CHAPTER Zero :
Introduction to Orthogonal Frequency
Division Multiplexing Technique
PREFACE
Introduction
The History of OFDM
OFDM and Multicarrier Transmission
The Applications of OFDM
; DAB
; HDTV
; Wireless LAN
; IEEE 802.16
; IEEE 802.20
Introduction
Due to the high data rate transmission and the ability to against
frequency selective fading, orthogonal frequency division multiplexing (OFDM) is a promising technique in the current broadband wireless communication system.
OFDM technology is to split a high-rate data stream into a number of lower rate streams that are transmitted simultaneously over a number of subcarrier. Because the symbol duration increases for the lower rate parallel subcarrier, the relative amount of dispersion in time causes by multipath delay spread is decreased.
In OFDM systems, the spectrum of individual subcarrier is overlapped with minimum frequency spacing, which is carefully designed so that each subcarrier is orthogonal to the other subcarriers. The bandwidth efficiency of OFDM is another advantage.
Introduction
OFDM can be viewed as either a modulation technique or a multiplex technique.
Modulation technique
; Viewed by the relation between input and output signals
Multiplex technique
; Viewed by the output signal which is the linear sum of the modulated signals
Introduction
The employment of discrete Fourier transform to replace the
banks of sinusoidal generator and the demodulation significantly reduces the implementation complexity of OFDM modems.
Introduction
Intersymbol interference is eliminated almost completely by
introducing a guard interval with zero padding (or cyclic prefix) in every OFDM symbol.
Introduction
In the guard time , the OFDM symbol is cyclically extended to avoid intercarrier interference.
Introduction
The way to avoid ICI
Introduction
The advantages of OFDM
; Immunity to delay spread and multipath
; Resistance to frequency selective fading
; Simple equalization
; Efficient bandwidth usage
The disadvantages of OFDM
; Synchronization
; Need FFT units at transmitter, receiver
; Sensitive to carrier frequency offset
; High peak to average power ratio
The History of OFDM
The idea, which was proposed in mid-1960s, used parallel data transmission and frequency division multiplexing (FDM).
In the 1960s, the OFDM technique was used in several high frequency military system
; KINEPLEX
; ANDEFT
; KATHRYN
The History of OFDM
In 1971,Weinstein and Ebert applied the Discrete Fourier
Transform (DFT) to parallel data transmission systems as part of modulation and demodulation process.
; FFT-based OFDM
In the 1980s, OFDM was studied for high-speed modems, digital mobile communication, and high-density recording.
; Pilot tone is used to stabilize carrier and frequency control
; Trellis code is implemented COFDM
In 1980, Hirosaki suggested an equalization algorithm in order to suppress both intersymbol and intercarrier interference caused by the channel impulse response or timing and frequency errors.
In 1980. Hirosaki also introduced the DFT-based implementation of Saltzburg’s O-QAM OFDM system.
The History of OFDM
In the 1990s, OFDM was exploited for wideband data communications :
Fix-wire network
; High-bit-rate digital subscriber line (HDSL)
; Asymmetric digital subscriber line (ADSL)
; Very-high-speed digital subscriber line (VDSL)
Digital audio broadcasting (DAB)
Digital video broadcasting (DVB)
High-definition television (HDTV) terrestrial broadcasting
; There exist three mechanisms about the digital terrestrial television broadcasting system in European (COFDM), North America (8-VSB), and Japan (BST-OFDM).
Wireless LAN
; HIPERLAN2 (European)
; IEEE 802.11a (U.S.A)
; IEEE 802.11g (U.S.A)
The History of OFDM
Now, OFDM technique has been adopted as the new European DAB standard, and HDTV standard.
A candidate of 4G mobile communication
OFDM/UWB (802.15.3a)
IEEE 802.16 broadband wireless access system
IEEE 802.20 mobile broadband wireless access (MBWA)
The History of OFDM (data communication)
IEEE-802-16 OFDMA (802.20)
WWAN
802.20
The History of OFDM (4G
communication )
The History of OFDM (4G
communication )
OFDM and Multicarrier Transmission
OFDM is a special case of multicarrier transmission , where a single data stream is transmitted over a number of lower rate subcarrier.
Single carrier transmission
; The concept of single-carrier is that each user transmits and receives data stream with only one carrier at any time.
Multicarrier transmission
; The concept of multi-carrier transmission is that a user can employ a number of carriers to transmit data simultaneously.
OFDM and Multicarrier Transmission
Single and multicarrier transmission
OFDM and Multicarrier Transmission
Orthogonality
OFDM and Multicarrier Transmission
Orthogonality
OFDM and Multicarrier Transmission
Orthogonality f1
f2
f3
f
Time Domain
Frequency Domain
OFDM and Multicarrier Transmission
Power spectrum density of OFDM signals with N=4,1024 Amplitude spectrum for an OFDM signal with N
subcarriers
Power spectrum with rectangular pulse waveform (solid line) and with raised-cosine pulse waveform (dashed
OFDM and Multicarrier Transmission
Multicarrier CDMA systems
; CDMA + OFDM system
Frequency domain spreading
; The spreading operation in the frequency domain
; It spreads the original data streams using a given spreading code, and then modulates a different subcarriers with each chip
The Applications of OFDM
DAB
HDTV
Wireless LAN
IEEE 802.16
IEEE 802.20
The Applications of OFDM - DAB
Digital Audio Broadcasting (DAB)
; DAB is a digital technology offering considerable advantages over today's FM radio, both to listeners and broadcasting.
; DAB's flexibility will also provide a wider choice of programs, including many not available on FM.
; A single station might offer its listeners a choice of mono voice
commentaries on three or four sporting events at the same time, and then combine the bitstreams to provide high-quality sound for the concert which follows.
The Applications of OFDM –Wireless LANs
Wireless LANs
; HIPERLAN2 (European)
; IEEE 802.11a (U.S.A)
; IEEE 802.11g (U.S.A)
; IEEE 802.11n (U.S.A)
The Applications of OFDM –Wireless LANs : HIPERLAN2
ETSI has developed a new WLAN technology called HiperLAN type 2 (HiperLAN2).
HiperLAN2 provides:
; High and scalable capacity as the number of users increase in the system
; Managed bandwidth with predictable performance for each user and application
; Robust protocols that also optimize the overall throughput of the available radio resource, making it the most spectrum-efficient WLAN technology operating at 5 GHz
; A high level of security QoS capabilities to support virtually any type of service or application
; Ease-of-use through a set of auto-configuration tools.
; HipperLan2 standard achieves its 54 Mbps data rate through a OFDM technique.
The Applications of OFDM –Wireless LANs : IEEE 802.11a Standard
IEEE 802.11a, 1999
; 5GHz band
; Orthogonal frequency division multiplexing (OFDM)
; 6Mbps to 54Mbps
The Applications of OFDM –Wireless LANs : IEEE 802.11g Standard
IEEE 802.11g
; IEEE 802.11g will use the same 2.4 GHz radio spectrum as current 802.11b equipment, but with the higher data rates, packet structure, and
modulation technology of 802.11a.
; IEEE 802.11g standard achieves its 54 Mbps data rate through a OFDM technique.
; IEEE 802.11b and IEEE 802.11g are operated in the same 2.4 GHz
frequency band. When their devices are used in the same area, they will coexist, sharing the airspace between them.
The Applications of OFDM –Wireless LANs : IEEE 802.11g Standard
IEEE 802.11g : Operational modes
; 802.11b Mode
; OFDM Mode (Support of 6, 12 and 24 Mbit/s data rates is mandatory )
; PBCC-22 and PBCC-33 Modes (Optional)
; CCK-OFDM Modes (Optional)
The Applications of OFDM –Wireless LANs : IEEE 802.11g Standard
IEEE 802.11a/g :
P/S
Encoder Interleaver QAM mapper
Pilot
insertion S/P
IFFTIFFT FFTFFT
P/S windowingCP and D/A RF TX
Decoder De-
interleaver
QAM
demapper Equalizer S/P removalCP
RF RX A/D
Timing and frequency synchronization Binary
input
Binary output
The Applications of OFDM – IEEE 802.16
IEEE 802.16 broadband wireless access system
; Broadband wireless access (BWA) is a term referring to a range of fixed radio systems, used primarily to convey broadband services between users’ premises and core networks.
; The term “broadband” is usually taken to mean the capability to deliver significant bandwidth to each user (in ITU terminology, greater than around 1.5 Mbps, though many BWA networks support significantly higher data rates).
; A typical BWA network supports connection to many user premises within a radio coverage area.
; The IEEE 802.16 standard should provides the solution to access systems based on DSL, cable, and eventually fiber optics.
; The applications of IEEE 802.16
~ The range of applications is very wide and evolving quickly. It
The Applications of OFDM – IEEE 802.16
IEEE 802.16 wireless MAN background
; Target: FBWA (fixed broadband wireless access)
; Fast local connection to network
; Project development since 1998
The Applications of OFDM – IEEE 802.16
802.16 bit rate and channel size
The Applications of OFDM – IEEE 802.20
IEEE 802.20 mobile broadband wireless access
; Mission and goal
~ The mission of IEEE 802.20 is to develop the specification for an
efficient packet based air interface that is optimized for the transport of IP based services.
~ The goal is to enable worldwide deployment of affordable, ubiquitous, always-on and interoperable multi-vendor mobile broadband wireless access networks that meet the needs of business and residential end user markets.
The Applications of OFDM – IEEE 802.20
IEEE 802.20 mobile broadband wireless access
; MBWA Scope
~ Specification of physical and medium access control layers of an air interface for interoperable mobile broadband wireless access
systems, operating in licensed bands below 3.5 GHz, optimized for IP- data transport, with peak data rates per user in excess of 1 Mbps.
~ It supports various vehicular mobility classes up to 250 Km/h in a MAN environment and targets spectral efficiencies, sustained user data rates and numbers of active users that are all significantly higher than achieved by existing mobile systems.
; The applications of MBWA
~ This allows applications including, but not limited to, video, full graphical web browsing, e-mail, file uploading and downloading without size limitations (e.g., FTP), streaming video and streaming
The Applications of OFDM –
IEEE 802.20
The Applications of OFDM – IEEE 802.20
IEEE 802.20 mobile broadband wireless access
; Peak data rates
~ These peak data rate targets are independent of channel conditions, traffic loading, and system architecture.
§ HomeWork
在IEEE資料庫搜尋IEEE 802.11a標準文件同時繳交一份該文件
閱讀報告。
Due date: 本章結束後當週星期五。
