mMIMO technologies needs to be accompanied by the development of beamforming networks capable of feeding large MIMO antennas, which if based on mmWave electronics would cause increased power consumption, complexity and cost. On the contrary, optical beamformers can be integrated on photonic chips for enhanced performance and minimized footprint, exploiting either optical phase shifters or more appropriately True Time Delay elements interleaved for instance between different cascade stages of 1x2 splitters in a tree-like architectures or arranged in two-dimensional (2D) phased array matrixes [8],[9]. Although optical phase shifters provide simpler implementation, the True Time Delays (TTDs) are preferred since they support squint-free (frequency-independent) beamsteering that is essential for broadband applications, which the actual control mechanism may vary from optical ring resonators (ORRs) [8], active unit cells [9] or even plasmonic phase modulators [10].
Optical Add/Drop Multiplexers (OADMs) have been widely employed in optical long-haul and metro networks as the main units for performing wavelength selectivity and reconfigurability of the network [11]-[13]. Considering that typical OADMs comprise a wavelength demultiplexer, a wavelength-selective unit and a wavelength multiplexer, these devices are capable of supporting Add and Drop functionalities simply by inserting new wavelength channel(s) in an existing WDM-based channel stream or removing a number of wavelength channels from the WDM stream for rerouting at different output ports. The OADMs are categorized into Reconfigurable (ROADM) and Fixed devices [11] depending on their ability to provide reconfigurability through their wavelength-selective unit or not, respectively, with the first type being preferred due to the network flexibility and bandwidth re-allocation they offer. ROADMs can exploit various switching mechanisms, such as Thermo-Optic (TO) switches in PLCs, Liquid Crystals on Silicon (LCoS) and Micro- Electro-Mechanical Systems (MEMS) [11]. By employing silicon photonics as an alternative photonic integrated ROADM technology, significant benefits can be achieved, such as CMOS-compatibility, reduced cost, energy and footprint requirements as well as improved reconfiguration times and controllability via Software Defined processes [13], which can be applied to the Hot-Spot scenario, so as to enable dynamic lightpath re-allocation that allows to add or remove wavelength channels, and consequently bandwidth, with reduced CAPEX and OPEX [12].
4. MAC LAYER RESOURCE ALLOCATION FOR CONVERGED 5G FH
The new optical FH architecture will also require advances in access control mechanisms that will control the optical/wireless resource allocation and introduce packetization for interfacing the access and backhaul equipment.
Towards a direct negotiation of the wavelength, frequency and time resources, the proposed FH can rely on medium- transparency, such as the MT-MAC protocol described in [14],[15]. Regarding the Urban area, the RRH antennas are constantly steered towards the PCs. The MT-MAC protocol creates transmission windows for the UL and DL communication between the BBU Box and the PCs. Synchronization is maintained by frequent exchange of POLL type packets. More specifically, the operation is initiated by the BBU-Box, which transmits Schedule POLLs (S-POLL) to each PC. After receiving the responses of the PC, including the buffer status and requests for certain levels of Quality of Service, the BBU-Box will execute the transmission scheduling algorithm, which involves two steps. Firstly, the BBU
transmits a control packet indicating the transmission schedule, the directionality of the antenna (to configure the control of the beamformer) for the forthcoming time-windows and the intermediate guard-spaces. Then the BBU sends a Data POLL (D-POLL) towards the PCs requesting the UL/DL data traffic demands, while also carrying the transmission window scheduling for the certain PC, the power levels, the sleep-times and the channel state. The PCs, after receiving the D-POLL, start transmitting their buffered UL-data and/or wait for DL data, and then piggyback the new status of their buffers, so as to assist the next scheduling. The wireless DL/UL data packets are encapsulated in payload agnostic MT-MAC frames with Ethernet support, while at the fiber part of the FH consecutive MT-MAC frames form a large Superframe for scheduled fiber-transmission towards/from the BBU-Box.
Regarding the hot-spot scenario, the MT-MAC protocol undertakes to control also the access-part of the FH network, i.e.
all the communication between the BBU-Box and the final end-point users of a stadium, as well as to control the wavelength resources, i.e. the number of wavelengths W available for fiber transmission, and the number of RRHs R that surround the hot-spot and, hence their wavelength-to-RRHs ratio denoted as WR. As the ROADM of the RRH is responsible to carry out the wavelength selectivity, a control packet needs to be transmitted by the BBU-Box towards the ROADM’s controller indicating its configuration over the next time-span. Afterwards, the MT-MAC will have to perform a full sector beam over the access network by transmitting S-POLL packet to the end-point wireless users. The users in-turn respond after a random delay, termed as back-off timer, by sending their buffers and requests for levels of Quality-of-Service. The BBU-Box, after receiving the users’ responses, reconstructs the time-scheduling for the next superframe, and assigns a permanent ID to each end-point user, so as to prioritizing their responses to the future rounds of S-POLL packets. When the random back-off timers of two or more users are equal, the users will respond at the same time to the S-POLL packets, causing their responses to collide, thus necessitating an additional polling round of S-POLL packets. When all users have been identified correctly, the MT-MAC protocol will resume to its normal data-packet scheduling operation following the same principle of operation as the one applied for the Urban area.
The performance of the proposed MT-MAC has been validated in terms of throughput and latency values for 40 RRH units connected through a 1km-long fiber to a single BBU-Box as expected for 5G applications in [15]. The MT-MAC protocol has been shown to support Gb/s transmission at packet switched networks with less than 1 ms latencies under various network conditions of a WR ratio of 30%, 50%, 80% and 100% 40 RRHs connected to the BBU-Box. The key results are depicted in Figure 3(a), describing the obtained delay for various network loads. The graph reveals that for small network loads the obtained delay is typically less than 1 ms, regardless of the WR ratio, while stable performance is maintained as the normalized load keeps increasing until a certain threshold. At that threshold value, when the normalized network load reaches the network capacity (defined by the WR ratio), the obtained delay values increase rapidly, revealing that the packets remain buffered for a longer time. Finally, the latency has also been evaluated for increasing fiber-lengths and distances between the BBU-Box and the end-point users, spanning from a few hundred of meters up to 10 km, and under various normalized workload conditions ranging from 10% up to 95%, as illustrated in Figure 3(b). As the simulation measurements reveal, for small and medium network loads up to 50%, indicated with a dark-blue plot-line, the delays remain always below the 0.5 ms value, regardless of the fiber-transmission length. For higher network loads, the obtained delay is similar for all network loads, i.e. sub-ms latency values are obtained only for up to a certain distance, which is approximately 8 km, 4.5 km and 2 km for a normalized load of 60%, 70% and 80%
respectively, revealing the impact of accumulating time-propagation delays. As such, for a typical fiber FH-network of less than 2 km that is widely deployed in metropolitan city networks, the MT-MAC still achieves the sub-1ms latency.
Figure 3. (a) Delay versus normalized load for 40 RRHs and 1 BBU Box when the Wavelength to RRH (WR) ratio is 100%
(40 λs), 80% (32 λs), 50% (20 λs) and 30% (12 λs), (b) Delay versus fiber length 100% WR and 10-95% normalized load.
5. CONCLUSIONS
This communication describes a FH architecture relying on cost-effective linear analog-optical transceivers, integrated optical beamformers and mmWave mMIMO antennas for spectrally efficient communication between the BBU-Box and the RRH-unit, being able to carry multiple parallel frequency aggregated native-wireless data-streams. Moreover, the introduction of ROADMs at the RRH units allows for enhanced wavelength selectivity in the FH network, while the complete network operation and medium access control can be monitored by means of a centralized MT-MAC protocol.
Initial measurements and evaluation of the overall performance have verified multi-Gb/s data transmission over a packet switched network with latency values within the 1ms value for typical fiber-lengths of a few km.
ACKNOWLEDGEMENT
This work is supported by the H2020 5GPPP Phase II project 5G-PHOS, Contract Number 761989
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