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Trường Đại học Bách Khoa Hà Nội Khoa Điện tử Viễn thông Thông tin di động c om Mobile Communications TS Đỗ Trọng Tuấn Bộ môn Kỹ thuật thông tin co ng Hà Nội, 10-2010 cu u du on g th an Mạng thông tin di động 3G UMTS / W-CDMA (Universal Mobile Telecommunications System) ξ Nguyên lý trải phổ sử dụng mã UMTS 2 CuuDuongThanCong.com https://fb.com/tailieudientucntt Multiple Access Schemes FDMA CDMA TDMA Cod e Tim e … N Frequenc y Frequency Division Multiple Access (FDMA), different frequencies for different users Time Division Multiple Access (TDMA), same frequency but different timeslots for different users, example Global System for Mobile Communication (GSM) GSM also uses FDMA co ng Code Division Multiple Access (CDMA), same frequency and time but users are separated from each other with orthogonal codes du on g th an u Shannon Equation cu example Nordic Mobile Terminal (NMT) systems c om C = B log2 (1 + SNR) B = Bandwidth C= Channel SNR = Signal-to-noise ratio CuuDuongThanCong.com https://fb.com/tailieudientucntt CDMA • Multiple users occupying the same band simultaneously by having different codes is known as Code Division Multiple Access or CDMA • This leads to universal frequency reuse • CDMA is a Spread Spectrum technique • Direct Sequence - CDMA is used in mobile communication Spread Data co ng Code Carrier Modulation c om X Data du on g th an Spread Spectrum u Means that the transmission bandwidth is much larger than the information bandwidth i.e transmitted signal is spread to a wider bandwidth cu Benefits Bandwidth is not dependent on the information signal More secure communication Reduces the impact of interference (and jamming) due to processing gain Classification Direct Sequence (spreading with pseudo noise (PN) sequence) Frequency hopping (rapidly changing frequency) Time Hopping (large frequency, short transmission bursts) Direct Sequence is currently commercially most viable CuuDuongThanCong.com https://fb.com/tailieudientucntt Spread Spectrum Where does spread spectrum come from First publications, late 40s First applications: Military from the 50s Rake receiver patent 1956 Cellular applications proposed late 70s Investigations for cellular use 80s IS-95 standard 1993 (2G) 1997/1998 3G technology choice 2001/2002 Commercial launch of WCDMA technology co ng c om du on g th an Direct Sequence u In direct sequence (DS) user bits are coded with unique binary sequence i.e with spreading/channelization code cu The bits of the channelization code are called chips Chip rate (W) is typically much higher than bit rate (R) Codes need to be in some respect orthogonal to each other Length of a channelization code defines how many chips are used to spread a single information bit and thus determines the end bit rate Shorter code equals to higher bit rate but better Signal to Interference and Noise Ratio (SINR) is required Also the shorter the code, the fewer number of codes are available Different bit rates have different geographical areas covered based on the interference levels CuuDuongThanCong.com https://fb.com/tailieudientucntt Direct Sequence Transmission (Tx) side with DS Information signal is multiplied with channelization code => spread signal Receiving (Rx) side with DS Spread signal is multiplied with channelization code Multiplied signal (spread signal x code) is then integrated (i.e summed together) If the integration results in adequately high (or low) values, the signal is meant for the receiver co ng c om du on g th an cu u Direct Sequence 10 CuuDuongThanCong.com https://fb.com/tailieudientucntt co ng c om Direct Sequence du on g th an 11 Transmitted signal before spreading Power density (Watts/Hz) cu u Processing gain and Spreading Despread narrowband signal Spread wideband signal Received signal before despreading Power density (Watts/Hz) R Frequency W Interference for the part we are interested in Frequency 12 CuuDuongThanCong.com https://fb.com/tailieudientucntt Received signal after despreading but before filtering Power density (Watts/Hz) Processing gain and Spreading Transmitted signal Interference c om Frequency co ng Received signal after despreading and after filtering Power density (Watts/Hz) Frequency du on g th an 13 Processing gain and Spreading u Spread spectrum systems reduce the effect of interference due to processing gain Processing gain is generally defined as follows: cu The number of users takes negative effect on the processing gain The loss is defined as: Lp = 10*log10k, where ’k’ is the amount of users Processing gain when the processing loss is taken into account is G[dB]=10*log10(W/R), where ’W’ is the chip rate and ’R’ is the user bit rate Gtot=10*log10(W/kR) High bit rate means lower processing gain and higher power OR smaller coverage The processing gain is different for different services over 3G mobile network (voice, web browsing, videophone) due to different bit rates Thus, the coverage area and capacity might be different for different services depending on the radio network planning issues 14 CuuDuongThanCong.com https://fb.com/tailieudientucntt Processing gain and Spreading co processing gain 10 log10(3.84e6/12.2e3) = 25 dB For speech service the required SINR is typically in the order of 5.0 dB, so the required wideband signal-tointerference ratio (also called “carrier-to-interference ratio, C/I ) is therefore “5.0 dB minus the processing” = -20.0 dB In other words, the signal power can be 20 dB under the interference or thermal noise power, and the WCDMA receiver can still detect the signal Notice: in GSM, a good quality speech connection requires C/I = 9–12 dB .c om Processing gain is what gives CDMA systems the robustness against self-interference that is necessary in order to reuse the available MHz carrier frequency over geographically close distances Examples: Speech service with a bit rate of 12.2 kbps ng du on g th an 15 cu u Principle of spreading 16 16 CuuDuongThanCong.com https://fb.com/tailieudientucntt ng c om Principle of spreading co 17 du on g th an 17 Trải phổ giải trải phổ cu u Spreading 001 encoder 1-11-1 010 encoder 11-1-1 -1-111 11-1-1 -1-111 110 encoder 1-1-11 1-1-11 1-1-11 -111-1 -11-11 -11-11 1-11-1 -1 -1 -1 1 -3 -1 -1 -1 18 CuuDuongThanCong.com https://fb.com/tailieudientucntt Trải phổ giải trải phổ Spreading 001 encoder 1-11-1 010 encoder 11-1-1 -1-111 11-1-1 -1-111 110 encoder 1-1-11 1-1-11 1-1-11 -111-1 -11-11 -11-11 1-11-1 -1 -1 -1 1 -3 -1 -1 -1 De-Spreading -1-1-13 11-31 -1-13-1 11-1-1 -1 -1 -1 1 -3 -1 -1 -1 -1-1-13 11-31 -1-13-1 -4 -4 001 decoder -4 -4 010 decoder 4 -4 110 co ng 1-1-11 decoder c om -1-1-13 11-31 -1-13-1 1-11-1 du on g th an 19 -1 -1 -1 11 11 -1 -1 -1 -1 11 11 -1 -1 -1 -1 11 11 -1 -1 -1 -1 11 11 -1 -1 -1 -1 11 11 -1 -1 -1 Data1 cu u -1 Code1 -1 1 -1 -1 -1 -1 1 -1 -1 10 -1 -1 -1 Data1*Code1 1 1 1 1 1 1 1 1 1 1 Code2 1 1 -1 -1 -1 1 1 1 1 1 -1 -10 -10-1 1 1 0-1 -1 0 2 0 2 -1 0 1 -1 Data2 Channel Channel Data2*Code2 Received data Rx - Received data ? Rx * Code1 -2 -2 0 -2 -1 -1 0 -1 1 -1 -1 Rx*Code1 Data de-spreaded Data despreaded Rx * Code2 2 0 2 -1 0 1 -1 1 1 1 0 1 RX*Code2 Data de-spreaded Data despreaded 20 CuuDuongThanCong.com https://fb.com/tailieudientucntt -1 -1 -1 11 11 -1 -1 -1 -1 11 11 -1 -1 -1 -1 11 11 -1 -1 -1 -1 11 11 -1 -1 -1 -1 11 11 -1 -1 -1 -1 Data1 -1 1 -1 -1 -1 -1 1 -1 -1 10 -1 -1 -1 1 1 1 1 1 1 1 1 1 1 1 1 -1 -1 -1 Code1 Channel Data1*Code1 Code2 Data2 1 1 1 1 1 -1 -10 -10-1 1 1 0-1 -1 Data2*Code2 -2 0 2 0 2 01 -2 -1 -2 0 21 01 -1 Received data Channel -1 -1 c om Rx * Code1 -2 -2 0 -2 -1 -1 0 -1 1 Rx*Code1 Data de-spreaded Data despreaded Rx * Code2 1 1 0 1 Data despreaded co RX*Code2 Data de-spreaded ng 2 0 2 -1 0 1 -1 du on g th an 21 WCDMA System u WCDMA is the radio interface for UMTS systems 3.84 Mcps (Megachips per second) Maps to MHz due to pulse shaping and small guard bands between the carriers cu Wide bandwidth, Users share the same MHz frequency band and time High bit rates UL and DL have separate MHz frequency bands With Release ’99 theoretically Mbps both UL and DL 384 kbps highest implemented Fast Power Control (PC) => Reduces the impact of channel fading and minimizes the interference 22 CuuDuongThanCong.com https://fb.com/tailieudientucntt co ng c om Codes in UMTS du on g th an 23 Codes in WCDMA u Channelization Codes (=short code) Codes from different branches of the code tree are orthogonal Length is dependent on the spreading factor Used for cu channel separation from the single source in downlink separation of data and control channels from each other in the uplink Same channelization codes in every cell / mobiles and therefore the additional scrambling code is needed Scrambling codes (=long code) Very long (38400 chips = 10 ms =1 radio frame), many codes available Does not spread the signal Uplink: to separate different mobiles Downlink: to separate different cells The correlation between two codes (two mobiles/NodeBs) is low Not fully orthogonal 24 CuuDuongThanCong.com https://fb.com/tailieudientucntt Codes in UMTS Channelization codes separate different connection c om Scrambling codes separate cells/sectors co ng Downlink du on g th an 25 cu u Codes in UMTS Channelization codes separate data/control channels Scrambling codes separate different mobiles Uplink 26 CuuDuongThanCong.com https://fb.com/tailieudientucntt Codes in UMTS Channelization codes separate data/control channels Channelization codes separate different connection Scrambling codes separate cells/sectors c om Scrambling codes separate different mobiles Uplink co ng Downlink du on g th an 27 Channelization Codes (=short codes) Defines how many chips are used to spread a single information bit and thus determines the end bit rate cu u Codes in UMTS Used for: Length is referred as spreading factor Downlink: Separation of downlink connections to different users within one cell Uplink: Separation of data and control channels from same terminal Same channelization codes in every cell / mobiles additional scrambling code is needed 28 CuuDuongThanCong.com https://fb.com/tailieudientucntt Codes in UMTS Scrambling codes (=long codes) Very long (38400 chips), many codes available Does not spread the signal Used for Downlink: The correlation between two codes (two mobiles/NodeBs) is low co ng to separate different mobiles c om Uplink: to separate different cells/sectors Codes used cu u du on g th an 29 30 30 CuuDuongThanCong.com https://fb.com/tailieudientucntt co ng c om Codes and their Use 31 du on g th an 31 Codes in WCDMA u The relation between downlink physical layer bit rates and codes cu DPDCH : Dedicated Physical Data Channel 32 CuuDuongThanCong.com https://fb.com/tailieudientucntt Orthogonal Variable Spreading Factor Codes OVSF code C4(1) 1111 Recursive rule C2(1) 11 C4(2) 11-1-1 C1(1) C1(0) 1 C4(3) c om 1-11-1 C2(2) 1-1 C4(4) 1-1-11 co ng C2N (2i 1) CN (i), CN (i) CN (i) C2N (2i) CN (i), CN (i) du on g th an 33 cu u Channelisation code generation process 34 CuuDuongThanCong.com https://fb.com/tailieudientucntt co ng c om An example of the generation of two channelisation codes du on g th an 35 cu u An example of the generation of two channelisation codes 36 CuuDuongThanCong.com https://fb.com/tailieudientucntt co ng c om The orthogonality property of Walsh codes du on g th an 37 cu u The orthogonality property of different length OVSF codes 38 CuuDuongThanCong.com https://fb.com/tailieudientucntt co ng c om The cross correlation of two different Walsh codes with a non-zero offset du on g th an 39 cu u An example cross correlation function of two eight-chip Walsh codes 40 CuuDuongThanCong.com https://fb.com/tailieudientucntt co ng c om The cross correlation of codes from the same branch of the code tree u du on g th an 41 cu Channelisation code allocation rules 42 CuuDuongThanCong.com https://fb.com/tailieudientucntt An example autocorrelation function of a co ng c om Walsh code du on g th an 43 cu u The autocorrelation correlation properties of scrambling codes 44 CuuDuongThanCong.com https://fb.com/tailieudientucntt co ng c om The cross correlation properties of scrambling codes cu u du on g th an 45 CuuDuongThanCong.com https://fb.com/tailieudientucntt ... Frequenc y Frequency Division Multiple Access (FDMA), different frequencies for different users Time Division Multiple Access (TDMA), same frequency but different timeslots for different users,... gain is different for different services over 3G mobile network (voice, web browsing, videophone) due to different bit rates Thus, the coverage area and capacity might be different for different... for cellular use 80 s IS-95 standard 1993 (2G) 1997/19 98 3G technology choice 2001/2002 Commercial launch of WCDMA technology co ng c om du on g th an Direct Sequence u In direct sequence
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