WiMAX Technology Overview: An SEO-Optimized Title

WiMAX Technology Overview

Version: 1
Date: May, 2008
Author: Sergio Cruzes

WiMAX Technology Overview Page 1 of 27

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 W IMAX 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 narrowband
data 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 and
desirable for both fixed and mobile clients. Finally, recent advances in Radio Frequency
technology, coding algorithms, Medium Access Control (MAC) protocols, and packet
processing techniques have made it possible to achieve the high bandwidths of Wi-Fi
networks over the extended coverage areas of cellular networks. This fusion, which is
realized in the IEEE 802.16 architecture, not only addresses the traditional last mile
problem, but also supports nomadic and mobile clients on the go. This architecture
enables a deployment model, where high-speed Internet access is provided over large
portions of urban areas and along major freeways. In this model, laptops and PDAs
operate as Subscriber Stations (SS’s) allowing users to connect to the network in
parks, buildings, or wherever they may be.

So, WiMAX is a wireless communication technology that travels much farther than
today’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 distances
at 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 the
promise of WiMAX.

WiMAX (Worldwide interoperability for Microwave Access) is defined as an existing
new 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 Introduction
Mobile WiMAX is a broadband wireless access technology based on IEEE 802.16-2004
and IEEE 802.16e-2005 air-interface standards.

WiMAX was developed to address the challenges and high cost of deploying copper
wires and optical fibers. It is possible to mention some applications such as:
- Backhaul: uses point-to-point antennas to connect aggregate subscriber
sites to each other and to base stations across long distances.
- Last mile: uses point-to-multipoint antennas to connect residential or
business 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 The
Innovator´s Dilemma from Clayton M. Christensen. This book is about the failure of
good companies to stay atop their industries when they confront certain types of
market and technological change. In this book, it is described how disruptive
technologies have precipitated the failure of leading products and their well-managed
companies. “Disruptive technologies bring to a market a very different value proposition
than had had been available previously. Product based on this technology are typically
cheaper, simpler, smaller, and, frequently, more convenient to use”. We can mention
some examples such as the personal computer and transistors (relative to vacuum
tubes).
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 is
bypassed.
- Disruption for Cable TV and Satellite TV Companies: a technology called TV
over Internet Protocol (TvoIP) does for cable TV what VoIP does for
telephone companies.
- Disruption for Cell Phone Companies: VoIP technologies may be used for
mobile telephony to replace incumbent cell phone technologies.
- Disruption for the Backhaul Industry: The building of big and expensive fiber
optic 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 and
where alternative technologies in switching and access offer a competitor lower barriers
to 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 transmission
technique that has gained great attraction recently and recognized as an excellent
modulation 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), which
enables channels to overlap without losing their individual characteristics
(orthogonality). This is a more efficient use of the spectrum and enables the channels
to be processed at the receiver more efficiently.
All signal processing in OFDM is performed in the frequency domain and before
transmission 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 very
susceptible to phase and frequency offsets.

To understand how OFDM works, it is useful to start with the FDM (Frequency Division
Multiplexing) 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 closely
spaced to each other without causing interference, removing guard bands between
adjacent sub-carriers. This is possible because the sub-carriers are orthogonal (the
peak of one sub-carrier coincides with the null of an adjacent sub-carrier. Figure 2.2
shows 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 Amplitude
Modulation (QPSK, 16-QAM, 64-QAM).

OFDM needs less bandwidth than FDM to carry the same amount of information which
means 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 into
many N sub-carriers, the bit period is N times greater than the original signal. So, the
multipath delay needs to be N times greater than the multipath delay which would
cause 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.

Total Symbol
Ts Period

Cyclic
Prefix
Data Payload

Tg Tu Useful Symbol
Period
Tg

Figure 2.3: OFDM Symbol Duration

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


Like OFDM, OFDMA employs multiple closely sub-carriers, but the sub-carriers are
divided 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 units
depending on their channel conditions and data requirements. Using sub-
channelization, a WiMAX CPE concentrates the total available power to a smaller
number of sub-channels instead of dividing the total amount of power to the entire
channel bandwidth. So, there is high energy per sub-channel and the CPE may move
farther 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, the
maximum possible number of sub-channels may be used to provide better rates. When
mobile is located far from BTS, it is necessary to reduce the number of sub-channels
used 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)

P ilot
D ata DC S ub-c a rriers
S ub -ca rrie rs S ub -c arrier

G u a rd
S u b-c arriers

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 equal
to 48 data tones (sub-carriers).
The most commonly used channel bandwidth values in E system are multiple of 1.25
MHz. The N_FFT size scales with the channel bandwidth that means the wider the
channel bandwidth, the higher the N_FFT, so the bandwidth of each sub-carrier is
always 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 by
spreading 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


• 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 only
TDD.

TDD is a technique in which the system transmits and receives within the same
frequency channel, assigning time slices for transmit and receive modes. FDD requires
two separates frequencies generally separated by 50 to 100 MHz within the operating
band. 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 the
downlink and uplink are transmitted on separate channels

Frame Downlink T Uplink
Header Subframe G Subframe

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 allocate
downlink and uplink resources (e.g.: a 10 MHz channel may divided as 3.3 MHz for
uplink and 6.7 MHz for downlink).

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


2.2.7 Mobility
The mobile WIMAX suports optimized handover schemes with latencies less than 50
ms 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 each
end of the wireless link.

The use of multiple-input multiple-output (MIMO) antenna techniques along with flexible
sub-channelization schemes, adaptive modulation and coding enable the mobile
WiMAX 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 (commonly
known as the Alamouti Code). This technique captures diversity gains by sending a
single data stream in two parts out of two antennas, interleaved with
transformed/conjugated versions of the same information, so that the receiver has
higher probability of successfully extracting the desired signal.

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

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 signals
transmitted 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 the
transmit antennas and the columns represent time. Each element of the matrix
indicates which symbol is to be transmitted from each antenna and when. Figure 2.7
shows 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 complex
symbols are then fed into the Encoder, which maps the symbols onto the transmit
antennas according to the matrix above.

The code works with a pair of symbols at a time and it takes two periods to transmit the
two symbols. Therefore it has the same rate as the data stream that enters the encoder
but 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 data
rate 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 associate
antennas, up to twice the capacity of a single antenna system can be achieved. This is
particularly useful in urban deployments where long reach is less important than high
data rate at the end user device. In WiMAX, spatial multiplexing is possible using Matrix
B 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 the
matrix represent the transmit antennas; there is no time element because Matrix B
operates over a single time interval. Each element of the matrix indicates which symbol
is to be transmitted from which antenna. In this system, two symbols are transmitted in
a 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 an
efficient manner. Upon entering the network, each Subscriber Station (SS) creates one
or 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 Quality
of Services (QoS) differentiation. It performs link adaptation and Automatic Repeat
Request (ARQ) functions to maintain target Bit Error Rates (BER) while maximizing the
data throughput. The MAC layer also handles network entry for SS’s that enter and
leave the network, and it performs standard Protocol Data Unit (PDU) creation tasks.
Finally, the MAC layer provides a convergence sub layer that supports Asynchronous
Transfer 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 the
network entry process with the desired BS. The network entry process is divided into
DL channel synchronization, initial ranging, capabilities negotiation, authentication
message 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 the
network entry process, the SS creates one or more service flows to send data to the
BS. 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 system
acquisition and radio mobility handling procedures.

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

• Unsolicited Grant Services (UGS): UGS is designed to support Constant Bit
Rate (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 as
MPEG 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 regular
basis.

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

2.3.3 Scheduling and Link Adaptation
[3] The goal of scheduling and link adaptation is to provide the desired QoS treatment
to 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: scheduling
the usage of the airlink among the SS’s and scheduling individual packets at the BSs
and SS’s.
The airlink scheduler runs on the BS and is generally considered to be part of the BS
MAC layer. This scheduler determines the contents of the DL and UL portions of each
frame. When optional modes such as transmit diversity, AAS, and MIMO are used, the
MAC 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 these
modes


4 ARCHITECTURE
[4] While the 802.16 defines the Broadband Wireless Access (BWA) technology (it
limits 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 successful
interoperability between equipment from various vendors and between networks of
various operators. The WiMAX Forum has defined such architecture which is an All IP-
based. [30] This architecture contains procedures and protocols for how the network
will 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 enable
mobility between different network elements.
This network architecture has two elements: the Access Service Network (ASN) and
the Connectivity Service Network (CSN) which can be compared to the BSC and MSC
in a CDMA network.
A depiction of the network architecture is presented in the network reference model in
Figure 4.1.

Figure 4.1 – WiMAX end-to-end architecture

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


The ASN thus serves as management of the WiMAX radio links only, leaving much of
the high level management to the CSN.
The ASN is deployed by a business entity called Network Access Provider (NAP) which
provides a SS with L2 connectivity to a WiMAX radio network and connects users to a
Network Service Providers (NSP) managing a CSN. The ASN Gateway serves as the
interconnection between ASN and CSN.
This logical partition of the access network from the service network enables individual
access networks to be deployed, e.g. in the case of where several NAPs can form
cooperation 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 for
Internet access, routers, server or proxies for AAA, DHCP servers, Home Agent (HA),
and DNS servers. It also handles admission and policy control, mobility between ASN
and specific WiMAX services such as Location Based Services or Law Enforcement
Services.
The CSN is deployed by a business entity called NSP. WiMAX subscribers enter
contractual agreements on services, QoS, bandwidth, etc with the NSP and access
these services through the ASN it is currently homed in.
The user can then use the service providers network or roam to networks deployed by
other companies as long as the home network has a roaming agreement with the
visitor network. The foreign ASN uses its own management functions of the foreign
CSN, and proxies them to the home network, or communicates directly with the home
network CSN.

4.1 WiMAX Network Reference Model
The WIMAX Network Reference Model (NRM) is a logical representation of the network
architecture. The NRM identifies functional entities and reference points over which
interoperability is achieved between functional entities. The intent of the NRM is to
provide multiple implementation options for a given functional entity, and yet achieve
interoperability among different realizations of functional entities.

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


Figure 4.2 – WIMAX Network Reference Model

Profiles

The WiMAX Forum’s Network Working Group (NWG) is tasked to create higher level
networking specifications for WiMAX systems, beyond what is defined in the scope of
802.16. 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 a
centralized controller network element, called the ASN/Gateway. Profile B embeds the
ASN functionality inside the base station, such that an external ASN/GWY is not
needed.

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

It should be noted that all three profiles include exactly the same functionality. The only
difference is which network elements host that function, and therefore what message
flows 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 between
the MS and ASN with respect to the air interface (MAC and PHY) specifications
based on the WiMAX 802.16e-2005 standards.

Reference Point R2

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

Reference Point R3

Reference Point R3 consists of the set of control plane protocols between the ASN
and the CSN to support AAA, policy enforcement and mobility management
capabilities. 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 CSN
operated 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 by
the WiMAX Forum Network Working Group (NWG), Some vendors implemented a
proprietary Reference Point R6.

Reference Point R6 consists of the set of control and bearer plane protocols for
communication between the BTS and ASG. The bearer plane is the data path
between the BTS and ASG. The control plane includes protocols for data path
establishment, 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 target
BS have radio resource available and to notify the MS

• Basic network entry including user or MS, or both authentication and
authorization whereby an user or device establishes a session with the
WiMAX 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 within
the ASN

• User session tear-down where the user either gracefully (or ungracefully)
shuts down the MS and the WiMAX network elements take appropriate
actions 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 network
architecture.

Batteries


5 RF PLANNING
RF planning is a key to a successful wireless network deployment. It ensures that base
stations 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 at
individual subscriber locations.

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

The following sections will describe the main parameters that are needed to be defined
in 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 TDD
systems.

In cellular systems which are FDD, Transmitter frequency blocks are separated from
Receiver frequency blocks by an amount of 20 to 50 MHz. So, the possibility of a BTS
transmitter 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 the
same 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 is
not ideal neither the transmitter cavity filter. So, out-of-band signals are transmitted
causing interference with neighbor systems. In most of the cases a FDD system is
neighbor of a TDD WiMAX system. In this scenario the following possible interference
conditions should be analyzed:

• WiMAX Base Station transmitter interfering into a FDD frequency neighbor
receiver

• 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 rejection
values), transmitter spectral mask, receiver sensitivity for both systems (WiMAX and
neighbor)


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

2,500 2,5
2,506 2,536 2,566 2,578 2,694 2,700

guard WiMA WiMAX guard MMD guard
X
S
6 MHz 12 MHz 6 MHz

Figure 5.1 – Wind Telecom spectrum

5.2 Link Budget
A fundamental concept in any communications system is the link budget, or the
summation of all gains and losses in a communication system. The result of the link
budget is the transmit power required to present a signal with a given signal-to-noise
ratio (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 calculating
a 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 signal
level where the RF planning will be based on.

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

• 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


5.2 Propagation model
An important and crucial requirement for a Wireless project is to have an accurate
description 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 higher
than 2 GHZ. SUI models are recommended by the industry for WiMax applications by
WiMax Forum. SUI models do not correspond exactly to the traditional morphology
class models. They do represent certain morphology profiles based on the study
conducted by Erceg.

Some vendors decided to use SUI propagation model for the initial design as the best
alternative to a time prohibitive model tuning option.

The real life has shown that SUI models are optimistic. So, careful use of this model is
very important. I suggest that sales proposals state that coverage radius are
based 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 / flat
with moderate-to-heavy tree density or low-density suburban

3. Environment C: the minimum path loss category, mostly flat terrain with light tree
densities 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 in
several ways) and takes into account clutter classes and effective antenna heights in
order 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 for
suburban 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 CW
measurements) 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.

SPM 2.5 DR
Link Budget SUI Radius
Environment radius (km) SUI Models (km)
Urban mid 0.41
Urban low 0.58 A 0.86
Suburban
mid 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 Shakedown
Before the first BTS is turned on, the spectrum needs to be clear down to -111 dBm at
all locations within the network coverage area.

After a BTS has been installed and commissioned, the RF engineer needs to audit the
installation in order to verify that azimuths, tilts and installation in general are as
specified. After that, RF engineer needs to analyze the sweep tests. The sweep tests
consist 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. The
antenna is not attached for this test. The cable assembly should be terminated
with a calibrated short termination.

• Return Loss – No Antenna
with a calibrated 50 ohm load

• Distance to Fault – No Antenna
with a calibrated 50 ohm load

• Return Loss – With Antenna
Based on the antenna, top jumper, feeder cable and bottom jumper cable
assembly.

• Distance to Fault – With Antenna
Based on the antenna, top jumper, feeder cable and bottom jumper cable
assembly

The RF engineer needs to calculate the pass/fail values for each site. Find below an
example 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 the
verification of BTS call processing (network entry, network re-entry, initial and periodic
ranging, 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, Michael
Carlberg and Annelie Dammander, Master’s Thesis in Computing Science, UMEA
University, Dept of Computing Science, Sweden.


WiMAX Technology Overview: An SEO-Optimized Title

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

En vedette

En vedette (7)

Similaire à WiMAX Technology Overview: An SEO-Optimized Title

Similaire à WiMAX Technology Overview: An SEO-Optimized Title (20)

Plus de Sergio Cruzes

Plus de Sergio Cruzes (8)

WiMAX Technology Overview: An SEO-Optimized Title