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前言

本文记录在 GNU Radio+USRP 实现 OFDM 收发时，在接收端 QPSK 星座图映射无“抖动”问题的解决方法，

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一、遇到的问题

我遇到的问题是，现在搭建的 OFDM 模型在接收端做信道均衡时，接收端的 QPSK 星座图映射在有噪声的情形下并没有出现 “抖动” 现象。如下图所示，可以看到 OFDM 头和负载的 BSPK 和 QPSK 都是比较规则的。

经过初步的分析，问题应当是出在了接收端的 “信道均衡模块” (OFDM Frame Equalizer Module)， 这个模块不仅仅进行了信道估计和信道均衡，同时进行了最小距离分析并把含有噪声的点强行 “拉回” 星座图上的标准映射点。这个与我们一般意义下理解的“信道均衡” 是不同的，一般不会“拉回去” 。所以，现在的任务就是找出与 equalizer 相关的 .h 和 .cc 文件，阅读源代码，并对源代码进行修改。

二、解决方案

在 gnuradio/gr-digital/include/gnuradio/digital/ 文件夹下找到了所有与 equalizer 有关的 .h 文件
在 gnuradio/gr-digital/lib/ 文件夹下找到了所有上述.h 文件对应的.cc 源文件

最后直接定位到 ofdm_equalizer_simpledfe.cc

其源文件完整源码如下：

/* -*- c++ -*- */
/* Copyright 2012 Free Software Foundation, Inc.
 *
 * This file is part of GNU Radio
 *
 * GNU Radio is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 3, or (at your option)
 * any later version.
 *
 * GNU Radio is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with GNU Radio; see the file COPYING.  If not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street,
 * Boston, MA 02110-1301, USA.
 */

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include <gnuradio/digital/ofdm_equalizer_simpledfe.h>

namespace gr {
namespace digital {

ofdm_equalizer_simpledfe::sptr
ofdm_equalizer_simpledfe::make(int fft_len,
                               const gr::digital::constellation_sptr& constellation,
                               const std::vector<std::vector<int>>& occupied_carriers,
                               const std::vector<std::vector<int>>& pilot_carriers,
                               const std::vector<std::vector<gr_complex>>& pilot_symbols,
                               int symbols_skipped,
                               float alpha,
                               bool input_is_shifted)
{
    return ofdm_equalizer_simpledfe::sptr(new ofdm_equalizer_simpledfe(fft_len,
                                                                       constellation,
                                                                       occupied_carriers,
                                                                       pilot_carriers,
                                                                       pilot_symbols,
                                                                       symbols_skipped,
                                                                       alpha,
                                                                       input_is_shifted));
}

ofdm_equalizer_simpledfe::ofdm_equalizer_simpledfe(
    int fft_len,
    const gr::digital::constellation_sptr& constellation,
    const std::vector<std::vector<int>>& occupied_carriers,
    const std::vector<std::vector<int>>& pilot_carriers,
    const std::vector<std::vector<gr_complex>>& pilot_symbols,
    int symbols_skipped,
    float alpha,
    bool input_is_shifted)
    : ofdm_equalizer_1d_pilots(fft_len,
                               occupied_carriers,
                               pilot_carriers,
                               pilot_symbols,
                               symbols_skipped,
                               input_is_shifted),
      d_constellation(constellation),
      d_alpha(alpha)
{
}


ofdm_equalizer_simpledfe::~ofdm_equalizer_simpledfe() {}


void ofdm_equalizer_simpledfe::equalize(gr_complex* frame,
                                        int n_sym,
                                        const std::vector<gr_complex>& initial_taps,
                                        const std::vector<tag_t>& tags)
{
    if (!initial_taps.empty()) {
        d_channel_state = initial_taps;
    }
    gr_complex sym_eq, sym_est;

    for (int i = 0; i < n_sym; i++) {
        for (int k = 0; k < d_fft_len; k++) {
            if (!d_occupied_carriers[k]) {
                continue;
            }
            if (!d_pilot_carriers.empty() && d_pilot_carriers[d_pilot_carr_set][k]) {
                d_channel_state[k] = d_alpha * d_channel_state[k] +
                                     (1 - d_alpha) * frame[i * d_fft_len + k] /
                                         d_pilot_symbols[d_pilot_carr_set][k];
                frame[i * d_fft_len + k] = d_pilot_symbols[d_pilot_carr_set][k];
            } else {
                sym_eq = frame[i * d_fft_len + k] / d_channel_state[k];
                // The `map_to_points` function will treat `sym_est` as an array
                // pointer.  This call is "safe" because `map_to_points` is limited
                // by the dimensionality of the constellation. This class calls the
                // `constellation` class default constructor, which initializes the
                // dimensionality value to `1`. Thus, Only the single `gr_complex`
                // value will be dereferenced.
                
                d_constellation->map_to_points(d_constellation->decision_maker(&sym_eq),
                                               &sym_est);
                                               
                d_channel_state[k] = d_alpha * d_channel_state[k] +
                                     (1 - d_alpha) * frame[i * d_fft_len + k] / sym_est;
                frame[i * d_fft_len + k] = sym_est;
            }
        }
        if (!d_pilot_carriers.empty()) {
            d_pilot_carr_set = (d_pilot_carr_set + 1) % d_pilot_carriers.size();
        }
    }
}

} /* namespace digital */
} /* namespace gr */

我们需要对其进行修改，有关其修改内容及解决办法已放到文末，有需求的通信爱好者可以自取。

三、重新编译安装

首先进入安装的时候的 gnuradio 文件夹下，之后进入 build 文件夹下，在这个文件夹下开启 terminal
之后在 terminal 中输入下面命令：

cd workarea/gnuradio/build

cmake -DCMAKE_BUILD_TYPE=Release -DPYTHON_EXECUTABLE=/usr/bin/python3 ../
make -j4 
sudo make install
sudo ldconfig

四、验证

打开 OFDM 工程，运行可以看到下面界面：

1、发端效果图：

2、收端效果图

可以看到目前属于正常的状态

五、资源自取

链接：解决GNU Radio+USRP实现OFDM收发在接收端QPSK星座图映射无“抖动”问题

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我的qq：2442391036，欢迎交流！

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