2002034

Microwave transmission in mobile networksAldo Bolle and Andrea Nascimbene

The inherent reliability and cost-effectiveness of microwave technology havebeen given a dominant role in connectingmobile radio base stations (RBS). The roll-out of packet-data and third-generation mo-bile networks fundamentally changes thetraffic demands on transmission systems.Consequently, new microwave transmissiontechniques and solutions are required.

With the continuous growth of mobilesubscribers and mobile data communica-

tion, operators need enhanced microwavetransmission systems. In particular, en-hanced features are needed to handle chang-ing traffic patterns efficiently, to offer in-creased capacity, and to make optimum useof radio spectrum.

To achieve profitability, operators musthave flexibility and be able to respondquickly to dynamic market conditions.These requirements make microwave, withits ease of implementation, ideal for accesstransmission.

Today, 60% of all second-generationRBSs are connected via microwave technol-ogy. As voice and data traffic increases inmobile networks, PDH-based point-to-point microwave solutions can be comple-mented with ATM-based point-to-multipoint solutions and SDH equipmentto create a unified, fully integrated and cost-effective transmission solution that givesoperators the best network control and mostprofitable operation.

MINI-LINK portfolioThe MINI-LINK portfolio includes solu-tions for point-to-point as well as for point-to-multipoint operation. Terminals andsmart nodes (Figure 1) are used for imple-menting the building blocks in a network.

124 Ericsson Review No. 3, 2002

Microwave links became an enormous success with the roll-out of second-generation mobile networks. With close to 500,000 units deliveredto date, the Ericsson family of MINI-LINK microwave products has animportant role in mobile operator networks. Now, the advent of third-generation mobile networks is starting a new wave of deployment charac-terized by cost-effective and flexible roll-out, and short site-to-site dis-tance. Moreover, we are seeing a shift in focus from plain point-to-pointbit transport to a network view with optimized site solutions.

The authors address the launch of Ericsson’s microwave solution fortransmission in current second-generation and imminent third-generationmobile networks, showing how combined use of the point-to-multipointand point-to-point technologies provides the most cost-effective andspectrum-efficient solution.

High Capacity

Medium CapacityC-QPSK

Medium CapacityQAM

Point-to-point Point-to-multipoint

Traffic Node Radio shelf

Concentrationshelf(optional)

Figure 1The MINI-LINK portfolio.

Ericsson Review No. 3, 2002 125

MINI-LINK point-to-pointEricsson’s microwave point-to-point port-folio consists of MINI-LINK Medium Capacity and High Capacity terminals, andthe MINI-LINK Traffic Node (Fig-ures 2-3). Depending on the range and ca-pacity to be implemented, the MINI-LINKportfolio offers frequencies ranging from 7to 38 GHz, for hop lengths of several tensof kilometers to just a few kilometers, andtransmission capacities of up to 155 Mbit/s.Constant envelop offset – quadrature phase-shift keying (C-QPSK) and quadrature am-plitude modulation (QAM) schemes areavailable for the terminal configurations.The MINI-LINK Traffic Node, which is asmart node for point-to-point operation, hasbeen optimized for the aggregation nodes inthe network, thus providing the ideal ca-pacity and functionality to solve transmis-sion needs. It complements the terminalswith the additional features needed to pro-vide a complete and efficient site and net-work solution.

Figure 3MINI-LINK Medium Capacity terminal (left)and MINI-LINK High Capacity terminal(right).

Figure 2MINI-LINK Traffic Node.

AAL2 ATM adaptation layer type 2ATM Asynchronous transfer modeCPP Connectivity packet platformC-QPSK Constant envelop offset –

quadrature phase-shift keyingE1/E2/E3 ETSI digital multiplexing stage IP Internet protocolLAN Local area networkMIB Management information baseOC-3 ANSI digital multiplexing stagePDH Plesiochronous digital hierarchy

QAM Quadrature amplitude modulationRAN Radio access networkRBS Radio base stationSDH Synchronous digital hierarchySNMP Simple network management

protocolSTM-1 Synchronous transport module

level 1T1/T2 ANSI digital multiplexing stagesVC Virtual containerxDSL Digital subscriber line

BOX A, TERMS AND ABBREVIATIONS

MINI-LINK point-to-multipointThe MINI-LINK point-to-multipoint sys-tem (Figure 5) provides 37.5 Mbit/s datatransfer per sector. Each sector can be 90° inthe standard solution or 180°/360° in the“launch” solution, in accordance with the re-quired capacity and RBS density ratio. Thecapacity within a sector can be fixed or dy-namically allocated to each terminal, allow-

ing, in the latter case, reallocation of capac-ity within a few milliseconds. The system isthus very suitable for data traffic, both forbusiness access and backhaul in mobile sys-tems. It uses ATM to guarantee differentclasses of service. E1, ATM (over E1/T1,E3/T3 or STM-1/OC-3) and Ethernet in-terfaces are available. The system operateson frequencies from 24 to 31 GHz and usesthe C-QPSK modulation scheme.

Management systemThe third building block in the portfolio isthe MINI-LINK Manager (Figure 6), whichenables operators to manage a completeMINI-LINK microwave transmission net-work from a single screen. Network elementmanagement provides functionality formanaging faults, performance, configura-tions and security. Together with local craft terminals (LCT) and the element-management functionality embedded in the network elements, the MINI-LINK Manager gives operators the tools they needfor efficient and cost-effective operation of a MINI-LINK network.

MINI-LINK Manager has several exportinterfaces for easy integration into other network-management systems. It can be in-corporated into a total management solu-


Figure 4MINI-LINK outdoor radios.

Figure 5MINI-LINK BAS radio shelf.


tion for mobile systems, either as part of acomplete solution provided by Ericsson oras an integration with an existing manage-ment system.

MINI-LINK features

Bandwidth aggregation

The point-to-point and point-to-multi-point smart nodes are hub solutions devel-oped to support a large number of sites andfuture increases in capacity. Being scalable,the smart node enables the aggregation oftraffic bandwidth that originates from alarge number of end-nodes. At Medium Ca-pacity aggregation nodes, the bandwidth isaggregated into a medium-capacity inter-face (maximum 34 Mbit/s) Similarly, atHigh Capacity aggregation nodes, thebandwidth is aggregated into a high-ca-pacity interface (STM-1 or greater). Trafficfrom the aggregation nodes can be furthertransmitted either on microwave or opticallinks.

In a point-to-multipoint system, the airinterface is shared among multiple accessterminals. The shared media enables multi-plexing gains over the air, provided a packet-based infrastructure is employed.MINI-LINK point-to-multipoint is basedon ATM end-to-end, which enables multi-plexing gains and efficient usage of thebandwidth when second- and third-generation traffic is handled in the aggre-gation nodes.

Use of spectrum

Spectrum is a sparse resource. Besides thecontinuous development of radios in newlyallocated frequency bands, some importantnew features have been introduced in theMINI-LINK portfolio to deal with futureshortages of spectrum. To allow the opera-tor to increase transmission capacity withinan existing frequency spectrum, higher-order modulation methods (based on 16 and128 QAM) have been introduced in theMINI-LINK point-to-point portfolio.These new features give the operator addi-tional flexibility in balancing spectrum andpower efficiency in the network.

Point-to-multipoint systems (Figure 7)make efficient use of spectrum by • allocating capacity per ATM cell (ATM

granularity gain) instead of on a 2 Mbit/s-basis;

• ATM multiplexing in conjunction withfast dynamic capacity allocation. The net-work can be “oversubscribed” in terms of

number of registered users while stillmaintaining QoS; and

• delivering unused access capacity to otherservices, such as wireless LAN accesspoints or business access users, based onthe diversity gain of the daily traffic pro-file (daily profile gain), since the busyhours for residential users generally differfrom those of business users.

E1

E1

E1

VP

VP

VP

VP

Call admission controlled

VCAAL2 or

0

90

50

05 10 15

Link capacity needed [Mbit/s]

Base station aggregation 2 Mbit/s average 4.2 Mbit/s peak

Number of base stations

Figure 7Aggregation gain. The diagram shows the aggregated link capacity required by multiplebase stations per base station. The red line indicates aggregating link capacities. The yellow line represents peak load capacities, and the blue line, average traffic loads. Theaggregation gain increases as the number of base stations connected in the same sectorincreases.

Connection managementFault management

Fault management

Faultmanagement

Performance management

Performancemanagement

Performancemanagement

Configuration management

Configurationmanagement

Billing mediationNetwork-wide, cross-domain

Network elementmanagement

Networkmanagement

Network elements

Embeded elementmanager/Local craft terminal

Microwave-specific

Network-element-specific

MINI-LINK Manager

MSM

EEM

MINI-LINKpoint-to-point

MINI-LINKpoint-to multipoint

EEM

Figure 6MINI-LINK Manager—its role in network management.

Protection

The operator’s most important asset is end-user traffic. If service delivery is not reliable,end-users will change service providers.High-quality equipment that is comple-mented with additional protection mecha-nisms gives operators a means of deliveringhigh-quality services.

The MINI-LINK products are protectedagainst equipment failure and radio propa-gation anomalies. All hardware is duplicat-ed to support the configurations on one orboth sides of the radio connection. Thetransmitting equipment can be configuredto operate in hot standby or working stand-by transmission mode.

The MINI-LINK Traffic Node adds yet an-other level of protection—network or ringprotection. This functionality enables the op-erator to build reliable ring structures basedon any microwave capacity up to 155 Mbit/s.These protection mechanisms work at theE1/T1 level, protecting every or pin-pointedE1/T1s within the total payload.

The Traffic Node solution also includesline-protection mechanisms without theduplication of hardware. Instead, theE1/T1s to be protected are routed into twoseparate ports on the same interface board.

Ease of installation and visual impact

Speed of installation is always a business con-sideration, especially during the roll-out ofthird-generation networks in Europe. Microwave is less costly and time-consuming to deploy than copper leasedlines. The MINI-LINK portfolio has beenoptimized for simple installation with a com-pact, easy-to-carry outdoor unit (Figure 8).The single-cable interface between the in-door and outdoor unit, and the single-boltalignment fixture are well known. Thepoint-to-multipoint system is even less com-plex and therefore faster to install, since onlyone end of the link has to be installed. In ad-dition, new base stations or interfaces can beadded to the backhaul network configura-tion, literally in a matter of minutes, mini-mizing maintenance and upgrade costs.

The point-to-multipoint hub needs onlyone antenna (and a single cable between out-door and indoor equipment) per sector, re-gardless of the number of connected RBSs.This strongly minimizes the visual impact,especially in cities and towns where anten-na pollution is an important issue. More-over, fewer antennas means fewer sites (sim-pler site acquisition) and reduced installa-tion time and cost.

Data communication networks

The Ericsson network solution for trans-porting operation and maintenance (O&M)information from equipment to the man-agement center is based on IP communica-tion over Ethernet, with a distributed man-agement information base (MIB) architec-ture.

MINI-LINK provides efficient in-banddata communication between end-nodesand aggregation nodes. The MIB is physi-cally located in each network element.Using the simple network managementprotocol (SNMP), operators can access O&Minformation in the MIB remotely from a net-work management system. They can also ac-cess the information locally, on site, bymeans of the local craft terminal. The ter-minal software can be upgraded remotelyfrom a central location, or locally using alaptop connected to the terminal.

Each MINI-LINK Traffic Node and HighCapacity terminal holds its own IP router forextending the data communication networkthroughout the transmission network, andtransporting O&M information on otherequipment via external service channels.

Combined solutions forthe mobile transport RANIn dense areas, point-to-multipoint has clearadvantages over point-to-point transmis-sion. As a simple rule of thumb, point-to-multipoint becomes an interesting optionwhen four or five RBSs can be seen from onelocation. However, the two technologies are,and will be, used in combination. Point-to-point microwave, which is typically de-ployed in areas with fewer RBSs, can becombined with point-to-multipoint toovercome distances or interference.

The combination of Ericsson’s point-to-point and point-to-multipoint product fam-ilies results in the most cost-optimized andspectrum-efficient solution for second- andthird-generation networks (Figure 9).

E1/T1 aggregation via point-to-pointlinks is typically suitable in small hubs wherethe number of directions (or connected RBSs)is limited and spectrum is not an issue (therequired bandwidth is very likely to be a por-tion of that required to deploy the large hub).

ATM aggregation, typically via point-to-multipoint, is more suitable in large hubswhere the number of directions (or con-nected RBSs) is great and spectrum effi-ciency is a must (since it determines the sizeof the frequency blocks required).


Traditionally, operators have deployedmobile backhaul networks using a combina-tion of point-to-point microwave and leasedlines. A determinant when choosing betweenmicrowave and leased lines is the individualoperator’s needs in terms of network controland transmission quality.

Typical leased-line contracts have oftenguaranteed availability figures around 98.7%,which corresponds to a potential of four orfive days downtime per year. Microwave net-works (which are often used to relink theentire connection between the end-RBSsand the switch site) are dimensioned for99.95% availability or better, which corre-sponds to four hours or less of downtime peryear.

In conclusion, the availability of amicrowave network is very much a planningissue. By selecting high-quality products incombination with proper network planning,availability is normally the same as or betterthan that of fiber or copper networks.

BOX B, PLANNING MICROWAVE

TRANSMISSION, AVAILABILITY AND

QUALITY

Figure 8Installing MINI-LINK.


The hubs are connected to each other, tothe switch site, or both, via point-to-pointsystems in accordance with the requiredrange, capacity and available spectrum.

E1/T1 multiplexing nodesThe E1/T1 multiplexing node is the currentsolution for present-day second-generationnetworks. In all likelihood, it will also be themost efficient solution for operators who planto add third-generation services in environ-ments where second-generation traffic willcontinue to dominate. This is also the typicalsolution for operators who want to reuse as much of the existing network as possi-ble (by exploiting spare capacity on the microwave links or on STM-1/OC-3 rings).This aggregation strategy might also be jus-tified by the price structure for leased lines.The benefits of a network based on E1/T1multiplexing nodes are low initial invest-ments and secure upgrade with minimum dis-turbance to existing traffic.

Figure 9 exemplifies how a combinationof point-to-multipoint and point-to-pointlinks can efficiently serve the Medium Ca-pacity and High Capacity aggregationnodes.

The Low Capacity and Medium Capacityaggregation nodes typically handle fromtwo to four radio base stations—that is, fromtwo to four directions. These nodes are gen-erally deployed where RBS density is lowand the RBS-to-RBS distance is great.

In the southbound direction (Figure 9),the Medium Capacity aggregation nodes in-terconnect the end RBSs through MINI-LINK point-to-point; in the north-bound direction, the connection can bemade via MINI-LINK point-to-multipoint(Low Capacity ATM aggregation nodes) orpoint-to-point terminals (Medium Capaci-ty E1/T1 aggregation nodes), depending oncapacity, protection and range require-ments.

Ordinarily, the High Capacity aggrega-tion nodes are located in suburban or urbanareas where RBS density is high. During op-eration, error-free transport over microwavelinks is guaranteed by large fading marginsand forward error correction mechanisms,which make microwave links highly suit-able for ATM and IP transport. In these sitespoint-to-multipoint is likely to connect theend RBSs. Those RBSs that are outside thepoint-to-multipoint coverage range are con-nected through point-to-point links.

When E1/T1 traffic is aggregated, the MINI-LINK point-to-multipoint system

for second-generation traffic connects to theMINI-LINK Traffic Node through nxE1,which, in turn, provides a single STM-1VC12 interface to the switch site.

The main drawbacks of the E1/T1 multi-plexing solution can be limited expansionand greater long-term cost of operating thenetwork.

ATM aggregating nodesWhen third-generation traffic dominatesover second-generation traffic, ATM aggre-gating nodes can be used to provide the mostcost-effective network solution. Networksbased on ATM aggregating nodes are like-ly to be typical for greenfield operators andfor incumbent operators who want to over-lay the existing network or to replace exist-ing leased-line connections.

The MINI-LINK point-to-multipointhub provides port aggregation, aggregatingtraffic from point-to-multipoint and point-to-point terminals. It also provides a very ef-ficient and cost-effective solution for cellu-lar backhaul applications. It can also aggre-gate traffic from leased lines and xDSL lines.In the northbound direction, a single ATM-over-STM-1 VC4 interface provides a veryclean and cost-effective solution that opti-mizes backbone capacity, switch site com-plexity and cost.

This solution can also be used in combi-nation with Ericsson’s RBS and RXI prod-ucts, providing a complete Ericsson mo-bile and transport network (Figure 10). In

Low Capacity or Medium CapacityE1 aggregation node

Low Capacity ATMaggregation node

2G/3G RBS sites

Point-to-point links

PMP radio shelf

Point-to-multipointcoverage

Traffic Node

NxE1 or STM-1 VC12 to BSC/RNCHigh Capacity E1aggregation node

Figure 9Example of site solutions based on E1 multiplexing and the combining of point-to-multipoint and point-to-point.

PMP radio shelfTraffic Node

ATM over E3 or STM-1 to BSC/RNCHigh Capacity ATMaggregation node

RBS/RXI

PMP radio shelfTraffic Node

ATM over E3 or STM-1 to BSC/RNCHigh Capacity ATMaggregation node

Figure 10Top: Example of High Capacity aggrega-tion node handling ATM.Bottom: Example of combined CPP andMINI-LINK point-to-multipoint (PMP) radioshelf.

addition to the benefits of ATM aggrega-tion, the solution brings optimized statis-tical multiplexing gain, thanks to theAAL2 switching functionality of the con-nectivity packet platform (CPP, formerlycalled Cello packet platform). Because

ATM virtual-path multiplexing and portaggregation are performed in the MINI-LINK point-to-multipoint radioshelf, the AAL2 functionality is achievedwhile optimizing costs (no increase innumber of boards).


The use of short-haul microwave radio hasevolved from scattered cable replacements tothe forming of complete microwave-basedtransmission networks. The requirements puton the products have shifted from optimizationof the terminal or hop level to optimization ofthe network level. In a microwave network, onecan define logical nodes (or physical sites) withdistinct characteristics. The logical buildingblocks are the end-node and aggregationnode. Any microwave network can be imple-mented as a combination of end-nodes andaggregation nodes (Figure 11).

To address the network aspects, Ericsson’sproducts are optimized for the different typesof network node. Therefore, the MINI-LINKportfolio comprises compact, cost-effectiveaccess terminals and smart nodes that featureadvanced traffic routing and multiplexing. TheMINI-LINK portfolio includes access terminalsand smart nodes for point-to-point and point-to-multipoint operation.

Typical building blocks of a microwave network

End-nodeThe end-node is the smallest building block. Bydefinition, it supports transmission in only onedirection. In most cases, the capacity of the end-node ranges from 2x2 up to 34 Mbit/s. Ordinarily no redundancy is required at end-nodesites and therefore the normal microwave con-figuration is 1+0. Point-to-point and point-to-multipoint end-nodes are foreseen. The end-node should support traffic interfaces rangingfrom multiple E1/T1s to Ethernet. Ideally, in apoint-to-multipoint system, the end-node willprovide an ATM interface for third-generationbackhaul, to take better advantage of the sharedair interface.

Low Capacity and Medium Capacity aggregation nodesThe Low Capacity and Medium Capacity aggre-gation nodes have a northbound microwave linkthat carries traffic up to 34 Mbit/s. In the south-bound direction these nodes have a limitednumber of subtended end-nodes.

Ericsson’s solution to the Medium Capacityaggregation node has been to design smart,cost-effective Traffic Nodes that can aggre-

gate all traffic from the southbound links intoanother microwave link in the northbounddirection. The solution supports protected andnon-protected configurations of the MediumCapacity aggregation node. The solution alsosupports dropping and insertion of local traffic.

High Capacity aggregation nodeThe High Capacity aggregation node has anorthbound transmission link with a trafficcapacity of 155 Mbit/s or greater. The north-bound media can be either optical ormicrowave. The topology in the northbounddirection can be ring or point-to-point. Sincethe High Capacity aggregation node supportsa considerable amount of traffic, it is assumedthat most of the sites will aggregate a substan-tial number of southbound links. Some end-nodes are directly connected to the HighCapacity aggregation node and some are con-nected through a Medium Capacity aggrega-tion node. Point-to-point, point-to-multipointand E1/T1 and ATM aggregating sites are sup-ported. The Ericsson solution to the HighCapacity aggregation node can be designed tobe very compact and cost-effective, as part ofan all-microwave solution that supports155 Mbit/s traffic capacity.

BOX C, NETWORK ARCHITECTURE

High-capacity links

Medium-capacity linksSwitch site

Low Capacity or Medium Capacityaggregation node

High Capacityaggregation node

Figure 11Network architecture.


ConclusionThe key issues for efficient support of themobile network infrastructure are:• A complete portfolio of point-to-point

solutions (any frequency, any capacity,PDH/SDH), explicitly designed for asmart network-oriented approach.

• An ATM-based point-to-multipoint solu-tion that provides a suitable combinationof high coverage and high capacity.

• A combination of these technologies toprovide the most cost-effective and spectrum-efficient microwave solu-tion.

• An integrated management system for theentire portfolio.

• Proven reliability and large productioncapability for secure roll-out.

Uniquely, the Ericsson MINI-LINK port-folio can meet all of these requirements.

One fundamental issue in microwave networkplanning is the efficient use of the frequencyspectrum. National authorities and internation-al committees regulate the availability of spec-trum. Point-to-point links typically require alicense per link, whereas licenses for point-to-multipoint systems are issued as regional ornational block allowances. In many cases,operators prefer block licenses since theyallow faster planning and deployment of thelinks.

In point-to-multipoint cellular deployments,a few locations inside the multipoint sectorcan experience interference from neighboringhubs. However, this effect can be minimizedby avoiding reuse of frequencies in neighbor-ing sectors or by combining point-to-multipoint with point-to-point technologies.

In Figure 12, the RBS-1 location is assumedto be affected by co-channel interferencefrom a remote point-to-multipoint hub if con-nected to the local hub through a point-to-multipoint terminal. If the RBS-1 is insteadconnected to the RBS-2 location by means of a point-to-point link, the antenna angulardiscrimination improves the carrier-to-interference ratio and guarantees error-freeoperation. It is worth noting that the point-to-point link can reuse part of the same point-to-multipoint spectrum, allowing for a very spectrum-efficient solution. Thanks to thecombined MINI-LINK point-to-point andpoint-to-multipoint solution, only a single28 MHz link is required for the complete net-work deployment (excluding the spectrum forthe northbound connections).

BOX D, IMPROVING SPECTRUM EFFICIENCY BY COMBINING POINT-TO-MULTIPOINT

AND POINT-TO-POINT SYSTEMS

Point-to-multipoint hub

Radio shelf

RBS-1

RBS-2

Point-to-point

Point-to-point

Point-to-multipoint

Remote hub co-channelinterference

Figure 12Operators can reuse frequencies by com-bining point-to-multipoint and point-to-point systems.

2002034

Documents