simulations/models/OFDM.py

import numpy as np
import tensorflow as tf
from tensorflow.keras import layers, losses
from matplotlib import pyplot as plt
from models.basic import AlphabetMod, AlphabetDemod, RFSignal
from models.custom_layers import OpticalChannel


class OFDM():

    def __init__(self, num_channels, channel_args, **kwargs):
        self.num_channels = num_channels
        self.channel_args = channel_args

        if "dc_offset" in kwargs:
            self.dc_offset = kwargs["dc_offset"]
        else:
            self.dc_offset = 50

        self.mod_schemes = None
        self.modulators = None
        self.demodulators = None
        self.bits_per_ofdm_symbol = None
        self.set_mod_schemes()

        self.tx_bits = None

    def set_mod_schemes(self, new_mod_schemes=None):
        if new_mod_schemes is None:
            self.mod_schemes = ["qpsk" for x in range(self.num_channels)]
        else:
            self.mod_schemes = new_mod_schemes
        self.modulators = {x: AlphabetMod(x, 0.0) for x in list(set(self.mod_schemes))}
        self.demodulators = {x: AlphabetDemod(x, 0.0) for x in list(set(self.mod_schemes))}
        self.bits_per_ofdm_symbol = 0

        for mod in self.mod_schemes:
            self.bits_per_ofdm_symbol += self.modulators[mod].N

    def transmit_and_equalize(self, tx_stream):

        optical_channel = OpticalChannel(**self.channel_args, num_of_samples=len(tx_stream))

        rx_stream = optical_channel(tx_stream)
        rx_stream_f = np.fft.fft(np.sqrt(rx_stream))
        f = np.fft.fftfreq(rx_stream_f.shape[0], d=1/(self.channel_args["fs"]))

        # TODO : Equalization needs to be tested
        mul = np.exp(-0.5j*(self.channel_args["dispersion_factor"])*(self.channel_args["fiber_length"])*np.square(2*np.pi*f))
        rx_compensated_f = rx_stream_f * mul
        rx_compensated = np.abs(np.fft.ifft(rx_compensated_f))
        return rx_compensated

    def find_optimal_schemes(self):
        # TODO : Choosing approapriate modulation scheme after a single pass of QPSK
        pass

    def encode_ofdm_symbol(self, bits):
        if bits.size != self.bits_per_ofdm_symbol:
            raise Exception("Number of bits passed does not match number of bits per OFDM symbol!")

        bit_idx = 0
        enc_stream_upper_f = []
        for mod in self.mod_schemes:
            num_bits = self.modulators[mod].N
            a = bits[bit_idx:bit_idx + num_bits]
            alph_sym = self.modulators[mod].forward(bits[bit_idx:bit_idx + num_bits]).rect[0]
            bit_idx += num_bits
            sym = alph_sym[0] + 1j * alph_sym[1]
            enc_stream_upper_f.append(sym)

        if len(enc_stream_upper_f) != self.num_channels:
            raise Exception("An error occurred! Computed upper sideband contains more channels than specified")

        enc_stream_upper_f = np.asarray(enc_stream_upper_f)
        enc_stream_lower_f = np.conjugate(np.flip(enc_stream_upper_f))
        enc_stream_f = np.concatenate((self.dc_offset, enc_stream_upper_f, enc_stream_lower_f), axis=None)
        enc_stream_t = np.fft.ifft(enc_stream_f)
        return np.real(enc_stream_t)

    def encode_bits(self, bit_stream):
        if bit_stream.size % self.bits_per_ofdm_symbol != 0:
            raise Exception("Bit stream size is not an integer multiple of bits per OFDM symbol!")

        self.tx_bits = bit_stream

        bit_blocks = bit_stream.reshape(-1, self.bits_per_ofdm_symbol)
        tx_stream = []
        for bits in bit_blocks:
            tx_stream.append(self.encode_ofdm_symbol(bits))
        tx_stream = np.asarray(tx_stream).flatten()
        return tx_stream

    def decode_ofdm_symbol(self, stream):
        if stream.size != (2*self.num_channels + 1):
            raise Exception("Number of samples passed is not equal to number of samples in one OFDM symbol!")

        stream_f = np.fft.fft(stream)
        stream_upper_f = stream_f[1:self.num_channels+1]

        bits = []
        for chan, mod in zip(stream_upper_f, self.mod_schemes):
            rf_sym = RFSignal(np.zeros((3, 3)))
            rf_sym.set_rect_xy(np.real(chan), np.imag(chan))
            bits.append(self.demodulators[mod].forward(rf_sym).astype(int))
        bits = np.asarray(bits).flatten()
        return bits

    def decode_bits(self, rx_stream):
        if rx_stream.size % (2*self.num_channels + 1) != 0:
            raise Exception("Number of samples passed is not an integer multiple of the samples per OFDM symbol")
        pass

        ofdm_symbols = rx_stream.reshape(-1, (2*self.num_channels + 1))
        rx_bit_stream = []
        for sym in ofdm_symbols:
            rx_bit_stream.append(self.decode_ofdm_symbol(sym))
        rx_bit_stream = np.asarray(rx_bit_stream).flatten()
        return rx_bit_stream

    def computeBER(self, n_ofdm_symbols):
        bit_stream = np.random.randint(2, size=n_ofdm_symbols * self.bits_per_ofdm_symbol)
        tx_stream = self.encode_bits(bit_stream)
        rx_stream = self.transmit_and_equalize(tx_stream)

        tx_stream_f = np.fft.fft(tx_stream)
        rx_stream_f = np.fft.fft(rx_stream)
        f = np.fft.fftfreq(tx_stream_f.shape[0], d=1/(self.channel_args["fs"]))

        plt.plot(f, np.real(tx_stream_f), 'x')
        plt.plot(f, np.real(rx_stream_f), 'x')
        plt.ylim([-5, 5])
        plt.show()

        # plt.plot(tx_stream)
        # plt.plot(rx_stream)
        # plt.show()

        rx_bit_stream = self.decode_bits(rx_stream)
        ber = np.sum(np.equal(bit_stream, rx_bit_stream).astype(int))/bit_stream.size
        print(ber)
        pass


if __name__ == '__main__':
    BANDWIDTH = 100e9
    channel_args = {"fs": BANDWIDTH,
                    "dispersion_factor": -21.7 * 1e-24,
                    "fiber_length": 10,
                    "fiber_length_stddev": 0,
                    "lpf_cutoff": BANDWIDTH / 2,
                    "rx_stddev": 0.01,
                    "sig_avg": 0.5,
                    "enob": 10}

    # TODO : For High Cardinality or number of OFDM Symbols the decoder fails and BER ~ 0.5

    ofdm_obj = OFDM(64, channel_args)
    ofdm_obj.computeBER(100)

    pass