Mobile Cdma Wll

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Mobile CDMA Technologies for Rural WLL

Range and Capacity Trade-offs

WHITE PAPER 033-100xxx-001, Issue 1 2005, SR Telecom Inc. 1

Abstract

Operators rolling out mobile CDMA-based networks for rural WLL

applications can expect to find that the final network design is far larger,more complex, and more expensive than they would expect, relative to

the capacity of the network. This is because CDMA mobile technology

(CDMA2000 1XRTT and its derivatives) is optimized for maximum

capacity for mobile applications in dense urban environments. However,

rural areas are characterized not only by low subscriber density, but also

by widely scattered clusters of subscribers. Under these conditions

CDMA will not provide economical coverage, and in fact optimized fixed

wireless solutions like SR Telecoms SR500 and symmetry systems can

be much more cost-effective to deploy in rural areas.

Migrating to a lower frequency band (i.e. 450MHz) may at first glance

appear to alleviate the limitations of CDMA technology. However,

changing frequency will not usually reduce the number of base stations

required, since range in CDMA networks is limited by the traffic loading

on the cell, not by the propagation characteristics of the radio.

Before deciding on a wireless technology for rural areas, operators need

to carefully study real-life deployment scenarios, to evaluate the full costs

of base stations, core networks, civil infrastructures, and backhaul

networks. In short, if all aspects of a solution are considered in the

business case, rural operators will often be best served by a self-

backhauling SR500 network. For more dense networks, a dedicated FWA

solution like symmetry can provide greater capacity, better flexibility, anda higher quality of service than mobile-based solutions.


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Overview

CDMA 450MHz is often proposed as a rural WLL solution, based on the

idea that propagation at 450MHz is superior; therefore coverage using

this technology can be achieved very economically in rural areas.

Vendors of the technology, who claim and demonstrate 50km range,

support this view. However, it is critical for operators evaluating thistechnology to understand that range in CDMA systems is limited by noise

and mutual interference, not by propagation and path loss. CDMA

450MHz can achieve 50km range, but in single-user conditions, or under

very light system loading. When the base stations are sufficiently loaded

to amortize their cost, the achievable range will be much less. Typically,

one base station must be installed in each served community, even if the

network operates at 450MHz. Moreover, being a mobile network, CDMA-

450 requires a complex core network mandated by mobility standards.

Operators of fixed rural CDMA networks will find that they have to:

Over-invest in the CDMA core network

Integrate mobile network nodes into their fixed line networks

Build extensive, high-performance backhaul networks

Deploy an excessive number of base stations

Use dedicated, high-power phones and directional antennas for

remote subscribers

The result is that a rural CDMA-450 network can be much more

expensive than operators would expect based on the nominal traffic

capacity required by the network. Dedicated fixed wireless access (FWA)

systems are much more economical to deploy in rural areas, and areparticularly more cost-efficient to expand, because of their stable

coverage under increasing traffic conditions.

Major Cost Elements of CDMA Networks

The CDMA Core Network

Many core network nodes are required just to get the network CDMA

operational and provide a mobility infrastructure. In a mobile network,

which consists of hundreds of thousands of users in a single city,the cost

of the core network nodes can be amortized over many subscribers and it

forms a relatively small part of the price per line. In a rural network, which

may consist of 50,000 lines spread over an entire country, the cost of the

core network is a significant part of the price per line. In addition, many of

these nodes are typically deployed on regional basis, to allow for

administrative concerns, failure-tolerance, and to minimize backhaul

requirements. Since multiple instances of each node will be required for a

national network, even if the capacity is low, the price per line for a rural

CDMA network will be even higher.


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Operators planning a CDMA-450 network should budget for the core

network when evaluating the costs of such a deployment. The required

nodes for standards-compliant CDMA network include:

Gateway Mobile Switching Center (GMSC) for connection to the

PSTN.

Even if the interface to the PSTN is done on a V5.2 interface, a

Gateway function will be required.

Mobile Switching Center (MSC) and Visitor Location Register (VLR),

to route calls to mobile subscribers and maintain location data

Home Location Register (HLR) to store basic subscriber information

Base Station Controller (BSC), to interface the base stations to the

MSC

A sophisticated Operations and Maintenance Center (OMC)

In addition, the operator will require new or adapted service network

nodes, such as a billing center, and mobile Intelligent Networkplatform.

If the operator wishes to take advantage of CDMA 1XRTTs data

capabilities, it will also be necessary to add several data-centric nodes:

Packet Data Serving Node (PDSN)

Accounting, Authorization, Authentication server (AAA)

Home Agent (HA)

Its important to note that the data nodes are typically provided by a third

parties, such as Cisco or Hewlett-Packard (HP), and are substantial

additional cost to the network. These nodes consist of large routers and

servers, which require sophisticated IT support.

Beyond the CAPEX associated with the CDMA core network, the addition

of all these nodes to a fixed network will also have an impact on an

operators OPEX, since these nodes require buildings, power,

maintenance, and operating personnel. Most importantly, these nodes are

extremely software intensive, and operators can expect to pay for yearly

network-wide software upgrades.

Backhaul Requirements

Mobile CDMA networks are designed around the availability of very

stable, synchronous, optical backhaul networks. Each CDMA mobile base

station requires one to four E1 links to connect it to the host BSC. These

links have strict synchronization requirements to ensure that all base

stations operate synchronously to allow the correlation algorithms

required for code division multiplexing to work. Stability requirements on

the backhaul links are on the order of 4 x 10-11

parts per million. If a base

station loses synchronization with the BSC, it will stop transmitting and all

subscribers in that cell will lose service. Thus, a synch loss, due to a

microwave fade for example, is a major outage that will require a base


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station restart. For this reason, base stations are typically backhauled

using more stable optical links.

The fiber optic infrastructure required for the stable backhaul links is

relatively easily provided in urban environments, where distances are

short, and optical fiber can usually be run in existing conduits. Most urban

centers already have existing fiber rings spanning the city. In the rural

areas, running fiber to base stations located on hilltops or in small townswill be an expensive proposition due to the distances involved, the costs

of digging trenches, laying fiber in new ducts, and terminating the fiber.

In remote rural areas, operators will need to rely on microwave links to

backhaul the CDMA base stations. Multiple tandem microwave links may

be required to reach the most remote areas. However, these microwave

links must be of very high quality, such as SDH radio, in order to ensure

that the stability requirements of the CDMA network are met. The

provision of a long-range, potentially multi-hop, SDH radio link to every

base station is major cost in a rural CDMA deployment.

The large number of base stations required makes the situation evenworse. As explained below, even at 450MHz, the number of base stations

required to cover a rural area will be far higher than the nominal number

of base stations indicated by a simple capacity calculation.

CDMA Network Planning

Rural Traffic Loading Issues

Experience shows that even rural, low-capacity CDMA networks require

one or more base stations at each served community. This is true

independentof frequency, because CDMA systems range is determined

by traffic loading, not by propagation conditions. Cell range in real-world

applications is limited by the mutual interference of the subscribers in a

cell, not by the RF propagation.

CDMA network planning is based on power control, not frequency

coordination as in TDMA network planning. Operators should be

concerned that budgetary planning takes into account the loadedrange of

the cells. Simply dividing the offered traffic load (say 50,000 subscribers

at 0.07E each) by the nominal Erlang capacity of a base station will

grossly underestimate the number of base stations required, since CDMA

must trade off capacity to enhance its range to a useful level for rural

applications.

Cell Breathing

Advocates of CDMA-450 often claim a range of up to 50 km. However,

CDMA is an interference limited system, not a propagation limited

system. The 50km claimed range is true only under very light loading

conditions. As soon as more users begin to connect to the base station,

the range will decrease. Implementing CDMA in a lower frequency band

Traffic loading limits range,

not propagation.


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(i.e. 450MHz) will not usually increase coverage, since range is limited by

the loading on the cell, not by the propagation characteristics of the radio.

A single-carrier CDMA cell has a traffic capacity of 20 Erlang. However, it

cannot provide its full capacity and its full range simultaneously. At 50%

cell loading, the range may be reduced by 20%. Range drops quickly over

50% loading a fully loaded cell has radius of 1.5Km, even at 450MHz.

Figure 1: Cell breathing Cell radius varies with traffic loading (not to scale)

The Near / Far Problem

CDMA systems must manage two key resources: Erlang capacity, and

power. A difficult power management problem (the so-called near/far

problem) arises when the base station attempts to service a distant

subscriber. The base station may assign the entire available power

budget to the distant subscriber, thereby preventing subscribers closer to

the base station (the near subscribers) from accessing the network, even

if spare Erlang capacity is available. Isolated, distant subscribers play

havoc with the CDMA base stations power budget, drastically reducing

the base stations traffic carrying capacity. Yet this is the typical problem

in rural networks: how to serve isolated, distant locations. With CDMA

technology, the solution is typically to install a base station at each served

community.

Network Expansion Issues

The maximum cell radius is a configurable parameter at the CDMA base

station, which is deliberately restricted to prevent distant subscribers from

trying to access the system and causing a near/far situation that would

affect the subscribers in the main coverage area. If a subscriber outside

the configured radius attempts to access the system, his request will be

rejected. Otherwise, the increased power and interference caused by this

one subscriber could cause all the other subscribers on the system to

Unloaded CDMA Cell Fully Loaded CDMA Cell

50 km radius

1.5 km radius


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drop off. To reach new subscribers outside a cells configured range, it

will most likely be necessary to install a new base station and associated

backhaul. This can make network expansion extremely costly. Repeaters

cannot be used because they will cause more interference in the mother

cell; thereby reducing its range and reducing coverage to already served

communities.

In a rural setting, it may be possible to cover several small villages with asingle cell, as long as the loading caused by the villages is similar, and a

base station site roughly equidistant to all the villages can be found (to

avoid the near/far problem). However, if capacity at the villages

increases, it will be impossible to simply add capacity at the base station

site, since adding capacity will decrease the range, so that some areas

that formerly received service will no longer be covered. Typically,

expansion of capacity will require locating a base station in each village,

with the associated backhaul. This kind of expansion can dramatically

increase the number of cells and drive up network costs after the initial

rollout.

CPE Considerations

One of the potential advantages of CDMA is a low-cost, zero-install CPE.

However, for rural areas, operators should not imagine that this phone

consists of a small handset. A recent article in Wireless Week by a

prominent CDMA network-planning consultant explains what kind of

terminal is required for rural applications:

A fixed terminal typically provides for connecting an ordinary

telephone and an external antenna that can be located to make the

most of marginal coverage. Because coverage is generally uplink-

limited, the fixed terminals transmitter is typically much morepowerful than a portable handsets. Power is provided from a plug-in

AC supply, generally with battery backup1

Obviously, this CPE requires a more involved installation than a handset

or a desktop telephone. Moreover, it is customized, with a high-power

amplifier that increases costs and power consumption. The responsibility

for power and battery backup is an issue, which can add substantially to

operators maintenance expenses and processes.

Its also very important to note that a high-power CPE shooting into a

distant base station is a classic example of the near/far problem. While

this subscriber may have access to this cell, subscribers near the basestation may be washed out by the distant subscriber, and may not be

able to access the network.

1Wireless for Universal Access, by Elliott Drucker, Wireless Week, August 15, 2004 (www.wirelessweek.com)

Expanding coverage is complex

and expensive.


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Conclusion

The key criteria in deploying a rural network include the deployment of a

flexible, economical backhaul network. In addition, since rural networks

are small, initial network start up costs must be minimal. The technology

selected must be easy to plan, and easily expand to handle isolated

pockets of subscribers. To foster economic development, the networkmust provide adequate Internet access, and should offer an economical

migration path to broadband, at least for selected subscribers.

CDMA networks do not meet any of the key criteria outlined above for

deploying an economical rural universal access network. Using CDMA-

450 for rural access appears to have natural appeal. After all, lower

frequencies propagate better than higher frequencies; every engineer

understands this concept. However, the planning aspects of CDMA, such

as capacity/range trade-offs and the near/far problem, are less

understood by operators, particularly those with no experience in the

technology. Before coming to the conclusion that fixed CDMA-450 is an

economical technology for rural networks, operators must conduct careful

network engineering to understand how many base stations will really be

required to serve all of the targeted communities.

In addition, CDMA-450, like all mobile technologies requires a complex

and expensive core network. Adding data service to the network will

make the core network even more complex. Broadband service requires

substantial upgrades of both the base stations and the core network.

SR Telecoms point-to-multipoint systems have been optimized for 20

years to address the challenges of rural network deployments. They are

self-backhauling, offer low initial costs, integrate easily into fixed-

networks, and offer carrier-class voice and data performance. In short, if

all aspects of a solution are considered in the business case (core

network, civil works, backhaul network, installation, maintenance) an

optimized rural technology will often prove to be operators most

economical solution.


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Acronym Glossary

3G: Third Generation's application

ADPCM: Adaptive Differential Pulse Code Modulation

BTS: Base Terminal Station

EDGE: Enhanced Data Rates for Global Systems for Mobile

Communications Evolution

FGSM: Fixed Global System for Mobile communications

FWA: Fixed Wireless Access

GPRS: General Packet Radio Service

GSM: Global System for Mobile communications

PMP: Point-to-Multipoint

PSTN: Public Switched Telephone Network

R&D: Research and Development

TRU: Transceiver Unit

TRX: Transceiver / Receiver

WLL: Wireless Local Loop

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Inc. All rights reserved 2005. All other trademarks are property of their

owners. Information subject to change without notice.

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