1. Department of Electrical and Computer Engineering
ECE 4371, Fall, Introduction to Telecommunication Engineering/Telecommunication Laboratory
                                                           
Zhu Han
Department of Electrical and Computer Engineering
Class 19
Nov. 5th, 2014

2. Outline
OFDM
Motivation
System Model
Applications

3. Introduction
A method of encoding digital data on multiple carrier frequencies.
OFDM has developed into a popular scheme for wideband digital communication
Wireless and wire
Digital television and audio broadcasting, DSL Internet access, wireless networks, powerline networks, and 4G mobile communications
A large number of closely spaced orthogonal sub-carrier signals are used to carry data on several parallel data streams or channels.

4. Introduction OFDM is a specialized FDM
Additional constraint
All the carrier signals are orthogonal to each other
Sub-carriers are orthogonal to each other
Cross-talk between the sub-channels is eliminated
Inter-carrier guard bands are not required
Greatly simplifies the design of
Transmitter and the receiver;

5. Basic Idea
Divide a high bit- rate stream into several low bit- rate streams ( serial to parallel)
Robust against frequency selective fading due to multipath propagation

6. OFDM
Low symbol rate modulation schemes suffer less from inter-symbol interference caused by multipath propagation
Utilizes all carriers to transmit its data as coded quantity at each frequency carrier, which can be QAM, etc..

7. Orthogonal frequency-division multiplexing
Special form of Multi-Carrier Modulation.
Divide a high bit-rate digital stream into several low bit-rate schemes and transmit in parallel

8. Modulation techniques: Single-carrier vs. Multi-carrier
Drawbacks
Selective Fading
Very short pulses
ISI is comparatively long
Poor spectral efficiency because of band guards
Advantages
Flat Fading per carrier
N long pulses
ISI is comparatively short

9. Modulation techniques: Single-carrier vs. Multi-carrier
Compare communication system by power efficiency and bandwidth efficiency.
Power efficiency describes the ability of communication system to preserve BER of the transmitted signal at low power levels.
Bandwidth efficiency reflects how efficiently the allocated bandwidth is utilized and is defined as the throughput data rate per Hertz in a given bandwidth.
The bandwidth efficiency of OFDM with using optical fiber channel is defined as
Factor 2 is because of two polarization states in the fiber.
where R_{s} is the symbol rate in Gsps, and B_{OFDM} is the bandwidth of OFDM signal.

10. Modulation techniques: Single-carrier vs. Multi-carrier
There is only 1 dBm increase in receiver power, but we get 76.7% improvement in bandwidth efficiency with using multicarrier transmission technique.
There is saving of bandwidth by using Multicarrier modulation with OFDM.
Bandwidth efficiency of multicarrier system is larger than single carrier system.
Transmission Type
M-QAM
No. of Subcarriers
Bit rate
Fiber length
Power at the receiver (at BER of 10−9)
Bandwidth efficiency 1.
Single-carrier 64 1
10 Gbit/s
20 km
-37.3 dBm
6.0000 2.
Multi-carrier
128
-36.3 dBm

11. Key features - Advantages
High spectral efficiency as compared to other double sideband modulation schemes, spread spectrum, etc.
Using close-spaced overlapping sub-carriers
Can easily adapt to severe channel conditions without complex time-domain equalization. Simpler equalization to flat fading channel.
Overall channel divided into multiple narrowband signals that are affected individually as flat fading sub-channels.
Single carrier systems has high complexity of the channel equalization applied across the whole channel.
OFDM is that using multiple sub-channels, the channel equalization becomes much simpler.
Against narrow-band co-channel interference.
Adequate channel coding and interleaving
Possible to recover symbols lost due to the frequency selectivity of the channel and narrow band interference.
Robust against inter-symbol interference (ISI) and fading caused by multipath propagation.
Low data rate on each of the sub-channels
Efficient implementation using Fast Fourier Transform (FFT).
Low sensitivity to time synchronization errors.

12. Key features - Disadvantages
Sensitive to Doppler shift.
Sensitive to frequency synchronization problems.
High peak-to-average-power ratio (PAPR)
An OFDM signal has a noise like amplitude variation and has a relatively high large dynamic range, or PAPR.
Impacts the RF amplifier efficiency as the amplifiers need to be linear and accommodate the large amplitude variations and these factors.
Amplifier cannot operate with a high efficiency level, suffers from poor power efficiency.
Loss of efficiency caused by cyclic prefix/guard interval.

13. OFDM
Sensitive to frequency synchronization problems.

14. Outline
OFDM
Motivation
System Model
Applications

15. Frequency Response Transformation to time domain:
Data coded in frequency domain
Transformation to time domain:
each frequency is a sine wave
in time, all added up
Channel frequency response

16. OFDM Effective Diagram
z[n]=H[n]s_n+w[n]

17. OFDM Transmitter and Receiver

18. Data on OFDM
The data to be transmitted on an OFDM signal is spread across the carriers of the signal, each carrier taking part of the payload.
This reduces the data rate taken by each carrier.
The lower data rate has the advantage that interference from reflections is much less critical.
This is achieved by adding a guard band time or guard interval into the system.
This ensures that the data is only sampled when the signal is stable and no new delayed signals arrive that would alter the timing and phase of the signal.

19. Guard Time and Cyclic Prefix
Since the duration of each symbol is long, it is feasible to insert a guard interval between the OFDM symbols.
A Guard time is introduced at the end of each OFDM symbol for protection against multipath.
The Guard time is “cyclically prefix” to avoid Inter-Carrier Interference (ICI) - integer number of cycles in the symbol interval.
Guard Time > Multipath Delay Spread, to guarantee zero ISI & ICI.

20. Cyclic Prefix The prefixing of a symbol with a repetition of the end
Receiver is typically configured to discard the cyclic prefix samples, the cyclic prefix serves two purposes.
As a guard interval
Eliminates the inter-symbol interference from the previous symbol.
As a repetition of the end of the symbol
The receiver will integrate over an integer number of sinusoid cycles for each of the multi-paths when it performs OFDM demodulation with the FFT.
For the cyclic prefix to be effective
The length of the cyclic prefix must be at least equal to the length of the multipath channel.

21. Transmitted Symbol To have ISI-free channel, Tsymbol>>τ
In OFDM, each symbol has T =Ts L >> τ
Guard interval between OFDM symbols Tg>> τ ensures no ISI between the symbols.

22. OFDM Bit & Power loading
Bit Loading
Map the rate with the sub-channel condition
Sub-carriers with low gain should be assigned less complex constellations and should carry fewer bits per symbol.
Extreme case: no bit should be assigned when channel gain close to zero
Sub-carriers with large gain should be assigned more complex constellations and should carry more bits per symbol.
Power Loading
Transmitter apply additional power to subcarrier.
Water-filling

23. OFDM Bit & Power loading
Bit loading and power loading are often complementary at the transmitter.

24. Outline
OFDM
Motivation
System Model
Applications

25. OFDM Applications Wireless
The wireless LAN (WLAN) radio interfaces IEEE a, g, n, ac and HIPERLAN/2.
The digital radio systems DAB/EUREKA 147, DAB+, Digital Radio Mondiale, HD Radio, T-DMB and ISDB-TSB.
The terrestrial digital TV systems DVB-T and ISDB-T.
The terrestrial mobile TV systems DVB-H, T-DMB, ISDB-T and MediaFLO forward link.
The wireless personal area network (PAN) ultra-wideband (UWB) IEEE a implementation suggested by WiMedia Alliance.
The OFDM based multiple access technology OFDMA is also used in several 4G and pre-4G cellular networks and mobile broadband standards:
The mobility mode of the wireless MAN/broadband wireless access (BWA) standard IEEE e (or Mobile-WiMAX).
The mobile broadband wireless access (MBWA) standard IEEE
the downlink of the 3GPP Long Term Evolution (LTE) fourth generation mobile broadband standard. The radio interface was formerly named High Speed OFDM Packet Access (HSOPA), now named Evolved UMTS Terrestrial Radio Access (E-UTRA).

26. OFDM Applications Cable
ADSL and VDSL broadband access via POTS copper wiring,
DVB-C2, an enhanced version of the DVB-C digital cable TV standard,
Power line communication (PLC),
ITU-T G.hn, a standard which provides high-speed local area networking of existing home wiring (power lines, phone lines and coaxial cables). [2]
TrailBlazer telephone line modems,
Multimedia over Coax Alliance (MoCA) home networking.

27. OFDMA OFDMA SC-FDMA A multi-user version of the OFDM
Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users
Combining OFDM with time division multiple access (TDMA) or time-domain statistical multiplexing communication.
SC-FDMA
A linearly pre-coded OFDMA scheme, in the sense that it has an additional DFT processing step preceding the conventional OFDMA processing.
Lower peak-to-average power ratio (PAPR)
Greatly benefits the mobile terminal in terms of transmit power efficiency and reduced cost of the power amplifier.

28. Diagram of SC-FDMA & OFDMA

29. OFDM Time and Frequency Grid
Put different users data to different time-frequency slots

30. SC-FDMA VS OFDMA

31. Comparison

32. ADSL Conventional voice band modem
Dial up modem converts bits into waveforms that must fit to the voice band (3.4kHz)
Large QAM modulation require. (960-QAM)
High transmission power and large complexity equalization.
No phone call when surfing the Internet.
ADSL completely bypass the voice telephone system by specializing in data service.
Voice traffic continue to use voice band below 3.4 kHz.
Achieved by Low pass Filter
Bandwidth much larger than 4kHz
Long copper wires suffer from attenuation at high frequencies
Connection distance is short (1-5 km)

33. ADSL
OFDM can cope with this frequency selective attenuation and with narrow-band interference
Achieves high-speed data connections on existing copper wires.
ADSL2 uses variable sub-carrier modulation, ranging from BPSK to 32768QAM.

34. Upstream and Downstream Subcarrier Allocation
The asymmetric data service
Higher downstream than upstream
More downstream subcarriers than upstream carriers.

35. ADSL However, DSL cannot be used on every copper pair;
Interference may become significant if more than 25% of phone lines coming into a central office are used for DSL.
OFDM is used in ADSL connections that follow the ANSI T1.413 and G.dmt (ITU G.992.1) standards, where it is called discrete multitone modulation (DMT).
OFDM is also used in the successor standards ADSL2, ADSL2+, VDSL, VDSL2, and G.fast.

36. Wireless local area networks (LAN) and metropolitan area networks (MAN)
OFDM is extensively used in wireless LAN and MAN applications, including IEEE a/g/n and WiMAX.
IEEE a/g/n operating in the 2.4 and 5 GHz bands, specifies a per-stream airside data rates ranging from 6 to 54 Mbit/s.
Four different modulation schemes are used: BPSK, QPSK, 16-QAM, and 64-QAM.
The multitude of choices allows the system to adapt the optimum data rate for the current signal conditions.

37. The IEEE a/g Standard
Belongs to the IEEE system of specifications for wireless LANs.
covers both MAC and PHY layers.
Five different PHY layers.
802.11a/g belongs to the High Speed WLAN category with peak data rate of 54Mbps
PHY Layer very similar to ETSI’s HIPERLAN Type 2

38. Outline
OFDM
Motivation
System Model
Applications
Thanks

39. Multiband OFDM - Simple to implement
- Captures 95% of the multipath channel energy in the Cyclic Prefix
- Complexity of OFDM system varies Logarithmically with FFT size i.e.
- N point FFT  (N/2) Log2 (N) complex multiplies for every OFDM symbol