OFDM Modulation

Concepts of OFDM:

• A type of multi-carrier modulation

• Single high-rate bit stream is converted to low-rate N parallel bit streams

• Each parallel bit stream is modulated on one of N sub-carriers

• Each sub-carrier can be modulated differently, e.g. BPSK, QPSK or QAM

• To achieve high bandwidth efficiency, the spectrum of the sub-carriers are closely spaced and overlapped

• Nulls in each sub-carrier's spectrum land at the center of all other sub-carriers (orthogonal)

• OFDM symbols are generated using IFFT

Advantages of OFDM:

• Robustness in multipath propagation environment

• More tolerant to delay spread:

• Due to the use of many sub-carriers, the symbol duration on the sub-carriers is increased, relative to delay spread.

• Intersymbol interference is avoided through the use of guard interval.

• Simplified or eliminate equalization needs, as compared to single carrier modulation.

• More resistant to fading. FEC is used to correct for sub-carriers suffer from deep fade.

Design challenges of OFDM modulation:

• Sensitive to frequency offset; need frequency offset correction in the receiver.

• Sensitive to oscillator phase noise- clean and stable oscillator required.

• Large peak to average ratio; amplifier back-off, reduced power efficiency.

• IFFT/FFT complexity; fixed point implementation to optimize latency and performance.

• Intersymbol Interference (ISI) due to multipath; use guard interval.

Inter-Carrier Interference Due to Frequency Offset

Select Tutorial: Understanding OFDM Modulation > Inter-Carrier Interference (ICI) due to Freq. Offset.

Guard Interval

• Multipath delays up to the guard time do not cause inter-symbol interference.

• Subcarriers remain orthogonal for multipath delays up to guard time (no inter-carrier interference).

Windowing

• To reduce spectrum splatter, the OFDM symbol is multiplied by a raised-cosine window, w(t) before transmission to more quickly reduce the power of out-of-band subcarriers.

• Preceding illustration shows spectra for 64 subcarriers with different values of the rolloff factor,b of the raised cosine window.

• Larger b, better spectral roll-off.

• However, a roll-off factor of b reduces delay spread tolerance by a factor of bT_S.

OFDM Transceiver Block Diagram

Effects of Link Impairments on OFDM Modulation

This section summarizes the evaluation of the effects of link impairment when using the WLAN Design Library and the WLAN DesignGuide.

The following WLAN DesignGuide menu is shown as it appears when you have configured your program for dialog box access vs. cascading menus.

Effects of Power Amplifier Nonlinearity

Select Evaluating OFDM Performance > Effect of Power Amplifier Non-Linearity > EVM/Constellation.

Following is the behavioral model used in the PA non-linearity simulation:

Here the output 1-dB Compression Point (dBc1out) is used along with the output Third-Order Intercept (TOIout) derived from it by adding 12 dB. The results can be evaluated for their effect on EVM (Error Vector Magnitude), Constellation diagram, spectrum and CCDF (Complementary Cumulative Density Function).

Here is a Constellation diagram at 6 dB backoff:

CCDF indicates the probability (starting from 100%) of the signal's peak value in dB. The CCDF plot for the power amplifier response, operated at 6 dB backoff from saturation, indicates signal clipping at 7.8 dB, compared to the unamplified signal's peak of 9.4 dB at 0.01%.

The bit error rate (BER) and packet error rate (PER) can also be measured against a particular impairment. For the non-linear PA, the BER can be shown to degrade when the amplifier is not sufficiently backed-off, as shown here.

Requirement for BER/PER Simulations

Due to the use of coding and the presence of non-linear impairments, a Monte Carlo BER simulation method must be used. Since a PSDU length of 1,000 bits is required, these simulation can be quite lengthy. Therefore, most of the saved datasets included with this DesignGuide reflect simulations performed with a much smaller length, e.g. 10 or 100, and will show degradation as the signal is more greatly impaired in some way. However, reliable estimates of the BER or PER for less-impaired signals will require multiple 1,000-bit packets to be simulated.

Effects of Frequency Offset

Frequency offset due to differences between the transmit and receive reference oscillators is expressed as a percentage of the 312.5 kHz sub-carrier frequency spacing. The receiver can perform frequency offset estimation and correction using preambles:

• Make use of short preamble for coarse frequency offset estimation and long preamble for fine frequency offset estimation.

• Short preamble symbol duration of 0.8us allows frequency correction up to 1/(2x0.8us)=±625kHz

• Assume RF frequency=5.8GHz, the tolerable frequency offset (worst case) =0.5x625k/5.8G=±53.8ppm > ±20ppm specified in 802.11a.

Effects of Oscillator Phase Noise

An N_Tones model is used to model the phase noise.

Effects of Fixed Point implementation of IFFT/FFT

The IFFT and FFT function in the transceiver will have a fixed bit-width. This might have an effect on the system performance. The WLAN DesignGuide provides a 64-point implementation which uses the bit width as a parameter, so it can be changed or swept. It uses a decimation in frequency, Radix-2 algorithm.

Effects of Multipath

Multipath propagation is simulated using the user-defined channel model.

This defines an impulse response.

The RMS delay spread (defined as follows) varies. Typical values are 100-200 nsec.