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OFC/NFOEC Technical Digest © 2013 OSA
Emerging Disruptive Wireless Technologies – Prospects and
Challenges for Integration with Optical Networks
Dalma Novak and Rod Waterhouse
Pharad, LLC; 797 Cromwell Park Drive, Suite V; Glen Burnie MD 21061
Abstract: We describe some emerging technologies that are being investigated for the realization
of next generation wireless networks capable of supporting multiple standards and meeting
capacity demands. The challenges associated with their efficient integration in a converged
wireless/optical network are also discussed.
OCIS codes: (060.2360) Fiber optics links and subsystems; (060.5625) Radio frequency photonics
1. Introduction
The convergence of optical and wireless networks continues to evolve, ever since the first reports of extending
wireless coverage areas using optical fiber as feeder links, that were proposed more than two decades ago [1, 2].
The benefits of creating integrated end-to-end network solutions that can provide reliable service for both fixed and
mobile users is now well-recognized.
Today wireless networks are evolving more rapidly than ever before. The demand for wireless access to highspeed data communications continues to increase at an unprecedented rate, driven by the proliferation of connected
high capacity devices such as tablets and smart phones as well as the increase in the number of broadband multimedia services available to the consumer. By 2015 it is forecast that more than 5 billion consumers will be exposed
to wireless connectivity leading to an aggregate mobile traffic of almost 7,000 Petabytes per month [3]. In addition,
the expected associated number of mobile backhaul connections that will be provided by optical fiber links will total
more than 3.5 million [4].
The realization of integrated optical/wireless networks that can reliably and cost-effectively support current and
future capacity demands, traffic growth rates, new services, as well as multiple wireless standards (GSM, CDMA,
LTE, WiMAX, HSPA+) is an ongoing challenge for carriers. In this paper we discuss some emerging technologies
that are being investigated as potential solutions to this problem and describe the implications for their successful
integration with optical networks.
2. Active Antenna Systems
Presently, traditional macro-cell infrastructure features a distributed base station architecture in which the radio
hardware is positioned in close proximity to the tower-mounted passive antennas. This remote radio head (RRH)
contains the RF circuitry (including amplifiers, diplexer and bandpass filters) as well as analog-to-digital/digital-toanalog converters and frequency conversion components. Meanwhile the base station server (BTS) or baseband unit
(BBU) comprising the digital baseband processing circuitry (the channel cards) is located separately and interfaces
with the RRH via a digital fiber optic link; CPRI (Common Public Radio Interface) and OBSAI (Open Base Station
Architecture Initiative) are two standards that have been developed for this serial link and a number of vendors
currently supply the associated optical transceivers for these wireless backhaul standards.
One architecture concept that is currently attracting significant interest for meeting the growing capacity
demands of wireless networks is the deployment of smaller sized cells that would coexist with, and complement
larger macro sized cells. These small cells (anywhere from 50 m to 5 km in size) could include micro-, pico-, or
femto-cells, as well as Wi-Fi deployments. They would provide additional capacity and data rates beyond the
capabilities of macro-cell sized networks by being deployed selectively where traffic demand is not being met, such
as coverage hotspots and inside buildings. Ultimately network control would be utilized to coordinate how a user is
most optimally connected to the wireless network infrastructure, considering the different cell layers and radio
technologies available.
As part of the trend towards smaller sized cells, the cell site hardware is becoming more advanced; active
antenna systems are key examples of this technology innovation. Active antenna systems are essentially an
extension of the distributed base station concept whereby the RRH functionality is now directly integrated with the
antenna elements [5]. In this emerging cell site technology, the active antenna is also becoming more ‘intelligent’
since its radiation patterns can be adapted to accommodate changing capacity demands and even multiple wireless
978-1-55752-962-6/13/$31.00 ©2013 Optical Society of America
OFC/NFOEC Technical Digest © 2013 OSA
standards in the one cell. The energy efficiency and performance benefits of active antenna system technology lead
directly to improvements in base station capacity and coverage, as has been reported in recent network trial
demonstrations [6 – 8]. Another key benefit of an active antenna architecture is the potential to significantly reduce
the footprint of the cell site.
Research is underway into a number of different aspects of realizing high performance active antenna systems
that can support current and emerging wireless air interface standards, spectrum bands and data bandwidths. Novel
technologies related to new efficient amplifier structures, adaptive beamforming technologies, RF/antenna
integration techniques, advanced baseband processing concepts, redundant subsystems, as well as wideband antenna
array designs, are all being actively investigated. Innovative antenna structures that can accommodate the next
generation form factors of cell sites and be compliant with system on chip (SoC) RF technologies are also being
pursued. New antennas that can support a variety of wireless standards will naturally require appropriate hardware
to carry out the digitization process required for interfacing with the BBU.
Antenna arrays that can support multiple frequency bands will play a key role in future implementations of
wireless base stations featuring active antenna systems [6 – 8]. A key requirement for these arrays is the ability to
operate efficiently over the entire frequency spectrum that would support current and new wireless services (2G, 3G,
and 4G). Beam steering capability over a wide field of view in a small factor, are additional requirements for the
wideband arrays, however the combination of all these factors presents a significant challenge. One promising new
radiator solution for implementation in next generation multi-radio active antenna systems is the balanced anti-podal
Vivaldi antenna (BAVA) structure [9]. These antenna elements are inherently wideband in nature, electrically small
and also modular, allowing easy formation into an array.
Due to its electrically small size, the BAVA radiating element does not encounter the typical scan blindness and
grating lobe issues associated with creating large arrays of wideband antenna elements, which leads to a very
efficient radiating system. It is also very suited to being integrated with SoC technologies for small cell
applications, where the RF hardware can be positioned behind an individual element or integrated into an active
antenna array to achieve a very compact footprint. Figure 1 shows an array of BAVA elements in a highly compact
single platform that we recently designed to efficiently cover the 700 – 3600 MHz frequency range, with very good
beam steering capabilities over multiple octaves [10].
Fig. 1 BAVA radiator for next generation active antenna systems: Single element and cross-section (left); Array of elements (right)
3. Integration of Small Cells with Optical Networks
Alongside the move towards smaller cell sites with integrated antennas and RRH functionality, the interconnections
between the active antenna systems and the baseband units are also evolving. In the traditional arrangement the
BBU is located in a cabinet at the base of the cell tower however the concept of a centralized architecture in which a
number of BBUs are remotely co-located together in a secure Central Office, is being actively investigated [11, 12].
In this scenario, the typical CPRI digital fiber-optic link between the active antenna system and the BBU would be
longer in length and the optical distribution network comprising the digital links constitutes the fronthaul of the
wireless network. A centralized BBU architecture will lead to savings in OPEX as well as improved performance
since there is no transmission delay between adjacent cells. If the co-located BBUs are also pooled together such
that the baseband processing resources can be effectively shared across a large number of cell sites in a “virtual”
configuration, the resulting cloud radio access network (C-RAN) can also enable CAPEX reductions while enabling
the connectivity between different wireless network layers to be optimized.
OFC/NFOEC Technical Digest © 2013 OSA
One of the key challenges associated with remoting pooled BBUs from the active antenna systems in next
generation wireless networks is the very high bit-rates that must be accommodated by the digital links since the data
rate will depend on the sampling frequency (proportional to the wireless data bandwidth) and sampling resolution
[13]. This problem becomes more pronounced with the trend towards using multiple transmit and receive antennas
at the cell site as a means to increase capacity. For example, an LTE network with 20 MHz bandwidth, 2 × 2 MIMO
in the downlink, and 3 sectors (RRHs) per cell site would equate to an aggregate data rate of more than 7 Gb/s after
digital sampling of the analog radio signals [12]. The implementation of active antenna systems with multiple
radios that support a diversity of wireless standards can lead to expected data rates well in excess of 10 Gb/s. In
addition, there are strict requirements on transmission latency and jitter that must be satisfied, which are even more
stringent for emerging higher data rate 4G wireless networks. Ultimately these constraints will limit the fiber
distances between the active antenna systems and the baseband processing hardware to a few tens of kilometers.
A variety of architectures for realizing the fronthaul optical distribution network are currently being evaluated
with regards to efficient fiber usage and cost, including point-to-point fiber links between the RRH and BBU and
multiplexing of the CPRI links via TDM, WDM or OFDMA PONs [11 – 13]. Depending on the required
bandwidth, a dedicated dense WDM (DWDM) network architecture may be required with dedicated wavelengths
assigned for one or more active antenna systems [13]. Also being investigated is the possibility of deploying fiberconnected small cells within existing macrocells that may already have sufficient baseband processing capacity and
are integrated with conventional optical transport links.
One approach to addressing the challenge of realizing high data rate fiber optic links for future wireless fronthaul
networks is the development of novel digital compression algorithms to reduce the bandwidth [11]. A complete
paradigm shift however would be to consider implementing analog RF optical links which would avoid the sampling
process altogether [13, 14]. An analog optical distribution network connecting the RRHs and BBUs would also
greatly simplify the cell site hardware and reduce power consumption since the ADC/DACs and frequency up/downconversion circuitry are no longer required. The main drawback with transporting the wireless signals as an
analog signal over fiber is the reduced dynamic range which limits the fiber transmission distance. This issue can be
mitigated somewhat through the use of power or gain control techniques in the wireless network, however the
achievable dynamic range will still limit the potential applications for this transport scheme. An analog based
connection would also have an impact on feasible architecture options for the optical distribution network that
interfaces multiple small cell sites to a centralized pool of BBUs.
4. Conclusions
Meeting current and future capacity demands and supporting multiple wireless standards continue to drive the
evolution of wireless networks. Active antenna systems and C-RANs are some of the emerging concepts being
investigated for these next generation wireless systems, and have the potential to significantly impact the evolution
of converged optical and wireless networks. A key challenge in successfully realizing the optical distribution
network for future wireless fronthaul will be the very large bit rates per cell site that must be supported.
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