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The Orthogonal Frequency Division Multiplexing (OFDM) system is already used in commercial applications and is capable to deal with Intersymbolic Interference (ISI) caused by multipath channels. This system gained popularity after the application of the Fast Fourier Transform (FFT) and its inverse (IFFT) to modulate the signal in many subcarriers. This paper discusses implementation aspects of an OFDM system; such system is characterized by considering real constraints, including the memory consumption and the processing time. The OFDM modulator, channel samples and OFDM demodulator were implemented entirely in the DSP TMS320C6678 platform. As a proof-of-concept, a 256-QAM OFDM BER performance is compared with theoretical values. Moreover, the memory size is not demanding, consuming very few resources. It was observed a very high number of DSP clock cycles needed for the OFDM signal modulation, corresponding to more than 4 times the number used in demodulating the signal.

Signal processing is deployed in lots of equipment such as radars, cell phones, missiles, space buses, radars, and so on. Basic mathematical operations can be performed by using discrete digital components, e.g., a full adder circuit, while subtraction can be implemented using the same circuit with the subtrahend being represented by two’s complement. Digital Signal Processing (DSP) platforms are integrated circuit boards designed aiming to optimize the implementability of the algorithms and mathematical operations including sums, multiplications and so on. The internal architecture of a floating DSP is more complex than a fixed point device [

Orthogonal Frequency Division Multiplexing (OFDM) is a transmission technique currently available in commercial applications, such as wireless networks (Wi-Fi 802.11) and cellular systems (LTE) [

Among the three losses associated to the wireless radio channel, namely path loss, shadowing and fading terms, the OFDM system offers resistance to fading, when the number of subcarriers is large enough such that the flat fading condition is achieved. The delay spread that occurs in channels with multipath can provoke some intersymbol interference (ISI). To completely eliminate this effect, OFDM uses the Cyclic Prefix (CP). The CP addition can be easily implemented digitally, because the CP addition is a simple vector concatenation.

In the literature, there are works discussing the implementation aspects of specific parameter of the OFDM system. Among them, [

This paper deals with the implementability of an OFDM transmission. For that, the DSP TMS320 platform receives data from a computer, data is processed and modulated using an OFDM modulator, the channel coefficients are generated and applied to the signal, the OFDM symbols are demodulated and sent back to the computer. The DSP TMS320C6678 platform deployed presents a high performance multicore processor with 8 cores, each core can reach 1.25 GHz, supporting fixed and floating point operations. It has I2C and SPI interface, a 64-bit DDR3 interface, 64 timers, 16 GPIO pins [

This work will be structured as following: in Section 2, the theory of the OFDM system is presented. Block diagrams, mathematical relations and some OFDM design parameters are stated. In Section 3, some implementation details and tools utilized are shown. In Section 4, simulations are presented and results discussed. In Section 5, the conclusions of the work are highlighted.

Notation:

A block diagram representing an OFDM transmitter is depicted in

The OFDM receiver is depicted in

OFDM systems use a great number of orthogonal subcarriers to transmit information; hence, the data rate on each subchannel is much less than the system data rate [

where

orthogonal if the integral on the interval of one period of the multiplication of the signals is zero. In [

The transmitted OFDM signal can be represented in time domain as:

where x[k] represents the k-th modulation symbol,

In

The received signal corrupted by multiplicative noise, considering only the effect of fading channel, i.e., assuming that the additive white Gaussian noise effect can be neglected (high SNR regime) in discrete-time domain can be expressed by the circular convolution of the signal s[n] and channel impulse response h[n]:

Hence, the original signal s[n] can be recovered in the frequency domain using frequency equalization: as:

where

To obtain a satisfactory performance, the OFDM must achieve flat fading condition. In other words, the channel coherence band

where W represents the OFDM system bandwidth and N the number of subcarriers. Note that the right side of Equation (6) considers a system without spectral superposition. So, the number of subcarriers is a parameter of the OFDM system and can be increased to achieve the flat fading condition.

Another channel parameter that exerts influence in the OFDM system design is the delay spread (

To eliminate the ISI caused by multipath channel, the CP is added to the signal vector. Considering h[n] the discrete channel impulse response with length

In this work, the Code Composer Studio (CCS) software was deployed as the Integrated Development Environment (IDE) to write, compile and debug the entire developed code. Another software installed was the BIOS Multicore Software Development Kit (MCSDK), which provided some boot utilities, chip support libraries, drivers, and basic

platform utilities [

There are some specific functions implemented in an optimized way for specific DSP platforms. For the TMDSEVM6678 platform, the TI C6000 DSPLIB library of signal processing provides some routines for signal processing. In this work, the FFT, IFFT and convolution operations were performed using the DSPLIB optimized functions. In the DSPLIB documentation [

To verify the DSP clock cycles consumed in executing a piece of the code, the CCS provides the necessary tools to this verification. The user can obtain the clock information going to the clock menu and enabling it, or using the Profiler tool. Another method is calling the function itoll provided by the header c6x.h, which returns the DSP clock count value that can be stored inside a variable.

Random Number GenerationIn [

in which the random variables of such transformations follow a normalized Gaussian distribution:

Finally, to obtain different values of mean (

Resulting in

In this section, some experiments using the DSP platform are described and analyzed. First, the 256-QAM OFDM algorithm is validated comparing the bit error rate (BER) values obtained via Monte Carlo simulation with the theoretical curves of a 256-QAM modulation. At the flat fading condition, the 256 QAM OFDM BER performance should be close as possible to the conventional 256-QAM BER performance. After this Tx-Rx OFDM code validation, an experiment sending an image data to the DSP platform was performed aiming to corroborate the effectiveness of the proposed OFDM DSP-based system implementation. The image data generated at PC was sent to the DSP platform, it was modulated and converted to OFDM symbols and sent to the receiver through a simulated wireless radio fading channel in discrete-time domain. Besides, the additive thermal noise effects were included at the receiver input; after that, the OFDM symbols were demodulated and sent back to the PC, where the original data were compared with the recovered data aiming to determine the average BER. As a figure-of-merit of implementability, the DSP resources allocated to the Tx-Rx OFDM execution were determined in therms of memory occupation and DSP clock cycles.

The OFDM system performance was measured verifying the bit error rate (BER) in two different channels: a) multipath fading channel; and b) pure additive white Gaussian noise (AWGN), just for noise power calibration purpose. Simulation parameters are summarized in

The BER performance simulated on the DSP platform and the theoretical curves of the 256-QAM are presented in

Parameter | Value |
---|---|

Modulation | 256 - QAM |

Subcarriers | N = 256 |

Channel | AWGN, SNR Î {0.0; 18} [dB], 4-paths Rayleigh fading channels |

Codification | Gray Code |

The image data sent in this experiment was the Lenna, an image commonly used in image processing [

The OFDM DSP-based system implementability was verified by measuring the DSP platform resource allocated to the entire code execution. To perform this task, the number of DSP clock cycles was counted. For modulation, it was taken in to account the 256-QAM modulation, IFFT calculation and CP addition. On the demodulation, it was considered the CP removal, FFT calculation, channel equalization and 256-QAM demodulation. The mean values measured are presented in table

The number of cycles of one OFDM symbol was different from the others due to QAM modulation and demodulation implementation. To demodulate the signal, the implemented code separates/classifies the constellation map into decision regions and

Parameter | Value (DSP Cycles) |
---|---|

256 QAM OFDM Modulation | 2,893,850.31 |

256 QAM OFDM Demodulation | 606,401.31 |

verified if the received symbol was inside or not a specific region to demodulate/de- mapping the symbol. On the modulation, it has two "for" loops to convert the information to components in phase and quadrature, and that part of the code was responsible for a great part of the total cycles consumed in the modulation process.

Even with more components needed to the signal demodulation, including mainly channel equalization, the number of cycles consumed in the OFDM demodulation process was smaller than the number necessary for the OFDM modulation.

To measure the code size, the file created by the compiler with extension .map was verified, as discussed in [

Consulting [

The signal processing in the OFDM system had an important role in simplifying its implementation using the FFT and IFFT to modulate the signal in multicarrier-based systems instead of using analogical huge number of discrete oscillators. In this work, a

Name | Origin | Length | Used |
---|---|---|---|

L2SRAM | 0x00800000 | 0x00080000 | 0x0000000 |

L1PSRAM | 0x00E00000 | 0x00007FFF | 0x0000000 |

L1DSRAM | 0x00F00000 | 0x00007FFF | 0x0000000 |

MSMCSRAM | 0x0C000000 | 0x00200000 | 0x006FB12 |

DDR3 | 0x80000000 | 0x10000000 | 0x0000000 |

256-QAM DSP-based baseband OFDM was implemented and analyzed in terms of BER performance, qualitative recovered image performance, DSP clock cycles and DSP memory requirement.

The 256-QAM OFDM BER system performance was obtained via Monte-Carlo simulation and these values were corroborated with a theoretical curve of a conventional 256-QAM modulation single-carrier system. In attaining the flat fading condition, the M-QAM OFDM performance converged to the M-QAM modulation and the code implemented on the DSP behaved as expected.

By verifying the clock cycles, the number of DSP cycles consumed to modulate the OFDM signal was greater than the number used to demodulate the signal. The reason is that the M-QAM modulation function was responsible for a great part of the cycles used. In the future work, assembly language optimization aspects should be implemented, while the processing gain concerning DSP clock cycles should be evidenced.

By analyzing the memory requirement of the entire implemented baseband OFDM, the amount memory allocated had size of 457,490 bytes of the memory block that corresponded to the address 0x0C000000 and was not a critical issue for the deployed DSP platform.

With the increase of new technologies, for example, Internet of Things, with countless chips communicating with each other, there is great appeal and interest of investigation around the viability of the implementation of sophisticated but efficient wireless transmission-modulation schemes on chip with scarce resources availability. This paper provides a multi-functional analysis of the implementability of an OFDM baseband system on a robust DSP platform, which can be extended for another DSP, FPGA or microprocessors platforms.

Thanks to Raul Ambrosio Valente Neto for the explanation on the random number generation. Colleagues at T & SP Lab at UEL University are grateful recognized for the conceptual discussions and hints.

Fukuda, R.M. and Abrão, T. (2016) OFDM System Implementation in DSP Platform TMS320C6678. Journal of Computer and Communications, 4, 26-36. http://dx.doi.org/10.4236/jcc.2016.411003