You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: STO_by_difference.m
    path of file: ./repos/MIMO-OFDM-Wireless-Communications-with-MATLAB
    
    the code of the file until where you have to start completion: function [STO_est,Mag]=STO_by_difference(y,Nfft,Ng,com_delay)
% STO estimation by minimizing the difference between CP and rear part of OFDM symbol
% 通过最小化CP和OFDM符号后部的差值，实现STO的估计
% estimates STO by minimizing the difference between CP (cyclic prefix) 

    
function [STO_est,Mag]=STO_by_difference(y,Nfft,Ng,com_delay)
% STO estimation by minimizing the difference between CP and rear part of OFDM symbol
% 通过最小化CP和OFDM符号后部的差值，实现STO的估计
% estimates STO by minimizing the difference between CP (cyclic prefix) 
%     and rear part of OFDM symbol
% Input:  y          = Received OFDM signal including CP|包括CP的OFDM接收信号
%          Ng         = Number of samples in CP (Guard Interval)|CP/GI内的采样数
%          com_delay = Common delay|公共时延
% Output: STO_est   = STO estimate|STO估计
%           Mag        = Correlation function trajectory varying with time
%                   相关函数的时变轨迹

% MIMO-OFDM Wireless Communications with MATLAB㈢   Yong Soo Cho, Jaekwon Kim, Won Young Yang and Chung G. Kang
% 2010 John Wiley & Sons (Asia) Pte Ltd

% http://www.wiley.com//legacy/wileychi/cho/

N_ofdm=Nfft+Ng; minimum=100; STO_est=0;
if nargin<4, com_delay = N_ofdm/2; end