Mobile 미디어 IT 기술 Multiplexing / Multiple Access Department of Electronics and IT Media Engineering 1
Contents 1. Multiplexing / Multiple Access Techniques Multiplexing Multiple Access 2. CDMA System Spread Spectrum Techniques DS-CDMA system 2G/3G Mobile Communication System (IS-95/UMTS/cdma2000) 3. OFDMA System
Multiplexing / Multiple Access Techniques 3
Block Diagram Digital Communication Analog message A/D conversion Source coding Channel coding Modulation Multiple access Transmitter Digital message From other source Channel Digital data output To other sink Analog data output D/A conversion Source decoding Channel decoding Demodulation Multiple access Receiver 4
Multiplexing simultaneous transmission of multiple signals across a single data link Division of link into n channels (frequency, time, code, space, etc.) 5
FDM process Multiplexing example Multiplexing: FDM 6
FDM process Demultiplexing example Multiplexing: FDM 7
Multiplexing: FDM Ex 1) Assume that a voice channel occupies a bandwidth of 4 khz. We need to combine three voice channels into a link with a bandwidth of 12 khz, from 20 to 32 khz. Show the configuration, using the frequency domain. Assume there are no guard bands. 8
Multiplexing: FDM Ex 2) Five channels, each with a 100-kHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 khz between the channels to prevent interference? Sol) For five channels, we need at least four guard bands. This means that the required bandwidth is at least 5 100 + 4 10 = 540 khz 9
Multiplexing: FDM Ex 3) Four data channels (digital), each transmitting at 1 Mbps, use a satellite channel of 1 MHz. Design an appropriate configuration, using FDM. Sol) We divide it into four channels, each channel having a 250-kHz bandwidth. Each digital channel of 1 Mbps is modulated so that each 4 bits is modulated to 1 Hz. One solution is 16-QAM modulation. 10
Multiplexing: FDM Ex 4) The Advanced Mobile Phone System (AMPS) uses two bands. The first band of 824 to 849 MHz is used for sending, and 869 to 894 MHz is used for receiving. Each user has a bandwidth of 30 khz in each direction. The 3-kHz voice is modulated using FM, creating 30 khz of modulated signal. How many people can use their cellular phones simultaneously?. Sol) Each band is 25 MHz. If we divide 25 MHz by 30 khz, we get 833.33. In reality, the band is divided into 832 channels. Of these, 42 channels are used for control, which means only 790 channels are available for cellular phone users. 11
TDM process Time division multiplexing Multiplexing: TDM 12
Multiplexing: TDM Ex 5) Synchronous TDM: the data rate for each input connection is 1 kbps. If 1 bit at a time is multiplexed (a unit is 1 bit), what is the duration of Sol) each input slot, each output slot, and each frame? The data rate of each input connection is 1 kbps the bit duration is 1/1000 s or 1 ms. The duration of the input time slot is 1 ms The duration of each output time slot is one-third of the input time slot. Output time slot is 1/3 ms. Each frame carries three output time slots. So the duration of a frame is 3 (1/3) = 1 ms. The duration of a frame is the same as the duration of an input unit. 13
Multiplexing: TDM Ex 6) Four channels are multiplexed using TDM. If each channel sends 100 bytes/s and we multiplex 1 byte per channel, show the frame traveling on the link, the size of the frame, the duration of a frame, the frame rate, and the bit rate for the link. Sol) Each frame carries 1 byte from each channel; the size of each frame, therefore, is 4 bytes, or 32 bits. The frame rate is 100 frames per second. The duration of a frame is therefore 1/100 s. The link is carrying 100 frames per second, and since each frame contains 32 bits, the bit rate is 100 32, or 3200 bps. So the duration of a frame is 3 (1/3) = 1 ms. The duration of a frame is the same as the duration of an input unit. 14
Multiple Access Channelization for multiple users Division of finite channel resources (frequency, time, code, space, etc.) FDMA Power TDMA Time Frequency Power CDMA Time Frequency Power Time Frequency 15
Spread Spectrum Techniques 16
Spread Spectrum Techniques Spread Spectrum Overview The signal occupies a bandwidth much in excess of the minimum bandwidth necessary to send the information. Spreading is accomplished by means of a spreading signal, often called a code signal, which is independent of the data. At the receiver, despreading is accomplished by the correlation of the received spread signal with a synchronized replica of the spreading signal used to spread the information. 17
Spread Spectrum Techniques The Beneficial Attributes of Spread-Spectrum Systems Interference Suppression Benefits Energy Density Reduction Fine Time Resolution Multiple Access 18
Concept of Spread Spectrum Spread Spectrum Same signal power Power Spectral Density (PSD) 는 spreading factor W/R 만큼감소 Spreading Factor: ratio between the bandwidth of the user signal and the transmit bandwidth 수신기에서는 spreading code 를알아야만 despreading 가능 R (Hz) PSD 1 Spread 1 W (Hz) R/W frequency frequency 19
Concept of Spread Spectrum Concept of Spread Spectrum PSD A N0 2 SNR>0 (a) Spreading PSD B same power P 2AB s N0 2 A/ PG SNR 0 W PG B (b) Despreading PSD A N0 2 SNR>0 (c) B 20
Concept of Spread Spectrum Thermal Noise level remains unchanged by despreading, due to noise being uncorrelated with the despreading code Despreading negative SNR at Rx input positive SNR at Rx output after despreading Processing Gain (Spreading Gain) Signal to Noise Ratio (SNR) improvement of W/R 21
Basic Types of SS System Direct-Sequence (DS) SS systems Spectrum spreading is achieved by multiplying the source with a pseudo-random signal. Frequency Hopping (FH) SS systems Spectrum spreading is achieved by hopping its carrier frequency over a (large) set of frequencies. The frequency-hopping pattern is pseudo-random. Time-Hopping (TH) SS systems A block of data bits is compressed and transmitted intermittently in one or more time slots within a frame that consists of a large number of time slots. A pseudo-random time-hopping pattern determines which time slots to be used for transmission during each frame. 22
Direct-Sequence (DS) SS systems bt () 1 Tb 2T b 3T b t (a) 1 T b ct () 1 Tb 2T b 3T b t (b) 1 T c b( t) c( t) 1 Tb 2T b 3T b t (c) 1 T c 23
Frequency Hopping (FH) SS systems Frequency Hopping (FH) SS systems the transmitter changes the carrier frequency according to a certain hopping pattern. The advantage is that the signal sees a different channel and a different set of interfering signals during each hop. This avoids the problem of failing communication at a particular frequency, because of a fade or a particular interferer. 24
Timing Hopping (TH) SS systems Time diagram of TH/SS system 25
Direct Sequence Spread Spectrum & CDMA System 26
Direct Sequence Spread Spectrum DSSS(Direct Sequence Spread Spectrum) Direct sequence is the name given to the spectrum spreading technique whereby a carrier wave is first modulated with a data signal x(t), then the data-modulated signal is again modulated with a high-speed (wideband) spreading signal g(t). The data pulse stream and the spreading pulse stream are first multiplied, and then the composite x(t) modulates the carrier In DSSS, we replace each data bit with n bits using a spreading code. In other words, each bit is assigned a code of n bits, called chips, where the chip rate is n times that of the data bit. BPSK carrier by x(t) s x ( t) 2Px( t) cos 0 t By the spreading signal g(t) s 0 ( t) 2Px( t) g( t) cos t 27
Direct Sequence Spread Spectrum DSSS(Direct Sequence Spread Spectrum) The data pulse stream and the spreading pulse stream are first multiplied, and then the composite x(t) modulates the carrier If the assignment of pulse value to binary value is then the initial step in the DS/BPSK modulation can be accomplished by the modulo-2 addition of the binary data sequence with the binary spreading sequence. 28
Direct Sequence Spread Spectrum DSSS(Direct Sequence Spread Spectrum) Modulation 29
Direct Sequence Spread Spectrum DSSS(Direct Sequence Spread Spectrum) Demodulation Demodulation of the DS/BPSK modulation is accomplished by correlating the received signal with a synchronize replica of the spreading signal The output signal from the correlator can be written as A 2Px( t T ˆ d ) g( t Td ) g( t Td ) cos 0( t Td ) Since g ( t) 1, the product g( t T ˆ d ) g( t Td ) will be unity if Tˆ d T d, that is, if the code signal at the receiver is exactly synchronized with the code signal at the transmitter. When it is synchronized, the output of the receiver correlator is the despread data-modulated signal. The despreading correlator is then followed by a conventional demodulator for recovering the data. 30
Direct Sequence Spread Spectrum DSSS(Direct Sequence Spread Spectrum) 31
Direct Sequence Spread Spectrum DSSS(Direct Sequence Spread Spectrum) 32
CDMA DSSS & CDMA System 서로다른사용자에게서로다른 PN 부호열을할당하여여러신호가동일한주파수대역을사용하여전송되더라도수신기에서는다중사용자신호로부터원하는사용자신호를추출하여정보데이터를복구할수있다. 이렇게사용자신호를부호열을사용하여구별이되도록한다중접속방식을부호분할다중접속 (Code Division Multiple Access: CDMA) 이라하며, 특히 DS 로대역확산이이루어지는경우 DS- CDMA 라한다. 33
DS-CDMA System DSSS & CDMA System bt () 1 ct () 1 dt bt () 2 ct () 1 ct () 2 A CDMA receiver can retrieve the wanted signal by multiplying the receive signal with the same code as the one used during transmission. The receive code must be perfectly time aligned with the transmit code. 34
DS-CDMA System: Tx DSSS & CDMA System 1 bt () 1 1 Tb 2T b b() t 1 t 2 1 Tb 2T b t c() t ct () t 1 2 t 1 b1( t) c1( t) t b2( t) c2( t) 1 t 1 1 b( t) c( t) b ( t) c ( t) 1 1 2 2 2 2 T b 2T b t
DS-CDMA System: Rx DSSS & CDMA System c1( t) t b ( t) c ( t) b ( t) c ( t) c ( t) 2 1 1 2 2 1 Tb 2T b t 2 Integrator output Tb 2T b t 36
DSSS & CDMA System N equal power SS signals (at RX input) After despreading : Co-users remain spread if their spreading code is uncorrelated with the desired signal code Co-users can be considered as additive noise P w /W P w /R Despread } N-1 co-users Wanted signal P int /W frequency 37
Performance Effect of Spread Spectrum Immunity to Jamming 수신기에서역확산은원하는광대역신호를협대역신호로변환시키는반면협대역의간섭신호는광대역으로확산시킨다. 따라서원하는신호의대역내에있는간섭신호의전력은 1/PG 배로줄어든다. 교란 (jamming) 을위해방사하는간섭신호의전력은유한하다. 통신을방해하기위해서는 SNR 이 0 db 이하가되도록해야한다. 교란이유효하게이루어지기위해서는수신기에서역확산을거쳐서 PSD 가 PG 배커진대상신호와간섭신호의 PSD 가동일한크기를가져야한다. 교란신호는역확산후에도 PSD 가변화없으므로광대역에서이크기의 PSD 를가져야한다. 그러므로간섭의총전력은 P I = PG P s 가되어야한다. 확산이득이매우큰경우유효한교란을주기위한방해신호의전력이너무커서실현이불가능하게된다. 38
Performance Effect of Spread Spectrum Bit Error Rate (BER) Performance AWGN 채널과같은이상적인환경에서는대역확산이시스템의성능에영향을주지않는다. 즉대역확산을통하여비트오율성능이개선되는효과는없다. 이것은 AWGN 잡음이무한대의전력을갖기때문이다. AWGN 잡음의 PSD 는수신기의역확산에의하여변화하지않는다. 대역확산통신시스템의성능이개선되는경우는교란이나간섭신호와같이전력이유한한경우이다. 유한전력의신호는역확산에의하여 PSD 레벨이낮아지지만무한전력의 AWGN 잡음은역확산에의해 PSD 레벨이변화하지않기때문이다. 이동통신과같이다중경로로수신되는환경에서는페이딩현상이발생되는데, 이경우에는대역확산통신시스템이협대역통신시스템에비해우수한성능을갖는다. 39
Multiple Access by Multi-Users DS CDMA system with N users Received Signal N r( t) 2 P b ( t t ) c ( t t )cos( t ) i1 i i i i i c i 여기서 P i 와 t i 는각각 i 번째사용자신호의송신전력과지연시간이며, 잡음은고려하지않았다. 사용자 1의수신기에서의신호처리에대해살펴보자. 수신신호에 c (t- i t i ) 을곱하여 despreading 하고 carrier signal 2 cos( ) c t 1 를곱하면다음과같다. 40
Multiple Access by Multi-Users DS CDMA system with N users x( t) r( t) 2 c ( t t )cos( t ) i2 1 1 c 1 2 Pb ( t t )cos ( t ) N 2 1 1 1 c 1 2 Pb ( t t ) c ( t t ) c ( t t )cos( t )cos( t ) i i i i i 1 1 c i c 1 Pb ( t t ) Pb ( t t )cos(2 t 2 ) 1 1 1 1 1 1 c 1 N Pb ( t t ) c ( t t ) c ( t t )cos( ) i2 N i2 i i i i i 1 1 1 i Pb ( t t ) c ( t t ) c ( t t )cos(2 t ) i i i i i 1 1 c 1 i Assuming that every user is synchronized t1 t 2 t N 0 41
Multiple Access by Multi-Users DS CDMA system with N users 한비트구간동안적분하면고주파성분은제거되고다음을얻는다. z Pb ( t) dt Pb ( t) c ( t) c ( t) dt T 1 1 1 b T i i i b i 2 P T Pb ( t) c ( t) c ( t) dt 1 b i i T i 1 b i2 1 N N P T N P R b i cic i2 1 (0) where R ( t) c ( t) c ( t t) dt c c i 1 T i 1 b 는두 PN 신호간의상호상관함수이다. 42
Multiple Access by Multi-Users DS CDMA system with N users 위의결정변수에관한식에서첫번째항은원하는신호성분이며, 두번째항은사용자간간섭성분이다. 이사용자간간섭성분의크기에따라적분기의출력값이변화하여비트판정오류를유발하게된다. 결정변수에포함된사용자간간섭의총합은사용자신호의전력, 사용자수, PN 부호열간상호상관값에의하여변화한다는것을알수있다. 그러므로 PN 부호열을설계할때는서로다른 PN 부호열간의상호상관값이작게설계하는것이바람직하다. 만일두 PN 부호열의상관값이 0 이라면사용자간간섭은 0 이된다. 그러므로사용자들에게할당하는 PN 부호열들이상호상관값이 0 이되도록할수있다면다른사용자신호의영향을받지않게된다. 43
Multiple Access by Multi-Users DS CDMA system with N users 예를들어 Walsh 함수들은서로직교하므로 CDMA 시스템에서사용자구별을위한대역확산신호로사용할수있다. 그러나 Walsh 함수들은 t = 0 인경우만상관값이 0 이므로모든사용자간동기가맞아야만간섭이없게된다. 이동전화시스템의순방향링크의경우기지국에서는여러이동국들에게전송되는신호가동기가맞아있으므로 Walsh 함수를사용할수있다. 그러나역방향링크의경우이동국들의위치가다르므로기지국에도달하는사용자신호들의동기가맞지않게되므로 Walsh 함수를사용하지않는다. 44
Walsh Code Multiple Access by Multi-Users Orthogonal Sequence W () t 0 W () t 1 W () t 2 W () t 3 W () t 4 W () t 5 W () t 6 W () t 7 45
Multiple Access by Multi-Users DS CDMA system with N users 역확산후타사용자로부터의간섭성분 ( 즉다사용자간섭 ) 다사용자간섭신호의 PSD 는 만일모든사용자가동일한전력 P s 를사용한다면 가된다. N I ( t) 2 Pb ( t) c ( t) c ( t)cos( t) MU i i i 1 c i2 S f P T 2 f f f f 여기서 MU c 2 2 MU ( ) sinc ( c) sinc ( c) P MU N Pi i2 P ( N 1) P MU s 46
Multiple Access by Multi-Users DS CDMA system with N users 다사용자간섭은백색잡음으로근사화할수있다. 이경우 PSD 는 ' N0 PMU Tc ( N 1) PT s c 2 2 2 된다. 그러므로 AWGN 잡음이있는환경에서 DS-CDMA 시스템의비트오율은 P b E b 2E b Q Q ' N0 / 2 N0 / 2 N0 ( N 1) PT s c 가되어사용자수 N 이증가할수록비트오율이증가한다. 그러나처리이득 PG 를증가시키면 T c 가작아져서다사용자간섭의영향을줄일수있다는것을알수있다. 47
Code Sequence 48
Orthogonal code Crosscorrelation = 0 Ex) Walsh-Hadamard code Code Sequence Pseudo-random Noise (PN) Code Random noise sequence와유사한통계적특성을가진코드 Mean 0, autocorrelation delta function 실제로는 deterministic하게발생 ( 주기신호 ) Ex) m-sequence Family of PN codes Crosscorrelation이 0에가까운 PN 코드의집합 Ex) Gold codes, Kasami codes 49
Walsh Code Generation of Walsh-Hadamard Matrix 0 0 H0 0 H1 0 1 0 0 0 0 0 1 0 1 Hk1 Hk 1 H2 Hk 0 0 1 1 Hk1 Hk 1 0 1 1 0 Example W0 Row vectors are orthogonal 50 H 3 0 0 0 0 0 0 0 0 W 0 1 0 1 0 1 0 1 1 W 2 0 0 1 1 0 0 1 1 W3 0 1 1 0 0 1 1 0 W 4 0 0 0 0 1 1 1 1 W5 0 1 0 1 1 0 1 0 W 0 0 1 1 1 1 0 0 6 W7 0 1 1 0 1 0 0 1
Properties of Walsh Code Walsh Code Cross correlation of Walsh function is zero, i.e. orthogonal. No interference among users. However, orthogonality is destroyed when user signals are not synchronized multi-user interference Multipath reception destroys orthogonality self interference 51
PN(Pseudo Noise) Sequence PN Sequence A random signal cannot be predicted; its future variations can only be described in a statistical sense. A PN signal is not random at all; it is a deterministic, periodic signal that is known to both the transmitter and receiver. Even though the signal is deterministic, it appears to have the statistical properties of sampled white noise. A PN signal appears, to an unauthorized listener, to be a truly random signal. 52
Properties of PN Sequence Properties of PN Sequence Balance Property Good balance requires that in each period of the sequence, the number of binary ones differs from the number of binary zeros by at most one digit. Run Property A run is defined as a sequence of a single type of binary digit(s). The appearance of the alternate digit in a sequence starts a new run. The length of the run is the number of digits in the run. Among the runs of ones and zeros in each period, it is desirable that about one-half the runs of each type are of length 1, about one-fourth are of length 2, one-eighth are of length 3, and so on. Correlation Property If a period of the sequence is compared term by term with any cyclic shift of itself, it is best if the number of agreements differs from the number of disagreements by not more than one count. 53
Properties of PN Sequence Advantages of PN Sequence Single PN codes with ideal autocorrelation properties Single code with different offsets (phases, lags) can be used for MA e.g. Maximum length shift register sequences (m-sequences) => Code synchronization is required Families of code sequences Codes with zero or small crosscorrelation are generated to be used for MA e.g. Walsh Hadamard Codes: orthogonal when synchronized => Code synchronization is required e.g. Gold codes, Kasami codes: optimal crosscorrelation properties Crosscorrelation is not zero, but is small for any PN offsets => Code synchronization is NOT required 54
Linear Feedback Shift Register LFSR with memory size n The output sequences can be classified as either maximal length or nonmaximal length. Maximal length sequences have the property that for an n-stage linear feedback shift register the sequence repetition period is p = 2 n 1 55
Linear Feedback Shift Register LFSR with memory size n Assuming that stage X 1 is initially filled with a one and the remaining stages are filled with zeros, that is, the initial state of the register is 1000. The succession of register states will be as follows: 1000 0100 0010 1001 1100 0110 1011 0101 1010 1101 1110 1111 0111 0011 0001 1000 Since the last state, 1000, corresponds to the initial state, we see that the register repeats the foregoing sequence after 15 clock pulses. The output sequence is obtained by noting the contents of stage X 4 at each clock pulse. 56
Linear Feedback Shift Register LFSR with memory size n The output sequence is seen to be 0 0 0 1 0 0 1 1 0 1 0 1 1 1 1 where the leftmost bit is the earliest bit. Balance property: there are 7 zeros and 8 ones in the sequence. Run property: consider zero runs-there are 4 of them. One-half are of length 1, and one-fourth are of length 2. The same is true for the one runs. Correlation property The shift register generator produces sequences that depend on the number of stages, the feedback tap connections, and initial conditions. 57
Linear Feedback Shift Register Correlation property Autocorrelation of periodic signal 1 1 T0 / 2 Rx( t ) x t x t dt K T ( ) ( t ) for t T0 / 2 0 1 T0 / 2 2 where K x t dt T ( ) T0 / 2 0 Autocorrelation of PN sequence 1 for k mn ( N : integer) N 1 1 Rc ( k) cn cn k 1 N n0 for k mn N As N r ( k) ( k) c 58
Multipath Effect in CDMA System 59
Multipath Effect in CDMA System Multi-path Environment 무선통신환경에서는수신기에입력되는신호가직접전파된직접파신호와인공환경, 혹은자연적인지형으로부터반사된간접파신호들로구성된다. 이와같이신호가여러개의경로를거쳐전파되는환경을다중경로 (multi-path) 전파환경이라한다. 여러경로를통해신호가수신된다는것은각경로신호의지연시간과크기가다르다는것을의미한다. 따라서다중경로환경에서는동일한신호가크기와지연시간이다른형태로더해져서수신된다. 60
Multipath Effect in CDMA System Multi-path Effect 경로의개수가 L 인경우수신신호는다음과같이표현할수있다. L r( t) r Ab( t t ) c( t t )cos( t ) l1 l l l c l 여기서 r l 은 l 번째경로의이득이며, t l 은 l 번째경로의시간지연이다. 수신기에서첫번째경로로수신되는신호에 PN 부호열을동기화시키는경우, despreading 하고 carrier signal 을곱한신호는다음과같이된다. L x( t) rl Ab( t t l ) c( t t l )cos( ct l ) c( t t1)cos( ct 1) l1 r Ab( t t )cos ( t ) 2 1 1 c 1 L r Ab( t t ) c( t t ) c( t t )cos( t )cos( t ) l2 l l l 1 c l c 1 61
Multipath Effect in CDMA System Multi-path Effect 여기서편의상제일먼저수신되는신호성분의지연시간은 0으로 ( 즉 t 1 = 0) 가정하자. 한비트구간동안적분하면적분기출력은다음과같이된다. L A z r1 b( t) dt rl Acos l b( t) c( t) c( t t l ) dt Tb 2 T b L 1 b l2 r AT rla cos lrc ( tl ) 2 l2 2 여기서 R c (t ) l 는 PN 신호 c(t) 의자기상관함수이다. 위의식의두번째항은자기자신의신호가다른경로로수신되어간섭으로작용하는성분이다. 이러한간섭을 self-interference라한다. 62
Multipath Effect in CDMA System Multi-path Effect PN 신호의자기상관함수는 t T c 에서근사적으로 0에가깝다. 따라서칩시간 T c 가지연시간에비해작은경우자기간섭의영향은매우작게된다. 그러므로주어진비트율의데이터에대하여처리이득 PG 를크게하면자기간섭의영향을작게할수있다. 63
Combiner Multipath Effect in CDMA System Rake Receiver for Multi-path Effect in CDMA System Different reflected waves arrive with different delays. A rake receiver can detect these different signals separately. These signals are then combined, using the diversity technique, for example, maximum ratio combining. Multipath Diversity t d X X C 1 (t) code C 1 (t) X C 1 (t-t d ) 64
Near-Far Problem in CDMA System 65
Near-Far Problem in CDMA System What is Near-Far Problem? 다중접속통신환경에서근거리에서전송된강한신호에의해원거리에서전송된약한신호가영향을받는현상 Near-Far Effect 다중사용자환경에서성능을열화시키는요소는잡음과 N-1 개의타사용자간섭신호이다. 앞서다중사용자환경에서의비트오율성능분석에서는모든사용자의신호가동일한평균전력을가지고수신된다고가정하였다. 그러나실제환경에서는송신전력이동일한경우에도송수신기간의거리가일정하지않으므로수신전력은차이가있게된다. 즉근거리의사용자로부터수신된신호의전력은크고, 원거리의사용자로부터수신된신호의전력은작다. 66
Near-Far Problem in CDMA System Near-Far Effect N - 1 개의타사용자중하나가수신기와매우가까운경우를고려해보자. 모든사용자신호는 P s 의평균전력으로수신되고근거리의사용자신호는 ap s 의평균전력으로수신된다고하자. 전자파의 propagation 특성은거리의지수승으로감쇠가증가한다. 감쇠지수는자유공간전파의경우 2 이며 ( 즉수신강도는거리의제곱에반비례하여작아지며 ), 환경에따라 2~5 의값을가진다. DS 시스템의기본개념은수신기에서역확산하면원하는신호의스펙트럼은크기가크고협대역인신호로변환되어백색잡음과같은형태로남아있는간섭신호보다전력이커지게되는것이다. 그러나근거리의간섭신호가크기가매우커서역확산후에도원하는신호의전력보다큰상황이라면 SNR 이 0 보다작아져서비트오율이증가한다. 67
Near-Far Problem in CDMA System Near-Far Effect DS-CDMA 수신기에서적분기출력 SNR 은다음과같이된다. SNR o E E 2PT N N N N apt N PT b b s b ' ' 0 / 2 0 / 2 0 / 2 0 s c ( 2) s c 비트오율은 Pb Q SNRo 이므로 a의값이매우큰경우 SNR o 이급격히감소하여비트오율이크게증가한다. 즉많은사용자로인한다중사용자간섭영향보다근거리에있는하나의사용자에의한간섭영향이더커질수있다. 이와같은 Near-Far 문제는 DS-CDMA 의단점이된다. Power Control 이중요한기술적 issue 68
Near-Far Problem in CDMA System Power Control 이와같은문제를해결하는방법으로 DS-CDMA 이동전화의역방향링크에서는전력제어를사용한다. 즉근거리의단말기는작은전력을사용하여전송하고원거리의단말기는큰전력을사용하도록한다. 이상적인전력제어는모든단말기의신호가기지국에도달할때는동일한전력으로들어오게하는것이다. 이러한전력제어는 DS-CDMA의단점인근원문제를해결하는방법인동시에불필요한전력사용을줄임으로써단말기의배터리사용시간을늘릴수있고시스템의용량도증가시킬수있는장점도된다. 69
Near-Far Problem in CDMA System Near-Far Problem in Other Multiple Access Schemes DS-CDMA에서는동일한주파수대역을동일한시간에서여러사용자가사용함으로써 near-far problem이발생한다. 그러나 TDMA 시스템에서는각사용자가시간대를달리하여동작하므로 near-far problem이발생하지않는다. 또한 FDMA에서는각사용자가다른주파수대역을사용하기때문에대역통과필터로써타사용자간섭신호를제거함으로써 nearfar problem이발생하지않는다. 70