10.4 Performance Analysis of the Proposed System
10.5.5 BER Performance
In this experiment, the BER performance of the proposed scheme in comparison to ZP-OFDM and conventional TDS-OFDM are evaluated using simulations. This evaluation is conducted in three cases i) sufficient guard interval without any IBI between OFDM data blocks with maximum tap number M128; ii) insufficient guard interval with a medium IBI between OFDM data blocks and M 64; iii) insufficient guard interval with a significant IBI between OFDM data blocks and M32. As shown in Figures 10.6, 10.7, 10.8, the BER performance tends to be saturated at high SNRs. These performance floor levels decrease as the guard interval increases. Such error floor is caused by IBI between OFDM data blocks in insufficient guard interval and it is reduced in case i). In practice, the error floor removed by using channel coding.
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As shown in Figs. 10.6, 10.7, 10.8, the proposed ZPNZ-OFDM outperforms the conventional TDS-OFDM. For ZP-OFDM in the single antenna receiver case, it outperforms the proposed system, but for the rich antenna receiver
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rN
ZPNZ-OFDM outperforms the conventional ZP-OFDM. UWA is a sparse channel and a large number of antenna receivers render the TR-channel to be impulse-like which reduces the mutual interference [10].Fig. 10.6. BER performance of OFDM systems with sufficient guard interval, M128.
Fig. 10.7. BER performance of OFDM systems with insufficient guard interval with medium IBI level, M64.
(a) Uncoded BER (b) Coded BER
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Fig. 10.8. BER performance of OFDM systems with insufficient guard interval with significant level of IBI, .
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M
10.6
Conclusion
A novel transmission scheme called a “double-side zero delimited pseudorandom noise OFDM (ZPNZ-OFDM)” has been proposed. In such a scheme a new design of training sequence inserted into the guard interval has been introduced to save energy, especially for long tap delay UWA channels. In the proposed new scheme, the PN sequence amplitude can be reduced to achieve a significant reduction of inter-block interference problems between OFDM data blocks and the training sequence. The new OFDM frame structure provides a significant signal–to– interfered–signal ratio. The proposed system design has been evaluated by using real channels measured from one sea-going experiment as well as a theoretical model. In both situations the merits of the proposed system have been confirmed. In particular, the ZPNZ-OFDM scheme outperforms the ZP-OFDM in terms of energy and spectrum, as well as BER performance. Also, ZPNZ-OFDM significantly outperforms the other TDS-OFDM proposed in this thesis in terms of energy and recived image quality.
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Chapter 11
Conclusion and Future Work
11.1
Conclusion
In this thesis, several signal processing techniques for underwater acoustic communication networks have been developed and investigated.
An efficient image transmission over the underwater acoustic channel has been proposed. One of the main challenges for underwater communication is the limited channel bandwidth. In this thesis, unequal error protection techniques have been proposed to achieve efficient data protection satisfying a high protection level with limited overhead information. These unequal error protection techniques are based on the classification of the image coded bits. This thesis proposed three types of unequal error protection techniques: (1) based on the modulation using HQAM; (2) based on the rate allocation to achieve an efficient distribution of the channel coder overhead information; (3) based on the guard interval length of the ZP-OFDM multicarrier modulation.
Based on the SPIHT coder, encoded image bits can be classified based on their effects on the quality of the reconstructed image at the receiver. The communication system proposed for underwater channel takes advantage of these features. In the unequal error protection using HQAM modulation method, these encoded bits are classified into two groups corresponding to high priority and low priority bits. HQAM controls the protection level in each group using the modulation parameter. In the unequal error protection using rate allocation method and different length guard intervals, encoded bits are classified into four groups: (1) significant bits; (2) sign bits; (3) set bits; and (4) refinement bits. In the rate allocation method these four groups are protected with different levels of protection using a Reed Solomon channel coder. In the unequal error protection based on the guard interval length, each of these four groups are transmitted over a ZP-OFDM multicarrier system with different zero padding length. Using these unequal error protection techniques, bandwidth can be saved and the PSNR of images reconstructed at the receiver can be improved.
However, multicarrier systems are very sensitive to the carrier frequency offset produced due to the transmitter- receiver mismatched oscillator or transmitter-receiver relative velocity. In this case, the multicarrier system sub- carrier orthogonality can be loose. This thesis included in depth analysis and discussion of the carrier frequency offset problem and proposed a pulse shaping function to mitigate the inter-carrier interference problem.
Energy and spectrum efficiency are very important for communication systems. In underwater communication these factors become even more critical, where channel bandwidth and transmission power are more restrictive than that in wireless communication. To achieve a high multicarrier system, an efficient energy multicarrier system has been proposed in this thesis. By replacing the first pseudorandom noise (PN) of the dual PN OFDM (DPN- OFDM) system with zero padding and using the second PN sequence only for synchronization and channel estimation, ZPN-OFDM yields significant improvement in energy efficiency and bit error rate performance. Compressive sensing is proposed for efficient channel estimation and removes the mutual interference of the
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training sequence in the OFDM data blocks. Furthermore the amplitude of the training sequence is reduced for more energy saving.
However, the extra guard interval used in the ZPN-OFDM decreases the spectrum efficiency. Also, as the data rate transmitted over the underwater channel increases, the channel delay spread increases and the efficient guard interval required to the OFDM multicarrier also increases. This will significantly decrease the spectral efficiency of the communication system. To address this problem, OFDM together with single-input multi-output (SIMO) system is proposed in this thesis for an underwater communication system to combat channel fading. While the OFDM itself provides a high spectral efficiency, the actual spectrum efficiency achieved using SIMO-OFDM in underwater communication is often much lower due to long guard intervals.
The proposed time reversal technique combined with SIMO-OFDM can convert the underwater multipath channel into an impulse like channel. In this case, a guard interval length shorter than the maximum channel tap delay can be used. Also, as the benefits of the time reversal SIMO-OFDM, the mutual interference between training sequence and OFDM data blocks in TDS-OFDM can be reduced and a short guard interval can be used. Furthermore, structural compressive sensing has been proposed to achieve accurate channel estimation and reduce the training sequence amplitude to reduce its mutual interference.
Another novel method for energy efficiency improvement has been proposed in this thesis. A novel training sequence, pseudorandom noise with zero insertion, for any of the known symbol padding orthogonal frequency division multiplexing (KSP-OFDM) systems is proposed, where zeroes are inserted into pseudo-noise bit sequence used in the KSP-OFDM training sequence to reduce the guard interval energy use in underwater acoustic channel with significant tap delay, without being detrimental to other pseudo-noise properties required for synchronization and channel estimation. This method is inspired by the observation that long guard intervals in OFDM systems caused by long delays in underwater acoustic channels can cause significant energy waste if KSP-OFDM is used. Zero insertion can be used to avoid this. Also, by using the new ZPNZ-OFDM scheme, high spectral and energy efficiency has been achieved, as well as a high BER performance.
In this thesis, another underwater acoustic (UWA) channel problem has been solved using the TDS-OFDM training sequence. Specifically, most of the UWA communication systems use the preamble signal to estimate the Doppler scaling factor. The problem is over time variant channel is need to resend this preamble. In this thesis, the TDS-OFDM training sequence was used to update the estimation of the Doppler scaling factor due to the time varying feature of the channel.
In this thesis, the TDS-OFDM technique provided for the underwater communication systems where, it adopts a pseudorandom noise (PN) sequence known to both the transmitter and the receiver as a guard interval as well as a training sequence (TS) for synchronization and channel estimation. In the ZPN-OFDM based on the good autocorrelation property of the PN sequences, the received PN sequence is in an IBI free region used in channel estimation and the frame timing and carrier synchronization. For the time reversal TDS-ODM system the Zadoff- Chu sequence (ZOS) is used as training sequence for frame time and carrier frame synchronization and channel estimation. Thanks to ZOS perfect autocorrelation properties. A preamble block is proposed for frame synchronization with Doppler scaling factor (DSF) estimation. Also, DSF updating is analyzed based on TDS- OFDM frequency domain pseudorandom noise training sequence. All proposed techniques have been illustrated
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using simulation experiments for a simulated channel as well as a real UWA channel measured from one sea experiment.