A Survey on DSP Implementation of OFDM UW Acoustics Modem

C = BWlog2 (1+SNR) (1)

Fig.no.4. Determining Signal Capacity

Synchronization for OFDM:

Synchronization is a vital part of this process. Signal acquisition is achieved in two steps:

1. Estimation of symbol timing:

   Signal recovery relies on training symbols which are composed of 2 identical halves in the time domain. Halves are made by transmitting arbitrary data at even frequencies and zeros at odd frequencies. Timing data is added as a prefix to actual data.

2. Detection of carrier frequency offset:

No two oscillators oscillate at the same rate. The oscillators in the receiver are slightly different than those in the transmitter. Decoding the original signal means accounting for the offset between the two oscillators.

Broadband applications has imposed great challenges for conventional single carrier transmissions, as the channel exhibits strong frequency selectivity that prevents effective channel equalization to remove inter-symbol- interference (ISI) at affordable complexity. Multi-carrier techniques divide the available bandwidth into a large number of overlapping sub bands, so that the symbol duration is long compared to the multipath spread of the channel.

Consequently, ISI may be neglected in each sub band, greatly simplifying the receiver complexity of channel equalization [11].

OFDM is an efficient multi-carrier implementation based on fast-Fourier-transform (FFT). Consider an OFDM transmission over a frequency selective channel, that is described by its discrete-time baseband impulse response vector h: = [h(0), . . . , h(L)]T , with L standing for the channel order. The channel impulse response includes the effects of transmit receive filters and physical multipath. By implementation an inverse FFT at the transmitter and an FFT at the receiver, OFDM converts an ISI channel into parallel ISI-free sub channels with gains equal to the channels frequency response values on the FFT grid. Specifically, let K denote the number of subcarriers in the OFDM system, x(p) as the transmitted symbol on the pth subcarrier, y(p) as the received symbol on the pth subcarrier, the equivalent channel input-output relationship can be described by:

y(p) = H(p)x(p) + v(p), p= 0, 1, . . .,K 1, (1)

where v(p) stands for the additive white Gaussian noise, and H (p) is the channels frequency response on the pth subcarrier:

(2)

Channel equalization amounts to scalar inversion:

s(p) = y(p)/H(p) (3)

on each subcarrier. The equalization complexity thus does not depend on the channel length. Precisely due to its low equalization complexity in the presence of highly-dispersive channels, OFDM has prevailed in recent broadband wireless systems. Those include digital audio/video broadcasting (DAB/DVB) standards in Europe, high-speed digital subscriber line (DSL) modems in the United States, digital cable television systems, and wireless local area networks [11].

1. Transreceiver Design

   The transmitter diagram is shown in Fig.5. We use a graphics user interface to input some text messages. The transmitter first converts the text data into a binary format. The binary data is then interleaved and coded by a simple (3,1) repetition code for error correction. The coded data is mapped to QPSK (quadrature phase-shift keying) symbols. The symbol stream is partitioned into blocks, and each block is OFDM modulated. During the OFDM modulation, we insert pilot symbols at every 4th subcarrier, which facilitates channel estimation at the receiver for coherent demodulation. The modulation is implemented at baseband and then up-shifted to passband for transmission. A synchronization sequence is inserted in front of the data packets during transmission.

   The receiver diagram is shown in Fig. 5. The receiver first applies bandpass filtering on the incoming data stream, then it

   looks for where the useful data begins via correlating the received data with the synchronization sequence template. Once the receiver has found a useful data packet, it downshifts the passband signal to baseband. We then estimate the carrier frequency offset (CFO) to correct any carrier mismatch between the transmitter and the receiver. After CFO compensation, we rely on pilot tones to estimate the channel frequency response. With coherent demodulation at each OFDM subcarrier, we decode the (3,1) repetition coding using maximum ratio combining. The receiver finally extracts the binary bits from the QPSK symbols and generates the text message [11].

2. Graphic User Interface

   There are two graphical user interfaces (GUIs), one for the transmitter and one for the receiver. These interfaces allow for configuration of basic parameters such as

   • Number of OFDM subcarriers

   • First carrier frequency

   • Guard time between data packets

   • Synchronization sequence duration

   • Center frequency for synchronization sequence

   • Bandwidth of synchronization sequence

   • Pause time between synchronization sequence and first data packet

   • Number of packets per transmission

   • Repetition coding rate

   • Number of edge channels deactivated

   The receiver has a few extra parameters including the Correlation threshold, the amplitude trigger and the channel length. The receiver also gives the user the option of checking the bit error rate and plotting each channel estimation graph during the CFO estimation process[11].

3. Packet Formation

The typed text messages have different lengths, while the packet has a fixed length. Hence, different number of packets may be required for different messages. To create an automatic message transmission, we design the packet format as follows. The binary sequence created by the transmitter consists of the text data along with 2 metadata bits (administrative bits) at the end. These metadata bits are known as the partial packet bit and the continuation bit, respectively. The continuation bit will tell the receiver whether the next transmission contains a continuation of the current transmission. The partial packet bit alerts the receiver that the text data was not able to fill the packet and extra meaningless data have been inserted, which must be removed at the receiver end. In this case, eleven extra bits must be added before the partial packet bit to identify the length of the useful data (and where the meaningless data begins). The format of the binary data in a packet can be seen in Figs. 6. On the transmitter end, the maximum allowable data length

User interface for Text

Bit interleaving

User interface for Text

Bit interleaving

Cross-correlation for synchronisation

Cross-correlation for synchronisation

BPF

Fig.no.5.Block diagram of Transmitter (Top) and Receiver (Bottom) [11]

determines what the metadata bits are set to. If the length of the binary data is equal to the maximum allowable data length minus two, the transmitter will set both of the metadata bits to zero. If the binary data length is greater than the maximum allowable data length minus 2, the transmitter will break the data up into multiple transmissions, set the partial packet bit to zero, and the continuation bit to one. If the data length is less than the maximum data length minus thirteen, meaningless data is added to the end of the sequence. The eleven identifying bits are added to the end of the meaningless data and the partial packet bit is set to one while the continuation bit is set to zero. If the data length is between the maximum allowable data length minus two and the maximum allowable data length minus thirteen,the binary sequence is zero padded (to make the data length equal to the maximum allowable data length, zeros are added between the end of the useful data and the meta data) and both the partial packet and continuation bits are set to zero. These zeros will not be removed by the receiver but will not affect the received data. The point to placing meaningless data in the partial packets (as opposed to just zeros) is to ensure that the peak-to-average ratio of the OFDM symbol is randomized [11].

Full Packet

Add synch sequence

Soft Decoding

User interface for text input

Soft Decoding

User interface for text input

OFDM

modulation

OFDM

modulation

QPSK mod and packet formation

QPSK mod and packet formation

OFDM Demod

OFDM Demod

Fig.no.7.Transmitter GUI [11]

Partial Packet

Fig.no.8.Receiver GUI [11]

Fig.no.6.Packet formation [11]

Section III

In this paper, we investigate the implementation of OFDM modems for underwater high data rate transmission. We first implemented OFDM in air and then in water and analyze the performance in both the cases. Thus it is very beneficial to use OFDM in underwater communications.

Our demonstration has two settings. In the first setting, we demonstrated multicarrier OFDM transmission and reception in air. The testbed is depicted in Fig.9, where a speaker together with a laptop serve as the transmitter, and a together with another laptop serve as the receiver. A typical micro-phone channel impulse response is shown in Fig. 10, where we clearly see the dominant line-of-sight path. In the second setting, we demonstrated OFDM transmission and reception in water. The testbed is depicted in Fig.11. The underwater speaker and hydrophone are used as transmitting and receiving devices, respectively. A typical channel impulse response is shown in Fig. 12, where we clearly see the reverberation effects. The last path is about 37 meters longer than the first path inside the water tank of 2 meters long, 0.5 meters wide and 0.5 meter deep. Hence, the transmitted signal has bounced back and forth between the hard surfaces of the water tank a plenty of times. We can also see the amplitude attenuation associated with each bounce [11].

TRANSMISSION IN AIR

The OFDM burst, which was generated in the 3-5 kHz band using MATLAB, was transmitted via soundcard using a speaker connected to the computer. The acoustic transmission impinged on microphone connected to the sound card in a second computer. This second computer acted as the OFDM receiver (implemented in MATLAB) and estimated the received bits. Hence the channel in effect consisted of a combination of two sound cards, speaker, air, and microphone. Several issues such as timing synchronization, carrier frequency offset estimation and correction, and channel equalization had to be taken into account for the system to be operational at usable bit error rates. Since the experiment was conducted in an indoor environment which contained tables, chairs, computers, and shelves, the received signal was subject to multipath. Recall that sound travels about five times slower over air when compared to underwater. This results in a larger propagation delay as well as a more pronounced Doppler effect at much slower speeds. Hence the over-the-air channel is in some ways less conducive for acoustic transmission when compared to its underwater counterpart [1].

CFO ESTIMATION

We will use the block-by-block receiver structure of [2]. The channel time variation within each OFDM symbol is modeled as a multiplicative process ej2t on the received signal after the multipath propagation, where is termed as carrier

frequency offset (CFO).We use null subcarriers to facilitate the finding of the CFO.The idea is as follows. We collect the baseband samples for each OFDM block. For each tentative , we compensate the CFO on the OFDM block and evaluate the FFT output on the null subcarriers. The energy of the null subcarriers is used as the cost function J (). If the receiver compensates the data samples with the correct CFO, the null subcarriers will not see the intercarrier interference (ICI) from neighboring data subcarriers [12].

= arg min J () (4)

Fig.no.9. Transmission in air [11]

Fig.no.10.Estimated Channel Impulse Response for In-Air Transmission (2048 Subcarriers) [11]

TRANSMISSION IN WATER

We demonstrated OFDM transmission and reception in water. The testbed is depicted in Fig.11.The underwater speaker and hydrophones are used as transmitting and receiving devices, respectively. A typical channel impulse response is shown in Fig. 12, where we clearly see the reverberation effects.

Fig.no.11 Transmission in Water [11]

Fig.no.12Estimated Channel Impulse Response for Underwater Transmission (2048 Subcarriers) [11]

Table.no.2. Comparison of OFDM parameters in Air and in Water

The table shows the basic difference in OFDM implementation techniques in air and in water.

Section IV CONCLUSION

In this paper, we investigate the implementation of OFDM modems for underwater high data rate transmission. There are many advantages of OFDM Acoustics modems in underwater communications. The OFDM basics give us the broad idea about OFDM techniques. We have first demonstrated OFDM in air and then in water. The channel impulse response shows a big difference in delay spread.The system will be able to transmit text.Clear line of sight between transmit receiver is observed.Integration into a fully functional Modem can be possible.

Transmission of other forms of data (voice, image etc.can be send. Integration into other network like Wi-Fi can be made possible. The implementation effort in this paper provides some useful insights towards realizing a practical OFDM modem for high-data-rate and reliable underwater acoustic communications.

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