Spread Spectrum Signal for Digital Communications

Transcription

1 Wireless Information Transmission System Lab. Spread Spectrum Signal for Digital Communications Institute of Communications Engineering National Sun Yat-sen University

2 Multiple Access Schemes Table of Contents Spread Spectrum Communications Generation of Pseudo-Noise (PN) Sequences Elementary Codes in WCDMA

3 Wireless Information Transmission System Lab. Multiple Access Schemes Institute of Communications Engineering National Sun Yat-sen University

4 Multiple Access Schemes Time Division Multiple Access (TDMA) Frequency Division Multiple Access (FDMA) Space Division Multiple Access (SDMA) Code Division Multiple Access (CDMA)

5 Multiple Access -- TDMA Partition the time axis into frame of n slots and assign slots in some fashion require synchronization between users. allow variable rate sources (e.g. assign multiple slots per frame to a user). Time Orthogonality!!

6 Multiple Access -- FDMA Partition the spectrum into a set of bands and assign a band to each user no-need for synchronization in time between users different RF carrier frequencies variable peak power in the total signal inflexible to variable data rate per terminal the idle channel cannot be used by other users to increase or share capacity low complexity to implement Frequency Orthogonality!!

7 FDMA Channels

8 TDMA Channels on Multiple Carrier Frequencies GSM GSM System

9 TDMA with Use of Frequency Hopping Technique Add Add the the frequency diversity by by frequency hopping to to reduce reduce the the frequency-selective interference.

10 Multiple Access -- SDMA Space Division Multiple Access ( SDMA ) serves different users by using spot beam antennas. These different areas covered by the antenna beam may be served by the same frequency ( in a TDMA or CDMA system) or different frequencies ( in an FDMA system ). Use array antenna to separate the simultaneously received signals of spatially separated subscribes by exploiting the directional selectivity of the mobile radio channel. The SDMA technique can be combined with each of the other multiple access techniques ( FDMA, TDMA, CDMA ) to increase the network capacity. 0

11 Multiple Access -- SDMA

12 Block Diagram of Array Signal Processor

13 Adaptive Antenna Array

14 Concentration of Power Density of a Transmitting Antenna

15 Isotropic Radiator An isotropic radiator is an ideal antenna which radiates power with unit gain uniformly in all directions, and is often used to reference antenna gains in wireless systems. Maximum radiated power available from a transmitter in the direction of maximum antenna gain, as compared to an isotropic radiator, is called the effective isotropic radiated power (EIRP): EIRP = P t G t

16 Antenna Gain Pattern Examples of Antenna Gain Pattern: G = sin( θ ) θ G = sin( θ ) θ

17 sin( θ ) /θ sin(x)/x db Angle = Antenna Gain Incident Angle [pi]

18 sin( θ ) /θ sin(x)/x db Angle =

19 ( sin( θ ) / θ ) (sin(x)/x) db Angle = Antenna Gain Incident Angle [pi]

20 ( sin( θ ) / θ ) (sin(x)/x) db Angle =

21 Antenna Array For an M-element linear array, the array pattern is given by: Where G = e j( M ) sin( / ) φ / Mφ sin( φ / ) φ = πd sinθ / λ d : inter - element spacing λ : wavelength θ : incident angle

22 Array Pattern Frequency = e+00 [Hz] M= M= M=0 Antenna Gain Incident Angle [pi]

23 M= M = ; Frequency = e+00 [Hz]

24 M= M = ; Frequency = e+00 [Hz]

25 M=0 M = 0; Frequency = e+00 [Hz]

26 Wireless Information Transmission System Lab. Spread Spectrum Communications Institute of Communications Engineering National Sun Yat-sen University

27 Spread Spectrum Communications Definition: The transmitted signal must occupy a bandwidth which is large than the information bit rate and which is independent of the information bit rate. Characteristics of Spread Spectrum Communications Demodulation must be accomplished, in part, by correlation of the received signal with a replica of the signal used in the transmitter to spread the information signal. Possess pseudo-randomness, which makes the signals appear similar to random noise & difficult to demodulate by receivers other than the intended ones.

28 Spread Spectrum Communications Advantages Jam resistance Low probability of intercept Resistance to multi-path fading Frequency sharing Channel sharing, soft capacity, soft blocking Soft handoff Disadvantages Self-jamming Near-far problem Implementation is more complex

29 Techniques for Spread Spectrum - Direct Sequence Spread Spectrum (DSSS) A carrier is modulated by a digital code in which the code bit rate is much larger than the information signal bit rate. These systems are also called pseudo-noise systems.

30 Techniques for Spread Spectrum - Time-hopped Spread Spectrum (THSS) The transmission time is divided into intervals called frames. Each frame is divided into time slots. During each frame, one and only one time slot is modulated with a message. 0

31 Techniques for Spread Spectrum - Frequency Hopping Spread Spectrum (FHSS) The carrier frequency is shift in discrete increments in a pattern generated by a code sequence. Fast-hop: frequency hopping occurs at a rate that is greater than the message bit rate. Slow-hop: the hop rate is less than the message bit rate.

32 Idealized Model of Baseband Spread- Spectrum System (DSSS System) Transmitter Channel Receiver

33 Waveforms in the Transmitter of DSSS

34 DSSS Technique in the Passband - Coherent Binary Phase-Shift Keying Transmitter

35 DSSS Technique in the Passband - Coherent Binary Phase-Shift Keying Receiver

36 Power Spectral Density

37 Power Spectral Density Relative to Narrow Band Interference (NBI)

38 Power Spectral Density After Despreading

39 Synchronization For proper operation, a spread-spectrum communication system requires that the locally generated PN sequence used in the receiver to despread the received signal be synchronized to the PN sequence used to spread the transmitted signal in the transmitter. A solution to the synchronization problem consists of two parts: acquisition and tracking. In acquisition, or coarse synchronization, the two PN codes are aligned to within a fraction of the chip in as short a time as possible. Once the incoming PN code has been acquired, tracking, or fine synchronization, takes place.

40 Synchronization Typically, PN acquisition proceeds in two steps: The received signal is multiplied by a locally generated PN code to produce a measure of correlation between it and the PH code used in the transmitter. An appropriate decision-rule and search strategy is used to process the measure of correlation so obtained to determine whether the two codes are in synchronism and what to do if they are not. For tracking, it is accomplished using phase-lock techniques, similar to those used for the local generation of coherent carrier references. 0

41 Wireless Information Transmission System Lab. Generation of Pseudo-Noise (PN) Sequences Institute of Communications Engineering National Sun Yat-sen University

42 Contents Hadamard Codes Systematic Linear Binary Block Codes Cyclic Codes Maximum-Length Shift-Register Codes (m-sequence) Preferred Sequences Gold Sequences

43 Hadamard Codes Hadamard code is obtained by selecting the rows of a Hadamard matrix. A Hadamard matrix M n is an n x n matrix that any row differs from any other row in exactly n/ positions. = = = M M M M M M M n n n n n and form a linear binary code of block length. n n M M n min We can generate Hadamard codes with block length, and, where is a positive integer. m m n d n m = = =

44 Example of Hadamard Codes Hadamard Code of Length

45 Correlation of Orthogonal Codes Correlation properties of orthogonal codes are very sensitive to synchronization. Orthogonality of OVSF codes (or Hadamard codes) is achieved when codes are synchronized. Orthogonality of OVSF codes (or Hadamard codes) may not be maintained when codes are not synchronized.

46 Orthogonality is achieved when synchronization is maintained.

47 Orthogonality is maintained when codes are not synchronized

48 Orthogonality isn't maintained when codes are not synchronized

49 Systematic Linear Binary Block Code Any generator matrix G of an (n,k) code can be reduced by row operations (and column permutations) to the systematic form. A linear systematic (n,k) binary block encoder may be implemented by using a k-bit shift register and n-k modulo- adders tied to the appropriate stages of the shift register.

50 0 Systematic Linear Binary Block Code Consider a (,) code with generator matrix: m m m m C C C ] [ Word :C Code P] [I m m m m m m m m m m m m m m m m x x x x x x x x x c c c x x x x G + + = + + = + + = = = =

51 Cyclic Codes Cyclic codes are a subset of the class of linear codes that satisfy the following cyclic shift property: if C=[c n-,c n-,,c,c,c 0 ] is a code word of a cyclic code, then [c n-,c n-,,c,c 0,c n- ], obtained by a cyclic shift of the elements of C, is also a code word. All cyclic shifts of C are code words. Reference: Digital Communications by John G. Proakis, rd edition.. &... or th edition.. &...

52 Pseudo-Noise Sequences A pseudo-noise (PN) sequence is a periodic binary sequence with a noiselike waveform that is usually generated by means of a feedback shift register.

53 Pseudo-Noise Sequences A feedback shift register consists of an ordinary shift register made up of m flip-flop (two-state memory stages) and a logic circuit that are interconnected to form a feedback circuit. With a total number of m flip-flops, the number of possible state of the shift register is at most m. A feedback shift register is said to be linear when the feedback logic consists entirely of modulo- adders. The all-zero state is not permitted. As a result, the period of a PN sequence produced by a linear feedback shift register with m flip-flops can t exceed m -. When the period is exactly m -, the PN sequence is called a maximal-length-sequence or simply m-sequence.

54 Properties of Maximal-Length Sequences In each period of a maximal-length sequence, the number of s is always one more than the number of 0s. Among the runs of s and of 0s in each period of a maximallength sequence, one-half the runs of each kind are of length one, one-fourth are of length two, one-eighth are of length three, and so on as long as these fractions represent meaningful numbers of runs. This property is called the run property. By a run, we mean a subsequence of identical symbols (s or 0s) within one period of the sequence. Autocorrelation property: the autocorrelation function of a maximum-length sequence is periodic and binary-valued.

55 Properties of Maximal-Length Sequences Convert 0 and -. Let c(t) denote the resulting waveform of the maximal-length sequence. The period of the waveform c(t) is: T b =NT c, where T c is the duration assigned to symbol or 0 and N= m -. The autocorrelation function of a periodic signal c(t) of period b T / T b is: ( τ) () ( τ) Rc = c t c t dt T Tb / b

56 Properties of Maximal-Length Sequences The autocorrelation function of the maximal-length sequence is: R c τ ( ) N + τ NTc = N τ T c otherwise

57 Properties of Maximal-Length Sequences Periodicity in the time domain is transformed into uniform sampling in the frequency domain. + N ( ) ( ) n n Sc f = δ f + sinc δ f N N n= N NTc n 0

58 Example of Maximum-Length Shift- Register Codes Systematic Code

59 Maximum-Length Shift Register

60 Maximal Length Shift Register (m=) 0

61 Maximal Length Shift Register (m=)

62 Maximal-Length Sequences of Shift- Register Lengths (-)

63 Maximal-Length Sequences of Shift- Register Lengths (-) Maximum-length shift-register codes exist for any positive value of m.

64 Problems with m-sequence Problems : jammer can determine the feedback connections by observing only m- chips from the PN sequence. Solution : Output sequences from several stages of the shift register or the outputs from several distinct m-sequences are combined in a nonlinear sequence that is considerably more difficult for the jammer to learn. Solution : Frequently changing the feedback connections and/or the number of stages in the shift registers.

65 Problems with m-sequence Problem : periodic cross correlation function between any pair of m-sequences of the same period can have relatively large peaks. Although it is possible to select a small subset of m sequences that have relatively smaller cross correlation peak values, the number of sequences in the set is usually too small for CDMA applications.

66 Correlation Properties of PN sequences Consider two PN sequences of period -=, one feedback shift register has the feedback taps [,] and the other one has the feedback taps [,,,]. Both sequences have the same autocorrelation function.

67 Peak Cross Correlation of m Sequences and Gold Sequences

68 Gold s theorem: Preferred Sequences Gold and Kasami proved that certain pairs of m sequences of length n (e.g. g (X) and g (X) ) exhibit a three-valued cross correlation function with values { -, -t(m), t(m)-} where: tm ( ) ( m+ )/ + odd m = ( m+ )/ + even m Two m sequences of length n with a periodic cross correlation function that takes on the possible values { -, - t(m), t(m)-} are called preferred sequences. The shift register corresponding to the product polynomial g (X) g (X) will generate m + different sequences, with each sequence having a period of m -.

69 Golden Sequences From a pair of preferred sequences, say a=[ a a a a n ] and b=[ b b b b n ], we construct a set of sequences of length n by taking the modulo- sum of a with the n cyclicly shifted versions of b or vice versa. Thus, we obtain n new periodic sequences with period n= m -. Together with the original sequences a and b, we have a total of n+ sequences, which are called Gold sequences. With the exception of the sequences a and b, the set of Gold sequences is not comprised of maximum-length shift-register sequences of length n. The cross correlation function for any pair of sequences from the set of n+ Gold sequences is three-valued with possible values { -, -t(m), t(m)-}. The off-peak autocorrelation function for a Gold sequence takes on values from the set { -, -t(m), t(m)-}.

70 Generator for a Gold Sequence of Period 0

71 Cross-Correlation Function Cross-correlation function of a pair of Gold sequences based on the two PN sequences [,] and [,,,].

72 Wireless Information Transmission System Lab. Elementary Codes in WCDMA Institute of Communications Engineering National Sun Yat-sen University

73 Elementary Codes in WCDMA Scrambling codes Uplink long scrambling codes Uplink short scrambling codes Downlink scrambling codes Channelization Codes Synchronization codes Downlink Synchronization Channel Uplink PRACH Preamble Reference: GPP TS. &.

74 Scrambling Codes Scrambling codes are used to scramble the spread sequences Multiply the spread chip rate More random and orthogonal Uplink Long scrambling codes Short scrambling codes Possibility of multi-user detection at base stations Downlink Long scrambling codes

75 Spreading for Uplink DPCCH and DPDCHs c d, β d DPDCH DPDCH c d, β d Σ I c d, β d DPDCH S dpch,n I+ jq c d, β d S DPDCH c d, β d DPDCH DPDCH c d, β d Σ Q c c β c j DPCCH

76 Configuration of Uplink Scrambling Sequence Generator MSB LSB x c long,,n y c long,,n

77 Uplink Long Scrambling Codes Two elementary codes: c long,,n and c long,,n. c long,,n and c long,,n are constructed from position wise modulo sum of 00 chip segments of two binary m-sequences, x and y. x and y are originated from two generator polynomials of degree. x sequence: generator polynomial: X +X + y sequence: generator polynomial: y +y +y +y+ The sequence c long,,n is a chip shifted version of the sequence c long,,n. c long,,n and c long,,n are Gold codes.

78 For code number, n Uplink Long Scrambling Codes n=[n n 0 ], with n 0 being the LSB Let x n (i)andy(i) denote the i -th chip of the sequence x n and y. Initial conditions x n (0)=n 0, x n ()=n,, x n ()=n, x n ()=n, x n ()= y(0)=y()= =y()= y()=

79 Uplink Long Scrambling Codes Recursive formulation, i=0,, - x n (i+) =x n (i+) + x n (i) modulo y(i+) = y(i+)+y(i+) +y(i+)+y(i) modulo Gold sequence z n z n (i ) = x n (i ) + y (i ) modulo, i = 0,,,, - Z n ( i) + if zn( i) = 0 = for i = 0,,, if zn( i) =.

80 Uplink Long Scrambling Codes c long,,n (i ) = Z n (i ), i = 0,,,, - c long,,n is a chip shifted version of the sequence c long,,n c long,,n (i ) = Z n ((i+ ) modulo ( )), i = 0,,,, - C i i i) = c long,, n( i) + j( ) clong, n( ) long, n(, 0

81 Uplink Short Scrambling Sequence Generator for Chip Sequence 0 d(i) mod mod n addition multiplication mod 0 b(i) mod + z n (i) Mapper c short,,n (i) c short,,n (i) a(i) mod

82 Uplink Short Scrambling Codes Two elementary codes: c short,,n and c short,,n chips Generation From the family of periodically extended S() codes The n:th quaternary S() sequence z n (i ), 0 n, is obtained by modulo addition of three sequences One quaternary sequence a (i ) Two binary sequences b (i ) and d (i )

83 Uplink Short Scrambling Codes z n (i) = a(i) + b(i) + d (i) modulo (i = 0.. ) Given a code number n =[n n n 0 ] quaternary sequence a(i): g 0 (x)= x +x +x +x +x+ Initial conditions a (0) = n 0 + modulo a (i) = n i modulo, i =,,,, Recursive formulation a (i) = a (i-) + a (i-) + a (i-) + a (i-) + a (i-) modulo, i =,,,

84 Uplink Short Scrambling Codes Binary sequence b(i): g (x)= x +x +x +x+ Initial conditions B (i) = n +i modulo, i = 0,,,, Recursive formulation b (i) = b (i-) + b (i-) + b (i-) + b (i-) modulo, i =,,,

85 Uplink Short Scrambling Codes Binary sequence d (i): g (x)= x +x +x +x + Initial conditions d (i) = n +i modulo, i = 0,,, Recursive formulation d (i) = d (i-) + d (i-) + d (i-) + d (i-) modulo, i =,,, z n (i) = a(i) + b(i) + d(i) modulo (i = 0.. )

86 Uplink Short Scrambling Codes z n (i) is extended to length chips z n () = z n (0) Mapping C short, n is C i) = c z n (i) c short,,n (i) c short,,n (i) ( i mod) + j( ) i short, n( short,, n short,, n c i mod

87 Spreading for All Downlink Physical Channels Except Synchronization Channel (SCH) I Any downlink physical channel except SCH S C ch,sf,m I+jQ S dl,n S P Q j

88 Spreading and Modulation for SCH and P- CCPCH Different downlink Physical channels (point S in Figure of previous page.) G G Σ P-SCH Σ G P S-SCH G S

89 Downlink Scrambling Codes codes are chosen from a total of - scrambling codes, numbered 0,, These chosen scrambling codes are divided into sets, each set has One primary scrambling code Code number, n=*i (i=0 ) secondary scrambling codes Code number, n=*i+k (k= )

90 Downlink Scrambling Codes primary scrambling codes Further divided into scrambling code groups Each group consisting of primary scrambling codes The j:th scrambling code group consists of primary scrambling codes **j+*k (j=0.. & k=0..) Each cell is allocated one and only one primary scrambling code. The primary CCPCH, primary CPICH, PICH, AICH, AP- AICH, CD/CA-ICH, CSICH and S-CCPCH carrying PCH are always transmitted using the primary scrambling code. The other downlink physical channels can be transmitted with either the primary scrambling code or a secondary scrambling code from the set associated with the primary scrambling code of the cell. 0

91 Configuration of Downlink Scrambling Code Generator 0 0 I Q 0 0

92 Downlink Scrambling Codes Constructed by combining two real sequences Each is constructed as the position wise modulo sum of two binary m-sequences, x and y Generator polynomials is of degree 00 chip segments (0 ms radio frame) Gold sequences x sequence: generator polynomial +X +X Initial: x (0)=, x()= x()=...= x ()= x ()=0 x(i+) =x(i+) + x(i) modulo, i=0,, -0, y sequence: generator polynomial +y +y + y 0 +y Initial: y(0)=y()= =y()= y()= y(i+) = y(i+0)+y(i+)+y(i+)+y(i) modulo, i=0,, -0

93 Downlink Scrambling Codes The nth Gold code sequence z n is z n (i) = x((i+n) modulo ( - )) + y(i) modulo, i=0,, - Mapping Z n ( i) + if z n ( i) = 0 = for i = 0,,, if z n ( i) = The n:th complex scrambling code sequence S dl,n is defined as: S dl,n (i) = Z n (i) + j Z n ((i+0) modulo ( -)), i=0,,,..

94 Channelization Codes Each CDMA channel is distinguished via a unique spreading code. These spreading codes should have low crosscorrelation values. In GPP W-CDMA, Orthogonal Variable SF (OVSF) codes are used. Preserve the orthogonality between a user s different physical channels. Scrambling is used on top of spreading.

95 Code-tree for Generation of Orthogonal Variable Spreading Factor (OVSF) Codes C ch,,0 =(,,,) C ch,,0 = (,) C ch,, = (,,-,-) C ch,,0 = () C ch,, = (,-,,-) C ch,, = (,-) C ch,, = (,-,-,) SF = SF = SF = The channelization codes are uniquely described as C ch,sf,k, where SF is the spreading factor of the code and k is the code number, 0 k SF-.

96 Channelization Codes As the chip rate is already achieved in the spreading by the channelization codes, the symbol rate is not affected by the scrambling. Another physical channel may use a certain code in the tree if no other physical channel to be transmitted using the same code three is using a code that is on an underlying branch, i.e. using a higher spreading factor code generated from the intended spreading code to be used. Neither can a smaller spreading factor code on the path to the root of the tree be used.

97 Channelization Codes The downlink orthogonal codes within each base station are managed by the radio network controller (RNC) in the network. The definition for the same code tree means that for transmission from a single source, from either a terminal or a base station. One code tree is used with one scrambling code on top of the tree. Different terminals and different base stations may operate their code trees independently of each other.

98 Generation of Channelization Codes C ch,,0 = = =,,0,,0,,0,,0,,,,0 ch ch ch ch ch ch C C C C C C ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) = ,,,,,,,,,,,,,,,,,0,,0,,0,,0,,,,,,,,,,,,0, : : : n n ch n n ch n n ch n n ch n ch n ch n ch n ch n ch n ch n ch n ch n n ch n n ch n ch n ch n ch n ch C C C C C C C C C C C C C C C C C C

99 Synchronization The receiver requires a replica of the PN code with a correct clock phase in order to de-spread the signal. Applications in downlink synchronization Initial Cell Search (power on) Capture the slot & frame timing of the chosen BS and establish links. Target Cell Search (handoff) To re-synchronize to a new BS. In WCDMA, there is no global synchronization between BSs. Application in uplink synchronization PRACH preamble code detection.

100 Downlink Synchronization Base stations always transmit PN codes in synchronization channel - T c Synchronization code MS is synchronized to BS with the aid of synch. code In GPP, a synchronization code of chips is used. 00

101 Structure of Synchronisation Channel (SCH) Secondary SCH ac s i,0 Slot #0 Slot # Slot # Primary SCH ac p ac p ac p chips 0 chips ac s i, One 0 ms SCH radio frame ac s i, 0

102 Cell Search Procedures During the cell search, the UE searches for a cell and determines the downlink scrambling code and frame synchronisation of that cell. The cell search is typically carried out in three steps:. Slot synchronization. Frame synchronization/code-group identification. Scrambling-code identification 0

103 Slot Synchronization During the first step of the cell search procedure the UE uses the SCH s primary synchronisation code to acquire slot synchronisation to a cell. In GPP W-CDMA, Primary SCH is the same for every BS At each slot beginning, P-SCH codes of are transmitted and then turned off until the next slot. A matched filter could be used to match the code such that the strongest path from a certain BS could be captured. Due to path propagation loss, the strongest path usually comes from the nearest BS. 0

104 Slot Synchronization To reduce the noise effect, slot-wise accumulator could be used for accumulating the output power in several slots. Slot timing is then acquired by finding the maximum peak of accumulator output within one observation interval. One observation interval = slot period 0

105 Slot Synchronization Matched filter (c p ) Slot-wise accumulation Find maximum Timing modulo T slot T slot Two rays from BS i One ray from BS j Multipath channel!! 0

106 Primary Synchronization Codes Primary SYN code (PSC), C psc Constructed as a so called generalised hierarchical Golay sequence. Good aperiodic auto-correlation properties. Define a = <x, x, x,, x > = <,,,,,, -, -,, -,, -,, -, -, > PSC, C psc = ( + j) <a, a, a, -a, -a, a, -a, -a, a, a, a, -a, a, -a, a, a>, The leftmost chip in the sequence corresponds to the chip transmitted first in time 0

107 Frame Synchronization and Code Group Identification During the second step of the cell search procedure, the UE uses the SCH s secondary synchronisation code to find frame synchronisation and identify the code group of the cell found in the first step. This is done by correlating the received signal with all possible secondary synchronisation code sequences, and identifying the maximum correlation value. 0

108 Secondary Synchronization Codes codes: {C ssc,,,c ssc, } Complex-valued with identical real and imaginary components. Constructed from position wise multiplication of a Hadamard sequence and a sequence z The Hadamard sequences are obtained as the rows in a matrix H constructed recursively by: H H 0 k = () H = H k k H H k k k 0 Rows are numbered from the top (row): the all ones sequence.

109 Secondary Synchronization Codes z = <b, b, b, -b, b, b, -b, -b, b, -b, b, -b, -b, -b, -b, -b> b = <x, x, x, x, x, x, x, x, -x, -x 0, -x, -x, -x, -x, -x, -x > The k-th SSC, C ssc,k, k =,,,, C ssc,k = ( + j) <h m (0) z(0), h m () z(), h m () z(),, h m () z()> m = (k ) C ssc,k (0) is the first transmitted chip in time. 0

110 Code Allocation of SSCs The secondary SCH sequences are constructed such that their cyclic-shifts are unique. A non-zero cyclic shift less than of any of the sequences is not equivalent to some cyclic shift of any other of the sequences. Also, a non-zero cyclic shift less than of any of the sequences is not equivalent to itself with any other cyclic shift less than. 0

111 Allocation of SSCs for secondary SCH Group Group 0 Group Group 0 Group Group Group Group Group 0 0 Group 0 Group 0 Group 0 0 Group Group 0 Group 0 Group 0 Group 0 0 Group 0 0 Group Group 0 Group Group Group 0 0 Group 0 Group 0 0 Group Group Group Group Group 0 0 Group 0 0 Group 0 # # # # #0 # # # # # # # # # #0 slot number Scrambling Code Group

112 Allocation of SSCs for secondary SCH 0 0 Group 0 Group 0 0 Group Group 0 0 Group 0 0 Group 0 0 Group 0 0 Group Group 0 0 Group Group 0 0 Group Group 0 0 Group 0 Group Group Group Group 0 0 Group Group 0 0 Group 0 0 Group Group 0 0 Group 0 Group Group 0 0 Group 0 0 Group Group Group Group 0 0 Group # # # # #0 # # # # # # # # # #0 slot number Scrambling Code Group

113 Frame Synchronization and Code Group Identification After the receiver knows the slot timing in the first step of cell search, the receiver can correlate the received signal with matched correlators. Each matched to Secondary Synchronization codes. However, the receiver has no knowledge about the slot position in the frame and in what group.

114 Frame Synchronization and Code Group Identification Solution: Exhaustive search for the slot position and the sequence of S-SCH codes. Accumulate these output samples extensively slot-byslot over all possible 0 combinations. sequences and slot positions. Choose one from 0 candidates to determine the code group and frame timing of the chosen BS. Since the cyclic shifts of the sequences are unique, the code group as well as the frame synchronisation is determined.

115 Scrambling-code Identification During the third and last step of the cell search procedure, the UE determines the exact primary scrambling code used by the found cell. The primary scrambling code is typically identified through symbol-by-symbol correlation over the CPICH with all codes within the code group identified in the second step. Recall that scrambling codes are in the derived code group. After the primary scrambling code has been identified, the Primary CCPCH can be detected. And the systemand cell specific BCH information can be read.

116 Uplink PRACH Preamble Detection The purpose of PRACH detection is to provide Node B the information of incoming requests, as well as the reference timing for delay estimation. The random-access transmission is based on a Slotted ALOHA approach. The UE can start the random-access transmission at the beginning of a number of well-defined time intervals, denoted access slots. There are access slots per two frames and they are spaced 0 chips apart.

117 Uplink RACH Access Slot Numbers and Their Spacing radio frame: 0 ms radio frame: 0 ms 0 chips Access slot #0 # # # # # # # # # #0 # # # # Random Access Transmission Random Access Transmission Random Access Transmission Random Access Transmission

118 Structure of the Random-Access Transmission The random-access transmission consists of one or several preambles of length 0 chips and a message of length 0 ms or 0 ms. Preamble Preamble Preamble Message part 0 chips 0 ms (one radio frame) Preamble Preamble Preamble Message part 0 chips 0 ms (two radio frames)

119 RACH Preamble Code Construction Each preamble is of length 0 chips and consists of repetitions of a signature of length chips. There are a maximum of available signatures. The random access preamble code C pre,n, is a complex valued sequence. It is built from a preamble scrambling code S r-pre,n and a preamble signature C sig,s as follows: C pre, n, s ( k) = S r pre, n ( k) C sig, s ( k) e π π j( + k ) where k=0 corresponds to the chip transmitted first in time., k = 0,,,,0

120 PRACH Preamble Scrambling Code The scrambling code for the PRACH preamble part is constructed from the long scrambling sequences. There are PRACH preamble scrambling codes in total. The n-th preamble scrambling code, n = 0,,,, is defined as: S r-pre,n (i) = c long,,n (i), i = 0,,, 0; 0

121 PRACH Preamble Scrambling Code The PRACH preamble scrambling codes are divided into groups with codes in each group. There is a one-to-one correspondence between the group of PRACH preamble scrambling codes in a cell and the primary scrambling code used in the downlink of the cell. The k-th PRACH preamble scrambling code within the cell with downlink primary scrambling code m, k = 0,,,, and m = 0,,,,, is S r-pre,n (i) as defined above with n = m + k.

122 PRACH Preamble Signature The preamble signature corresponding to a signature s consists of repetitions of a length signature P s (n), n=0. This is defined as follows: C sig,s (i) = P s (i modulo ), i = 0,,, 0. The signature P s (n) is from the set of Hadamard codes of length.

123 PRACH Preamble Signatures Preamble Signature 0 Value of n 0 P 0 (n) P (n) P (n) P (n) P (n) P (n) P (n) P (n) P (n) P (n) P 0 (n) P (n) P (n) P (n) P (n) P (n)