Lecture 15. Signal Transmission Radio Spectrum. Duplexing Channel Sharing or Multiplexing Modulation. Elec 1200

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1 Signal Transmission- Modulation Lecture 15 Signal Transmission Radio Spectrum Multiple Users Duplexing Channel Sharing or Multiplexing Modulation Elec 1200

2 Signal Transmission In a communications system signals or messages need to be transmitted from one place to another We say the signals are passed through a channel (e.g. free space, copper wires, optical fibers) We call this signal transmission Channel Channels operate only at certain frequencies and suffer from distortion, interference and noise In the previous lectures we assumed the channel operated at the frequencies you needed Elec 1200 Slide 2

3 Radio Waves often form part of the Channel Radio waves have different frequencies High frequency-the waves vary quickly Low frequency-the waves vary slowly The set of all frequencies from 0Hz to infinity is known as the radio spectrum and is used for many different applications Elec 1200 Slide 3

4 Some Radio Channels in Hong Kong AM: 535kHz to 1605kHz; FM: 88MHz to 108MHz Stereo Elec 1200 Slide 4

5 Radio Spectrum The radio spectrum is very crowded! Elec 1200 Slide 5

6 Sharing and Regulation Radio waves travel or propagate through a common channel that everybody shares That is for a particular frequency only one person, user or company can use it- otherwise there will be interference and chaos! Elec 1200 Slide 6

7 Hong Kong Mobile Phone Allocations Three / MHz / MHz / / MHz / MHz (3G) and MHz SmarTone /952.5/960MHz / MHz (DCS1800) / / MHz (3G) and MHz PCCW Mobile CSL / MHz / MHz / / MHz / MHz (3G) / MHz / MHz WLAN and Bluetooth / GHz / MHz (3G) GHz and MHz GPS/Mobile Satelite China Mobile / MHz and MHz2655MHz Elec 1200 Slide 7

8 Two-Way Communication Protocols Simplex Communication flow can only Simplex occur in one direction Broadcast radio or TV Half Duplex Communication flow can occur in both directions, but not at the same time Full Duplex Communication link can support simultaneous two- way communications.. Half Duplex Walkie-Talkies, CB radio Elec 1200 Slide 8

9 Regulation of Radio Spectrum The government effectively owns the radio spectrum and regulates it In some cases the government sells the spectrum to a user or company The government of different countries must coordinate the regulation of the spectrum Elec 1200 Slide 9

10 OFCA and the ITU In Hong Kong this regulation is performed by the government office of OFCA (The Office of the Communications Authority) OFCA web site located at http// and a lot of useful information is contained there Generally you must obtain a license from OFCA to make use of the radio spectrum OFCA will also coordinate spectrum with the world authority ITU (International n ti n Telecommunications n Union) n) The ITU is an organization under the United Nations The ITU is headquartered in Geneva, Switzerland is an international organization within which h governments and the private sector coordinate global telecom networks and services. Web site http// / Elec 1200 Slide 10

11 Regulation of Radio Spectrum In most countries this process has been performed by auction- the government will sell the spectrum to the highest bidder This is thought more efficient since it allows the spectrum to be allocated by the free market In Europe 3G spectrum was sold for around HK$300 Billion in Germany and UK! Considered extremely expensive at the time! In 2008 US 700MHz spectrum (old TV band- 60MHz BW) was auctioned for HK$160 Billion In G bands at 2.6GHz were auctioned in UK for HK$24 billion In G bands were auctioned for HK$1.54B in HK Elec 1200 Slide 11

12 Channel Sharing Many users share one common channel Elec 1200 Slide 12

13 Channel Sharing To share a single common channel such as the radio spectrum we must work out some method of sharing it In essence multiple users share a common channel and it is often known as multiplexing The most obvious way is simply to allocate every user a separate frequency band. That is users divide the common channel into frequency bands Hence we refer to this type of channel sharing as Frequency Division Multiplexing (FDM) The key in this is how we translate a baseband signal to a different frequency band and this is known as frequency translation or modulation Elec 1200 Slide 13

Baseband Signals When we refer to signals and messages we often use the term baseband signal This term includes digital data (001100.

14 Baseband Signals When we refer to signals and messages we often use the term baseband signal This term includes digital data ( ), voice, video and nearly all signals we deal with A typical baseband signal can be pictured as: You can think of a baseband signal as any signal that is centered around DC Baseband Message BW Elec 1200 frequency Slide 14

15 Frequency Division Multiplexing (FDM) We first translate baseband signal to center frequency fo Baseband Message BW modulation 2BW Modulated message frequency f 0 frequency Then translate second message to f 1 so we share the channel (FDM) 2BW 2BW BW f 0 f 1 Elec 1200 Slide 15

16 Understanding modulation The idea of modulation is to multiply l (i.e. mix) a baseband signal with a carrier signal Modulation concept can be easily seen through h the trigonometric formula (product to sum sin and cosine) assuming a sinusoidal signal 2 cos(2 π f t )cos(2 π f t ) = cos(2 π ( f f ) t ) + cos(2 π ( f + f ) t ) 2 s 0 0 s 0 + s t signal carrier diff. sum The signal at f s is translated by a carrier frequency f 0 (f 0 >> f s ). The translation involves both the sum and difference frequency Elec 1200 Slide 16

17 A simple example Consider modulating a 1 Hz sinusoidal signal to a carrier frequency at 10 Hz m(t) = cos(2πt) X 1 1 cos( 2πt ) cos(2π 10t) = cos(2π 9t) + cos(2π 11 t) cos(2π10t) time Elec 1200 Slide 17

18 Complex exponentials of a signal The baseband signal m(t) = cos (2πf s t) M(f) 1 f s f 1 cos θ = 2 j θ j θ ( e + e ) This can be verified by knowing Euler s formula θ e j = cos θ + sin m(t) = cos (2πf s t) = ½ ( exp (j2πf s t) + exp (-j2πf s t) ) ½ M(f) j θ Elec f s f s f Slide 18

19 Using complex exponentials & full frequency spectra The baseband signal m(t) = cos (2πf s t) = ½ exp(j2πf s t) + ½ exp(-j2πf s t) The carrier c(t) = cos (2πf 0 t) = ½ exp(j2πf 0 t) + ½ exp(-j2πf 0 t) m(t) x c(t) = ¼ [ exp(j2π(f 0 +f s )t) + exp(j2π(f 0 -f s )t) + exp(j2π(-(f 0 -f s )t) + exp(j2π(-(f 0 +f s )t) ] -f s f s -f 0 -f s -f 0 +f s f 0 -f s f 0 +f s -f s f s -f 0 f 0 Message (BW = f s ) Modulated signal (BW = 2f s ) Modulation, Slide 19

20 Picture analysis in the frequency domain The message f s is translated along the frequency axis to f 0 ±f s modulation f 0 -f s f 0 +f s f s f 0 Message Modulated signal Note that the bandwidth BW of the message is f s and that of the modulated signal is 2f s Elec 1200 Slide 20

21 Picture analysis in the frequency domain A message with continuous spectrum and bandwidth BW is translated along the frequency axis to f 0 ± BW modulation 2BW BW f 0 Message Modulated signal The modulated signal has a bandwidth twice that of the baseband signal. Modulation, Slide 21

22 Frequency Division Multiplexing (FDM) We first translate baseband signal to center frequency fo Baseband Message BW modulation 2BW Modulated message frequency f 0 frequency Then translate second message to f 1 so we share the channel (FDM) 2BW 2BW BW f 0 f 1 Elec 1200 Slide 22

23 De-Multiplexing We must also retrieve or de-multiplex l the signals too Multiplexed messages 2BW f 0 2BW f 1 frequency Bandpass Filter Elec BW 2BW f0 frequency Band Pass filtering i De-modulation frequency Demultiplexed Baseband Message BW Slide 23

24 Summary Sharing a common channel is very important in communications Sharing in Radio channels is regulated because radio channels can be accessed by anyone One common way to share channels is Frequency Division Multiplexing (FDM) Key yparts of FDM are frequency translation and filtering Modulation is used for signal transmission Frequency translation ti to a desirable band Modulation is a straightforward process involves multiplication (mixing) Elec 1200 Slide 24

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