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4G Mobile (IMT Advanced) System and Applications
Core Technologies for 4G: OFDM Pusan, HAEUNDAE CENTUM HOTEL. Korea April 2011 Jongseob Baek J
[email protected] b b k@ il
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Contents Backgrounds Broadband wireless channels Basic concept of FDM SC System OFDM System Application of OFDM System (OFDMA) CP-SC System Application of CP-SC System (SC-FDMA)
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Backgrounds
Digital communication systems require each channel to operate at a specific frequency and with a specific bandwidth.
In fact, communication systems have evolved so that the largest amount of data can be communicated through a finite frequency range.
This lecture will focus on the recent evolution of communication systems into using various mechanisms for effectively using the frequency spectrum.
More specifically, the lecture will describe how frequency division multiplexing (FDM) and orthogonal frequency division multiplexing (OFDM) are able to effectively utilize the frequency spectrum.
In addition,, this lecture will compare p the properties p p of single-carrier g ((SC)) and OFDM, and then it describe why OFDM systems are currently being implemented in some of the newest and most advanced communication systems.
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Broadband Wireless Channel
What is the broadband wireless channel? Broadband wireless channel could be defined by understanding a delay spread (multipath propagation) and Doppler spread (a fading phenomenon). Under a delay spread consideration, a radio signal travels over two or more paths from a transmitter to a receiver. receiver Thus, delay spread can cause changes in the received signal level by either adding or subtracting delayed signals (reflected signals) from the received signal level. Delay spread is frequency dependent, which means that it’s properties will vary depending on a used frequency(-band). frequency( band) Delay spread is important for lower frequencies and near line of sight (NLOS) transmission.
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Broadband Wireless Channel Whereas, Doppler spread is not usually a challenge on systems that use higher frequencies as these systems tend to use highly directional (high-gain) antennas for LOS transmission.
Under Doppler spread consideration, a radio signal level varies according to mobility y of either mobile terminal or surrounding g environments. In usual, a fading degree depends on a mobile speed of such associated terminals and the used frequency(-band). It is also depends on a angle of signal reception from transmitter.
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Broadband Wireless Channel
Delay spread determines a frequency frequency-selectivity selectivity Selective vs. Non-selective
Doppler pp spread p determines a time-selectivity y Fast fading vs. Slow fading
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Broadband Wireless Channel
Frequency selectivity (caused by multipath) Multipath channel is usually modeled by a finite impulse response (FIR) filter Ts < maximum delay (Ts: symbol duration) which causes an inter-symbol interference (ISI)
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Broadband Wireless Channel
Frequency selectivity (caused by multipath) Ts >> maximum delay, in which the ISI effect could be mitigated
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Broadband Wireless Channel
Time selectivity (caused by Doppler effect) Doppler frequency:
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Broadband Wireless Channel
Frequency-selective Frequency selective fading (doublely selectivity) channel Frequency selectivity + time selectivity 4-path Rayleigh fading channel- time domain
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Broadband Wireless Channel
Frequency-selective Frequency selective fading channel 4-path Rayleigh fading channel- frequency domain
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The basic concept of FDM
The concept of FDM involves the allocation of each channel to a unique frequency range.
This frequency range prescribes both the center frequency and channel width (bandwidth), which means these channels are non-overlapping.
Consequently, multiple channels (or users) can operate concurrently by using different channels in terms of the frequency domain.
Due to such a simple property, FDM is commonly used in a variety of communications such as Global System for Mobile Communications protocol (GSM), Time division multiple access (TDMA), code division multiple access (CDMA), WIMAX, and LTE.
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The basic concept of FDM
FDM with/without guard guard-band band which involves assigning non non-overlapping overlapping frequency ranges to different signals (or to each "user" of a medium)
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SC System
Let us consider that a single channel (individual channel) is utilized by SC system
Sequential signal is transmitted with a short Ts (< max. delay spread)
Signal are transmitted with a constant power level, which leads to a low peak-to-average power ratio (PAPR)
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SC System
The standard structure of SC transmission
Pulse-shaping filter implemented with a square root-raised cosine (SRRC) filter (Finite impulse response) It is used to eliminate interference from adjacent channels in the frequency domain Whereas, it causes an inter-symbol interference (ISI) by overlapping subsequent symbols on the same channel
Non-linear Non linear amplifier
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SC System
The structure of SC reception The receiver performs reverse process to the SC transmission Receive signal suffers from ISI effect due to short Ts
Basically, ISI effect can be mitigated through a channel equalizer implemented with a linear/non-linear FIR filter
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SC System
Understanding of channel equalization in terms of frequency frequency-domain domain
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SC System
Equalizer properties used for SC system Time-domain equalizer (TDE) implemented a FIR filter has a high computational complexity The total tap-length depends on the maximum delay spread and multipath power
Efficient implementation could be archived through an adaptive filtering scheme Adaptive filtering would degrade the equalizer performance over a multipath fading channel, since the adaptation could not follow the fading degree
The such problems could be resolved by using a frequency-domain equalizer (FDE) which requires additional receiver signal processing (FDE), processing.
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Key properties of SC System
Summary SC signal guarantees a low PAPR by controlling the transmit power in linear region of amplifier which means it can cover a wide-range service area with a high power emission and without additional repeater, such as relay
Channel equalization has a high computational complexity and low tracking capability to multipath fading channel SC receiver is robust to frequency and timing offset effects, since it just uses single carrier
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OFDM System
OFDM is a subset of FDM in which a single channel utilizes multiple sub subcarriers on adjacent frequencies
In addition, the sub-carriers in an OFDM system are overlapped to maximize spectral efficiency
Ordinarily, overlapping adjacent channels can interfere with one another. However, sub-carriers in an OFDM system are precisely orthogonal to one another
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OFDM System
Thus sub-carriers Thus, sub carriers are able to be overlapped without interfering and to maximize spectral efficiency without causing adjacent channel interference (in ideal system condition)
In addition, these sub-carriers are able to be detected correctly, since the maximum power of each sub-carrier corresponds directly with the minimum power off each h adjacent dj channel. h l
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OFDM System
Basic concept of OFDM: Rectangle pulse pulse-shaping shaping on the time time-domain domain (a) DC centered spectrum with equally spaced zeros (b) Shift spectrum with linear phase on DC pulse: move spectrum to first spectral zero
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OFDM System
Basic concept of OFDM: Rectangle pulse pulse-shaping shaping on the time time-domain domain Real and imaginary parts of complex exponential time series: Integer number of cycles per interval
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OFDM System
Basic concept of OFDM: Rectangle pulse pulse-shaping shaping on the time time-domain domain Spectra of complex exponential time series: Integer number of cycles per interval
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OFDM System
Basic concept of OFDM: orthogonal transmission
Serial-to-parallel (S/P): once the bit-stream composing of N data symbols has been divided among the individual sub-carriers, each sub-carrier is modulated as if it was an individual
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OFDM System
Basic concept of OFDM: orthogonal reception
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OFDM System
Efficient OFDM transmission using inverse discrete Fourier transform (IDFT)
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OFDM System
OFDM transceiver structure using inverse fast Fourier transform (IFFT) and FFT at transmitter and receiver, respectively
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OFDM System
OFDM converts the frequency frequency-selective selective channel to frequency frequency-flat flat channel in terms of each frequency-bin. Which means that OFDM system has more robust transmission Property than SC system in such a channel.
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OFDM System
OFDM system still suffers from ISI effect the ISI affects orthogonality between subcarriers, which leads to severe inter(sub)carrier interference (ICI) problem The advantages of robustness to frequency-selective channel and spectral efficiency do not be guaranteed anymore.
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OFDM System
Insertion of guard guard-interval interval (GI) between OFDM symbols to prevent the ISI effect The GI length is larger or equal to the maximum delay spread of a channel GI insertion with zero-padded (ZF) symbol No ISI, but ZF still affects the orthogonality after FFT operation, since it broken the continuity of sub-carriers.
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OFDM System
Insertion of cyclic cyclic-prefixed prefixed (CP) symbol between OFDM symbols instead of ZP symbol It completely eliminates ISI and ICI. It maintains subcarrier orthogonality.
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OFDM System
Key blocks of OFDM Transceiver Pilot insertion to estimate channel information and the amount of syn. offset Non-linear amplifier effect to IFFT output
Relation between symbol timing offset and ISI effect Relation between frequency offset and orthogonality One-tap channel equalization on the frequency-domain
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OFDM System
Pilot insertion Channel information estimation on the time-/-frequency domain Compensates symbol timing and frequency offsets
Representative pilot patterns Block-type pilot pattern arrangement Comb-type pilot pattern arrangement Scattered-pilot pattern arrangement
Cf) Known-symbol insertion Known Known-symbol symbol is sometimes appended in the front of frame block composing of several OFDM symbols Known-symbol can be used instead of CP symbol to improve synchronization and channel estimation
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OFDM System
Block-type Block type pilot pattern arrangement The channel estimation can be performed by either periodically inserting pilot tones into all sub-carriers (frequency axis) It is usually used in a severe frequency-selective channel Channel varies slowly enough so that the channel estimation will have a good accuracy
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OFDM System
Comb-type Comb type pilot pattern arrangement The number of pilots used for channel estimation is usually much smaller than the number of sub-carriers This method is usually used in systems having significant channel variation over a short period of time
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OFDM System
Scattered-pilot Scattered pilot pattern arrangement Block-type pilot pattern + comb-type pilot pattern This method is commonly be used in systems having significant doubly selective channels
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OFDM System
High PAPR problem IFFT output shows Gaussian distribution, approximately. High-peak random signals (a high PAPR problem) are often observed, which are distorted on the non-linear region of amplifier.
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OFDM System
OFDM Input and output of non non-linear linear amplifier
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OFDM System
ISI effect according to symbol timing offset Symbol timing offset corresponds to the starting position of FFT window. Cases of timing offset within CP symbol
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OFDM System
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OFDM System
ICI effect according to frequency offset Due to oscillator mismatch or Doppler Shift Breaking orthogonality -> ICI problem -> performance degradation
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OFDM System
Efficient receiver processing according to the use of CP symbol
Convert linear channel matrix
to circular matrix
after removing CP
symbol
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OFDM System
Simple one one-tap tap frequency frequency-domain domain equalization (FDE) realization Each subcarrier can be processed independently, which means that only onetap multiplier is sufficient in term of implementation. Consequently, it is obvious that the FDE used for OFDM system has lower computational complexity than TED for SC system
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OFDM System
An estimation of channel frequency response (CFR) using comb comb-type type pilot arrangement Calculate the channel estimates at the pilot subcarriers
Interpolate the estimates for the other subcarriers Channel, B andwidth, and S am ples 1 0.5 0 -1
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OFDM System
Constellations before and after performing FDE
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Key properties of OFDM System
Robustness to frequency selectivity, selectivity one one-tap tap equalizer
Bandwidth efficiency due to the overlapping orthogonal subcarriers
Simultaneous elimination of ISI and ICI ((inter carrier interference)) by y CP symbol
High peak-to-average power ratio (PAPR)
Sensitivity to Doppler: Channel variation within one OFDM symbol duration incurs inter-carrier interference (ICI)
Applications: ADSL, Digital Video Broadcast (DVB), Digital Audio Broadcast (DAB), Digital terrestrial multimedia broadcasting (DTMB) (DTMB), Wireless LAN (IEEE 802.11a), 802 11a) Wireless MAN (WiMax IEEE 802.16), Down-Link [base station to mobile] 3GPP LTE, etc
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Application of OFDM system
Orthogonal Frequency Frequency-Division Division Multiple Access (OFDMA) A multi-user version of the popular OFDM modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual This allows simultaneous low data rate transmission from several users.
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CP-SC System
Cyclic-prefixed Cyclic prefixed SC system It preserves the advantages of SC system, i.e., low PAPR, wide-range service coverage This allows efficient receiver processing, especially the application of FDE, which h h is caused d by b the h use off CP C symbol b l Convert linear channel matrix to circular matrix after removing CP symbol
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Application of CP-SC system
Single-carrier Single carrier FDMA (SC (SC-FDMA) FDMA) Like other multiple access schemes (TDMA, FDMA, CDMA, OFDMA), it deals with the assignment of multiple users to a shared communication resource It has an additional FFT/IFFT processing preceding the conventional OFDMA processing Multiple M lti l access among users iis made d possible ibl b by assigning i i different diff t users, different sets of non-overlapping Fourier-coefficients The distinguishing g g feature of SC-FDMA is that it leads to a single-carrier g transmit signal, in contrast to OFDMA which is a multi-carrier transmission scheme. Owing to its inherent single carrier structure, a prominent advantage of SCFDMA over OFDM and OFDMA is that its transmit signal has a lower PAPR.
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Application of CP-SC system
Single-carrier Single carrier FDMA (SC (SC-FDMA) FDMA)
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Key Transmission technologies of 4G
Some of the key technologies used in 4G communication systems, systems i.e., ie WiMAX and LTE, include OFDM, frequency reuse, adaptive modulation, multi-input multi-output (MIMO), and so on.
The use of OFDM reduces the effects of multipath and delay spread, which is especially important for lower frequencies and near line of sight (NLOS) transmission i i
The use of OFDM enables high data bandwidths to be transmitted efficiently
The use of OFDM allows efficient access schemes for the uplink and downlink WiMAX : Orthogonal Frequency Division Multiple Access (OFDMA) is used in both the downlink and uplink LTE: OFDMA is used for the down link, while Single Carrier-Frequency Division Multiple Access (SC-FDMA) is used in the uplink.
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Key Transmission technologies of 4G LTE: SC SC-FDMA FDMA is used in view of the fact that its peak to average power ratio is small and the more constant power enables high RF power amplifier efficiency in the mobile handsets - an important factor for battery power equipment.
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Thank you !!
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