Explanation of the Communication Model of the physical layer, and explanation of the Physical Layer Model

Explanation of the Communication Model of the physical layer, and explanation of the Physical Layer Model

Introduction

As the underlying layer of wireless communication protocols, the physical layer plays a crucial role in communication performance. It can be seen that almost all technical innovations that achieve cross-generation communication systems are basically on the physical layer, the underlying layer of the physical layer is the modulation and decoding of signals. The so-called frequency division multiplexing, time division multiplexing, and even the space division multiplexing represented by MIMO technologies all need to be built on the signal mediation function.

I. modulation and demodulation types

The purpose of modulation is to obtain a wireless signal that is easy to transmit. Therefore, all the signals after adjustment are analog signals.

Based on the pre-adjustment signal type, mediation can be divided into digital mediation and simulated mediation.

Based on the structure of the actual communication system, mediation can be divided into baseband mediation and RF mediation.

Baseband mediation-digital mediation and simulated mediation

RF mediation-analog mediation

The communication systems can be divided into digital communication systems and analog communication systems according to the baseband mediation method.

Ii. Communication Model

1. analog communication system

Analog waveform-<Analog modulation>-RF Waveform-<analog demodulation>-analog waveform

2. Digital Communication System

Digital Bit-<encoding modulation>-Digital Bit-<constellation ing/Modulation>-complex value symbol (I/Q) -<Pulse Forming filter * 2>-baseband waveform * 2-<IQ modulation>-RF Waveform-<IQ demodulation>-baseband waveform * 2-<sampling decision>-Reset symbol (I/Q) -<constellation inverse ing/Demodulation>-Digital Bit-<encoding and demodulation>-Digital Bit

The essence of RF modulation or Analog modulation is to shift the spectrum of the baseband signal to the RF frequency. The common method is to multiply the cos (2 * PI * fc * t) to achieve spectrum transfer.

The essence of digital mediation is Constellation ing and inverse ing. Therefore, digital mediation is also called Constellation mediation. Digital Modulation Based on the IQ method maps multiple digital bits into a complex symbol, then, the real imaginary part of the complex value symbol is formed into two baseband signals by two pulses, which are transmitted to the RF through the IQ Analog modulation spectrum.

In digital communication systems, the concept of Symbol Periods is important, because the duration of the waveform generated by the pulse after the complex value symbol mapped by multiple digital bits is the basic time particle that forms the sub-frame/frame of the system. In engineering, we also use an eye chart to observe the waveform changes in a single symbol cycle in the baseband waveform and determine whether there is inter-code crosstalk.

Iii. IQ mediation-Basics of digital/simulated mediation

IQ mediation is a very important engineering method. Currently, the mediation methods used by commercial communication systems are based on IQ mediation technology. IQ-based digital mediation is called Constellation mediation, IQ-based simulation mediation replaces single-band mediation based on Hilbert transformation because two baseband signals can be transmitted simultaneously at the same bandwidth.

IQ modulation principle:

Assume that the I-channel input is a and the Q-channel input is B, then the IQ modulation output is a * cos (2 * PI * fc * t) -B * sin (2 * PI * fc * t ).

Calculate the IQ input as a + j * B, and the IQ is modulated into a multiplier with the value e ^ j * 2 * PI * fc * t, then, the mathematical expression of IQ modulation is Re {(a + j * B) * e ^ j * 2 * PI * fc * t }.

IQ demodulation principle:

Acceptor I-Path First implements multiplication Re {(a + j * B) * e ^ j * 2 * PI * fc * t} * cos (2 * PI * fc * t ), we can see from the implementation that the spectrum of input a has gone through two spectrum moves in the same range, so the above signal can be obtained through low-pass filtering.

Similarly, the Q-path at the receiving end first implements the multiplication Re {(a + j * B) * e ^ j * 2 * PI * fc * t} *-sin (2 * PI * fc * t ), from the implementation perspective, we can see that the input B's spectrum has gone through two spectrum shifts in the same range. Therefore, B can be obtained after passing through the low-pass filtering of the above signal.

The advantage of IQ mediation lies in the multiplexing transmission of two channels of data.

Constellation mediation and constellation diagram:

If a and B are digital signals lasting for a period of time, IQ mediation is called Constellation mediation. At this time, the distribution chart formed by a + j * B on the complex plane is a constellation chart, the ing between code elements and complex values composed of multiple-digit word bits is Constellation ing.

Therefore, if N-bit binary BITs constitute a code element, a total of 2 ^ N code elements are combined. Therefore, 2 ^ N constellation points or 2 ^ N complex values are required to ensure a one-to-one relationship.

Mathematical principles implied in the constellation chart:

A * cos (2 * PI * fc * t)-B * sin (2 * PI * fc * t) can be obtained based on the sum difference product formula) = A * cos (2 * PI * fc * t + P). In the formula, the amplitude A and the initial phase P correspond to the modulus and phase angles of (a, B) in the complex plane respectively.

IQ simulation mediation:

If a and B are simulated signals, the IQ mediation is simulated mediation. Basically, IQ simulation mediation must be used in combination with Constellation mediation. Therefore, the two are collectively referred to as IQ mediation solutions.

Iv. Pulse Forming and eye View

Pulse Forming is an important part of generating baseband waveforms. In pulse forming, the so-called pulse refers to the impulse pulse formed by the modulated Digital Bit according to the sampling frequency, although the impulse pulse can also be regarded as a special analog signal, it is not suitable for wireless transmission. Therefore, the impulse pulse needs to be interpolated to turn it into a continuous analog signal, that is, the so-called "forming ", the pulse shaper is essentially a filter, so the forming waveform refers to its unit impulse response.

The most common waveform is the rectangular square wave. Therefore, the baseband waveform formed by the impulse pulse after the rectangular wave is a square wave pulse, but the square wave pulse cannot resist the crosstalk between codes in reality, that is, the front and back bitwise square wave levels will interfere with each other, resulting in serious waveform distortion. in actual application, the forming filter is often called a raised cosine filter.

In the project, the so-called eye chart uses an oscilloscope to display the ascending or descending waveform of the baseband code element period to determine whether there is inter-code crosstalk.

clear allclose allFd = 1Fs = 10r = 0.5delay = 3[num, den] = rcosine(Fd,Fs,'default',r,delay);x = randint(1,20)[y,ty] = rcosflt(x,Fd,Fs,'default',r,delay);% figure;% impz(num,1)% grid on;% figure% stem(num)% grid on;t = delay*Fd:(length(x)+delay -1) *Fdfigureplot(ty,y)hold onstem(t,x,'-r')grid on% figure% stem(x)k = 1for ii=1:length(x)     for jj=1:Fs/Fd          samp(k)=x(ii);         k=k+1;     end end  m = conv(samp,num)% figure% stem(ty,m)figureplot(y)grid onfigurefor k = 5:floor(length(y)/Fs/Fd)-5ss = y(k*Fs/Fd+1:(k+1)*Fs/Fd);    plot(ss);     hold on; end  figureplot(y)grid onfor k = 5:floor(length(y)/Fs/Fd)-5hold onstem(k*Fs/Fd+1,1)hold onstem((k+1)*Fs/Fd,1)end  figurefor k = t    ss = y(k*Fs/Fd+1:(k+1)*Fs/Fd+1)    plot(ss)    hold on end

V. UL/DL block diagram of LTE

1. OFDM/DL

Digital bit-<constellation mapping>-complex symbol-<RE mapping, serial to parallel>-rb diers-<IFFT, N> Nof subcarriers, parallel to serial>-N complex-<add CP>-N + L complex-<pulse shaping * 2>-baseband wave * 2-<IQ modulating>-RF wave

2. SC-FDMA/UL

Digital bit-<constellation mapping>-complex symbol-<DFT, Nof subcarriers>-complex symbol (frequency-domain)-<RE mapping, serial to parallel>-RB digoal-<IFFT, N> Nof subcarriers, parallel to serial>-N complex-<add CP>-N + L complex-<pulse shaping * 2>-baseband wave * 2-<IQ modulating>-RF wave