WiMAX Basic Sl v2 Sergio Cruzes

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WiMAX Technology Overview Page 1 of 27

WiMAX Technology Overview

Version: 1

Date: May, 2008

Author: Sergio Cruzes




1 INTRODUCTION .......................................................................................................... 3 2 MOBILE WIMAX .......................................................................................................... 4

2.1 INTRODUCTION .......................................................................................................................... 4 2.2 PHYSICAL LAYER....................................................................................................................... 5

2.2.1 OFDM and OFDMA .................................................................................................... 5 2.2.2 Uplink Sub-Channelization ......................................................................................... 7 2.2.3 OFDMA symbol structure ........................................................................................... 8 2.2.4 Duplex mode .......................................................................................................... 9 2.2.6 Security ...................................................................................................................... 9 2.2.7 Mobility ..................................................................................................................... 10 2.2.8 MIMO (Multiple Input Multiple Output) ...................................................................... 10

2.3 MAC LAYER............................................................................................................................ 12 2.3.1 Network Entry ........................................................................................................... 13 2.3.2 Quality of Service (QoS) .......................................................................................... 14 2.3.3 Scheduling and Link Adaptation ............................................................................... 14

4 ARCHITECTURE ....................................................................................................... 15 4.1 WIMAX NETWORK REFERENCE MODEL............................................................................... 16

5 RF PLANNING ................................................................................................................. 20 5.1 SPECTRUM AND GUARD-BANDS................................................................................................ 20 5.2 LINK BUDGET .......................................................................................................................... 21 5.2 PROPAGATION MODEL ............................................................................................................. 22

5.2.1 SUI Models ............................................................................................................... 22 5.2.2 SPM Models ............................................................................................................. 22 5.2.3 CW Measurements .................................................................................................. 23

5 RF OPTIMIZATION .................................................................................................... 25 7.1 RF SHAKEDOWN ..................................................................................................................... 25

6 REFERENCES ........................................................................................................... 27




1 INTRODUCTION

The success of cellular networks in the last decade and the integration of narrowbanddata solutions into these networks are the first indications that wireless solutions may

be able to solve the last mile, the consumer broadband problem. The emergence of Wi-Fi networks has demonstrated that high-bandwidth radio networks are feasible anddesirable for both fixed and mobile clients. Finally, recent advances in Radio Frequencytechnology, coding algorithms, Medium Access Control (MAC) protocols, and packetprocessing techniques have made it possible to achieve the high bandwidths of Wi-Finetworks over the extended coverage areas of cellular networks. This fusion, which isrealized in the IEEE 802.16 architecture, not only addresses the traditional last mileproblem, but also supports nomadic and mobile clients on the go. This architectureenables a deployment model, where high-speed Internet access is provided over largeportions of urban areas and along major freeways. In this model, laptops and PDAsoperate as Subscriber Stations (SS’s) allowing users to connect to the network inparks, buildings, or wherever they may be.

So, WiMAX is a wireless communication technology that travels much farther thantoday’s Wi-Fi. Theoretically, a WiMAX signal can travel up to 50 km [1].

The true promise of WiMAX is to provide faster wireless speeds over longer distancesat a lower cost to the carrier and ultimately to the customer.

Internet at home, Internet at office, Internet on the go. Internet everywhere, that’s thepromise of WiMAX.

WiMAX (Worldwide interoperability for Microwave Access) is defined as an existingnew technology that delivers high speed access over the air. It comes in two flavors:

Fixed and Mobile.

This article will focus the discussion on the Mobile WiMAX.




2 MOBILE WIMAX

2.1 IntroductionMobile WiMAX is a broadband wireless access technology based on IEEE 802.16-2004and IEEE 802.16e-2005 air-interface standards.

WiMAX was developed to address the challenges and high cost of deploying copperwires and optical fibers. It is possible to mention some applications such as:

- Backhaul: uses point-to-point antennas to connect aggregate subscribersites to each other and to base stations across long distances.

- Last mile: uses point-to-multipoint antennas to connect residential orbusiness subscribers to the base station

- Large coverage access: uses base stations, subscriber stations, and Wi-Fi

solutions, such as mesh networks, to cover a large area.At the end of year 2000, I received from Alfredo de Cardenas, a book called TheInnovator´s Dilemma from Clayton M. Christensen. This book is about the failure ofgood companies to stay atop their industries when they confront certain types ofmarket and technological change. In this book, it is described how disruptivetechnologies have precipitated the failure of leading products and their well-managedcompanies. “Disruptive technologies bring to a market a very different value propositionthan had had been available previously. Product based on this technology are typicallycheaper, simpler, smaller, and, frequently, more convenient to use”. We can mentionsome examples such as the personal computer and transistors (relative to vacuumtubes).

WiMAX seems to fit the above criterias.

As mentioned by Frank Ohrtman, in his book WiMAX Handbook, WiMAX is a- Disruption for Telephone Companies as WiMAX replaces the access portion

of the PSTN. With Voice over Internet Protocol (VoIP), the PSTN isbypassed.

- Disruption for Cable TV and Satellite TV Companies: a technology called TVover Internet Protocol (TvoIP) does for cable TV what VoIP does fortelephone companies.

- Disruption for Cell Phone Companies: VoIP technologies may be used formobile telephony to replace incumbent cell phone technologies.

- Disruption for the Backhaul Industry: The building of big and expensive fiberoptic networks marked the end of the1990s and the beginning of 2000s.WiMAX is a mean of simply expanding these networks.

Also, consumers will only enjoy the benefits of competition in the local loop when andwhere alternative technologies in switching and access offer a competitor lower barriersto entry and exit in the telecommunications market.




2.2 Physical Layer

2.2.1 OFDM and OFDMA

Orthogonal frequency division multiplexing (OFDM) is a multi-carrier radio transmissiontechnique that has gained great attraction recently and recognized as an excellentmodulation for wireless data networks.

OFDM is what put the max in Wimax [1].

The key issue of OFDM is the division of the transmission channel into several sub-channels.

OFDM is based on a mathematical process called Fast Fourier Transform (FFT), whichenables channels to overlap without losing their individual characteristics(orthogonality). This is a more efficient use of the spectrum and enables the channelsto be processed at the receiver more efficiently.

All signal processing in OFDM is performed in the frequency domain and beforetransmission the signal is transformed to the time domain. OFDM is very tolerant to

intersymbol interference and it is spectrally efficient. On the other hand, OFDM is verysusceptible to phase and frequency offsets.

To understand how OFDM works, it is useful to start with the FDM (Frequency DivisionMultiplexing) technique as depicted in the Figure 2.1

Figure 2.1 – FDM: space is included between two adjacent sub-carriers.

Similar to FDM, OFDM also uses multiple sub-carriers but the sub-carriers are closelyspaced to each other without causing interference, removing guard bands betweenadjacent sub-carriers. This is possible because the sub-carriers are orthogonal (thepeak of one sub-carrier coincides with the null of an adjacent sub-carrier. Figure 2.2shows an OFDM signal.




Figure 2.2: OFDM signal

In a OFDM system, a very high data rate stream is divided into multiple parallel low

data rate streams. Each smaller data stream is then mapped to individual data sub-carrier and modulated using some Phase Shift Keying Quadrature AmplitudeModulation (QPSK, 16-QAM, 64-QAM).

OFDM needs less bandwidth than FDM to carry the same amount of information whichmeans higher spectral efficiency. Also, OFDM is more robust to NLOS (non line-of-sight) environments.

The effect of ISI (inter symbol interference) is eliminated as the signal is divided intomany N sub-carriers, the bit period is N times greater than the original signal. So, themultipath delay needs to be N times greater than the multipath delay which wouldcause ISI in the original signal. In addition each subcarrier period is divided into a cyclic

prefix period and a data payload period as depicted in the Figure 2.3.

Figure 2.3: OFDM Symbol Duration

The main purpose of adding CP to OFDM symbols is to also help to combat the effectof multipath.

Data PayloadCyclic

Prefix

gT

uT

sT

gT Useful Symbol

Period

Total Symbol

Period

Data PayloadCyclic

Prefix

gT

uT

sT

gT Useful Symbol

Period

Total Symbol

Period




Like OFDM, OFDMA employs multiple closely sub-carriers, but the sub-carriers aredivided into groups of sub-carriers. Each group is named a sub-channel. The sub-carriers that form a sub-channel do not need to be adjacent.

2.2.2 Uplink Sub-Channelization

Sub-channelization defines sub-carriers that can be allocated to the mobile unitsdepending on their channel conditions and data requirements. Using sub-channelization, a WiMAX CPE concentrates the total available power to a smallernumber of sub-channels instead of dividing the total amount of power to the entirechannel bandwidth. So, there is high energy per sub-channel and the CPE may movefarther away from the BTS. However, throughput rate is decreased as few sub-channels are used.

So, when mobile is located close to BTS there is high CINR per sub-channel, themaximum possible number of sub-channels may be used to provide better rates. Whenmobile is located far from BTS, it is necessary to reduce the number of sub-channelsused to improve the CINR per sub-carriers and concentrate the power in fewer sub-carriers. Figure 2.4 shows how sub-channelization works.

(a) (b)

Figure 2.4: Subchannelization

(a) CPE available power is shared among all sub-carriers

(b) CPE available power is concentrated in only one sub-carrier




2.2.3 OFDMA symbol structure

There are three types of OFDMA sub-carriers:

• Data sub-carriers for data transmission

• Pilot sub-carriers for estimation and synchronization purposes

• Null sub-carriers for no transmission at all, used for guard bands (left and right)and DC carrier (used at the center of the channel)

Figure 2.5 shows the WiMAX channel (1024 FFT)

Figure 2.5: WiMAX channel with 1024 sub-carriers (1024 FFT)

Active (data and pilot) sub-carriers are grouped into subsets of sub-carriers called sub-channels. The WiMAX OFDMA PHY supports channelization in both DL and UL. The

minimum frequency-time resource unit of sub-channelization is one slot, which is equalto 48 data tones (sub-carriers).

The most commonly used channel bandwidth values in E system are multiple of 1.25MHz. The N_ FFT size scales with the channel bandwidth that means the wider thechannel bandwidth, the higher the N_FFT, so the bandwidth of each sub-carrier isalways the same regardless of channel bandwidth:

- 1.25 MHz, N_ FFT = 128- 5 MHz, N_ FFT = 512- 10 MHz, N_ FFT = 1024- 20 MHz, N_ FFT = 2048

The sub-carriers forming one sub-channel may be, but not need to be, contiguous.

Different ways of grouping sub-carriers into channels in 802.16 are called permutations.There are three main permutations:

• FUSC – Full Usage of Sub-channels (DL only): achieves frequency diversity byspreading the sub-carriers over the entire band

• PUSC – Partial Usage of Sub-channels (UL and DL)o Groups the sub-carriers into tiles to enable fractional frequency reuse

scheme (FFRS)o Still has distribution of sub-carriers across band for each sub-channel

GuardSub-carriers

PilotSub-carriersData

Sub-carriersDC

Sub-carrier

GuardSub-carriers

PilotSub-carriersData

Sub-carriersDC

Sub-carrier




• AMC – Adaptive Modulation and Coding (UL and DL)o Uses adjacent sub-carriers for each sub-channel for use with beam

forming

2.2.4 Duplex mode

The IEEE 802.16e-2005 air-interface supports TDD (Time Division Duplexing) and FDD(Frequency Division Multiplexing). The initial version of the WiMAX profile supports onlyTDD.

TDD is a technique in which the system transmits and receives within the samefrequency channel, assigning time slices for transmit and receive modes. FDD requirestwo separates frequencies generally separated by 50 to 100 MHz within the operatingband. TDD provides an advantage where a regulator allocates the spectrum in an

adjacent block. With TDD, band separation is not needed, as is shown in Figure 3.1.Thus, the entire spectrum allocation is used efficiently both upstream and downstream.

In FDD systems, the downlink and uplink frame structures are similar except that thedownlink and uplink are transmitted on separate channels

Figure 3.1 - A TDD subframe (TG is a guard-band)

One of the great advantages of the TDD mode is the freedom to dynamically allocatedownlink and uplink resources (e.g.: a 10 MHz channel may divided as 3.3 MHz foruplink and 6.7 MHz for downlink).

2.2.6 Security

The mobile WiMAX incorporates the most advanced security features that are currentlyused in wireless access systems. These include Extensible Authentication Protocol(EAP) based authentication, Advanced Encryption Standard (AES) basedauthentication encryption, and Cypher-based Message Authentication Code (CMAC)and Hashed Message Authentication Code (HMAC) based control message protectionschemes.

FrameHeader

DownlinkSubframe

TG

UplinkSubframe




2.2.7 Mobility

The mobile WIMAX suports optimized handover schemes with latencies less than 50ms to ensure real-time applications such as VoIP.

2.2.8 MIMO (Multiple Input Multiple Output)

MIMO describes systems that use more than one radio and antenna system at eachend of the wireless link.

The use of multiple-input multiple-output (MIMO) antenna techniques along with flexiblesub-channelization schemes, adaptive modulation and coding enable the mobileWiMAX technology to support both downlink and uplink high data rates

Matrix A MIMO

[2] Matrix A MIMO implements the rate 1 Space-Time Coding scheme (commonlyknown as the Alamouti Code). This technique captures diversity gains by sending asingle data stream in two parts out of two antennas, interleaved withtransformed/conjugated versions of the same information, so that the receiver hashigher probability of successfully extracting the desired signal.

Matrix A MIMO delivers higher link robustness, reducing fade margin by 5 to 6 dB, withlittle degradation as subscriber mobility increases. The impact on end-user data rate issmall.

The figure 2.6 depicts a 2 x 2 Matrix A MIMO operation:

Figure 2.6 – 2 x 2 Matrix A MIMO

The signal received by one of the antennas at the receiver is a mixture of the signalstransmitted from both of the transmit antennas.

The matrix is represented by:




X is the output of the encoder and S1 and S2 are the input symbols into the encoder. ‘*’denotes a complex conjugate of the symbol. The rows of the matrix represent thetransmit antennas and the columns represent time. Each element of the matrixindicates which symbol is to be transmitted from each antenna and when. Figure 2.7shows how this matrix works.

Figure 2.7 – Matrix A MIMO Tx Module

On the left hand side the binary bits enter a modulator, which converts binary bits into“symbols” according to the modulation to the modulation scheme. These complexsymbols are then fed into the Encoder, which maps the symbols onto the transmitantennas according to the matrix above.

The code works with a pair of symbols at a time and it takes two periods to transmit thetwo symbols. Therefore it has the same rate as the data stream that enters the encoderbut the error performance of the system is improved du to the coded information

transmitted by the system.




Matrix B MIMO

[2] For channels with a rich multipath environment it is possible to increase the datarate by transmitting separate information streams on each antenna in the DL direction.

Using sophisticated receiver technology, the different streams can be separated and

decoded. For example, using 2 transmit and 2 receive Tx/Rx chains and the associateantennas, up to twice the capacity of a single antenna system can be achieved. This isparticularly useful in urban deployments where long reach is less important than highdata rate at the end user device. In WiMAX, spatial multiplexing is possible using MatrixB MIMO.

The following matrix defines how the code works:

X is the output of the encoder and S1, S2 are the input into the encoder. The row of thematrix represent the transmit antennas; there is no time element because Matrix Boperates over a single time interval. Each element of the matrix indicates which symbolis to be transmitted from which antenna. In this system, two symbols are transmitted ina 1-symbol time duration thus providing a two-fold capacity increase.

2.3 MAC Layer

[3] The IEEE 802.16 MAC layer performs the standard Medium Access Control (MAC)function of providing a medium-independent interface to the 802.16 Physical Layer.

The main focus of the MAC layer is to manage the resources of the airlink in anefficient manner. Upon entering the network, each Subscriber Station (SS) creates oneor more connections over which their data are transmitted to and from the Base Station(BS). The MAC layer schedules the usage of the airlink resources and provides Qualityof Services (QoS) differentiation. It performs link adaptation and Automatic Repeat

Request (ARQ) functions to maintain target Bit Error Rates (BER) while maximizing thedata throughput. The MAC layer also handles network entry for SS’s that enter andleave the network, and it performs standard Protocol Data Unit (PDU) creation tasks.Finally, the MAC layer provides a convergence sub layer that supports AsynchronousTransfer Mode (ATM) cell and packet based networks.




2.3.1 Network Entry

In order to communicate on the network an SS needs to successfully complete thenetwork entry process with the desired BS. The network entry process is divided intoDL channel synchronization, initial ranging, capabilities negotiation, authenticationmessage exchange, registration, and IP connectivity stages. The network entry state

machine moves to reset if it fails to succeed from a state. Upon completion of thenetwork entry process, the SS creates one or more service flows to send data to theBS. Figure 2.8 depicts the network entry process.

Figure 2.8 – Network entry process.




2.3.2 Quality of Service (QoS)

The fundamental premise of the IEEE 802.16 medium access control (MAC)architecture is QoS.

Functions are the responsibility for connection establishment between the SS and the

network, scheduling of bandwidth among different users based on QoS, initial systemacquisition and radio mobility handling procedures.

[10] The 802.16 MAC provides QoS differentiation for different types of applicationsthat might operate over 802.16 networks. The 802.16 standard defines the followingtypes of services:

• Unsolicited Grant Services (UGS): UGS is designed to support Constant BitRate (CBR) services, such as T1/E1 emulation, and Voice Over IP (VoIP)without silence suppression.

• Real-Time Polling Services (rtPS): rtPS is designed to support real-time

services that generate variable size data packets on a periodic basis, such asMPEG video or VoIP with silence suppression.

• Non-Real-Time Polling Services (nrtPS): nrtPS is designed to support non-real-time services that require variable size data grant burst types on a regularbasis.

• Best Effort (BE) Services: BE services are typically provided by the Internettoday for Web surfing.

2.3.3 Scheduling and Link Adaptation

[3] The goal of scheduling and link adaptation is to provide the desired QoS treatmentto the traffic traversing the airlink, while optimally utilizing the resources of the airlink.Scheduling in the 802.16 MAC is divided into two related scheduling tasks: schedulingthe usage of the airlink among the SS’s and scheduling individual packets at the BSsand SS’s.

The airlink scheduler runs on the BS and is generally considered to be part of the BSMAC layer. This scheduler determines the contents of the DL and UL portions of eachframe. When optional modes such as transmit diversity, AAS, and MIMO are used, theMAC layer must divide the UL and DL subframes into normal, transmit diversity, AAS,

and MIMO zones, to accommodate SS’s that are to be serviced using one of thesemodes




4 ARCHITECTURE

[4] While the 802.16 defines the Broadband Wireless Access (BWA) technology (itlimits itself to Physical and MAC layers), it does not define end-to-end network

architecture. A standards-based end-to-end architecture is essential for successfulinteroperability between equipment from various vendors and between networks ofvarious operators. The WiMAX Forum has defined such architecture which is an All IP-based. [30] This architecture contains procedures and protocols for how the networkwill support e.g. mobility, security, internetworking and authentication to a WiMAX SS.In summary, it is defined the end-to-end architecture and the features to enablemobility between different network elements.

This network architecture has two elements: the Access Service Network (ASN) andthe Connectivity Service Network (CSN) which can be compared to the BSC and MSCin a CDMA network.

A depiction of the network architecture is presented in the network reference model inFigure 4.1.

Figure 4.1 – WiMAX end-to-end architecture

The ASN consists of one or several ASN Gateways and BSs, supplying WiMAX radiocoverage to a geographical area. A ASN manages MAC access functionality such aspaging, locating, Radio Resource Management (RRM), mobility between BSs and actsas a proxy for authentication and network mobility (Mobile IP) messages destined forthe CSN from the SS.




The ASN thus serves as management of the WiMAX radio links only, leaving much ofthe high level management to the CSN.

The ASN is deployed by a business entity called Network Access Provider (NAP) whichprovides a SS with L2 connectivity to a WiMAX radio network and connects users to aNetwork Service Providers (NSP) managing a CSN. The ASN Gateway serves as theinterconnection between ASN and CSN.

This logical partition of the access network from the service network enables individualaccess networks to be deployed, e.g. in the case of where several NAPs can formcooperation or contractual roaming agreement with each other or one or several NSP.

A CSN is a set of network functions that provides IP connectivity to WiMAX SSs,authentication functions, billing, mobility to the ASN. The CSN contains gateways forInternet access, routers, server or proxies for AAA, DHCP servers, Home Agent (HA),and DNS servers. It also handles admission and policy control, mobility between ASNand specific WiMAX services such as Location Based Services or Law EnforcementServices.

The CSN is deployed by a business entity called NSP. WiMAX subscribers enter

contractual agreements on services, QoS, bandwidth, etc with the NSP and accessthese services through the ASN it is currently homed in.

The user can then use the service providers network or roam to networks deployed byother companies as long as the home network has a roaming agreement with thevisitor network. The foreign ASN uses its own management functions of the foreignCSN, and proxies them to the home network, or communicates directly with the homenetwork CSN.

4.1 WiMAX Network Reference Model

The WIMAX Network Reference Model (NRM) is a logical representation of the networkarchitecture. The NRM identifies functional entities and reference points over whichinteroperability is achieved between functional entities. The intent of the NRM is toprovide multiple implementation options for a given functional entity, and yet achieveinteroperability among different realizations of functional entities.

Figure 4.2 shows an illustration of the WiMAX network reference model and itsreference points.




Figure 4.2 – WIMAX Network Reference Model

Profiles

The WiMAX Forum’s Network Working Group (NWG) is tasked to create higher levelnetworking specifications for WiMAX systems, beyond what is defined in the scope of802.16. The NWG has delivered three reference architecture models, from which

vendors and service providers can select their preferred solution.

These three “Profiles” are termed Profiles A, B and C. Profile A and C both use acentralized controller network element, called the ASN/Gateway. Profile B embeds theASN functionality inside the base station, such that an external ASN/GWY is notneeded.

In Profile A, the ASN/GWY includes Radio Resource Management (RRM), resulting ina cellular-like architecture, with ASN/GWY performing functions similar to a BSC orRNC. In Profile C, the RRM function is performed on the Base Station. In Profile B, allfunctions are in the Base Station, with no external controller.

It should be noted that all three profiles include exactly the same functionality. The onlydifference is which network elements host that function, and therefore what messageflows are needed. The result is a set of interfaces specifications, R1 through R8.




Reference Point R1

Reference Point R1 consists of the protocols of protocols and procedures betweenthe MS and ASN with respect to the air interface (MAC and PHY) specificationsbased on the WiMAX 802.16e-2005 standards.

Reference Point R2

Reference Point R2 consists of protocols and procedures between the MS and theCSN that is associated with authentication, services authorization and IP hostconfiguration management. This reference point is logical in that it does reflect adirect protocol interface between the MS and the CSN.

Reference Point R3

Reference Point R3 consists of the set of control plane protocols between the ASNand the CSN to support AAA, policy enforcement and mobility managementcapabilities. It also encompasses the bearer plane methods (such as tunneling) to

transfer data between the ASN and the CSN .

Reference Point R4

Reference Point R4 consists of the protocols and procedures between two ASG.

Reference Point R5

Reference Point R5 consists of the protocols for interworking between the CSNoperated by the home NSP and CSN operated by the visited NSP.

Reference Point R6

During the implementation of the Reference Point R6, as it was not fully defined bythe WiMAX Forum Network Working Group (NWG), Some vendors implemented aproprietary Reference Point R6.

Reference Point R6 consists of the set of control and bearer plane protocols forcommunication between the BTS and ASG. The bearer plane is the data pathbetween the BTS and ASG. The control plane includes protocols for data pathestablishment, modification and release in accordance with MS mobility events.Reference Point R6 facilitates the following functions:

• BS and ASG mutual discovery and communication path establishment

• Use of the ASG radio resource controller (RRC) to determine which targetBS have radio resource available and to notify the MS

• Basic network entry including user or MS, or both authentication andauthorization whereby an user or device establishes a session with theWiMAX network.

• Establishment of an initial service flow between the MS, BS, and ASG .




• ASN anchored mobility where the MS moves from one BS to another withinthe ASN

• User session tear-down where the user either gracefully (or ungracefully)shuts down the MS and the WiMAX network elements take appropriateactions to clean-up the user and MS context within the network.

Reference Point R8

Reference Point R8 consists of the protocols and procedures between two BTS. .Some WiMAX vendors do not currently include this reference point in its networkarchitecture.

Batteries




5 RF PLANNING

RF planning is a key to a successful wireless network deployment. It ensures that basestations are located and configured such that an operator has adequate coverage,

capacity and service availability in areas targeted within their business case.

The RF planning processes are designed to provide outputs that allow service quality atindividual subscriber locations.

Before starting the RF planning, the engineer needs to collect a lot of information inorder to determine the threshold levels the design will use.

The following sections will describe the main parameters that are needed to be definedin order to start the RF planning.

Based on the following sections, it is determined the RF threshold values that will be

used in the RF planning together with a validated propagation model.

5.1 Spectrum and Guard-bands

A very accurate definition of the spectrum and guard-band are necessary for TDDsystems.

In cellular systems which are FDD, Transmitter frequency blocks are separated fromReceiver frequency blocks by an amount of 20 to 50 MHz. So, the possibility of a BTStransmitter frequency interfere into another BTS receiver frequency is very low.

Mobile WiMAX systems use TDD. So, since in TDD systems both DL and UL use thesame channel, some analysis should be done regarding the WiMAX neighbor systems.The interference is explained by the fact that the spectral mask of a RF transmitter isnot ideal neither the transmitter cavity filter. So, out-of-band signals are transmittedcausing interference with neighbor systems. In most of the cases a FDD system isneighbor of a TDD WiMAX system. In this scenario the following possible interferenceconditions should be analyzed:

• WiMAX Base Station transmitter interfering into a FDD frequency neighborreceiver

• WiMAX mobile station transmitter interfering into a FDD frequency neighbor

receiver

• Neighbor FDD transmitter interfering into a WiMAX BTS receiver

• Neighbor FDD transmitter interfering into a WiMAX mobile station receiver

For this analysis, it is important to obtain filter characteristics (filter plot with rejectionvalues), transmitter spectral mask, receiver sensitivity for both systems (WiMAX andneighbor)




For Wind Telecom WiMAX deployment, it was assigned 60 MHz for the WIMAX systemwith guar-band of 6 MHz for the lower band (between WiMAX system and otheroperator and 12 MHz guard-band between Wind Telecom WIMAX and its MMDSsystem). Figure 6.1 depicts the Wind Telecom spectrum.

Figure 5.1 – Wind Telecom spectrum

5.2 Link BudgetA fundamental concept in any communications system is the link budget, or thesummation of all gains and losses in a communication system. The result of the linkbudget is the transmit power required to present a signal with a given signal-to-noiseratio (SNR) at the receiver to achieve a target BER.

For any wireless protocol, it is sufficient to consider factors such as path loss, noise,receiver sensitivity, and gains and losses from antennas and cables. Before calculatinga link budget, factors such as the frequency band must be determined.

The result of the link budget is the Maximum Allowed Path Loss or the threshold signallevel where the RF planning will be based on.

The following parameters should be agreed with customer in order to do the link budgetcalculation:

• Channel bandwidth (5 MHz or 10 MHz)

• Reuse factor

• Mobility or fixed environment

• Customer Mobile station (PCMCIA, indoor unit, outdoor unit)

• Coverage Area Reliability

• Target data rate at cell edge

• Average BTS antenna height

• BTS antenna gain

• TTLNA

2,694

WiMA

2,506 2,536

WiMAX

2,566

MMD

2,578

guard guard

2,500

guard

2,700

WiMA

2,5

WiMAX MMD

2,500 2,700

6 MHz 12 MHz 6 MHz




5.2 Propagation model

An important and crucial requirement for a Wireless project is to have an accuratedescription of the wireless channel.

A wireless channel is mainly characterized by path loss (including shadowing),

multipath delay spread, fading, co-channel and adjacent channel interference.

5.2.1 SUI Models

SUI model has been derived for the propagation modeling with the frequencies higherthan 2 GHZ. SUI models are recommended by the industry for WiMax applications byWiMax Forum. SUI models do not correspond exactly to the traditional morphologyclass models. They do represent certain morphology profiles based on the studyconducted by Erceg.

Some vendors decided to use SUI propagation model for the initial design as the bestalternative to a time prohibitive model tuning option. The real life has shown that SUI models are optimistic. So, careful use of this model isvery important. I suggest that sales proposals state that coverage radius arebased on SUI models for every proposal that predict cell radius with SUI models.Real coverage radius will be determined during the project implementation.

SUI Terrain Types

The SUI models are divided into three types of terrains, namely A, B and C.

1. Environment A: the maximum path loss category, hilly terrain with moderate-to-heavy tree densities or obstructed urban.

2. Environment B: intermediate path loss category, hilly with light tree density / flatwith moderate-to-heavy tree density or low-density suburban

3. Environment C: the minimum path loss category, mostly flat terrain with light treedensities or rural environment.

5.2.2 SPM Models

The Standard Propagation Model is a model (deduced from the Hata formulae)particularly suitable for predictions in the 150-3500 MHz band over long distances (1 <d < 20 km). This model uses the terrain profile, diffraction mechanisms (calculated inseveral ways) and takes into account clutter classes and effective antenna heights inorder to calculate path loss




The model may be used for any technology. It is based on the following formula:

Lmodel = K1 + K2log(d) + K3log(HTxeff) + K4.DiffractionLoss + K5log(d).log(HTxeff) + K6(HRxeff)+ Kclutterf(clutter) with,

K1: constant offset (dB)

K2: multiplying factor for log(d)

K3: multiplying factor for log(HTxeff)

HTxeff: effective height of the transmitter antenna (m)

K4: multiplying factor for diffraction calculation. K4 has to be a positive number.

Diffraction loss: loss due to diffraction over an obstructed path (dB)

K5: multiplying factor for log(d).log(HTxeff)

K6: multiplying factor for HRxeff

HRxeff: effective mobile antenna height (m)

Kclutter: multiplying factor for f(clutter)

f(clutter): average of weighted losses due to clutter

This model may be fine tuned using data collected in CW (continuous wave)measurements using the iplanner RF planning tool.

5.2.3 CW Measurements

CW measurements were done in a medium to big city. It was defined two models forsuburban areas (low and mid) and two models for urban areas (low and mid).

The following chart compares the path losses obtained with SPM (based on CWmeasurements) and with generic SUI models.

Figure 6.1 – Path loss comparison between SPM and SUI models




The Table 6.1 compares cell radius based on SPM tuned models against SUI models.

Environment

SPM 2.5 DRLink Budgetradius (km) SUI Models

SUI Radius(km)

Urban mid 0.41

A 0.86Urban low 0.58Suburbanmid 0.73 B 1.18

Suburban low 0.82 C 1.38

Table 6.1 – Cell radius comparison between SPM and SUI

It is possible to note the great difference between cell radius based on SPM and SUI.This implies a bigger difference when comparing the cell coverage areas.




5 RF OPTIMIZATION

7.1 RF ShakedownBefore the first BTS is turned on, the spectrum needs to be clear down to -111 dBm atall locations within the network coverage area.

After a BTS has been installed and commissioned, the RF engineer needs to audit theinstallation in order to verify that azimuths, tilts and installation in general are asspecified. After that, RF engineer needs to analyze the sweep tests. The sweep testsconsist of the following activities:

• Antenna Return Loss

Based on the sweep test equipment connected directly to the antenna. The

results should be compared against antenna specifications.

• Insertion Loss – No Antenna

Based on the top jumper, feeder cable and bottom jumper cable assembly. Theantenna is not attached for this test. The cable assembly should be terminatedwith a calibrated short termination.

• Return Loss – No Antenna

Based on the top jumper, feeder cable and bottom jumper cable assembly. Theantenna is not attached for this test. The cable assembly should be terminatedwith a calibrated 50 ohm load

• Distance to Fault – No Antenna

Based on the top jumper, feeder cable and bottom jumper cable assembly. Theantenna is not attached for this test. The cable assembly should be terminatedwith a calibrated 50 ohm load

• Return Loss – With Antenna

Based on the antenna, top jumper, feeder cable and bottom jumper cableassembly.

• Distance to Fault – With Antenna

Based on the antenna, top jumper, feeder cable and bottom jumper cableassembly

The RF engineer needs to calculate the pass/fail values for each site. Find below anexample of Return loss trigger values for an specific RF cabling:




Feeder Cable Length (m) Typical Retun Loss (dB) VSWR

8 >= 19.2 1.25

9 >= 19.2 1.25

10 >= 19.3 1.24

11 >= 19.4 1.24

12 >= 19.5 1.24

13 >= 19.6 1.23

14 >= 19.7 1.23

15 >= 19.8 1.23

If sweep tests results are within recommended values, the RF engineer can start theverification of BTS call processing (network entry, network re-entry, initial and periodicranging, subscriber basic capabilities negotiation, registration, FTP tests on DL and UL)



6 REFERENCES

[1] WiMAX Handbook – Building 802.16 Wireless Networks, Frank Ohrtman – McGraw-Hill Communications

[2] Multiple Antenna Systems in WiMAX, Airspan white paper

[3] IEEE 802.16 Medium Access Control and Service Provisioning – Intel

[4 WiMAX – A Study of Mobility and a MAC-layer Implementation in GloMoSim, MichaelCarlberg and Annelie Dammander, Master’s Thesis in Computing Science, UMEAUniversity, Dept of Computing Science, Sweden.

Documents

WiMAX Basic Sl v2 Sergio Cruzes