deaobcokCum
`
`.
`
`“a
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`Vii. 1..
`la
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`v2
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`

`WiMedia UWB
`TECHNOLOGY OF CHOICE FOR
`WIRELESS USB AND BLUETOOTH
`
`Ghobad Heidari
`Olympus Communication Technology of America USA
`
`@WI LEY
`
`John Wiley and Sons Ltd Publication
`
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`

`

`This edition first published 2008
`Sons Ltd
`02008 John Wiley
`
`Registered nffice
`Sons Ltd The Atrium Southem Gate Chichester West Sussex P019 8SQ United Kingdom
`
`John Wiley
`
`For details of our global editorial offices for customer services and for information about how to apply for
`permission to reuse the copyright msterial in this book please see our website st www.wiley.com
`
`The right of the author to be identified as the author of this work has been asserted irs accordance with the
`Copyright Designs and Patents Act 1988
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`and product names used in this book are trade names service marks trademarks or registered trademarks of
`is not associated with any product or vendor mentioned in this boot This
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`is designed to provide accurate and authoritative information in regard to the subject matter
`publication
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`is not engaged in rcodesing pmfesaional
`covered It is sold on the understanding that the publisher
`profeaaional advice or other expert assistance is required the services of
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`should be
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`catalogue record for this book is available from the British Library
`
`ISBN 978-0470-51834-2 H/B
`
`Set in ll/l4pt Tunes by Aptara Inc New Delhi
`India
`Printed in Great Britain by CPI Antony Rowe Chippenham Wiltshire
`
`SC
`UNiv
`
`SUPPLIER
`
`ORDER No
`
`DATE
`
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`iRARY
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`to3____
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`

`36
`
`41
`
`WiMedia UWB
`
`The WiMedia MAC is designed to be
`convergence platform to allow multiple
`independent MAC clients to coexist
`in the medium and provide the necessary
`QoS without causing interference to each other For now CW-USB High Rate
`over UWB WLP are planned to use WiMedia MAC as their
`Bluetooth and
`
`platform
`Hence the WiMedia MAC and PHY are designed to provide very high through
`put low power true ad hoc peer-to-peer WPAN capability with guaranteed QoS
`and
`of
`high focus on coexistence and mobility The short range about 10
`the PHY has the benefit of allowing spatial reuse of the HWB frequencies in short
`distances
`
`1.7 Terminology
`
`these
`
`throughout
`
`WiMedia specifications currently include PHY MAC sublayer MPI WLP PHY
`certification and Platform certification These specifications are privately held by
`they are published in Ecma International
`the WiMedia Alliance membership until
`or other standard bodies
`CW-USB is
`standard developed and published by the USB-IF This protocol
`builds on the WiMedia PHY and MAC as platform
`In this book we will
`focus our attention on the specifications
`included in
`standard WiMedia PHY layer and MAC sublayer
`ECMA-368
`as well
`as the CW-USB specification
`The focus of this section is to give the
`and terminology to understand the language of
`necessary background
`standards We will also describe the terminology used consistently
`this book
`WiMedia specifications frequently use the nomenclature of 150/051-IEEE 802
`Basic Reference Model
`which is also known simply as the 051 model for the
`hierarchical communication architecture Figure 1.13 depicts this model Without
`some basic understanding of this model and the terminology used in it
`to follow some parts of the ECMA-368 specification
`two layers PHY and the Data Link Layer
`The focus of this chapter is on the first
`DLL The latter
`into the MAC sublayer and Link Layer Control
`is further split
`LLC sublayer The ECMA-368 specithiation limits itself strictly to PHY and the
`MAC sublayer
`two layers Figure 1.14 illustrates the PHY MAC
`Further expanding the first
`and Device Management Entities DMEs the different Service Access Points
`relationships to the MAC Client
`SAPs and their
`It also shows the ter
`minology used to refer to the frames or packets of data at different
`
`it
`
`is difficult
`
`layersf
`
`sublayers
`
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`

`Introduction
`
`37
`
`4---
`
`-fr
`
`Device
`
`Device
`
`Figure 1.13
`
`ISO/OSI-IEEE 802 reference model
`
`Figure 1.14 shows the following
`
`The DME is the layer-independent
`device manager that controls the device as
`whole It can access all
`layers as needed This entity is not part of the OSI model
`is considered to be in different dimension
`but exists in every implementation It
`to the 051 model DME functionality is implementation dependent
`SAPs are formally defined to indicate the corresponding points of data or control
`communication between different layers as well as the DME
`PHY consists of two sublayers i.e the PHY Medium-Dependent PMD and the
`Physical Layer Convergence Protocol PLCP and management entity i.e the
`Physical Layer Management Entity PLMIE
`The PMD sublayer is the entity responsible for the physical transmission and
`peer PMD of another device
`reception of data over the wireless channel
`that defines PHYs service interface to the MAC
`The PLCP is the sublayer
`sublayer It makes the interface to the MAC independent of the PMD
`It has an interface to the DME
`The PLME is responsible for PHY control
`called the PLME SAP Through this SAP the DME can provide management
`services to the PLME
`The MAC sub-Layer Management Entity MLME is responsible for MAC con
`It has an interface to the DME called the PLME SAP Through this SAP the
`DME can provide management services to the MLME
`The MAC Service Data Unit MSDU is
`frame/unit of data that is passed from
`the MAC Client to the MAC through the MAC SAP
`The MAC Protocol Data Unit MPDU is the frame of data that the MAC proto
`from PHY to prepare for the MAC
`eol prepares for PHY to transmit or receives
`
`trol
`
`to
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`4-
`
`ci
`
`cc
`
`-J
`
`ncj
`
`flSVkI
`
`12
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`

`Introduction
`
`39
`
`Client As such it may contain the same data payload as the MSDU plus the
`the MAC protocol may decide that the
`MAC header
`In many cases however
`payload size received from the MAC SAP is not appropriate for transmission
`over the medium In this case the MAC protocol may aggregate several MSDUs
`into one MPDU or vice versa fragment an MSDU into multiple MPDUs The
`the MPDU to
`receiving MAC sublayer would then de-aggregate or defragment
`produce the required MSDU for its client
`The PLCP Service Data Unit PSDU is
`frame of data that is passed from the
`MAC sublayer to PHY specifically to the PLCP sublayer for transmission over
`the medium The PSDU is the same as the MPDU
`The PLCP Protocol Data Unit PPDU is the packet
`that is transmitted over the
`Medium It contains the PSDU PLCP Header and PLCP Preamble In the case
`of WiMedia PY the PLCP Header contains the MAC Header PHY Header
`the MAC Header and PHY Header The PLCP Preamble
`and FEC to protect
`and channel estimation sequences
`contains packet synchronization
`
`is
`
`concurrent clients as
`
`is not shown in Figure 1.14 is MAC Command Data Unit MCDU
`What
`which unlike the MSDU does not originate from MAC Client Instead it
`generated internally by the MAC protocol as part of its MAC-level command and
`control with peer MAC sublayers in other devices Thus the payloads of Beacon
`Command and Control frames defined in Section 4.4 of WiMedia MAC are con
`sidered MCDUs
`there is an additional sublayer called the MUX
`In ECMA-368 Appendix
`sublayer This sublayer has been defined in between the MAC sublayer and its
`way of providing coexistence among multiple protocols
`running on the WiMedia platform By adding this sublayer to Figure 1.14 we get
`Figure 1.15 shown only for one device
`In this figure MAC Clients
`are simultaneously able to operate with and
`get service from the MAC sublayer by way of the MUX sublayer
`very sim
`MUX Header
`to any MIJX service data units data
`ple service that adds
`frames from MAC Clients through the MUIX SAP before sending them to the
`MAC sublayer
`through the MAC SAP The IM1JX Header essentially identifies
`the data frame belongs to The peer MUX sub-
`the protocol MAC Client that
`recognizes to which protocol an incoming data frame be
`layer in Device
`longs by examining the MUX Header and routes the frame to the appropriate
`MUX SAP
`The OSI model of network communications
`is familiar to most communications
`practitioners from the seven-layer model shown in Figure 1.13 The OSI model
`specifies layer interactions as well Figure 1.16 shows the prototypical
`interactions
`between OSI
`
`layers
`
`device Station
`
`in this case generates
`
`request
`
`intended
`
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`

`

`40
`
`WiMedia UWB
`
`MAC
`Client
`
`MAC
`Client
`
`Figure 1.15 MUX sublayer as in between MAC and its concunent
`
`clients
`
`for Station
`
`and is intended
`In the figure the request
`is generated by Layer
`The request will progress down through the layers
`for Layer
`in Station
`until the PHYs exchange the message between devices Normally each layer adds
`header to the message as it progresses The message arrives at Station
`up through the layers each of which strips off any protocol information until
`message arrives at Layer
`Layer
`an indication to Layer
`
`rises
`
`the
`
`generates
`
`Station
`
`Request
`
`Confirm
`
`erN
`
`Station
`
`Layer NH
`
`Pt-tv Message
`
`PHY Message
`
`Intheation
`
`Response
`
`Figure 1.16 OSI protocol interactions
`
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`

`

`Introduction
`
`41
`
`response The response
`which then processes the request packet and generates
`confirm at Station
`follows the message path in reverse until it generates
`Layer
`
`N1
`
`tion
`
`In the WiMedia standard and many other networking standards it
`is possi
`intended for Sta
`and is not
`ble that
`is simply message to Layer
`request
`In this case the request will be acted on by Station As Layer
`without PIIY-to-PHY interaction Similar
`confirm is generated by this Layer
`indicationresponse pairs may be simply Layer
`interactions
`to Layer
`The WiMedia PHY never responds directly to messages from another WiMedia
`to the MAC to handle confirms and responses across the network
`PHY it
`is left
`Similar to other standards ECMA-368 has adopted the use of the words shall
`should and may strictly to mean as follows
`
`and the
`
`shall refers to mandatory requirements based on which conformance tests are
`
`designed
`should refers to recommendations implementers are allowed not to follow them
`may gives the permission to ad
`
`Other related words such as must will can etc are not used in any way to
`refer to any of the above meanings In keeping with the reader-friendly spirit of
`this book we will not necessarily adhere to the shallshouldmay nomenclature
`of the standard however
`to refer to an 8-bit number or sequence
`The standard also uses the word octet
`The word byte is avoided since it has been used in other contexts albeit infre
`quently to refer to numbers or sequences of fewer than bits Nevertheless in this
`book we will
`interchangeably use the words byte and octet to mean the same thing
`an exactly 8-bit long quantity
`The words device and host can mean different concepts
`in different contexts In
`this book we use the lower case device to literally mean an appliance whereas
`the upper case Device is mainly used in the USB/W-USB context
`to distinguish
`from Host By the same token the lower case host refers to the application
`that is the ultimate source or destination of data being transmitted On the other
`hand the upper case Host is strictly reserved for USB/W-USB master of protocol
`controlling the Devices
`The words PHY PHY layer and Physical
`layer are all synonymous So are
`MAC MAC sublayer and Medium Access Control We may also refer to the MAC
`and PHY sections of ECMA-368 as WiMedia MAC and WiMedia PRY respec
`
`it
`
`tively
`to the MAC chapter Chapter
`In going from the PHY chapter Chapter
`this book the reader will notice that there will be
`shift in our terminology In the
`
`of
`
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`42
`
`WiMedia HWB
`
`PHY chapter we will be focusing on packets whereas in the MAC chapter we will
`be switching to the termframes Although in other standards e.g CW-USB these
`they mean the same in ECMA-368 packet refers to
`two terms may be used as if
`the PPDU including the PLCP preamble
`non-data-bearing signal Frames on
`the other had are restricted to data units including any headers that get passed
`on the other hand intermixes
`form layer to layer in the 051 model Chapter
`the terms packet and frame to be consistent with the terminology of the CW-USB
`standard
`
`the PHY chapter
`there are references to the word symbol in differ
`Throughout
`ent contexts In order to avoid confusion we adopt the terminology given below in
`relation to the use of this word throughout
`the book The meaning of each of the
`terms will become clear in the PHY chapter
`
`zero-postfix
`
`an OFDM symbol plus
`symbol
`OFDM symbol
`the 128-sample output of the inverse fast Fourier transform
`IFEF in the modulator
`Quadrature Phase Shift Keying QPSK or Dual-Carrier Modu
`data symbol
`lation DCM symbol used at the input to the lEFT to create an OFDM symbol
`modulated symbol
`data symbol
`
`same as
`
`References
`
`Fema International Standard ECMA-368 High Rate Ultra Wideband PITY and MAC Standard 2nd edition
`December 2007
`
`http//www.ecma-biternational.org/publications/standards/Ecma-368.htm
`
`Barrett T.W History of UltrawideB and UWB radar
`communications pioneers and innovators
`Progress in Electromagnetics Symposium 2000 PIERS2000 Cambridge MA July 2000
`the speed of 802.11
`Bluetooth SIG Bluetooth
`technology
`http//www.bluetooth.com/
`to harness
`TECHNOLOGtTOJIARINESSJFBLSPEBJILOR8O21
`l.htm Febru
`
`Bluetooth/PressfSICl/BLUETOOTH
`ary 112008
`and Shetty
`Testing raises concerns over 802.11-based
`Aiello
`high-speed Bluetooth httpllwww
`Wireless Net DesignLine March 18 2008
`ma International Standard ECMA369 MACPRY Interfnce for BCMA-368 2nd edition httpffwww
`
`wirelessnetdesignline.com/howto/206903929
`
`December 2007
`ecma-international.org/publicationsfstandards/Ecma-369.htm
`revision 1.0 May 12 2005
`Agere Systems Inc et at Wireless Universal Serial Bus specification
`Open Systems Interconnection Basic Reference Model
`ISO/IEC 7498-11994 Information Technology
`The Basic ModeL
`
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`

`

`Physical Layer
`
`Robert
`
`Short PhD
`
`The Physical Layer PHY is the lowest
`layer in the 051 reference model see Fig
`ure 1.13 The PHY is the portion of the system that converts data into waveforms
`suitable for transmission across the physical medium In the case of UWB the
`medium is simply the air between the devices along with the frequency spectrum
`allocated by the regulatory bodies The waveform selected by the WiMedia com
`frequency-hop jed OFDM radio signal In this chapter we wifi describe
`mittee is
`both the waveform and related issues in detail
`We will present
`very high-level overview of the PITY in this chapter
`intro
`ducihg some of the basic concepts and terminologies We then explore each of the
`major sections in turn We use the terminology of the WiMedia standard to sim
`plify the readers understanding of the standard We wifi introduce the terminology
`here but discuss and redefine as we proceed
`this chapter we will adhere to the terminology of Section 1.7
`Throughout
`Figure 3.1 shows the structure of the PITY at the most basic level The PITY
`simply translates data that is received from the MAC to waveforms that may be
`transmitted over the UWB radio channel The purpose of this chapter is to describe
`that translation in simple but complete terms
`We will describe the entire PITY architecture in some detail but it
`is important
`to note that the WiMedia standard does not require any structure or architecture it
`only specifies the air interface All of the architectural detail found in the standard
`in which to describe the required PHY
`or this chapter is simply to provide
`to nomi
`functionality Most
`implementers however seem to find it convenient
`nally follow the structure described in the standard Similarly the standard rarely
`
`context
`
`WiMedia UWB Technology of Choice for Wireless ff513 and Bluetooth Ghobad Heidari
`62008 John Wiley
`Sons Ltd
`
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`

`62
`
`WiMedia T.JWB
`
`Air Interface
`
`Interface
`
`to MAC
`
`PHY
`
`Translates MAC Data
`
`to Air Interface
`
`Figure 3.1
`
`Basic block diagram of the PHY layer
`
`requires any particular implementation either for the receiver or the transmitter
`The emphasis both in this chapter and in the standard is on the transmitteL
`The PHY is partitioned into several layers as shown in Figure 3.2 In this section
`we will briefly introduce the layers and interfaces and we will progress to very de
`tailed descriptions as we proceed through the chapter There are two upper inter
`interface and the data interface The management
`faces the management
`interface
`i.e the PLME SAP as its definition suggests is
`into the PHY for
`side channel
`the purpose of controlling that layers operation The data interface i.e the PHY
`SM is the path through which data is passed between the MAC and the PHY
`We wifi defer any discussion of the WiMedia PHY management sublayer
`to
`later section At this time we are concerned with the data path Data to be
`transmitted is passed to the PHY via the PITY SAP and consists of
`data segment
`the PSDU
`MAC header and some control
`information data length etc.
`The PLCP sublayer presents the PHY SAP interface to the MAC in form that is
`digital waveform i.e
`independent of the air interface and formats the data into
`form that is dependent on the air interface The result of the PLCP operation
`
`into
`
`Management
`Interface
`
`PLME-SAP
`
`Data
`
`Interface
`
`PHY-SAP
`
`PRY Layer Management Entity
`PLME
`
`Air Interface
`
`Figure 3.2 PHY sublayers and interfaces
`
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`

`Physical Layer
`
`PLCP Al Digital
`
`Data
`
`Preamble
`-FEC
`-Scramble
`-OFDM Modulate
`PHY-SAP _____________________________
`
`Interface
`
`63
`
`Air Interface
`
`Digital
`PPDU
`
`PMD
`
`Analog and Digital
`-Frequency Hop
`-Power Control
`-Etc
`
`Analog
`PPDIJ
`
`__________________________
`
`Figure 3.3 PHY data path
`
`is passed to the PMD where it
`is converted to an analog waveform The lower
`interface is the antenna Of course received data follows the
`interface the air
`
`reverse path
`Note that the WiMedia standard does not specifically define the functions as
`signed to the PMD sublayer It does however specify that certain functions are in
`the PLCP so for the purposes of this document the PM is assumed to be every
`thing not specifically included in the PLCP by the standard In fact the standard
`is not entirely clear or consistent within itself nor is it entirely consistent with
`other published literature as to the form and function of the various sublayers As
`
`noted before the architectural boundaries are for reference so it
`
`is only important
`
`to define the boundaries for exposition
`The PLCP sublayer transforms the MAC header and data PSDU into PPDU
`The PPDU includes the header and data but after error correction scrambling
`and other processing has been applied or removed during the receive process
`see Figure 3.3 The PSDU is
`sequence of ls and Os
`sequence of bits i.e
`The PPDU at
`the output of the PLCP is
`digital waveform or
`sequence of
`finite-precision numbers The digital PPDU is converted to an analog PPDU by
`the PMD andtransmitted over the air Note that the OSI model defines the PPDU
`as the actual bits carried by the waveform rather than the waveform itself but the
`WiMedia standard clearly refers to the PPDU as waveform
`to further partition the PLCP into an outer sublayer and an inner
`It will be useful
`sublayer The outer sublayer includes scrambling error correction and other func
`tions that operate on bits while the inner sublayer converts bits to finite-precision
`waveforms using 017DM OFDM waveforms are composed of
`sequence of dis
`tinct entities known as symbols and each symbol is 312.5 ns long in the WiMedia
`waveform At the end of each symbol the PHY layer may hop to
`new center fre
`quency Again we will describe all of this in considerable detail in the following
`is important to understand that the frequency hopping occurs each
`sections but it
`symbol or every 312.5 us in the WiMedia PHI
`
`As explained in Section 1.7 throughout
`
`this book we will be consistently
`
`using the term symbol in different
`
`contexts
`
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`

`64
`
`Ti
`
`WiMedia UWB
`
`Preamble
`
`Header
`
`Payload
`
`Figure 3.4 PHY packet structure
`
`The PM translates the digital PPDU into analog waveforms that are transmit
`ted and of course received by the antenna The PM includes the data con
`verters frequency hopping RF up-conversion gain control etc Put another way
`the PLCP sublayer is related to the digital baseband in the PITY and the PMD sub-
`layer refers to the analog-to-digital conversion ADC/digital-to-analog conversion
`DAC and the RE portion of the PHY
`The WiMedia system is
`packet radio system i.e all information is transmitted
`packet generated by the PHY is
`waveform that consists of three
`in packets
`major parts as shown in Figure 3.4 The preamble conveys no information but
`waveform of predetermined length and structure that is used to allow the re
`the existence of the packet and to estimate the parameters needed
`ceiver to detect
`for accurate demodulation The header is
`short burst of data part of it
`is provided
`by/for the MAC sublayer and part of it
`is constructed by the PHY with informa
`tion such as data rate scrambling seed etc Of course the payload contains the
`information that one device is conveying to another
`The details of the PLCP and the PM themselves are strongly dependent on the
`
`is
`
`Section 3.1 gives
`
`final waveforms presented at the antenna
`quick summary of the PHY features and capabilities Sec
`tutorial on OFDM and
`combination of
`tion 3.2 is
`detailed description of the
`WiMedia OFDM waveform structure We then proceed to build the WiMedia PITY
`standard from the bottom up which is precisely the opposite of the approach taken
`in the standard We discuss the RE in Section 3.3 then FEC and related functions
`in Section 3.5 PHY perfor
`in Section 3.4 and finally put together an entire packet
`look at the PHY
`mance requirements are discussed in Section 3.6 followed by
`in Section 3.7 We will then return to Fig
`responsibilities for range measurement
`ures 3.2 and 3.3 in much more detail expanding on each of the interfaces the data
`paths and the sublayers in Section 3.8
`
`3.1 Feature Summary
`
`3.1.1 Packet Radio
`
`The WiMedia PRY is
`radio system Information is transmitted in chunks
`packet
`standard mode for transmitting individual packets and
`The standard includes
`mode for transmitting bursts of packets
`throughput Each packet
`
`for greater
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`Physical Layer
`
`65
`
`includes
`
`preamble for acquisition and parameter estimation
`information and
`veys basic modulation and packet
`payload section that carries
`data
`
`header that con
`
`3.1.2 MB-OFDM
`
`The PHY uses 01DM to transmit information 128 subcaniers are used 100 of
`which carry data 12 of which are pilots and 10 of which are reserved as guards
`The remaining subcarriers are zeros The WiMedia OFDM uses
`zero-postfix or
`zero-padded suffix instead of
`cyclic prefix to mitigate the effects of multipath
`The subcarriers are modulated using QPSK or
`new technique called 13CM
`variation on 16 quadrature amplitude modulation QAM Higher order modula
`tion types such as 64 QAM are not used because of the low power required by
`the regulatory agencies
`The PHY requires the ability to frequency hop over multiple bands of frequency
`The center frequency is selected from set of three available center frequencies or
`bands which are in turn selected from set of band groups the band groups span
`the entire 7.5 GHz of the UWB spectrum
`The WiMedia PHY uses
`technique called spreading in which data is placed on
`multiple 01DM subcarriers to gain diversity and thereby improve performance in
`fading channels
`
`3.1.3 Error Correction and Variable Data Rate
`
`code with puncturing The combination
`The WiMedia PITY uses
`convolutional
`of spreading and variable code rates gives the PHY
`selection of data rates al-
`lowing for very high data rates in good channels over short distances and good
`performance in poor channels and over longer distances The available payload
`data rates are 53.3 80 106.7 160 200 320 400 and 480 Mbits/s In addition
`to error correction interleaving and scrambling are an integral part of the packet
`structure
`
`3.2 WiMedia OFIM
`As discussed before except for the interface presented to the MAC the processing
`performed by the PHY is strongly dependent on the waveforms presented at the
`tutorial on the 01DM waveform structure We will
`air interface This section is
`motivate the use and structure of the OFDM modulation describe the basics of
`OFDM and discuss some of the interesting features that are unique to the WiMedia
`system
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`66
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`WiMedia TJWB
`
`3.2.1 Frequency-selective Fading
`
`tradi
`
`to transmit
`
`UWIB waveform as one
`that the worlds regulatory agencies are defining
`Recall
`that has minimum of 500 MHz of bandwidth If we attempt
`tional single-carrier waveform through an indoor channel
`then in many situations
`perhaps even most severe distortion of the signal will occuL In Figure 3.5 bi
`nary phase-shift keying BPSK signal is modulated to GHz transmitted through
`typical UWB channel and then down-converted back to baseband The pulse dis
`tortion is severe and since the resulting pulse is much longer than the transmitted
`interference ISI will occur Figure 3.6 shows the
`pulse significant intersymbol
`same signal in the frequency domain The deep notches in the spectrum along
`phe
`with the associated rapid phase changes create the pulse distortion and is
`nomenon known as frequency-selective
`fading This type of fading is
`significant
`problem even in narrowband waveforms but the wide bandwidths associated with
`U\VB make the problem particularly severe
`channel equal
`The traditional method for dealing with this type of 151 is to use
`izer Such equalizers are well known and understood but require adaptive signal
`processing to be effective Adaptive signal processing requires time to converge
`and special preamble structures are often needed to allow for convergence
`in
`packet radio environment The equalizers are often very large and if
`the equaJiza
`is lost As we will see OFDM
`tion process fails on
`then the entire packet
`packet
`provides an alternative to the equalizer structure In particular
`it will replace the
`
`1.5
`
`-0.5
`
`0.01
`
`0.02
`
`0.03
`
`0.04
`
`0.05
`
`Timemicroseconds
`
`Figure 3.5 Transmit
`
`and receive pulses for
`
`500 MHz single-can-ier waveform over
`
`UWB
`
`channel
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`JM
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`Physical Layer
`
`67
`
`Transmittbdspectrurit
`
`...r
`
`7\f
`
`10
`
`-20
`
`4-30
`
`-40
`
`-50
`
`-60
`
`-70
`3200
`
`3400
`
`3600
`
`3800
`
`4000
`
`4200
`
`4400
`
`4600
`
`4800
`
`FrequencyMT-lz
`
`Figure 3.6
`
`Transmit and receive spectrum for
`
`500 MHz sthgle-carrier waveform over
`
`UWB
`
`channel
`
`different set of complex structures OFDM is not necessarily su
`equalizer with
`perior to single-carrier modems but does offer
`different set of tradeoffs
`Observing the effect of lengthening the pulse motivates the OFDM approach
`Figures 3.73.10 show the impact of the channel on BPSK signal transmitted as
`before Figures 3.7 and 3.8 show
`pulse that has been increased in length by
`factor of 10 over the 500 MHz waveform reducing the bandwidth to 50 MHz
`
`RI
`All
`
`Transmitted Puls
`
`Received Pulse
`
`1.5
`
`0.5
`
`-0.5
`
`0.01
`
`0.02
`
`0.03
`
`0.04
`
`0.05
`
`0.06
`
`0.07
`
`0.08
`
`0.09
`
`--
`
`Timenaicroseconds
`
`Figure 3.7 Transmit and receive pulses for
`
`50 MHz single-carrier waveform
`
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`68
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`WiMedia UWB
`
`10
`
`-10
`
`-20
`
`-30
`
`-50
`
`-60
`
`-70
`
`-80
`3920
`
`3940
`
`3960
`
`3980
`
`4000
`
`4020
`
`4040
`
`4060
`
`4080
`
`FrcquencyMHz
`
`Figure 3.8 Transmit and receive spectra for
`
`50 MHz single-carrier waveform
`
`Figures 3.9 and 3.10 show the results of the pulse stretched by
`factor of 100 that
`bandwidth of MHz Clearly as the length of the pulse increases
`is pulse with
`the distortion reduces and the relative amount of 1ST decreases Of course in the
`process the amount of information that may be canied is also reduced and since
`the bandwidth is less than 500 MHz such systems are no longer UWB waveforms
`
`1.5
`
`0.5
`
`10
`
`-0.5
`
`0.05
`
`0.1
`
`0.15
`
`0.2
`
`0.25
`
`0.3
`
`0.35
`
`0.4
`
`0.45
`
`Timemicroseconds
`
`Figure 3.9 Transmit and receive pulses for
`
`MHz single-carrier waveform
`
`II
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`Physical Layer
`
`69
`
`10
`
`-10
`
`-20
`
`tt
`
`I-
`
`j-30
`
`-40
`
`50
`
`-60
`3992
`
`3994
`
`3996
`
`3998
`
`4000
`
`4002
`
`4004
`
`4006
`
`4008
`
`FrequencyMIHz
`
`Figure 3.10
`
`Transmit and receive spectra for
`
`MHz singlecarrier waveform
`
`If we add more carriers however we are able to increase both the bandwidth
`and the information-carrying capacity of the system Figure 3.11 shows
`system
`in which five MHz pulses are modulated onto subcarriers spaced 100 MHz apart
`Clearly the bandwidth is nominally 500 MHz and the information capacity is 11
`times the single-carrier version Since the pulses are all 100 times longer than
`
`10
`
`-10
`
`-20
`
`-30
`
`-60
`
`-70
`
`-so
`3400
`
`tilt
`
`.1
`
`Received Spectrum
`
`vy
`
`JIA
`
`3600
`
`3800
`
`4000
`
`4200
`
`4400
`
`4600
`
`PrequencyMHz
`
`Figure 3.11
`
`Transmit and receive spectra for multicanier waveform
`
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`WiMedia TJWB
`
`single-carrier 500 MHz pulse the effects of signal distortion and 1ST are reduced
`on each subcarrier
`
`is important to pack as many subcarriers as possible into the
`Just as clearly it
`system in order to maximize the information-carrying capacity of the multicanier
`system We will decrease the spacing between the subcarriers to the smallest pos
`the subcarriers do not
`interfere with each other
`sible value but will require that
`in the demodulation process Subcarriers that do not interfere with each other are
`orthogonal subcarriers and the minimum spacing to ensure orthogonality is the
`bandwidth of the pulse that is for the MHz pulses used in this example the
`minimum spacing is MHz With the spacing reduced to the minimum we have
`an OFDM waveform with the transmit spectrum shown in Figure 3.12 In our ex
`ample we have stretched the pulse out by
`factor of 100 when compared with
`single-carrier system but packed 100 subcarriers into the same spectrum Each of
`the subcarriers is subject to amplitude distortion but the phase distortion per sub-
`carrier is relatively small in other words we have reduced the frequency-selective
`fading ones The collection of modulated
`fading problem to
`collection of flat
`single pulse time is an OFDM symbol
`pulses for
`It might seem that further lengthening the pulse and adding more carriers would
`number of reasons to limit
`provide even greater benefit However
`the
`there are
`process First the channels do vary with time Any motion in the room will cause
`change and it
`is not generally desirable to adapt
`the system during the reception of
`an OFDM symbol so the pulses need to be significantly shorter than the channel
`variability Second adding carriers also increases the system complexity so we
`
`Transmitted Spectrum
`
`ci
`
`p.4
`
`3400
`
`3600
`
`3800
`
`4000
`
`4200
`
`4400
`
`4600
`
`FrequencyMHz
`
`Figure 3.12 Transmit spectrum for an OFDM waveform
`
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`Physical Layer
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`71
`
`are further driven to add only as many as needed There are other reasons that are
`beyond the scope of this text
`
`3.2.2 Mathematical Framework
`detailed mathematical treatise on OFDM but it
`This text is not intended to be
`basic framework in which to describe our system This
`will be necessary to have
`simplified mathematical view of OFDM
`section provides
`The complexity of OFDM waveforms generally precludes an analog implemen
`tation so we assume digital system for the generation and reception of our wave
`forms To that end our mathematics will assume sampled waveforms In general
`sample rate which we will denote f3 and an asso
`sampled waveforms require
`1/f3 The WiMedia PHY usesf3
`ciated sample interval At
`528 Msamples/s
`We wifi descri