DESIGN ISSUES OF SPACE TIME BLOCK CODED (STBC) OFDM SYSTEMS

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KAMALJIT SINGH BHATIA, IJRIT 213 International Journal of Research in Information Technology (IJRIT) International Journal of Research in Information Technology (IJRIT)International Journal of Research in Information Technology (IJRIT) International Journal of Research in Information Technology (IJRIT) www.ijrit.comwww.ijrit.comwww.ijrit.comwww.ijrit.com ISSN 2001-5569

DESIGN ISSUES OF SPACE TIME BLOCK CODED (STBC) OFDM SYSTEMS

KAMALJIT SINGH BHATIA₁, HARSIMRAT KAUR_2,GURSHARAN SINGH₃, KULWINDER SINGH4

1 Department of Electronics Engineering, Sri Guru Granth Sahib World University, Fatehgarh Sahib, Punjab, India

2 Department of Electronics and communication Engineering, CTIEMT, Jalandhar city 3&4 Department of Electronics and communication, BMSIET, Muktsar

E-mail: [email protected]

Abstract. We propose and demonstrate experimentally the bit error rate (BER)

performance of Space-time block coded spatial modulation (STBC-SM) system. The proposed detection scheme firstly detects the transmitted symbols for each possible pair of transmit antennas and secondly chooses the most likely pair of transmit antennas and transmitted symbols for STBC-SM. The second objective of this paper is to present an asymptotic bound to quantify the average bit-error rate performance of MQAM STBC-SM over different channels.

1. Introduction: In recent years, orthogonal frequency division multiplexing (OFDM) has been proposed in optical fiber transmission systems to mitigate transmission impairments such as chromatic dispersion and polarization mode dispersion (PMD). In order to transmit the OFDM signal on an optical link, there are two solutions. One is the coherent detection OFDM (CO- OFDM) scheme, and the other is the intensity modulation and direct detection (IM/DD) scheme.

The IM/DD scheme has been widely applied owing to its simple and low complexity [1-2]. A

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IJRIT International Journal of Research in Information Technology, Volume 2, Issue 3, March 2014, Pg: 213-219

KAMALJIT SINGH BHATIA, IJRIT 214

real-valued positive OFDM signal is required for optical IM/DD systems. Thus, the Hermitian symmetry (HS) property of the input data of the inverse fast Fourier transform (IFFT) must be satisfied in order to generate a real OFDM signal. Similar to a wireless OFDM system, the high peak-to-average power ratio (PAPR) is the main disadvantage of the optical IM/DD OFDM system, which can degrade the performance of the system [3]. For reducing the effect of the high PAPR on the performance of the optical OFDM system, many methods have been proposed for PAPR reduction [4-6]. Recently, PAPR reduction techniques using precoding have been proposed to reduce the PAPR of OFDM signals for optical OFDM systems. In the literature [6], some precoding methods, such as Hadamard precoding, discrete Hartley transform, discrete Cosine transform (DCT), have been researched in optical wireless IM/DD channel by simulation methods. Rf. [7] employed Hadamard precoding to improve the bit error rate (BTR) of the system with the PAPR reduction. In [8], the discrete Fourier transform (DFT) precoding technique was studied for CO-OFDM systems. The experimental results show that the BER performance with DFT precoding can be improved in the conventional CO-OFDM systems. Rf.

[9] proposed a combined Hadamard and companding technique for PAPR reduction. Among the PAPR reduction methods, clipping is the most simple. The nonlinearity of clipping is generally regarded as an impairment to a communication system. However, there is less attention on the benefits of the clipping technique for baseband signal in communication. RF. [10] analyzed the effect of clipping in the error performance of OFDM in frequency selective fading channels by simulation. The analysis results showed that the degradation in the error rates is very small for clipping with a clipping ratio (CR) of 6 dB in the two-path channel. In the literature [11], the authors have shown that clipping can improve the overall communication system performance, under the peak power constraint.

2. System Set-up:

In this section, we study the performance of such a scheme with two receive antennas (i.e., a 2x2 system) with and without channel estimation. In the realistic scenario where the channel state information is not known at the receiver, this has to be extracted from the received signal. We assume that the channel estimator performs this using orthogonal pilot signals that are prepended to every packet [3]. It is assumed that the channel remains unchanged for the length of the packet (i.e., it undergoes slow fading).

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KAMALJIT SINGH BHATIA, IJRIT 215 Fig1. Conceptual diagram of Spatial encoded communication system

A simulation similar to the one described in the previous section is employed here, which leads us to estimate the BER performance for a space-time block coded system using two transmit and two receive antennas.

For the 2x2 simulated system, the diversity order is different than that seen for either 1x2 or 2x1 systems in the previous section as shown in Fig 2. Note that with 8 pilot symbols for each 100 symbols of data, channel estimation causes about a 1 dB degradation in performance for the selected Eb/No range. This improves with an increase in the number of pilot symbols per frame but adds to the overhead of the link.

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Fig 2: BER Performance with pilot carrier

In this comparison, we keep the transmitted SNR per symbol to be the same in both cases. The accompanying functional script, OSTBC2M_E.m aids further experimentation for the interested users.

3. Orthogonal Space-Time Block Coding and Further Explorations

In this final section, we present some performance results for orthogonal space-time block coding using four transmit antennas (4x1 system) using a half-rate code, G4, as per [4]. We expect the system to offer a diversity order of 4 and will compare it with 1x4 and 2x2 systems, which have the same diversity order also.

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KAMALJIT SINGH BHATIA, IJRIT 217 Fig 3: BER Analysis for different scenario

To allow for a fair comparison, we use quaternary PSK with the half-rate G4 code to achieve the same transmission rate of 1 bit/sec/Hz. Since these results take some time to generate, we load the results from a prior simulation. The functional script OSTBC4M.m is included, which, along with MRC1M.m and OSTBC2M.m, was used to generate these results. The user is urged to use these scripts as a starting point to study other codes and systems.

As expected, the similar slopes of the BER curves for the 4x1, 2x2 and 1x4 systems indicate an identical diversity order for each system (Fig 3).Also observe the 3 dB penalty for the 4x1 system that can be attributed to the same total transmitted power assumption made for each of the three systems. If we calibrate the transmitted power such that the received power for each of these systems is the same, then the three systems would perform identically. Again, the theoretical performance matches the simulation performance of the 4x1 system as the total power is normalized across the diversity branches.

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