L10b-OFDM

1. What is the OFDM?

Each carrier has a very narrow bandwidth (i.e. 1 kHz)

Number of subcarriers: 100 - 8000

2. Why OFDM?

- Solves most of the problems with FDMA and TDMA

- Channel coding simplified: anti ICI and ISI scheme in OFDM less complex, no equalizer needed (equalization much more complex in CDMA)

Interleaving and error coding results in robust performance

- Robust method against channel non-ideality (multi-path, AWGN, in-band interference)

Each carrier's symbol rate is low. ¡æ A high tolerance to multipath delay spread.

The delay spread must be very long to cause significant inter-symbol interference (e.g. > 500 us)

- OFDM is mainly useful for communication over multipath channels with long delay spread and at high SNR

For such channels ISI becomes the major impairments.

- Problems with single-carrier signaling scheme:

For heavily multi-path channel, a single carrier receiver requires a complicated equalizer or special design (frequency domain equalizer)

Single carrier design cannot achieve capacity with known equalizer implementation for broadcasting channels.

- VSLI semiconductor advances ¡æ OFDM implementation economical

Simple implementation using FFT

- High spectral efficiency: due to no guard bands

- Ideally suited to MIMO due to easy matching of transmit signals to the uncorrelated RF channels

- Availability of OFDM in existing signal processing/chipsets

- Easy combining it with OFDMA multiple access scheme

- DSP advantage: OFDM uses IFFT/FFT ¡æ superior scalability over the channel dispersion and data rate

3. Disadvantages of the OFDM

- Requires highly linear amplifiers

Pure OFDM creates high PAR: SC-FDMA is used in UL

- Sensitive to Doppler effect which creates interference between subcarriers

- Guard-time introduces overhead

- More complex than CDMA for handling inter-cell interference at cell edge

3. OFDM Trends

1) History

- An OFDM concept was proposed by Weinstein and Ebert in 1971.

- OFDM already widely used in non-cellular technologies

- Was considered by ETSI for UMTS in 1998. CDMA was favored since OFDM requires large amounts of baseband processing which was not available in late 1990s.

2 Mbps in 1997

11 Mbps in 1999

54 Mbps in 802.11a

- Popularity

Terrestrial TV broadcasting in Japan and Europe: DVB-T

ADSL high-speed modem

WLAN (IEEE 802.11a/g/n)

WiMAX (IEEE 802.16d/e)

- Data rates to 54 Mbps at 2.4 GHz WiFi

- Single international standard at 5GHz: 5 GHz IAG, IEEE 802.11 5GSG

3. Pros and Cons of the OFDM

1) Pros

2) Cons

1. Why OFDM?

-

10.1.2 PAPR (peak to average power ratio)

1) PAPR

- PAPR: The maximum power of a sample in a given OFDM transmit symbol divided by the average power of that OFDM symbol. 

- PAPR occurs when in a multicarrier system the different sub-carriers are out of phase with each other. At each instant they are different with respect to each other at different phase values. When all the points achieve the maximum value simultaneously; this will cause the output envelope to suddenly shoot up which causes a ¡®peak¡¯ in the output envelope.

- Due to presence of large number of independently modulated subcarriers in an OFDM system, the peak value of the system can be very high as compared to the average of the whole system. 

- In LTE system, OFDM signal PAPR is approx. 12dB.

2) Power amplifier (PA) backoff:

- PA back off:

Average power: 40 dBm

PAPR: 12 dB

PA saturation point: 52 dBm

PA operating point: 40 dBm

PA back off: 12 dB

- Large back off: low efficiency

3) CFR (crest factor reduction)

- CFR: Reducing PAPR

- CFR algorithms:

Clipping and filtering (CF)

Peak windowing (PW)

Peak cancellation (PC)

Selective mapping (SLM)

- After CFR, PAPA is reduced to 7.5 dB from 12 dB.

S : aggregator

DPD (digital pre-distorter)

DAC (digital to analog converter)

HPA (high power amplifier)

- Selective mapping technique

Figure: OFDM procedure with selective mapping

- Peak cancellation algorithm

- Peak windowing algorithm

10.1.3 OFDM

1) OFDM signal: Tx chain

2) OFDM signal: Rx chain

3) OFDM signal in time domain

4) OFDM signal in frequency domain

[Keywords

MCCDMA (Multi-carrier CDMA): OFDM + CDMA

DS-CDMA (direct sequence CDMA)

COFDM (coded OFDM): FEC applied before signal transmission. In practice, every OFDM system is a COFDM system.

FDMA:

- Each user is allocated a single channel.

- Bandwidth of each channel is 10-30 kHz for voice communication.

- Minimum bandwidth for speech is only 3 kHz.

- Channel separation: channel filtering, allowance for Tx and Rx frequency drifts.

- Almost 50% of the spectrum is wasted due to guard bands

TDMA:

- Multiple users access the same channel by transmitting in their data in time slots.

- Thus, many low data users can be combined together to transmit in a single channel which has a bandwidth sufficient for a required number of users.

- In this sense, the spectrum may be used efficiently.

- As in FDMA, there should be some guard time slot between allocating time slots for each users to allow for propagation delay variations and synchronization errors.

- This results in a decrease in spectrum efficiency.

- The symbol rate of the channel is high resulting in problems with multipath delay spread.

OFDM:

- OFDM splits the available bandwidth into many narrow band channels (100 - 8000)

- Subcarriers are orthogonal to each other in frequency domain allowing them to be spaced very close together.

- Each subcarrier has an integer number of cycles over a symbol period.

- Each subcarrier in an OFDM signal has a very narrow bandwidth (1 kHz) resulting in low symbol rate.

- This results in the signal having a high tolerance to multipath delay spread, because in this case, the delay spread must be very long to cause significant ISI (e.g. > 500 us)

- All the carriers must be carefully controlled to maintain the orthogonality of the carriers.

[OFDM techniques

- Spectrum efficiency enhancement techniques

Bit and power loading

Pulse shaping

Channel shortening

[OFDM in wireless communication

- Applications

802.11a/g WiFi

HiperLAN2

802.16 WiMAX

DAB in Europe, Asia, Australia

DVB-T in Europe, Asia, Australia

DSL

[OFDM for optical communication

CO-OFDM (coherent OFDM): coherent detection + OFDM modulation

resilient to fiber PMD

nonlinearity mitigation

dispersion

chromatic

PMD (polarization mode dispersion)

MIMO-OFDM, 2x2 (by polarization diversity?)

nonlinearity in RF-to-optical up-converter

DSP, receiver-based

SPM (self-phase-modulation)

phase noise, Gordon-Mollenauer

transmission speed, 100 Gbps per channel

dynamically reconfigurable network

OADM (optical add/drop multiplexer)

fast link setup

dispersion compensation

conventional meticulous per-span optical dispersion compensation

direct up/down conversion: low cost high-speed circuit design possible

polarization diversity detection

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Shieh, Orthogonal frequency division multiplexing for optical communications