1. ELEG 6203: "Wireles Networks" Wireless Networks December 04,2003
Power Spectral Density Approach to Optimal Lattice OFDM Design for Time-Frequency Dispersive Wireless Channels
- It is known that there are two kind of dispersive occurred in the wireless channels:
1. Time dispersive due to multi-path propagation which results in ISI
2. Frequency dispersive due to Doppler spread which causes ICI
- OFDM can combat only ISI by introducing cycle prefix that is a copy of the last part of the OFDM symbol, which is prepended to the transmitted symbol.
Our objective is to minimize the joint ISI/ICI
Wireless Networks December 04,2003
23:12:31
Department of Electrical Engineering

2. ELEG 6203: "Wireles Networks"
December 04'2003
Cycle Prefix
A cyclic prefix is a copy of the last part of the OFDM symbol, which is prepended to the transmitted symbol
Although the cyclic prefix introduces a loss in signal-to-noise ratio (SNR), it is usually a small price to pay to mitigate interference.
Wireless Networks December 04,2003
23:12:31
Department of Electrical Engineering

3. ELEG 6203: "Wireles Networks"
December 04'2003
OFDM
Orthogonal Frequency-Division Multiplexing
The basic principle of OFDM is to split a high-rate DataStream into a number of lower streams that are translated simultaneously over a number of subcarriers.
Wireless Networks December 04,2003
23:12:31
Department of Electrical Engineering

4. ELEG 6203: "Wireles Networks" Wireless Networks December 04,2003
Continuous-time OFDM
is the impulse response of the time-varying channel H and is zero-mean stationary white noise.
Wireless Networks December 04,2003
23:12:31
Department of Electrical Engineering

5. ELEG 6203: "Wireles Networks" Wireless Networks December 04,2003
Discrete-time OFDM
Compared to the continuous-time model, the modulation and demodulation are replayed by an inverse DFT (IDFT) and a DFT, respectively and the channel is a discrete-time convolution.
Wireless Networks December 04,2003
23:12:31
Department of Electrical Engineering

6. ELEG 6203: "Wireles Networks" Wireless Networks December 04,2003
The transmission functions of an OFDM system consist of translations and modulations of a single pulse shape , i.e.,
Here T is the symbol period and F is the carrier separation
Wireless Networks December 04,2003
23:12:31
Department of Electrical Engineering

7. ELEG 6203: "Wireles Networks" Wireless Networks December 04,2003
OFDM Signal
1. where X be the data symbols to be transmitted.
2. the channel is a discrete-time convolution.
3. zero-mean stationary white noise
Wireless Networks December 04,2003
23:12:31
Department of Electrical Engineering

8. ELEG 6203: "Wireles Networks" Wireless Networks December 04,2003
How the transmitted data are recovered
the transmitted data are recovered by computing the inner product of the received signal with the functions , i.e., .
Wireless Networks December 04,2003
23:12:31
Department of Electrical Engineering

9. Wireless Networks December 04,2003
Lattice OFDM
Wireless Networks December 04,2003
23:12:31

10. What we want The time-frequency localization of  ,
The distance between adjacent time-frequency lattice points, and
TF =1, i.e., maximal spectral efficiency.
Wireless Networks December 04,2003
23:12:31

11. ELEG 6203: "Wireles Networks" Wireless Networks December 04,2003
Optimal OFDM
Dilation Operator
Chirp Operator
Wireless Networks December 04,2003
23:12:31
Department of Electrical Engineering

12. Power Spectral Density
Wireless Networks December 04,2003
23:12:31

13. Wireless Networks December 04,2003
Simulation
SLPR is 8.62% and 6.25%
Wireless Networks December 04,2003
23:12:31

14. ELEG 6203: "Wireles Networks" Wireless Networks December 04,2003
Concerns
PAPR 2. Synchronous
1. Large PAPR demands on linear amplifiers. Nonlinearities in amplifiers may cause both ISI and ICI in the system.
-Clipping filtering or, selective mapping coding
2. Time-varying fading, freq. Offset and timing mismatch can destroy the orthogonality of the subchannels.
- Timing offset estimation, and frequency offset estimation
Wireless Networks December 04,2003
23:12:31
Department of Electrical Engineering

15. ELEG 6203: "Wireles Networks" Wireless Networks December 04,2003
Discussion
Provides robustness of the OFDM systems
improvement of approximately 1dB-2dB Signal-to Noise-plus Interface Ratio (SNIR )
PSD approach provides the robustness of the OFDM against ISI and ICI in wireless channels
Wireless Networks December 04,2003
23:12:31
Department of Electrical Engineering

16. Thank You
Wireless Networks December 04,2003
23:12:31