Fixed and Mobile Broadband Wireless Access Networks Employing Optically Beam-formed Antennas

J. Martí, J.L. Corral, V. Polo and J. Herrera E.T.S.I. Telecomunicación

Camí de Vera s/n 46071 Valencia (SPAIN)

e-mail: jmarti@dcom.upv.es Abstract Broadband wireless access networks that operate in the millimetre-wave frequency band are envisaged to fulfil the

bandwidth requirements of broadband multimedia services. At these frequencies, temporary obstacles and adverse

meteorological conditions may drastically impair on the system performance by intercepting the required line-of-sight

radio wave propagation, which results in degrading the system. Base station antennas with beamforming capabilities

are envisaged to address dynamically changes in the network features and resources in order to overcome the service

unavailability arising from man-made and natural impairments. Traditional DSP-based and RF/IF beamforming, which

have limited performance for millimetre-wave frequency operation. The OBANET project is aimed to the design,

implementation and test of optically beam-formed antennas, which are the most promising alternative to such beam-

forming approaches, due to the huge bandwidth, low-loss and weight provided by optical fibre devices. In addition,

OBANET project aims at designing and testing specific coverage area management strategies as well as their

associated technologies for performance optimisation in broadband adaptive broadband wireless access networks in

two scenarios: fixed and mobile broadband radio accesses in the 40 GHz.

I. Introduction Broadband wireless access networks that operate in the millimetre-wave frequency band are envisaged to

fulfil the bandwidth requirements of broadband multimedia services, which are expected to experience a

tremendous growth in the next five years [1]. The provision of these services requires high-performance and

high-reliable communication networks for both the access and the transport parts [2]. At these frequencies,

temporary obstacles and adverse meteorological conditions may drastically impair on the system

performance by intercepting the required line-of-sight radio wave propagation, which results in degrading

the system performance, and even more, making the system unavailable [3].

Base station antennas with beamforming capabilities are envisaged to address dynamically changes in the

network features and resources in order to overcome the service unavailability arising from man-made and

natural impairments [4-6]. Traditionally, beamforming is achieved by means of DSP-based and RF/IF signal

processing, which have limited performance for millimetre-wave frequency operation. Optically beam-

formed antennas [7] are the most promising alternative to such beam-forming approaches, due to the huge

bandwidth, low-loss and weight provided by optical fibre devices [8]. The IST-2000-25390 OBANET

project (http://ist-obanet.upv.es) is aimed to design and test base-station antennas with beamforming

capabilities implemented in the optical domain.

OBANET project aims at designing and testing specific coverage area management strategies as well as their

associated technologies for performance optimisation in adaptive wireless access networks in two scenarios:

fixed and mobile broadband radio accesses in the 40 GHz band. High-performance optoelectronic devices

and photonic integrated circuits composing the optical beamformers will be developed, using dense

wavelength division multiplexing (DWDM) and photonic integrated circuit (PIC) technologies. In addition,

coverage area management strategies and control/monitoring protocols for fixed and mobile radio accesses

will be developed within OBANET project. An specific medium access control protocol for adaptive fixed

and mobile broadband access networks will be proposed.

II. System description. Figure 1 depicts the general network architecture of a broadband wireless access network for both fixed and

mobile broadband radio accesses. The main difference between fixed and mobile wireless access network

scenarios rely on the usual shape and size of the cells composing the coverage service area. Some other

service planing parameters such as sectors per cell, radio-wave polarisation, size of the cell cluster (group of

cells using different frequencies), frequency reuse plans, frequency band, etc, are also intrinsically different

in each scenario.

Different system performance degradation mechanisms are expected in fixed and mobile access networks,

mainly due to the user mobility of in the latter one. Thus, different coverage area management strategies are

addressed in both wireless access networks to overcome quality-of-service (QoS) drops.

CONTROLSTATION

BASE STATION

OPTICAL FIBRE

CELL

COVERAGE AREA

Figure 1. Typical scenario for a cell-divided service area in wireless access networks.

Next, individual scenarios for fixed and mobile networks are considered.

II.A Broadband adaptive fixed wireless access networks (BAFAN) BAFAN operating at the 40.5-42.5 GHz band is the main target scenario of this project. It is well known that

the system performance of conventional Multipoint Video Distribution Systems (MVDS) is sharply limited

when line-of-sight radio-wave propagation between base station and customer antennas is intercepted (e.g.

ACTS-project CRABS). Moreover, intensive rainfalls in some cells of the service area may also severely

impair on the system performance. Centralised management strategies based on varying dynamically the

base station antenna pattern (adaptive beam-forming and beam-steering) to counter system performance

degradations will be proposed and implemented. These strategies are mainly oriented to:

(i) overcome quality-of-service loss due to man-made (e.g. provisional obstacles intercepting

the required line-of-sight radio propagation between base station and service users) or

natural (e.g. intensive rainfalls whose propagation losses may not be compensated for using

power transmitting strategies) impairments.

(ii) use efficiently the network resources (available bandwidth and capacity) in its different

deployment phases. The base station antenna pattern will be adapted to specific network

environmental propagation issues.

BAFAN operate at millimetre-wave frequency bands, which offer larger channel allocation. At these

frequency bands the service area is divided in small cells or sectors (pico-sectors or pico-cells). Therefore,

many of base stations are required for service provision, which impose a stringent limitation on the base

station equipment cost. Low/moderate base station cost may be achieved by using telecom-market-oriented

photonic integrated technology for implementing antenna beam-formers.

Figure 2 illustrates four 90-degrees sectorised cells in a BAFAN. The plot on the left shows the cross-

polarised base station antenna beams as a normal operating mode. In the plot on the right is sketched how the

base station antennas may vary their patterns features to counter three different performance degradation

mechanisms: interference sources by means of antenna beam-steering, increasing the number of sectors of

cells to allow a moderate increase of the radio-wave link power budget (polarisation diversity transceiver

must be used) and generating highly directive dedicated beams. The latter may be used to compensate for

scattering losses introduced by provisional obstacles intercepting the line-of-sight link between base station

and user antennas.

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NORMAL SITUATION

90º Sector

Highly-DirectionalUser

Antennas

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ANOMALOUS SITUATIONS

BEAMSTEERING

Figure 2. Normal (left) and anomalous (right) operating conditions in BAFAN. II.B Broadband adaptive mobile wireless access networks (BAMAN) Mobile broadband systems operating in the millimetre-wave (mm-wave) band (f>30 GHz) are envisaged to

support real-time interactive multimedia services based on high-definition TV. This is the second scenario

addressed by this work. Mm-wave mobile and movable broadband terminals will have a transmission

capacity exceeding 100 Mbit/s. Owing to the high attenuation introduced by the atmosphere, specially at the

oxygen molecular absorption band (>17 dB/km), the service area is divided in pico-cells (diameters about

100 metres), which allow efficient frequency reuse plans with very high frequency repetition rate. Excess

attenuation losses due to multipath and scattered radio-waves limit severely the performance such mobile

systems, which may result in a transmission capacity reduction for users sited on coverage area borders

(dynamically assigned capacity is supposed in these systems.

In order to overcome these situations beam division multiple access (BDMA) was proposed (RACE-project

MBS, [9]). Dedicated base station antenna beams for certain users or users groups in a pico-cell may ensure

high quality-of-service, as required by demanding broadband IP multimedia applications. In addition, it is

desirable that the beam features dynamically adapt to user and systems requirements (beam width-directivity,

beam direction, etc). These base station antenna features are target in this project. MBS project envisioned to

provide between 34 Mbit/s and 155 Mbit/s to each user. A hybrid fibre-radio architecture with an ATM radio

section seems to be particularly well suited for MBS services provision [2].

II.C. Proposed scenario. What we propose in OBANET is to demonstrate the feasibility of optical beamforming in a test platform

with specifications that emulate a realistic scenario with the following characteristics:

� Four 90º sectors per cluster/BS

� A directive beam that switches between users on a time-slot/frame basis

� Multi-level modulation formats.

� Cell size:

• BAFAN: 1 km

• BAMAN: up to 100 m

� Duplexing technique: TDD

� Multiple Access Scheme: TDMA

� Bit rates up to 155 Mbit/s

Figure 3 shows a schematic of the envisaged scenario: a single beam is switched between users on a time-

slot frame-interval basis. A basic frame structure is depicted in the same figure.

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u2

CONTROL CONTROL

u1, t1

Switching time

u2, t2 uN, tN

uN

BS

tN

t1

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CONTROL CONTROL

u1, t1

Switching time

u2, t2 uN, tN

uN

BS

tN

Figure 3.- Proposed scenario. III MAC definition. Usually MAC protocols are not designed to cope with directed adaptive antennas and beam switching such

as the proposed in the previously defined scenarios [10]. The main problem is that broadcast and multiple

access capabilities are always required for all stations in the coverage area. Thus protocols have to be

adapted to the scenario which results in a larger protocol overhead and less efficiency. This disadvantages

should be overcome with the benefits of adaptive antennas.

The MAC protocol definition for this project must be based in the following directives:

• The definition should be in agreement with near term and future standards for wireless and

mobile access networks: ETSI BRAN HIPERACCESS [11], MBS Dynamic Slot Allocation [9]

and specially IEEE 802.16.1 [12]).

• The definition must be adapted for SDMA, adaptive antennas and beam-switching techniques.

• The modifications required for the protocol adaptation to OBANET scenarios should be as much

compliant as possible with the standard.

• The definition must be applicable for both fixed and mobile OBANET scenarios (if possible).

• The definition should be as flexible as possible to allow different approximations to the

scenarios.

The proposed beam-switching technique is optimal for a time based MAC, so the TDD duplex and TDMA

multiple access are chosen. A wide range of benefits can be obtained for this choice [13], [14]:

• TDD has been used for a large time in the design of point to multipoint access networks.

• A time based MAC is optimum for transport of digital data, which means asymmetric, burst and

unpredictable traffic. And also for dynamic bandwidth requirements.

• TDD only needs one RF channel and no duplexor is needed.

• Less stringent power control due to reduced inter-user interference because of dedicated slots.

• The burst transmission allows measurement slots for mobile assisted hand-over and energy save.

• Both TDD and TDMA techniques are contemplated in the IEEE 802.16.1 standard.

On the other hand, time-based techniques suffer the following disadvantages:

• High data rates implies need for equalisation.

• Requires guard times, at least the round-trip propagation delay between stations.

• There is a complexity inherent to slot allocation and synchronisation.

The 802.16.1 frame (see fig. 4) has been adopted for the MAC definition. In the case of TDD, the upstream

and downstream transmission share the same frequency, but they are separated in time. A TDD frame also

has a fixed duration and contains one downstream and one upstream sub-frame and is divided into an integer

number of PS. The framing is adaptive in the sense that the downstream and upstream dedicated bandwidth

can be modified depending upon traffic requirements.

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Figure 4. IEEE 802.16.1 Proposed Frame Structure.

IV Demonstration platform

OBANET is intended to demonstrate a Photonic Integrated Beamformer (PIB) operating at the 40 GHz band

on a test-bed.

To our knowledge, this is the first time that a wireless access system employing optical beam-formed

networks is going to be demonstrated. First, the demonstration will be carried out using discrete commercial

devices and design parameters for PIB system will be extracted from such discrete beam-former.

Using the design specifications derived from the discrete beam-former experiments, photonic integrated

beam-formers will be implemented, for the first time, using a technology traditionally devoted to the

manufacturing of DWDM components. Also innovative techniques devoted to the optimisation of the joint

integration of optical and electrical microwave monolithic integrated circuits (MMIC) on the same chip will

be developed, mainly focusing in the photodetection process.

Finally, once the PIB is fully manufactured, it will be integrated on the demonstration platform and its

performance evaluated.

Figure 4. Laboratory platform to carry out the performance evaluation of PIB.

VI Conclusions The use of optical beam-forming techniques to provide telecom-market oriented services will push the emergence of novel commercial applications of optical beam-forming networks. In addition, the combination of DWDM technologies, already employed in modern communication networks and photonic integrated circuits will significantly reduce the production cost, which provides a scale advantage. Acknowledgements The authors acknowledge the European Commission for funding the project IST-2000-25390. References [1] M. Peruyero, “The Current Status of Broadband Wireless Aceess and Ideas on Future Development”, Broadband Fixed Wireless Access Workshop, 4th Concertation Meeting IST Area IV.5 Mobile/Wireless/Satellite, Brussels, March 2001. [2] ACTS Guideline BAC-G1, ”Access Network Strategies for Broadband Services Provisioning”, Draft A, September 1997. [3] H. Xu, T. S. Rappaport, R. J. Boyle, J. H. Schaffner, “38-GHz Wide-Band Point-to-Multipoint Measurements Under Different Weather Conditions”, IEEE Communications Letters, Vol. 4, No. 1, pp. 7-8, January 2000. [4] A. O. Boukalov, S. G. Häggman, “System Aspects of Smart-Antenna Technology in Cellular Wireless Communications – An Overview”, IEEE Transactions on Microwave Theory and Techniques, Vol. 48, No. 6, pp. 919-929, June 2000. [5] J. H. Winters, “Smart Antennas for Wireless Systems”, IEEE Personal Communications, pp. 23-27, February 1998. [6] M. Chavero, V. Polo, F. Ramos, J. Marti, “Impact of vegetation on the performance of 28 GHz LMDS transmission", in Proc. IMS 1999 Symposium, paper WE2D-5, vol. 3, pp. 1063-1066, Anaheim, USA, 1999. [7] H. Zmuda, E. N. Toughlian, “Photonic Aspects of Modern Radar”, MA: Artech House, Boston, 1999. [8] R. D. Esman, M. Y. Frankel, J. L. Dexter, L. Goldberg, M. G. Parent, D. Stilwell, D. G. Cooper, “Fiber-Optic Prism True Time-Delay Antenna Feed”, IEEE Photonics Technology Letters, Vol. 5, No. 11, pp. 1347-1349, November 1993. [9] Mobile Broadband System, Requirements for the Air Interface, Deliverable R2067/DB/2.2.2/DS/P/043.bl, RACE Project 2067, Nov. 1994. [10] M. Horneffer and D. Plassmann, Directed Antennas in the Mobile Broadband System, RACE Mob. Tel. Summit, Cascais, P., Nov. 1995. [11] ETSI BRAN ETR, Broadband Radio Access Networks HIPERACCESS, ETS 300 207-1, 1998. [12] IEEE WirelessMAN, Draft Standard for Air Interface for Fixed Broadband Wireless Access Systems, IEEE 802.16.1/D1, Dec. 2000. [13] T. S. Rappaport, Wireless Communications: principles and practice., Prentice-Hall, 1996. [14] D. J. Goodman , Wireless Personal Communications Systems, Addisson-Wesley, 1997.