Prof. P. Magesh Kannan et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 5, Issue No. 2, 315 - 318
Performance Analysis of Optical OFDM on Fading Channel with Adaptive Interleaving VLSI Division, School of Electronics Engineering, VIT University, Vellore, India E-mail: mageshkannan.p@vit.ac.in
employed in each carrier. Additionally, the introduction of channel coding into the system is inevitable to enhance the reliability. Aiming at low latencies, the codeword size is limited to an OFDM symbol, i.e., a codeword spans all carriers. The channel “experienced” by a codeword, i.e., the different channel conditions of the carriers, can be well modeled by a Rayleigh fading channel. A well-known coding scheme tailored to fading channels is bit-interleaved coded modulation (BICM) [2]. The employed bit interleaver is crucial for the performance of BICM and its superiority over other coded modulation schemes (e.g., trellis coded modulation (TCM) [3]) in fading environments. In the initial publication on BICM [4], Zehavi used three independent bit interleavers for transmission with 8-PSK (phase shift keying). Regarding a triple of binary symbols mapped onto an 8-PSK symbol, each of the three binary symbols has undergone independent interleaving, so-called intralevel interleaving. Caire et al. rejected this approach in [2], arguing that there were no reasons justifying it, flexibility would be limited, analysis complicated, and unequal error protection introduced. Instead they proposed the application of a global interleaver.
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Abstract: A multi-antenna transmitter includes an adaptive bit interleaver for orthogonal frequency division multiplexed (OFDM) communications. The adaptive bit interleaver permutes a variable number of coded bits per OFDM symbols (Ncbps). The variable number of coded bits is calculated based on individual subcarrier modulation assignments for orthogonal subcarriers. The advantages of OFDM systems are high spectrum efficiency, longer symbol duration against inter-symbols interference (ISI) effect in multi-path environments, and narrow transmission band of each sub-carrier within coherence bandwidth such that the communication channel can be viewed as flat and not frequency selective. The interleaver may add padding bits to the interleaver matrix to fill any remaining positions. In the simulation results, we found that better system performance can be achieved by good concatenated codes design for OFDM based WLAN systems under the fixed power and bit-error-rate requirements.
Vijaya Kumar Kirubanandam PG Student – VLSI Design, SENSE VIT University, Vellore, India E-mail: vijayprem17@gmail.com
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Prof. P. Magesh Kannan
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Keywords: Inter-Symbols Interference (ISI), orthogonal frequency division multiplexed (OFDM), adaptive interleaving, bit-error-rate (BER), bitinterleaved coded modulation (BICM).
I. INTRODUCTION
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In order to exploit optical bandwidth more efficiently, novel transmission technologies are intensively studied. High-speed optical transmission systems are impaired by inter channel and intra channel nonlinearities, polarization mode dispersion and chromatic dispersion, and require precise dispersion compensation techniques. To increase the transmission capacity, narrower channel spacing is pursued and novel modulation formats are investigated. We consider very high data rate transmission over frequency selective channels. The occurring inter symbol interference (ISI) is treated by using orthogonal frequency division multiplexing (OFDM) [1]. To support the requested high data rates large signal constellations have to be
ISSN: 2230-7818
II. SYSTEM DESCRIPITION The first blocks in the transmitter are interleaving and coding. All OFDM systems use some form of error correction or detection because, if there is frequency selective fading in the channel, some of the parallel data streams will experience deep fading. The coding is usually preceded by interleaving because, as shown in Fig. 1, a number of adjacent OFDM subcarriers may fall within the frequencies which are experiencing fading. In most broadcast applications of OFDM such as digital audio broadcasting (DAB) and digital video
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Prof. P. Magesh Kannan et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 5, Issue No. 2, 315 - 318
broadcasting (DVB) there are two layers of interleaving and coding so that a very low overall bit error rate (BER) can be achieved even over a very noisy channel.
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OFDM is invariably used in conjunction with channel coding (forward error correction), and almost always uses frequency and/or time interleaving. Frequency (subcarrier) interleaving increases resistance to frequencyselective channel conditions such as fading. For example, when a part of the channel bandwidth fades, frequency interleaving ensures that the bit errors that would result from those subcarriers in the faded part of the bandwidth are spread out in the bit-stream rather than being concentrated. Similarly, time interleaving ensures that bits that are originally close together in the bit-stream are transmitted far apart in time, thus mitigating against severe fading as would happen when travelling at high speed. However, time interleaving is of little benefit in slowly fading channels, such as for stationary reception, and frequency interleaving offers little to no benefit for narrowband channels that suffer from flat-fading (where the whole channel bandwidth fades at the same time). The reason why interleaving is used on OFDM is to attempt to spread the errors out in the bit-
ISSN: 2230-7818
stream that is presented to the error correction decoder, because when such decoders are presented with a high concentration of errors the decoder is unable to correct all the bit errors, and a burst of uncorrected errors occurs. A similar design of audio data encoding makes compact disc (CD) playback robust.
A classical type of error correction coding used with OFDM-based systems is convolutional coding, often concatenated with Reed-Solomon coding. Usually, additional interleaving (on top of the time and frequency interleaving mentioned above) in between the two layers of coding is implemented. The choice for Reed-Solomon coding as the outer error correction code is based on the observation that the Viterbi decoder used for inner convolutional decoding produces short errors bursts when there is a high concentration of errors, and Reed-Solomon codes are inherently well-suited to correcting bursts of errors. Newer systems, however, usually now adopt near-optimal types of error correction codes that use the turbo decoding principle, where the decoder iterates towards the desired solution. Examples of such error correction coding types include turbo codes and LDPC codes, which perform close to the Shannon limit for the Additive White Gaussian Noise (AWGN) channel. Some systems that have
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Prof. P. Magesh Kannan et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 5, Issue No. 2, 315 - 318
If during transit more than a certain amount of data has been lost then the data cannot be correctly decoded by your router. Short bursts of noise on the line can cause these data packets to become corrupt and the modem has to re-request data which in turn can slow down the overall rate at which data is transmitted.
Frequency (GHz) Area Power (nW)
3.4 24050 Leakage: 127.022 Dynamic: 993206.574 Total: 993333.596
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Interleaving is a method of taking data packets, chopping them up into smaller bits and then rearranging them so that once contiguous data is now spaced further apart into a non continuous stream as shown in figure 2. Data packets are reassembled by your modem.
Table 1 Frequency, Area and Power of Interleaver in OFDM
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implemented these codes have concatenated them with either Reed-Solomon (for example on the MediaFLO system) or BCH codes (on the DVB-S2 system) to improve upon an error floor inherent to these codes at high signal-to-noise ratios.
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Interleaving is the other mechanism used in OFDM system to combat the increased error rate on the weakened subcarriers. Interleaving is a deterministic process that changes the order of transmitted bits. For OFDM systems, this means that bits that were adjacent in time are transmitted on subcarriers that are spaced out in frequency. Thus errors generated on weakened subcarriers are spread out in time, i.e. a few long bursts of errors are converted into many short bursts. Error correcting codes then correct the resulting short bursts of errors. III. SIMULATION RESULT: The proposed architecture has been captured by using Verilog HDL. All the internal components of the design were synthesized placed and routed using cadence tools is shown in figure 4. Thus the synthesize result of the interleaver is given in table 1.
ISSN: 2230-7818
IV. CONCLUSIONS: In this paper coding and interleaving techniques was studied and their performance was evaluated. It has been shown that, with adaptive interleaving the real and imaginary components of signals experience different independent fading channels and additional diversity gain can be obtained. In the simulation results, we found that under the fixed
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Prof. P. Magesh Kannan et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 5, Issue No. 2, 315 - 318
power and bit-error-rate requirements, better system performance can be achieved for OFDM based WLAN systems if good concatenated codes are designed. REFERENCES
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[1] J. A. C. Bingham. Multicarrier modulation for data transmission: An idea whose time has come. IEEE Communications Magazine, pages 5–14, May 1990. [2] G. Caire, G. Taricco, and E. Biglieri. Bit-Interleaved Coded Modulation. IEEE Transactions on Information Theory, 44(3):927–946, 1998. [3] G. Ungerb¨ock. Channel coding with multilevel/phase signals. IEEE Transactions on Information Theory, 28(1):55– 67, 1982. [4] E. Zehavi. 8-PSK Trellis Codes for a Rayleigh Channel. IEEE Transactions on Communications, 40(5):873–884, 1992
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