A 10 Mbps QPSK on a 11GHz carrier signal with comparison between the effects

of Eb/No Bit Error Rate (BER) of the digitally modulated signal .

SUBMITTED BY: SAMI UR REHMAN

SUBMITTED TO:

Dr. MUNIR AHMAD TARAR

COURSE: EE-874 - Microwave Communication System Design

SUBMISSION DATE: 2nd Nov 2011

Objective to Achieve

The objective of this simulation is to simulate the effects of Free Space Loss (FSL) and antenna gain on a

digitally QPSK-modulated signal in the form of a noise added to a channel and then calculates its effects

on Bit Error Rate (BER) using Advanced Design System (ADS). The comparison between the theoretical

and calculated curves of BER vs. Eb/No gives us an idea of how well the calculated values simulate the

theoretical values.

Background of the Task

The task is to evaluate the performance of a communication link operating at 11GHz carrier frequency

with digital phase modulation scheme of Quadrature Phase Shift Keying (QPSK) at the symbol rate of

10Mbps. All parameters of link budget are given to us. We are first to calculate the theoretical Eb/No for

a given BER and then simulate the whole system with ADS to see how close our theoretical value is to

the simulated value. The eventual result we should see is that by increasing noise power (No) we

decrease Bit Energy (Eb) and hence the system output has increased BER, and if we decrease the noise

power No we increase the Eb and hence decrease the BER.

The following study gives a brief insight on the theoretical side of the probability of bit error calculation

of this simulation.

Figure A : Constellation plot for QPSK constellation

The scaling factor of is for normalizing the average energy of the transmitted symbols to 1,

assuming that all the constellation points are equally likely.

Theoretical Calculations

Lt

Po Gt EIRP FSL+La IRL Gr Lr

RSL

Data Given:

Distance= 12 miles Bit Rate (Rb)=10Mbps BER= 10^-7 Modulation scheme= QPSK Po=1W F=11GHz Transmission losses=Lt=Lr=2.2 dB To find:

FSL=? Eb/No (theoretical) = ? Antenna Gain= ?

Calculations:

For FSL

Formulae to calculate the FSL is given by: FSL = 36.58 + 20log(Dsm) + 20log(FMHz) Where Dsm is the distance is statutes miles. Plugging in the values: FSL= 36.58 + 20log (12) + 20log (11000) FSL= 139 dB Theoretical Eb/No There are certain methods to calculate theoretical Eb/No. One way is to use hit and trial method by putting approximate values of unknown variables in the equation of Eb/No and come with its value which corresponds to required BER i.e. 10^-7. Another way is to take help from the given chart and work out the Eb/No which corresponds to required BER. In our case Eb/No turns out to be 11.5 dB.

Tx Rx

Source : http://en.wikipedia.org/wiki/Eb/N0

Antenna Gain: Now: Eb/No = 11.5 dB = Eb (dB) – No(dB) ---------------------------------------------A Where Eb = RSL – 10log(Rb) ------------------------------------------------------------------B Since Rb= 10Mbps And RSL = Po + Lt +Gt + FSL + La + Gr + Lr ------------------------------------------- - C We calculated FSL = 139 dB We are given Lt = Lr = Transmission Line Losses = 2.2 dB We are given Po = 0dB Putting all this data in C, we get RSL = -143.4 dBW + Gt + Gr --------------------------------------------------------D And

No = -204 dBW + NF -----------------------------------------------------------------E Since noise figure is not given, we can ignore it So, Eb/No (dB) = 11.5 dB = RSL - 70 + 204 dBW RSL = - 122.5 dBW Put this value of RSL in eq D -122.5 = -143.4 + Gt + Gr Assume Gt=Gr G = 10.45 dB

System Design

The following is the complete schematic of the QPSK system with the BER measurement blocks.

Complete System Design

Blocks Description

The following is a short description of the major simulation models used in this project.

Generating

binary data

with 10Mbps

Antenna

Gain

blocks

Noise Density

Block

FSL Modeling

in Loss Block Data splitted

for reference

Sweeping Eb/No

to calculate the

graph

DATA FLOW CONTROLLER (DF)

The essential and intrinsic block is the Data Flow controller for a timed signal processing simulation such

as this project. The major parameters used in this project for this controller are the Default Time Start,

Default Time Stop and Default Seed. Using this controller you can also display outputs of all type of data

which is flowing through the simulation model blocks.

PARAMETER SWEEP CONTROLLER

This is responsible for designating the sweep parameter by adding the Sweep variable and defines its

sweep values. Another important parameter which is hidden is to include the controller for which sweep

has to be performed which is DF (Data Flow Controller) in this project.

DATA GENERATOR

The Data source generates a user-defined or random baseband NRZ waveform with a given Bit Time and T-Step.

QPSK MODULATOR

QPSK modulator takes the in-phase I (t) and quadrature Q (t) baseband input signals from the Data generator (above) and generates a QPSK modulated timed signal at its output with a carrier frequency of 2.4GHz as QPSK modulator using an internal oscillator.

NOISE DENSITY

The AddNDensity channel block is used in another simulation where NDensity parameter denotes the

noise spectral density in dBm/Hz.

QPSK DEMODULATOR

The QPSK demodulator demodulates the timed output signal and the additive white Gaussian noise has an external oscillator input that generates the reference carrier signal used to demodulate the RF signal. The in-phase and quadrature components I (t) and Q (t) of the modulated signal are extracted.

Simulation & Results

As you can see below the ADS simulator processes each symbol. The parameters of the BER measurement block can be varied to process the symbol using the Default Time Start and Stop or the Default Numeric Start and Stop parameter of the data flow controller (DF).

Simulation Window

Following is a graphwhich gives a comparison between the Bit Error Rate (BER) calculated theoretically

using the probability of error (blue curve) and the simulated result (red curve).

Figure (B) QPSK BER Rate Comparison between the Theoretical and Simulation results

Figure (G) is a graph which also provides a comparison between the Bit Error Rate (BER) calculated theoretically using the probability of error (blue curve) and the simulated result (red curve).This result is a process of using the AddNDensity channel block which is a ADS library block hence it has better convergence between the theoretical BER (blue curve) and the simulated BER (red curve) with the same simulation parameters.

Figure (C): QPSK BER Rate Comparison between the Theoretical and Simulation results

I. Challenges Faced This simulation covers a lot of theoretical ground from probability to digital communication systems

hence numerous challenges were encountered. I summarize them in points, which are the following:

a. ADS software has a lot of functional blocks with various parameters and usages which need to

be understood with their theoretical implications.

b. Noise probability function generated by a block and then added into the modulated channel is a

challenge because simply adding wouldn’t work unless the output of the QPSK input is

recognizable by the demodulator. Hence the addition performed requires understanding of

timed and complex signals.

c. The results compiled do require understanding the probability of error calculation of QPSK,

without which the results cannot be displayed properly.

II. Conclusion: In this exercise we analyzed the impact of changing BER on digital modulation scheme which in our case is QPSK. The comparison between the theoretical and simulated values shows that there is an impact of noise on the symbols recovered by the demodulator but it depends on the Eb/No ratio. Lower the ratio higher is the probability of bit error or Bit Error Rate (BER) and higher the ratio lower is the Bit Error Rate (BER).