KINERJA SISTEMDEMODULASI DIGITAL

Amplitudo Shift Keying

Dosen :Hary Nugroho, MT

Pengantar

• Untuk komunikasi data jarak jauh, diperlukan teknik modulasi.• Modulasi merupakan teknik untuk “menumpangkan” infomasi dalam

bentuk bit-stream kepada sinyal-pembawa yang sesuai dgn media transmisinya.

Pengolahan Sinyal

Base-bandModulator UP

Converter

Bit-Stream

Data Input

Down Converter DeModulator Decission

Circuit

Perbandingan Besaran Kinerja

• Besaran yang mencerminkan kinerja sistem komunikasi digital adalah BER [Bit Error Rate] atau P(Є) [Probability of error].

• BER atau P(Є) adalah hasil kerja fungsi Decision Circuit menurut fungsi terhadap variabel Eb/η.

• Besaran yang mencerminkan kinerja pra-deteksi dan demulator adalah S/N dan C/N.

• Perlu ditegaskan hubungan antara kinerja sistem pradeteksi (S/N) dengan kinerja fungsi Decision Circuit (Eb/η) yaitu :

R

B

N

SE Nb .N

b

B

RE

N

S.

Modulasi digital• Dalam modulasi digital, suatu sinyal analog di-modulasi berdasarkan aliran data digital..

Perubahan sinyal pembawa dipilih dari jumlah terbatas simbol alternatif. • Teknik yang umum dipakai adalah :

– Phase Shift Keying (PSK), digunakan suatu jumlah terbatas berdasarkan fase.– Frekeunsi Shift Keying (FSK), digunakan suatu jumlah terbatas berdasarkan frekuensi.– Amplitudo Shift Keying (ASK), digunakan suatu jumlah terbatas amplitudo.

• Gambar dibawah ini merupakan bentuk sinyal termodulasi :

Amplitudo Shift Keying (ASK)• Amplitude shift keying - ASK - in the context of digital communications is a

modulation process, which imparts to a sinusoid two or more discrete amplitude levels.

• For a binary message sequence there are two levels, one of which is typically zero.

Bandlimiting is generally introduced before transmission, in which case these discontinuities would be ‘rounded off’. The bandlimiting may be applied to the digital message, or the modulated signal itself.

One of the disadvantages of ASK, compared with FSK and PSK, for example, is that it has not got a constant envelope. This makes its processing (eg, power amplification) more difficult, since linearity becomes an important factor. However, it does make for ease of demodulation with an envelope detector

Waveform

• Bentuk gelombang AM yang dipengaruhi sinyal digital baseband NRZ-L, adalah

tfmA

tfA

VA

tfVAtS

c

c

cm

2cos]1[

2cos]1[

2cos][)(

Di mana m adalah index modulasi

Dengan nilai V/A dan besaran kurang dari atau sama dengan 1

Bila Index modulasi adalah 100%, maka amplitudo berubah antara 2A untuk bit 1 dan 0 untuk bit 0. tipe komunikasi ini dinyatakan dengan istilah On-Off Keying (OOK)

A = Amplitudo Carrier

Power dari carrier : Pc = A2/2Rata-rata Total power dari ASK = A2 Nilai ini diperoleh dari asumsi data bersifat random yang setara dan tipe modulasi OOK, dimana Power = 2A2 saat bit ‘1’ dan Power = 0 saat bit ‘0’

Pembatasan Bandwidth• As already indicated, the sharp discontinuities in the ASK waveform imply a wide

bandwidth. A significant reduction can be accepted before errors at the receiver increase unacceptably. This can be brought about by bandlimiting (pulse shaping) the message before modulation, or bandlimiting the ASK signal itself after generation

original TTL message

ASK

bandlimited message

Demodulasi• It is apparent that the ASK signal has a well defined envelope. Thus it is amenable

to demodulation by an envelope detector.• With bandlimiting of the transmitted ASK neither of these demodulation methods

(envelope detection or synchronous demodulation) would recover the original binary sequence.

• Thus further processing - by some sort of decision-making circuitry for example - would be necessary.

Matched Filter Detector

• Receiver mendeteksi dua kemungkinan sinyal yang diterima, kondisi ‘1’ dan ‘0’• Perbedaannya diambil antara dua filter output dan dibandingkan dengan suatu

threshold .

Tugas utama detektor tersebut adalah menentukan nilai output berdasarkan nilai threshold-nya

Detector Performance• Dalam menghitung nilai kinerja sistem deteksi ASK, harus ditentukan rata-rata

energi per bit dan korelasinya.• Energi adalah perkalian daya dengan waktu : E = P.T, dimana P = A2/2 dan T

adalah perioda bit

Rata-rata Energi per bit untuk OOK adalah :

Probability of error

4)(

2bb

c

TAQ

EQBERP

Example

• Informasi Biner, di transmisikan pada kecepatan 10 kbps menggunakan teknik OOK, Frekuensi carrier adalah 10 MHz dan Amplitudo carrier yang diterima adalah 10-2 volt. Sementara kerapatan daya noise yang mempengaruhi adalah 5 x 10-10 watt / Hz.

R = 10 kbps

T = 1/104 = 10 -4 s

A = 10-2 volt

η = 5 x 10-10 watt/Hz

Jawab :

Frequency Shift Keying (FSK)

• As its name suggests, a frequency shift keyed transmitter has its frequency shifted by the message.

• Although there could be more than two frequencies involved in an FSK signal, in this experiment the message will be a binary bit stream, and so only two frequencies will be involved.

• The output from such a generator is illustrated in Figure 1 below

Conceptually, and in fact, the transmitter could consist of two oscillators (on frequencies f1 and f2), with only one being connected to the output at any one time.

Bandwidth

• Practice is for the tones f1 and f2 to bear special inter-relationships, and to be integer multiples of the bit rate. This leads to the possibility of continuous phase, which offers advantages, especially with respect to bandwidth control.

• FSK signals can be generated at baseband, and transmitted over telephone lines (for example). In this case, both f1 and f2 would be audio frequencies. Alternatively, this signal could be translated to a higher frequency. Yet again, it may be generated directly at ‘carrier’ frequencies.

Glossary

• Tb : Periode • R : bit rate• Eb : energi bit