Orthogonal Frequency Division Multiplexing OFDM

DSSS

FHSS Single Channel Ground and Airborne Radio System (SINCGARS)

OFDM  ADSL/DSL/VDSL  DAB  DVB  HDTV  HD Radio  Power Line communication  Wireless LAN  IEEE m Mobile WiMax  IEEE Mobile Broadband Wireless Access (MBWA)  4G LTE A 2008 Kyocera desktop wireless modem for IEEE provides an Ethernet interfaceKyocerawireless modem Ethernet

Working Group on Broadband Wireless Access Standards Digital Mobile TV Telia branded Samsung LTE modem HTC Thunderbolt the second commercially available LTE smartphone LLaptop is connected to a wireless access point using a PC card wireless card.wireless access pointPC card An example of information displayed by an AM HD station locking. A WiMAX USB modem for mobile internet OFDM Satellite Radio (digital)

 GPS (European Galileo and Russian GLONASS satellite Navitation Systems  Cordless Phones operating at 900 MHz, 2.4 GHz, 5.8 GHz bands  IEEE b, 2.4 GHz WiFi,  Automatic Meter Reading  IEEE (as Phy and MAC layer of Zigbee)  Radio Controlled Model Vehicles. APPLICATIONS OF DSSS

Applications of FHSS  WLAN  Bluetooth  Single Channel Ground and Airborne Radio System  Some Walkie Talkies

Modulations  IS-95 CdmaOne…QPSK..Downlink…OQPSK..Uplink  Wireless LANS…1 MBPS..BPSK…2 MBPS…DQPSK  Bluetooth…GFSK…  Zigbee…Binary phase-shift keying (BPSK) is used in the 868 and 915 MHz bands,…Binary phase-shift keying  Zigbee…Offset quadrature phase-shift keying (OQPSK) in2.4 GHz band. Zigbee…Offset quadrature phase-shift keying  GSM Mobiles…GMSK  Caller ID…Auto Metering…FSK

Plan of Lecture What is OFDM? History of OFDM Basics of OFDM Advantages of OFDM Disadvantages of OFDM Applications of OFDM Simulation of OFDM

OFDM OFDM is a Multicarrier Communication Partial channel information is known at the txer No ISI,ICI

OFDM  OFDM = Orthogonal FDM  Carrier centers are put on orthogonal frequencies  ORTHOGONALITY - The peak of each signal coincides with trough of other signals  Subcarriers are spaced by 1/Ts

OFDM FDM  OFDM Spectral Efficiency

History of OFDM

OFDM TXER AND RCVR Serial-to- Parallel Converter Signal Mapper IFFT Parallel- to-Serial Converter Guard Interval Insertion Serial Data Input x bits D/A & Low pass Filter Up- Converter Down- Converter A/D Guard Interval Removal Serial-to- Parallel Converter FFT One-tap Equalizer Signal Demapper Parallel- to-Serial Converter Serial Data Output x bits Channel Time Frequency Subchannels Fast Fourier Transform Guard Intervals Symbols

FFT-based OFDM System OFDM Transmitter x=[0,0,0,1,1,0,1,1, ….] x1=[0,0] x2=[0,1] x3=[1,0] x4=[1,1] d1=1 d2=i d3=-1 d4=-i …..

FFT-based OFDM System OFDM Transmitter DATA CP 3/3

OFDM Txer

After IFFT, a guard band (cyclic prefix) is added at the beginning Guard band duration must be > channel maximum delay Guard band prevents Inter- Symbol Interference (ISI) due to multipath OFDM Guard (Prefix)

Cyclic Prefix Explained Assume the time-domain signal x(n)=[1,2,3,2,1] Now we will transmit x(n) through channel h(n)=[3,2,1] When x(n) is transmitted through h(n), it undergoes linear convolution, which results in the following signal, x(n)*h(n)=[3,8,14,14,10,4,1]:

Cyclic Prefix Now lets see how it will look if we circularly convolve x(n) and h(n)=[7,9,14,14,10]. So we need to do something to the signal we transmit, x(n) so that we can obtain the circularly convolved signal. Lets add the CP to our signal x(n). I.e. we copy a portion from the end to the front of the signal. Since the channel delay is 3 time samples, we need atleast 2 samples as the CP. So our new signal with the CP will be, x’(n)= [2,1,1,2,3,2,1]. Now lets transmit this signal through the channel and see the result.

Cyclic Prefix.. So the signal with the CP gives [6,7,7,9,14,14,10,4,1] when it is linearly convoluted with channel h(n). Now we seem a similarity with the circularly convoluted signal x(n) and the linearly convoluted signal x’(n). So we see that by adding the CP to our original signal and transmitting through the same channel, we can obtain the desired circular convolution. After the reception of this signal, the receiver takes off the signal samples at either end extract the samples that correspond to the circular convolution and feed it to the DFT unit. So after the DFT operation, we get our desired result X(k)H(k) in frequency-domain. So by adding the CP, we could make the linear convolution give us the desired circularly convoluted signal. You can see that there are many signal samples we throw away. So there is a wastage of resources but CP not only serves this purpose. CP is used to eliminate Inter-Symbol-Interference (ISI) too.

General OFDM Modulation and Demodulation Complete OFDM Modulation

Without Guard ? Transmitted OFDM Symbols Received OFDM Symbols with ISI

With Zero Guard ? Transmitted OFDM Symbols Received OFDM Symbols without ISI FFT output is NOT phase shifted version of ideal OFDM Symbol

Correct Guard Output of FFT without fading is X(k), k=0, 1,…N-1 Output of FTT with fading =

Series and Parallel Concepts In OFDM system design, the series and parallel converter is considered to realize the concept of parallel data transmission.

Series and Parallel Concepts Series – In a conventional serial data system, the symbols are transmitted sequentially, with the frequency spectrum of each data symbol allowed to occupy the entire available bandwidth. – When the data rate is sufficient high, several adjacent symbols may be completely distorted over frequency selective fading or multipath delay spread channel.

Series and Parallel Concepts Parallel – The spectrum of an individual data element normally occupies only a small part of available bandwidth. – Because of dividing an entire channel bandwidth into many narrow subbands, the frequency response over each individual subchannel is relatively flat. – A parallel data transmission system offers possibilities for alleviating this problem encountered with serial systems. Resistance to frequency selective fading

Modulation/Mapping The process of mapping the information bits onto the signal constellation plays a fundamental role in determining the properties of the modulation. An OFDM signal consists of a sum of sub-carriers, each of which contains M-ary phase shift keyed (PSK) or quadrature amplitude modulated (QAM) signals. Modulation types over OFDM systems – Phase shift keying (PSK) – Quadrature amplitude modulation (QAM)

Orthogonality Digital communication systems – In time domain In frequency domain OFDM – Two conditions must be considered for the orthogonality between the subcarriers. 1. Each subcarrier has exactly an integer number of cycles in the FFT interval. 2. The number of cycles between adjacent subcarriers differs by exactly one. 2/2

Orthogonality Example of four subcarriers within one OFDM symbolSpectra of individual subcarriers Time domain Frequency domain

Adjacent Symbol Interference (ASI) Symbol Smearing Due to Channel

Guard Interval Inserted Between Adjacent Symbols to Suppress ASI

Cyclic Prefix Inserted in Guard Interval to Suppress Adjacent Channel Interference (ACI)

Guard Interval and Cyclic Extension OFDM symbol – OFDM symbol duration.

Guard Interval and Cyclic Extension Two different sources of interference can be identified in the OFDM system. – Intersymbol interference (ISI) is defined as the crosstalk between signals within the same sub- channel of consecutive FFT frames, which are separated in time by the signaling interval T. – Inter-carrier interference (ICI) is the crosstalk between adjacent subchannels or frequency bands of the same FFT frame. 2/7

Guard Interval and Cyclic Extension For the purpose to eliminate the effect of ISI, the guard interval could consist of no signals at all. Guard interval (or cyclic extension) is used in OFDM systems to combat against multipath fading. :guard interval :multi path delay spread In that case, however, the problem of intercarrier interference (ICI) would arise. The reason is that there is no integer number of cycles difference between subcarriers within the FFT interval. 4/7

Guard Interval and Cyclic Extension

To eliminate ICI, the OFDM symbol is cyclically extended in the guard interval. This ensures that delayed replicas of the OFDM symbol always have an integer number of cycles within the FFT interval, as long as the delay is smaller than the guard interval.

Guard Interval and Cyclic Extension Effect of multipath with zero signals in the guard interval, the delayed subcarrier 2 causes ICI on subcarrier 1 and vice versa. Part of subcarrier #2 causing ICI on subcarrier #1 Guard timeFFT integration time=1/carrier spacingGuard timeFFT integration time=1/carrier spacing OFDM symbol time Subcarrier #1 Delayed subcarrier #2

Advantages  Immunity to delay spread and multipath  Resistance to frequency selective fading  Simple equalization  Efficient bandwidth usage

Disadvantages  Synchronization  Need FFT units at transmitter, receiver  Sensitive to carrier frequency offset  High peak to average power ratio

Applications

Some OFDM Systems

OFDM Systems

Summary OFDM is emerging as popular solution for wireless LAN, and also for fixed broad-band access The questions that remain to be answered are – Will OFDM be good when there is vehicular mobility? Pulse-shaping or large tone-widths reduce throughput – What about macro-cellular, non-LoS coverage issues? – What about OFDM deployment in unlicensed bands? – Will OFDM be cost-effective? If not right now, when? Analog (linear PA) with dynamic PAR control

Summary Space-Time processing for OFDM is a very hot area of current research The cost-effectiveness of many of these space-time techniques is not clear at present – Multiple RF/IF chains versus faster base-band (MIPS) costs Will 4G see a combination of OFDM, DS-CDMA & TDMA ? Key Question is: Where are those high-bit rate, high usage applications ? -- at low cost ?