Multi-Band OFDM and OFDM Simulation Software Using MATLABÒ Graphical User Interface Roslina Mohamad, Wan Mohd Hafeez, Wan Mohd Salleh and Nuzli Mohamad Anas
Abstract This paper presents the conventional and multiband Orthogonal Frequency Division Multiplexing (OFDM) simulation software named as Signal Modulation Simulator (SiMiSIM). A simulation system with Graphical User Interface (GUI) is built to interface user friendlier than contemporary interface or command line interface. Three basic frequency modulation techniques are given as options which are Binary Phase Shift Keying (BPSK), Quadrature Phase-Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM). Each of the signals has to face Forward Error Correction (FEC) encoder to enhance the signal robustness at the receiver end. The simulation is able to simulate Bit Error Rate (BER) and Power Spectral Density (PSD) for research purpose as well as educational purpose.
Keywords Multiband OFDM Additive White Gaussian Noise Bit Error Rate Forward Error Correction
1 Introduction In early parallel transmission systems, a few non-overlapping sub-channels share the whole frequency band as shown in Fig. 1. Apparently the existence of guard band between two adjacent sub-channels is to provide non-overlapping subR. Mohamad (&) W. M. Hafeez (&) W. M. Salleh N. M. Anas (&) Faculty of Electrical Engineering, Universiti Teknologi MARA Malaysia, 40450 Shah Alam, Selangor, Malaysia e-mail:
[email protected] W. M. Hafeez e-mail:
[email protected] N. M. Anas e-mail:
[email protected]
James J. (Jong Hyuk) Park et al. (eds.), Computer Science and Convergence, Lecture Notes in Electrical Engineering 114, DOI: 10.1007/978-94-007-2792-2_29, Springer Science+Business Media B.V. 2012
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Fig. 1 Conventional nonoverlapping multi-carrier modulation [2]
Fig. 2 Overlapping multicarrier modulation [2]
channels as to eliminate the possible interference among adjacent sub-channels, which is known as Inter-Carrier Interference (ICI). This guard band constitutes a waste of spectrum. However in mid-1960s, spectral efficiency was improved by overlapping the sub-channels as shown in Fig. 2 had saved up to 50% of the spectrum used which was developed using OFDM technology. OFDM is not only a frequency multiplexing technique that mandates orthogonally among subchannels but also a special case for multi-carrier modulation. Moreover, OFDM can be regarded as either multiplexing technique or modulation scheme [1]. As compared in Figs. 1 and 2 respectively, apparently that bandwidth can be saved and be used for other sub-channels. In addition, another advantage of OFDM is multiple orthogonal carriers are transmitted simultaneously. By transmitting several symbols in parallel, symbol duration is increased proportionately, which reduces the effects of Inter Symbol Interference (ISI) caused by a dispersive Rayleigh-fading environment [2]. This paper objective is to describe the simulation functioning software called Signal Modulation Simulator, in short SiMiSIM, which developed using MATLAB Graphical User Interface Development Environment (GUIDE) to provide better easy access. SiMiSIM cater not only for OFDM transmission but also simulate the Multiband OFDM system. Various results concerning to the system performances are calculated such as the BER and other relevant information such as PSD and the signals constellation. Hence, it allows user to display multiple graph concurrently for easier comparisons among available modulation options which include PSK and QAM. The rest of this paper is organized as follow: A literature review of the project is explained in Sect. 2, emphasizes on both conventional and multiband OFDM system together with the introduction of SiMiSIM. Then, in Sect. 3, described system model and explained each module involved in baseband signal processing. Thorough explanation includes the data randomizer, random interleaver, channel coding and signal modulation used in this simulation. Section 4 discussed on methodology towards this simulation tool called SiMiSIM and briefed on the simulation flows. Section 5 discussed the result obtained from several examples and continued in the next section with the conclusion and future work recommendation on this project.
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2 Literature Review The motivation towards this simulation tool development, so-called SiMiSIM, is to ease the calculation of complex equation involved in bit error rate (BER) alongside the power spectral density (PSD) using GUI manner. Despite of conventional command line interface (CLI) approach, users are able to display various system performance results concurrently.
2.1 Conventional OFDM Orthogonal Frequency Division Multiplexing (OFDM) is a well-known technique to mitigate inter-symbol interference (ISI) due to multipath effects. For highly dispersive channels, OFDM is more efficient at capturing multipath energy provides higher spectral efficiency and inherent resilience to narrowband radio frequency (RF) interference. It also has excellent robustness in multipath environment while preserving the orthogonally among subcarriers. The concept is to divide a bandwidth into several parallel streams, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme at a lower rate.
2.2 Multiband OFDM The principle idea of multiband approaches is to divide the Ultra-Wideband (UWB) frequency band from 3.1 to 10.6 GHz into multiple smaller frequency bands or sub bands and uses multiple carrier frequencies to transmit the information. Each sub band has a bandwidth greater than 500 MHz to comply with the Federal Communication Commission (FCC) definition of UWB signal [3]. Multiband OFDM spectrum in UWB channel is shown in Fig. 3. Apparently it had been divided to S sub band and each sub band may have N subcarriers each.
2.3 Signal Modulation Simulator Signal Modulation Simulator (SiMiSIM) is a tool to simulate the performance of variants coding and modulation techniques in the context of OFDM transmission, both conventional and multiband OFDM system. SiMiSIM gives easy access and attractive user interfacing built using Graphical User Interface Development Environments (GUIDE) tool in MATLAB, where users are able to program the software themselves provided in m-file script format. Furthermore, ones can
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Fig. 3 Overlapping multicarrier modulation [2]
visualize the system performance in terms of BER, PSD and also the signal constellation graphically at both ends.
3 System Model This section discussed the transceiver module used in constructing SiMiSIM which includes the data randomizer, Forward Error Correction (FEC), bit-interleaving, signal modulation and also OFDM transmission. Each module is thoroughly explained in short while. Figure 4 depicted the system model where each module is representing in block diagram concatenate. Noted that, Additive White Gaussian Noise (AWGN) and Saleh–Valenzuela (S–V) has been used representing the channel modeling. Signal source enters the randomizer to have more evenly distributed power density and also to avoid too high peak power that may lead to distortion. Also in this process, it breaks up any adjoining long strings of zeros or ones and brings movement to the data stream [4]. The data are scrambled to convert the data bit sequence into a pseudo random sequence which is free from long repeated strings. Polynomial generator of pseudo random binary sequence (PRBS) used in data randomizer as shown in (1) [3]. g ¼ 1 þ D14 þ D15
ð1Þ
In (1), D represents a single bit delay. xn in PRBS is generated as in (2) and the scrambled data bit stream is then obtained using (3) [3] as derived below. xn ¼ xn14 xn15
ð2Þ
s n ¼ bn x n
ð3Þ
In information theory, FEC used as an error control system of data transmission, whereby systematically generated redundant data are added to its original messages. It is to increase the robustness of the signal due to the compression process and also makes the signal vulnerable to channel noise and interference [4, 5]. In
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Fig. 4 Transceiver architecture of OFDM
SiMiSIM, Convolutional encoder is used concatenated with Reed Solomon (RS) code to improve the BER and for more accurate decoding at the receiver [3, 4]. Reed Solomon (RS) code is non-binary cyclic codes with symbols made up of multiple bit sequences [6]. The codes are very useful for burst error correction where errors occur in a large sequence. RS code with symbols made up of m bit sequences where m corresponds to elements of the Galois Field, GF(qm) and is any positive integer greater than 2 [4]. The parameters of RS codes are as the following [4]: • Code length n ¼ 2m 1
ð4Þ
• Number of parity check symbols n k ¼ 2t
ð5Þ
K ¼ 2m 1
ð6Þ
Dmin ¼ n k þ 1
ð7Þ
• Number of data symbols
• Minimum distance
Random interleaving is used to rearrange data sequence using a fixed random permutation order. It enhances the error correcting capability of coding by constructing a long block code from small memory convolutional codes. By doing so, errors typically occurs in bursts rather than uniformly distributed in long codes can approach the Shannon capacity limit. In multi-carrier transmission, cyclic prefix of an exact copy of a segment of the OFDM symbol is located toward the symbol end [1] as to mitigate the ISI effects. Despite of redundancy it can effectively be avoided at the cost of power loss and bandwidth expansion but this inserting of cyclic prefix guard interval before each block of parallel data symbols will decreases the spectral efficiency of the OFDM system [7, 8].
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Fig. 5 SiMiSIM operations
Fig. 6 SiMiSIM after the simulation is completed
4 Methodology Initially, the baseband signal processing described in previous section is designed using MATLAB m-file prior converting to GUI based software. Figure 5 depicted the architecture and software flows. It involves displaying the transmitting power spectrum either both in time and frequency domain along with the
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Fig. 7 PSD in time domain
Fig. 8 PSD in frequency domain
signals constellation. Towards the end, the error rate analysis is visualized using semi-log plotting format in range of given signal-to-noise power ratio. In this simulation, either AWGN only or S–V channel modeling may be chosen as to simulate the realistic data transmission. Note that conventional or multiband OFDM transmission also can be simulated with various coding and modulation schemes. Options include both binary and quaternary PSK or multi-amplitude QAM of 16 and 64-level for the signal modulations while code rate of 1/2, 3/4 and 5/6 can be chosen for the convolutional encoder. Users also able to determine the length of cycle prefix and number of symbols involved. Ones would be able to compare various results obtained in the simulation with theoretical performance. BER plots of each type of modulation as well as their signal constellations can be easily visualized for academic and research purposes.
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Fig. 9 16-QAM bit error rate
Fig. 10 16-QAM scatter plot
Those visualizing figures are displayed side by side simultaneously in the Graph Preview section which includes the power spectrum of respective signal in both frequency and time domains. Figure 6 shows a snapshot of the SiMiSIM after a simulation is completed.
5 Results and Discussion Performance of 16-QAM with 3/4 Convolutional Encoder is used to discuss in this section. Guard period of rate 1/32 with 100 OFDM symbols is used throughout discussion. Hence in Figs. 7 and 8, the power spectrum density in both time and
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frequency domain is shown respectively. A multi-carrier modulation with different power is transmitted within the OFDM signal. While in Fig. 9, shown the BER calculated in SiMiSIM using semiology representation which follows the waterfall-like behavior for Shannon theory. Scatter plot of the 16-QAM transmission with AWGN corrupted signal is shown in Fig. 10. As a 16-QAM modulation is used, thus 4 constellations exist at each of quartet of the graph and making the total of the constellations is 16.
6 Conclusion SiMiSIM is a tool to simulate baseband signal processing involved in conventional and multiband OFDM systems in a graphical user interface manner. This tool able to simulates the error performance and other visualize figures relate to the signal processing which includes the signal constellation and the power spectrum. Easy access and attractive interface provided makes users able adapt and learn in short time. Furthermore, results obtained from simulations can be compared to theoretical or any other previous work for easy understanding. We would like to recommend of the future works of implementing the conventional and multiband OFDM system on a DSP hardware platform in order to gauge real-time system performances. Acknowledgments The authors would like to thank Ministry of Higher Education, Malaysia (MOHE) for supporting this research through grant number 600-RMI/ST/FRGS 5/3/Fst (120/2010) and 600-RMI/ST/FRGS 5/3/Fst(164/2010).
References 1. Chiueh T-D, Tsai P-Y (2007) OFDM baseband receiver design for wireless communications. Wiley, Singapore 2. Ghavami M et al (2007) Ultra wideband signals and systems in communication engineering, 2nd edn. Wiley, West Sussex 3. Siriwongpairat WP, Liu KJR (2008) Ultra-wideband communications systems multiband OFDM approach. Wiley, New Jersey 4. Vengadasalam JAL (2007) Performance analysis of digital television systems. Master of Engineering (Telecommunication), Faculty of Engineering, University Malaya 5. Mohamad R, Anas NM (2010) Performance analysis of convolutional interleaver on TMS320C6711 digital signal processing kit. In: Proceedings of the 2010 international conference on computer applications and industrial electronics, 5–8 December 2010 6. Sklar B (2001) Digital communications fundamental and applications, 2nd edn. Prentice-Hall, Upper Saddle River 7. Kattoush AH et al (2009) The performance of multiwavelets based OFDM systems under different channel conditions. Dig Sig Proc 20:472–482 8. Batra A et al (2004) Multi-band OFDM: a new approach for UWB. Presented at the 2004 IEEE international symposium on circuits and systems, Sheraton Vancouver Wall Centre Hotel, Vancouver, Canada
