Transmission Solutions and Architectures for Heterogeneous Networks Built as C-RANs

2012 7th International ICST Conference on Communications and Networking in China (CHINACOM) Transmission Solutions and Architectures for Heterogeneous Networks Built as C-RANs Zere Ghebretensaé, Kim Laraqui and Stefan Dahlfort* Ericsson AB Stockholm, Sweden (*San Jose, USA) Jingjing Chen, Yinggang Li and Jonas Hansryd Ericsson AB Mölndal, Sweden Filippo Ponzini, Luca Giorgi and Stefano Stracca Ericsson Telecomunicazioni SPA Pisa, Italy Abstract A novel end-to-end transport network solution is proposed to meet the operational and technical challenges of heterogeneous networks built as C-RANs with centralized base band processing and CPRI links. The flexibility of the proposed architecture to support distributed architectures and Ethernet links is also described and the results of a C-RAN proof of concept demonstration are discussed. Substantiated by the evolution of key optical technologies, including a novel WDM- PON based solution, we conclude that backhaul and metro network strategies need to flexibly support both centralized and distributed radio baseband solutions, as well as being multiservice capable. Additionally, as a complement to fiber, we propose use of the 70/80 GHz frequency band (E-band) to provide high performance microwave links for both centralized and distributed architectures. Keywords- Centralized baseband, WDM-PON, 70/80GHz E- band microwave, CPRI, C-RAN, Heterogenous network I. INTRODUCTION To meet the ever demanding expectations of mobile broadband users, improved and denser macro cells will be complemented by an increasing number of small cell sites. In these heterogeneous networks where small cell transmission solutions become more opportunistic, it is important to consider how these additional transmission requirements will influence the overall radio access network performance within the context of a centralized base band architecture. From a backhaul perspective, the addition of small cell sites puts increasing demands on the backhaul network, which need to be evaluated when considering an evolution to C-RAN architectures [1]. In this paper we propose a novel end-to-end transport network solution that can meet both the operational and technical challenges of a heterogeneous network built as a C- RAN with centralized base band processing and CPRI links, whilst at the same time supporting Ethernet links and distributed architectures. The proposed network solution circumvents service specific transport conundrums and we show how the scalability and inherent multiservice capability supports a future proof solution. For urban areas, where the Andrew R. Pratt Regional Portfolio Management Ericsson (China) Communications Co. Ltd Beijing, China availability of dedicated fiber is limited, we also propose an innovative WDM-PON based solution, with colorless transceivers supporting virtual point to point CPRI links between the baseband and radio units. Substantiated by the evolution of key optical components, we conclude that backhaul and metro network strategies, need to flexibly support both centralized and distributed radio baseband solutions, as well as being multiservice capable. Additionally, as a complement to fiber, we propose use of the 70/80 GHz frequency band (E-band) to provide high performance microwave links for both centralized and distributed architectures. II. HETEROGENEOUS NETWORKS AND LINK TECHNOLOGIES Within heterogeneous networks small cells are deployed within the proximity of macro cells to enhance the coverage, capacity and overall user experience. Within specific locations, assuming that 4-6 small cells will be associated with each macro cell, the total number of cells and the resulting volume of backhaul traffic will increase dramatically. Finding the appropriate backhaul solution in terms of capacity and cost will in many cases be a challenge for operators as there is no single backhaul link technology that can fit all situations. In order to understand the type of backhaul required for small cells, one has to consider the services that will be supported by the small cells. Small cell deployment will be scenario specific, which means that in some cases the operators will deploy small cells for best effort data offload for macro cells, while in other cases small cells are deployed for full blown QoS service support. When deployed for best effort data offload, the capacity, delay and delay variation requirements of the backhaul are relaxed allowing the deployment of packet switched backhaul solutions. In this case, operators can chose, from a plethora of link technologies, including P2P and P2MP fiber links, copper based VDSL2 links and a wide rage of wireless link technologies including WLAN, sub-6 GHz P2MP links, P2P microwave links, 60 GHz links and 70/80 GHz E- band links. 748 978-1-4673-2699-5/12/$31.00 2012 IEEE

When small cells are deployed to support full service, then they have to meet the same requirements as the macro cells. The backhaul requirements however, also depend on whether the cells are realized using distributed or centralized Radio Base Stations (RBSs). In the case of distributed RBSs with Ethernet interfaces, operators can deploy packet switched backhaul solutions, again supported by P2P and P2MP fiber links, VDSL2 links on bonded copper pairs and P2P wireless link technologies including microwave links, 60 GHz and 70/80 GHz E-band links. In both of the cases considered above, the backhaul of small cells can be easily supported by connecting the first leg of the small cells to the macro site, which functions as the hub or first aggregation point for the small cells. In cases where the small cells are realized using centralized RBSs however, operators need to deploy C-RAN solutions supported by high capacity and low delay links. In this case P2P fiber, P2MP WDM-PON and 70/80 GHz E-band links are the only realistic link technologies that can be used to connect the Remote Radio Units (RRUs) to centralized Base Band Units (BBUs). III. CONVERGED ACCESS METRO NETWORK ARCHITECTURE Today most operators have already started migration of their TDM networks to a converged packet switched Carrier Ethernet network using Ethernet or MPLS technologies. Since the majority of the traffic over the network is data traffic it makes sense to move to a technology that is cost optimized for packet transport, while still supporting TDM services, e.g., using pseudowire and circuit emulation services. The rapid modernization of current radio access networks, due to the introduction of Multi Standard Radios (MSRs), capable of supporting GSM, WCDMA and LTE within the same radio band with IP/ETH transport interfaces, has also contributed to the packet backhaul migration. However, in the longer term, even these packet switched network will not be able to cope with the increasing traffic demand placed on the backhaul network by new services and novel architectural solutions such as C-RANs, in a cost efficient way. This is because the cost of transport interfaces, as well as the switch technology of packet switched networks increases with bit rate, whilst the statistical multiplexing gain decreases, since most of the links will be carrying traffic close to their link capacity. Therefore it makes sense to look at alternative architectural solutions whose switching capacity and cost does not depend on the supported bit rates. The approach proposed in this paper is aligned with RFC3439, which recommends architectural and philosophical guidelines that minimize the network complexity by minimizing the number of layers, by locating service related functionality close to the edge of the network. Motivated by the recent developments in DWDM technology and the demand for energy efficient network solutions, we propose an all-optical, converged access metro transport network architecture as shown in Figure 1, which has the following key attributes: Low cost DWDM(-PON) colorless optics, for all equipment and locations, with possibilities for wavelength reuse in the uplink and downlink and supporting, narrow channel spacing across multiple bands. Mini (access) ROADM for high flexibility and ease of planning, enabling dynamic add/drop of wavelengths and supporting highly integrated photonics. Wavelength Selective Switch (WSS), enabled by advances in Liquid Crystal on Silicon (LCoS) technology, allowing for cost effective channel spacing with 100 s of wavelengths per fiber strand, whilst keeping energy consumption to a minimum i.e. in the order of 10 s of Watts for several Tbps capacity. Low cost and high density Photonic Integrated Circuit arrays for dense channels, with appropriate form factors in support of standard interfaces for low cost, minimum footprint and low power consumption. Control and management planes to support zero-touch connectivity handling of both client and server parts of the different service layer types connected. In this domain, we also include service fulfillment and assurance, e.g. performance monitoring solutions which are particularly challenging in the optical domain. The combination of the above features facilitates support of some of the key network requirements such as load balancing, auto configuration and path redundancy, without reverting to complex layered solutions. These features will be service agnostic, which bodes well for service diverse networks, both in term of supporting different service classes (i.e. remote radio heads, pico-rbss, enterprise switches and residential access), as well as in supporting the migration from packet based backhaul networks to C-RANs with centralized basebands. Figure 1. Proposed all-optical converged access metro network IV. ENABLING TRANSPORT TECHNOLOGIES It is well understood that the CPRI interface specification in C-RAN architectures imposes high speed, low delay link requirements on the P2P fiber, WDM-PON and wireless E- band link technologies considered. In the following we present a brief introduction to the CPRI specification and focus on the main features required of WDM-PON and E-band link technologies in the context of C-RAN deployment. A. CPRI Interface for Main-Remote Connection In centralized network architectures, a radio unit with active RF transceiver is remotely installed (the remote unit) and connected to a centrally located baseband processing unit (the main unit). Optical fiber is typically used for the physical mainremote connection and CPRI is the common interface protocol 749

that provides the connection between the base band unit and remote radios. The CPRI standard specifies the line bit rate at n x 614.4 Mbps, where n = 1, 2, 4, 5, 8, 10 and 16 [2]. The BER requirement for the user data (I/Q data) is set to 10-12, which is the same as typical commercial link deployments today. Delay calibration, to measure delay due to the transmission media is also required, as well as a requirement on maximum round-trip delay to ensure the quality of service (QoS). The delay specification for the CPRI interface is 200 μs, (400 us roundtrip-time), so delay is not normally an issue for CPRI microwave transmission. The CPRI standard however does not define a mandatory physical layer protocol and any transmission media can be used as long as it meets the stated specifications [2]. B. WDM-PON WDM-PON is a promising access technology that aims to bring the benefits of WDM technology in terms of high capacity, protocol transparency and end to end connectivity closer to the end user. For C-RAN deployments, the ability of WDM-PON to transport CPRI traffic is appealing due to the following advantages: High bit-rate: each WDM-PON wavelength can support up to 9830.4 Gbps the highest bit rate currently defined for the CPRI interface [2]. Stream segregation and protocol transparency: simultaneous transport of CPRI streams at different bit-rate and also data steams from different data protocols. High fiber spectral efficiency and low cost: The ability to re-use the same wavelength for UL and DL traffic in the same fiber and efficient fiber deployment. WDM-PON supports logical P2P topologies, but when deploying C-RANs the conventional physical topology, i.e., the tree topology, can be easily extended to ring topologies, allowing operators to re-use their existing infrastructure and to implement ring protection schemes, whilst providing seamlessly convergence of the access and metro networks. One of the attractive features of WDM-PON is that the ONUs can be made colorless, which means that operators do not need to perform specific wavelength planning; rather they simply plug SFPs into the optical network. Figure 2. Introduction of a transparent WDM Physical Layer (WDM PHY) to carry MU/RRU links over WDM Compared to conventional P2P links, WDM-PON transport is realized by introducing an additional sub-layer, referred to here as the WDM PHY sub-layer, responsible for any wavelength conversion within the network, as shown in Figure 2. In this way, the ONU remains colorless and there is no need for any changes in the baseband units and remote radio units in C-RAN deployments. Note however, this additional layer has to be thin enough not to introduce any additional delays (i.e. no electrical buffering). C. E-Band Microwave Thanks to the evolution of microwave technology and the availability of new frequency bands, e.g. 70/80 GHz (E-band), it is now possible to deliver fiber-like data rates by microwave links with high performance [3]. Where deploying fibers is not the best choice, high capacity microwave links provide a fast and flexible complimentary solution. 1) E-Band and its Licensing The so called E-band allocates frequencies from 71 GHz to 76 GHz and 81GHz to 86 GHz, which is by far the widest channel band ever allocated for fixed wireless services and was first open by FCC in 2003 [5]. The true benefit of using such a high frequency band is the availability of wide channel bandwidth, thus enabling high data rate transmission that is otherwise difficult to achieve in conventional microwave bands (6 to 42 GHz). The E-band has been available for commercial use in many of the major global markets and frequency allocation is consistent worldwide where the band is permitted but channelization varies. In the United States, the FCC has divided each 5 GHz band into four 1.25 GHz channels. The operator may use the channel as they wish and may also aggregate up to four channels, effectively using up the full 2x5 GHz bandwidth. In Europe, ETSI [5] has taken a more spectral efficient approach, splitting each band into nineteen 250 MHz channels with two 125 MHz guard bands at the top and bottom [6]. It is also allowed to aggregate up to eight channels, reaching 2 GHz effective channel bandwidth. Unlike conventional microwave bands that require point to point licensing and the unlicensed 60 GHz band, a novel lightlicensing approach for the 70/80 GHz band was introduced by FCC in 2005 to encourage cheap and fast link registration over the internet [5]. The fee for registering a link is $75 per link per year in the United States, and an issued link license is valid for 10 years. The United Kingdom adopts a similar licensing process. 2) System Characteristics and Architecture The E-band channel is sufficiently wide to transmit high data rates using a simple modulation method. For instance, Gigabit per second (Gbps) rates can be reached using a binary modulation scheme, typically OOK or BPSK with low spectral efficiency < 1 bit/s/hz [7]. However, in order to utilize the available spectra in the most efficient way, it is essential to move to higher spectral efficiencies. This is especially important for e.g. dense urban environments where a high density of links may be deployed. With the upcoming revision of ETSI EN 302 217-2-2 [8] spectral efficiency classes up to 6 bit/s/hz (64QAM) are defined. The 70/80 GHz radios are ideal for applications where high data rates are required rather than a very long hop length, since link distances are typically limited below 3 km with telecom grade availability. Although the 70/80 GHz frequency is located in an atmospheric absorption 750

window, rain attenuation only tends to limits the transmission distance at the high frequencies, where the size of large rain drops is comparable to the signal wavelength. The higher the rain intensity, the larger the drop size is. However, fog, cloud, sand, and small particles are much less of a concern. In most of the world, 70/80 GHz links are practically limited to 1 to 2 km with 99.999% availability, corresponding to a maximum 5 minute outage per year. If link availability is relaxed down to 99.9% (i.e. link down for approximately 8 hours per year), a distance of 5 to 7 km can be reached [9]. Figure 3. 70/80 GHz radio mounted on test site. Today, commercial E-band radio equipment is available that can provide Gbps capacity. Figure 3 shows a 70/80 GHz radio, from an outdoor test set-up in Mölndal, Sweden. The radio link has been running since 2009 for long term measurement on path attenuation associated with weather conditions [9]. The radio is an all-outdoor unit, where all the interfaces, radio modem and RF front-end circuitry are enclosed in a single box, as indicated in Figure 4. For GbE or higher data rate traffic, the interface to a 70/80 GHz radio is typically optical fiber based. advantage of being a very low delay and flexible wireless solution offering very high capacity with little or no restriction on what service or data it carries, so called protocol independent, which is desirable for interworking in the converged-access networks described in section-iii. 3) E-band Applications The most typical application of point-to-point microwave links today is in mobile backhaul networks connecting radio base stations and the switching sites. Roughly 50% of such connectivity is currently provided by microwave [9]. Driven by the evolution of radio access technologies, such as LTE and beyond, transport capacity for backhaul networks has to increase accordingly, where Gbps links for traffic aggregation is one application [11]. High capacity 70/80 GHz radios will play a significant role to strengthen microwave backhaul in next generation networks. For a rapid deployment of heterogeneous networks the 70/80 GHz band offers an ideal choice for high data rate over shorter distance. In addition, the use of 70/80 GHz links for fiber extension in the last mile is an economical solution compared to fiber trenching. Similarly, 70/80 GHz radios can also be used as back-up for short fiber links to enhance network availability. V. C-RAN LINK LEVEL PROOF OF CONCEPT A. WDM-PON Link Proof of Concept To verify the applicability of WDM-PON links to support the CPRI interface in an C-RAN architecture, we have built a test bed connecting 3 RRUs to a BBU using variable distances of fiber as shown in figure 5. Figure 4. Block diagram of an all outdoor 70/80 GHz radio architecture. Currently, most commercial 70/80 GHz radios use simple modulations for easy implementation and relaxed requirements on hardware. However, there is a trade-off between modulation, system performance and cost. A system with higher modulation offers a better spectral efficiency at a cost of increased system complexity and potentially a higher cost for design and production. Moreover, as a result of a high order modulation, receiver sensitivity is reduced because higher signal-to-noise ratio is required. The transmitter output power is also reduced to maintain a linear operation of the power amplifier. Eventually, system gain is degraded, leading to a shorter link distance. A simple modulation based system on the other hand can be implemented using mainly analog circuitry without digital processing devices (FPGA or ASIC) and mixed signal devices (A-D and D-A converters) [7]. This has the Figure 5. LTE C-RAN over WDM with only one link shown The WDM-PON used in the test-bed supports up to 10 Gbps, however for the proof of concept results reported here, only 2.5 Gbps CPRI links were used. The CPRI links connected the MU and RRU to a handheld LTE device (operating up to 60Mbit/s), allowing the bandwidth and backhaul capacity generated in the cell to be tested under different working conditions. In the test bed we could change not only the link distance but also the wavelength of the respective up and down link channels used for the CPRI links. This allowed us to explicitly test the robustness of the links to the imbalance between the up and down link delay. The results from this test are summarized in Table 1. As shown in the table, using adjacent wavelengths for uplink and downlink we are able to minimize the delay imbalance and obtain maximum performance, in terms of LTE cell throughput, up to 20 km. Choosing the maximum 751

wavelength separation between the up and down link channels i.e. using the two edges of the available C-Band bandwidth, we intentionally explored the worst case condition in terms of delay imbalance. The test results show that we are still able to reach the maximum cell throughput for up to 10 km of standard fiber. The overall round trip delay introduced by the WDMPON equipment was also measured to be about 0.2 μs, with an equipment contribution of about 50 ns / termination. Walk-off [nm] [100GHz Spacing] Feeder fiber (G652 Type) [km] 10 20 30 50% 31 39 10 31 39 20 33% TABLE I. Wavelengths Distance Cell Throughput [percentage] VI. C-RAN PROOF OF CONCEPT Using the WDM-PON and the E-band links described above, a C-RAN proof of concept was set-up and demonstrated at Ericsson s facility in Beijing. Three LTE RRUs were connected using WDM-PON links, while the forth RRU was connected using an E-band radio link, as shown in Figure 7. All the links support 2.5 Gbps CPRI interfaces. For the WDMPON, wavelength auto-configuration of the ONTs was achieved by using tunable transceivers and a configuration algorithm that operates at the physical layer. The wavelength assignment algorithm is based on a double wavelength sweep at each end of the link, combined with a hand-shaking between the two ONTs at the physical layer. This allows the physical link to self organize independent of protocol and without any additional delay, framing or overhead. LTE CELL THROUGHPUT PERFORMANCE VS CHANNEL SPACING AND DISTANCE B. Microwave Link Proof of Concept A wireless LOS main-remote link was demonstrated for the first time in an LTE trial in Beijing on December 19, 2011. The CRPI interface was carried by a high capacity microwave link between a main radio unit and a LTE remote radio unit. Microwave main-remote solutions offer great flexibility allowing operators to deploy remote radios where most needed, which is considered important for instances of small cell deployment in heterogeneous networks. Figure 7. C-RAN transport PoC demo set-up, at Ericsson Beijing VII. CONCLUSIONS In conclusion, a novel converged metro access network architecture has been proposed to support both centralized and distributed radio baseband solutions in heterogeneous networks. The proposed architecture has been validated in terms of recent advancements in optical as well microwave technologies and the results of a proof of concept demonstration have been used to show how the discussed technologies can be used to implement heterogeneous networks built as C-RANs. REFERENCES [1] [2] [3] Figure 6. Configuration of the installed CPRI microwave link. [4] In the trial, the installed microwave LOS link supported full duplex transmission up to 2.5 Gbps line rate using two 70/80 GHz radios. The radio system was implemented using a simple modulation with mainly analog circuitry, which offers very high capacity with no restriction on what service or data it carries, well suited for heterogeneous networks built with a mix of CPRI and Ethernet links. As shown in Figure 6, the baseband unit located inside the Ericsson building (main site) was connected via fiber to one roof mounted 70/80 GHz radio, while the remote radio unit (RRU) was placed about 200 meters away in a truck (remote site) fiber connected to another 70/80 GHz radio on top of the truck. The main-remote link was established over the microwave link and with the RRU configured as a small cell a download speed of 60Mbps was demonstrated to the end user equipment. [5] [6] C-RAN White paper V2.5, China Mobile Research institute, Oct 2011. CPRI specification V4.2 (2010-09-29) C. Plante, J. Wang, Opening Base Station Architectures Part2: An inside Look at CPRI, design article by Altera Corp., 2004. J. Hansryd, J. Edstam, Microwave capactiy evolution, Ericsson Review, 1/2011. http://wireless.fcc.gov/services/index.htm?job=about&id=millimeter_wave ECC recommendation (05) 07, Radio frequency channel arrangements for fixed service systems operating in the bands 71-76 GHz and 81-86 GHz, Dublin, 2009. [7] J. Hansryd, J. Chen, Y. Li, and B. E. Olsson, A simple DBPSK modem based on high-speed logical gates for a 70/80 GHz GbE microwave link, Proc. 71 IEEE Vehicular Technol. Conf., Taipei, Taiwan, 2010. [8] ETSI EN 302 217-2-2 (draft v2.1.1 to be published) [9] J. Wells, Multigigabit Microwave and Millimeter-Wave Wireless Communications, Artech House, 2010. [10] J. Hansryd, Y. Li, J. Chen, P. Ligander, Long term path attenuation measurement of the 71-76 GHz band in a 70/80 GHz microwave link, Proc. Fourth European Conference on Antennas and Propagation (EuCAP), April 2010. [11] J. Hansryd, P.E. Eriksson, High-speed mobile backhaul demonstrators, Ericsson Review, 2/2009. 752