Chapter 3 Communication Concepts 1
Sections to be covered 3.1 General Considerations 3.2 Analog Modulation 3.3 Digital Modulation 3.4 Spectral Regrowth 3.7 Wireless Standards 2
Chapter Outline Modulation AM,PM,FM Signal Constellations ASK,PSK,FSK QPSK,GMSK,QAM Spectral Regrowth Wireless Standards GSM IS-95 CDMA Wideband CDMA Bluetooth IEEE802.11 a/b/g 3
Modulation : The importance of modulation converts a baseband signal to a passband signal. varies certain parameters of a sinusoidal carrier according to the baseband signal. Carrier: A cos 0 ωct The modulated signal: A simple communication system consists of: a modulator/transmitter, a channel: attenuates the signal a receiver/demodulator. 4
Important Aspects of Modulation Critical attributes of each modulation scheme: Detectability: the quality of the demodulated signal for a given amount of channel attenuation and receiver noise Bandwidth Efficiency Power Efficiency 2-level 4-level The binary amplitude modulation: logical ONEs are represented by full amplitude; ZEROs by zero amplitude. The demodulation must distinguish between these two amplitude values. Four different amplitudes: The four amplitude values are closer to one another ; can be misinterpreted in the presence of noise. The signal is less detectable. 5
Important Aspects of Modulation Bandwidth Efficiency: the bandwidth occupied by the modulated carrier for a given information rate in the baseband signal. For example, the GSM phone system provides a total bandwidth of 25MHz for millions of users in crowded cities. Power Efficiency: The type of power amplifier (PA) that can be used in the transmitter. Some modulated waveforms can be processed by means of nonlinear power amplifiers, whereas some others require linear amplifiers. Nonlinear PAs are generally more efficient (Chapter 12). 6
Important Aspects of Modulation: Trade off Detectability, Bandwidth Efficiency and Power Efficiency trade with one another. 4-level modulation format is more bandwidth-efficient than that in 2-level it carries twice as much information for the same bandwidth. At the cost of detectability because the amplitude values are more closely spaced. At the cost of power efficiency because PA nonlinearity compresses the larger amplitudes. 7
Analog Modulation: Amplitude Modulation For a baseband signal x BB (t), an amplitude-modulated (AM) waveform: m is called the modulation index the bandwidth of x AM (t) is twice that of x BB (t) 8
Observations Except for broadcast AM radios, amplitude modulation finds limited use in today s wireless systems. Carrying analog information in the amplitude requires a highlylinear power amplifier in the transmitter. Amplitude modulation is also more sensitive to additive noise than phase or frequency modulation is. 9
Analog Modulation: Phase Modulation General form: ( ) = ( ) ω + θ( ) x t a t cos ct t ω c t +θ (t) : total phase. instantaneous frequency : the time derivative of the phase; ω c + dθ/dt is the total frequency dθ/dt is the frequency deviation. Amplitude is constant; The excess phase is linearly proportional to the baseband signal. m denotes the phase modulation index. If x BB (t)=0, then the zero-crossing points of the carrier waveform occur at uniformly spaced instants equal to integer multiples of the period, T c = 1/ ω c. For a time-varying x BB (t), the zero crossings are modulated while the amplitude remains constant. 10
Analog Modulation: Frequency Modulation The excess frequency, dθ/dt, is linearly proportional to the baseband signal. The instantaneous frequency is equal to ω c + mx BB (t). 11
Bandwidth: Phase & Frequency Modulation If x BB (t) = A m cos ω m t, then with spectral lines well beyond ω c ± ω m. What is the bandwidth of PM and FM signals? Various approximations have been derived. 12
If ma m /ω m << 1rad Narrowband FM Approximation ma m xfm ( t) = Ac cos ωct+ sinωmt ωm The spectrum consists of impulses at ω c (the carrier) sidebands at ω c ± ω m and -ω c ± ω m. As the modulating frequency, ω m, increases, the magnitude of the sidebands decreases. 13
Example of AM, PM and FM Modulation(Ⅰ) It is sometimes said that the FM (or PM) sidebands have opposite signs whereas AM sidebands have identical signs. Is this generally true? Solution: Equation suggests that cos(ω c - ω m )t and cos(ω c + ω m )t have opposite signs. For an AM signal, we have FM AM It appears that the sidebands have identical signs. 14 However
Example of AM, PM and FM Modulation(Ⅱ) In general, the polarity of the sidebands does not distinguish AM from FM. Writing the four possible combinations of sine and cosine in cosine&cosine sine&cosine sine&sine cosine&sine cosine&sine cosine&cosine sine&cosine sine&sine 15
Another example of Modulation (Ⅰ) The sum of a large sinusoid at ω c and a small sinusoid at ω c + ω m is applied to a differential pair. Explain why the output spectrum contains a component at ω c - ω m. Assume that the differential pair experiences hard limiting, i.e., A is large enough to drive I SS to each side. Solution: Decompose the input spectrum into two symmetric spectra. AM FM 16
Digital Modulation: ASK,PSK,FSK In digital communication systems, the carrier is modulated by a digital baseband signal. Analog: AM, PM, and FM Amplitude Shift Keying, Phase Shift Keying, and Frequency Shift Keying x x x ASK PSK FSK () t () t () t = A c = A = A c c cos ω t if data=one c 0 if data=zero A c cos ω t if data=zero c ( 0 ω ) ct+ cos 180 if data=one A c cos ω t if data=zero c ( ω + ω) cos t if data=one c 17
Digital Modulation: generating ASK,PSK ASK baseband binary data toggles between 0 and 1, product with the carrier. ASK PSK baseband data toggles between +0.5 and -0.5 (i.e., it has a zero average), product with the carrier. PSK 18 The sign of the carrier must change every time the data changes. the phase jumps by 180 0
PSK: The Spectrum of PSK and ASK Signal PSK The upconversion operation of PSK shifts the spectrum of x BB (t) to ±f c ; Spectrum of ASK is similar but with impulses at ±f c x ASK () t A = c cos ω t if data=one c 0 if data=zero ASK 19
Signal Constellation: Binary PSK and ASK fuzzy Ideal Noisy Plot the constellation of an ASK signal in the presence of amplitude noise. Noise corrupts the amplitude for both ZEROs and ONEs. fuzzy Ideal Noisy 20
Quadrature Modulation (quadrature PSK) To further reduce the bandwidth, divide a binary data stream into two streams: Since cosω c t and sin ω c t are orthogonal, the signal can be detected uniquely and the bits b 2m and b 2m+1 can be separated without corrupting each other. QPSK halves the occupied bandwidth; Pulses appear at A and B are called symbols rather than bits. The pulse appearing at A and B are called quadrature baseband signals and denoted by: I for in-phase ( 同相 ) Q for Quadrature ( 正交 ) 21
Example of Signal Constellation Due to circuit nonidealities, one of the carrier phases in a QPSK modulator suffers from a small phase error ( mismatch ) of θ Construct the signal constellation at the output of this modulator Solution: ( α1, α2) (1,1) (1,-1) (-1,1) (-1,-1) β1 β2 xt ( ) = α cosθacos ω t+ ( α α sin θ) Asinω t 1 c c 2 1 c c 22
Important Drawback of QPSK (Ⅰ) Important drawback of QPSK comes from the large phase changes at the end of each symbol. From symbol [-1,-1] to [+1,+1], the carrier experiences a 180 0 phase step, transition between two diagonally opposite points in the constellation. 23
Important Drawback of QPSK (Ⅱ) Square baseband pulses shaped baseband pulses The baseband pulses are usually shaped so as to tighten the spectrum. With pulse shaping, the output signal amplitude ( envelope ) experiences large changes each time the phase makes a 90 or 180 degree transition. Resulting waveform is called a variable-envelope signal. Need linear PA (less efficient than a nonlinear PA). Improvement with offset QPSK (OQPSK) and π/4-qpsk. 24
GMSK and GFSK Modulation (I) Constant-envelop modulation: FSK waveform has a constant envelope. FSK Square baseband pulses are applied to the VCO, producing a broad output spectrum due to the abrupt changes in the VCO frequency. How to tighten the spectrum? Smoother transitions between ONEs and ZEROs in the baseband signal. Pulse shaping: Gaussian filter, i.e., impulse response is a Gaussian pulse. Gaussian minimum shift keying GMSK The pulses applied to the VCO gradually change the output frequency, leading to a 25 narrower spectrum.
GMSK and GFSK Modulation (II) GMSK is used in GSM cell phones. The GMSK waveform can be expressed as: where h(t) denotes the impulse response of the Gaussian filter. Gaussian minimum shift keying (GMSK), modulation index m = 0.5 constant envelope signal GMSK allows optimization of PAs for high efficiency with little attention to linearity. Gaussian frequency shift keying (GFSK) is employed in Bluetooth, with modulation index m = 0.3 26
Solution: Example of GMSK Modulator Construction Construct a GMSK modulator using a quadrature upconverter. a Gaussian filter is followed by an integrator two arms that compute the sine and cosine of the signal at node A. The complexity of these operations is much more easily afforded in the digital domain than in the analog domain (Chapter 4). 27
Quadrature Amplitude Modulation (QAM) Draw the four possible waveforms for QPSK corresponding to the four points in the constellation. Each quadrature component of the carrier is multiplied by +1 or -1 according to the values of b 2m and b 2m+1. QAM ( quadrature amplitude modulation ) allows four possible amplitudes for sine and cosine waveform, e.g., ±1 and ±2, thus obtaining 16 possible output waveforms (grouping four consecutive bits, b 4m, b 4m+1, b 4m+2 and b 4m+3 ). 28
Quadrature Amplitude Modulation: Constellation Saves bandwidth---16qam occupies one-fourth the bandwidth of PSK. Denser constellation: making detection more sensitive to noise. Large envelope variation: need highly linear PA. Trade-offs among bandwidth efficiency, detectability, and power efficiency. 29
Spectral Regrowth: Constant vs. Variable Envelope Modulated signal Constant Envelope Suppose A(t) = A c third-order nonlinearity Shape of the spectrum near ω c remains unchanged Variable Envelope Where x I and x Q (t) are the baseband I and Q components (with a certain bandwidth) The output contains the spectra of x 3 I(t) and x 3 Q(t) centered around ω c. These components exhibit a broader spectrum than do x I (t) and x Q (t). The spectrum grows when a variable-envelope signal passes through a nonlinear system. 30
Spectral Regrowth: An Illustration Constant Envelope Variable Envelope Constant Envelope: Shape of Spectrum unchanged Variable Envelope: Spectrum grows 31
Wireless Standards: Common Specifications (Ⅰ) 1. Frequency Bands and Channelization: Each standard performs communication in an allocated frequency band 2. Data Rates: The standard specifies the data rates that must be supported 3. Antenna Duplexing Method: Most cellular phone systems incorporate FDD and other standards employ TDD 4. Type of Modulation: Each standard specifies the modulation scheme. 32
Wireless Standards: Common Specifications (Ⅱ) 5. TX output power: The standard specifies the power levels that the TX must produce 6. TX EVM and Spectral Mask: The signal transmitted by the TX must satisfy several requirements like EVM and spectral mask 7. RX Sensitivity: The standard specifies the acceptable receiver sensitivity, usually in terms of maximum BER 33
Wireless Standards: Common Specifications (Ⅲ) 8. RX Input Level Range: The standard specifies the desired signal range that the receiver must handle with acceptable noise or distortion 9. RX Tolerance to Blocks: The standard specifies the largest interferer that the RX must tolerate while receiving a small desired signal. Many standards also specify an intermodulation test 34
GSM: Air Interface GSM standard is a TDMA/FDD system with GMSK modulation Operate in different bands and accordingly called GSM900, GSM1800, and GSM 1900 Accommodate 8 time-multiplexed users Each channel is 200kHz Data rate per user is 271kHz TX and RX time slots are offset (by about 1.73ms) The total capacity is around 1000 (25MHz/200kHz*8) 35
GSM: an Example Example: GSM specifies a receiver sensitivity of -102 dbm. The detection of GMSK with acceptable bit error rate (10-3 ) requires an SNR of about 9 db. What is the maximum allowable RX noise figure? SNR P / P P / B/ P P / B / P NF = = = = SNR SNR SNR SNR The receiver is matched to the antenna, then, the available power of thermal noise in antenna is P RS = kt= -174 dbm/hz in sig RS sig, tot RS sen RS out out out out 36
GSM: Blocking Requirements With the blocker levels shown in above figure, the receiver must still provide the necessary BER. Desired signal at 3 db above the sensitivity level along with a single (unmodulated) tone at discrete increments of 600 khz from the desired channel. (Only one blocker is applied at a time.) 37
GSM: Intermodulation Requirements Desired channel 3 db above the reference sensitivity level. A tone and a modulated signal applied at 800-kHz and 1.6-MHz offset at -49 dbm and BER requirement must be satisfied. 38
GSM: TX Specifications Transmitter must deliver an output of at least 2 W (+33dBm) in the 900-MHz band or 1 W (+30dBm) in the 1.8-GHz band; Must be adjustable in steps of 2 db from +5 dbm to the maximum level; mask The output spectrum produced by a GSM transmitter must satisfy the mask. The maximum noise that the TX can emit in the receive band must be less than -129 dbm/hz. 39
GSM: Adjacent-Channel Interference Desired channel 20 db above the reference sensitivity level. Must withstand an adjacent-channel interferer 9 db above desired signal or and alternate-adjacent channel interferer 41 db above signal. 40
GSM: EDGE Enhanced Data Rates fro GSM Evolution (EDGE) is considered a 2.5thgeneration (2.5G) cellular system. Rate: 384kb/s, 8-PSK modulation; higher bandwidth efficiency Need pulse shaping, linear PA; lower power efficiency Requires a higher SNR: worse detectability 41
IS-95 CDMA: Air Interface Direct-sequence CDMA has been proposed by Qualcomm and adopted for North America as IS-95. Mobile unit: 9.6 kb/s spread to 1.23 MHz and modulated using OQPSK (nonlinear PA). Data rate can vary in four discrete steps: 9600, 4800, 2400, and 1200b/s. Coherent detection and pilot tone used. 42
Wideband CDMA: Air Interface BPSK for uplink, QPSK for downlink, nominal channel bandwidth 5MHz, rate 384 kb/s IMT-2000: total bandwidth 60 MHz, data rate 384 kb/s in a spread bandwidth of 3.84 MHz, channel spacing 5 MHz 43
Wideband CDMA: Transmitter Requirements Output power: -49 dbm to +24 dbm. Linear PA. Adjacent and alternate adjacent channel power 33 db and 43 db below main 44 channel.
Wideband CDMA: Receiver Requirements Blocking mask using a tone: Reference sensitivity: -107 dbm. Sinusoidal test for only out-of-band blocking Blocking test using a modulated interferer: For in-band blocking, blocker is modulated such that it behaves as another WCDMA channel, causing both compression and cross 45 modulation.
Wideband CDMA Receiver Requirements: Intermodulation Test & Adjacent Channel Test IMT-2000 intermodulation test: A tone and a modulated signal each at -46 dbm applied in the adjacent and alternate adjacent channels, desired signal at -104 dbm IMT-2000 receiver adjacent-channel test: Desired signal -93 dbm, adjacent channel -52 dbm 46
IEEE 802.11 a: Air Interface Channel spacing 20 MHz 47
IEEE 802.11 a: OFDM Channelization OFDM: 52 subcarriers with spacing of 0.3125 MHz, middle sub-channel and first and last 5 sub-channels are unused. 4 subcarriers are occupied by BPSKmodulated pilots. 48
IEEE 802.11 a: Transmission Mask TX must deliver a power of at least 40 mw (+16dBm). Pulse shaping: 16.6 MHz 49
IEEE 802.11 a: Data Rates, Sensitivities, Adjacent Channel Levels 11a/g receiver must operate properly with a maximum input -30 dbm 50
Bluetooth: Air Interface 2.4-GHz ISM band. Each channel carries 1 Mb/s, occupies 1 MHz 51