OFDM.zip_DSPF_sp_dotprod_DSPF_sp_ifftSPxSP_ofdm

Abstract

This report discusses the design and implementation of an OFDM modem for a simplex com-

munication between two PCs over a frequency selective channel. First a brief introduction is

provided by explaining the backrground and the specification of the project. Then the report

deals with the system model. Each blo ck of the OFDM system is described (IFFT/FFT, cyclic

prefix, modulation/demodulation, channel estimation, bit loading). In the following section, the

system architecture is anal ysed. The transmission protocol, as well as the system parameters

are explained in details. Then, the DSP implementation is discussed. Finally, the results are

provided in the last chapter.

Aknowledgment

We would like to thank our project assistant, Xi Zhang, for his help during the course.

Contents

1.3 Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.4 Project overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2 System Model 9

2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.1.1 Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.1.2 Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.1.3 Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2 OFDM System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2.1 Evoluti on of OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2.2 Introduction to OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.1 Frame structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.1.1 Transmission Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.1.2 Training sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.1.3 Length Frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.1.4 Pilots Frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.2 Convolution Encoder and Decoder . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.3 Mirror Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.4 Upsampling and pulse sh api ng . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.5 Synchroni zation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.6 Frequency offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.5 DSP transmitter implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.6 DSP Receiver Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.7 Changes compared to simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5 Graphical User Interface 32

5.1 Commu nicati on between the Host and the DSP . . . . . . . . . . . . . . . . . . . 32

5.2 Transmitter GUI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

5.3 Receiver GUI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

6 Simulation results 35

6.1 Channel estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

6.2 Bit loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

7.2 Final results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

8 Future work 41

8.1 Increase the bit rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

8.2 Peak to average ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

8.3 Improving the system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Conclusion 42

List of Figures

1.1 Point to point communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1 OFDM system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2 Bits flow through an interleaver . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.3 Basis functions in OFDM system . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.10 Adaptive bit loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.1 Preambu le of transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.2 Transmission protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.3 Structure of the training sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.4 A 1/2 - rate Convolution encoder . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.5 Trellis used in the Viterbi decoder . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.6 Mirror operation, producing a real valued output from IFFT . . . . . . . . . . . . 22

3.7 Impulse response of the root-raised-cosine filter . . . . . . . . . . . . . . . . . . . 23

3.8 The frames "slide" in the received buffer because of the sampling clock difference 23

3.9 The synchronization method uses correlation between the train ing sequence and

4.3 State machine at the transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.4 State machine at the receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.5 The bit allocation is constant over the transmission . . . . . . . . . . . . . . . . . 31

5.1 Interface at the transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5.2 Interface at the receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

6.1 Channel model in the Matlab simulation . . . . . . . . . . . . . . . . . . . . . . . 36

5