User equipment for receiving and transmitting communication signals

This invention generally relates to spread spectrum Time Division Duplex (TDD) communication systems using Code Division Multiple Access (CDMA). More particularly, the present invention relates to cell search procedure of User Equipmenti(UE) within TDD/CDMA commtinication systems.

Figure 1 depicts a wireless spread spectrum TDD/CDMA communication system. The system has a plurality of.base stations 30! to 30₇. Each base station 30! has ah associated cell 34! to 34₇ and communicates with user equipments (UEs) 32! to 32₃ in its cell 34!.

In addition to communicating over. different frequency spectrums, TDD/CDMA systems cany multiple communications over the same spectrum״ . The multiple signals are distinguished by their respective code sequences (codes). Also, to more efficiently use the spectrum, TDD/CDMA systems as illustrated in Figure ² use repeating frames 38 divided' into a number of time slots 36! to 36_n>, such as sixteen time slots 0 to 15. In such systems, a communication is sent in selected time slots 36! to 36_n using selected codes. Accordingly, one frame 38 is capable of carrying multiple communications distinguished by both time slot 36! to 36_a and code.

For a UE 32! to communicate with a base station 30!, time and code synchronization is required. Figure 3 is a flow chart of the cell search and synchronization process. Initially, the UE 32! must determine which base station 30! to 30₇ and cell 34! to 34₇ to communicate. In a TDD/CDMA system, all the base stations 30! to 30₇ are time synchronized within a base station cluster. For synchronization with UEs 32! to 32₇, each base station 30! to 30₇ sends a Primary ־ Synchronization Code (PSC) and several Secondary Synchronization Code (SSC) signals in the time slot dedicated for synchronization. The PSC signal has an associated chip code, such as an unmodulated 256 hierarchical code, and is transmitted in the dedicated time slot, step 46. To illustrate, a base station 30! may transmit in one or two time slots, such as for a system using time slots 0 to 15 in time slot K or slot K+8, where K is either 0,.., 7.

One technique used to generate a PSC signal is to use two 16 hierarchical sequences, such as XI and X2 in Equations 1 and 2.

¹⁵ XI = [1,1,-1,-1,1,-1,1,-1,-1,-1,-1,-1,1,1,1,-1] Equation!

X2 = [l, 1,-1,-1,-1,-1, 1,-1,1, 1,-1,1,1, 1,-1,1] Equation!

Equation 3 illustrates one approach to generate a 256 hierarchal code, y(i), using XI 20 andX2.

y(i) = XI (i mod 16) x X2 (i div 16), where i = 0,.., 255 Equation 3

Using y(i), the PSC is generated such as by combining y(i) with the first row of length 256 Hadamarad matrix, h₀, to produce C_p(i) as in Equation 4.

C_p(i) = y(i) x h₀(i), where i = 0,.. 255 Equation 4

י ' 5

Since the first row of the Hadamarad matrix is an all one sequence, Equation 4 reduces to Equation 5.

C_p(i) = y(i), where i = 0,.., 255 Equation 5

The C_p(i) is used to produce a spread spectrum PSC signal suitable for transmission.

To prevent the base stations' communications from interfering with each other, each base station 30! to 30₇ sends its PSC signal with a unique time offset, t_offset, from the time slot boundary 40. Differing time offsets are shown for time slot

42 in Figure 4. To illustrate, a-first base station 30! has a first time offset 44!, t_offset11 for the PSC signal, and a second base station 30₂, has a second time offset 44₂, t_offset12.

To differentiate the different base stations 30! to 30₇ and cells 34! to 34₇, each base station 30! to 30₇ within the cluster is assigned a different group of codes (code group). One approach for assigning a t_offset for a base station using an n* code group

44_n, t_offsew is Equation 6.

Ut,״ = ⁿ71־T_c Equation 6

T_c is the chip duration and each slot has a duration of 2560 chips. As a result, the offset 42_n for each sequential code group is spaced 71 chips.

Since initially, the UE 32! and the base stations 30! to 30₇ are not time synchronized, the UE 32! searches through every chip in the frame 38 for PSC 5 signals. To accomplish this search, received signals are inputted to a'matched filter which is matched to the PSC signal's chip code. The PSC matched filter is used to search through all the chips of a frame to identify the PSC signal of the base station 30! having.the strongest signal. This process is referred to as step-1 of cell search procedure.

After the UE 32! identifies the PSC signal of .the strongest base station 30!, the UE 32! needs to determine the time slot 36! to 36״ in which that PSC and SSC signals are transmitted (referred to as the Physical Synchronization Channel (PSCH) time slot) and the code group used by the identified base station 30!. This process is referred to as step-2 of cell search procedure. T0 indicate the code group assigned 15 to the base station 30! and the PSCH time slot index, the base station 30! transmits signals having selected secondary synchronization codes (SSCs), step 48. The UE 32! receives these SSC signals, step 50, and identifies the base station's code group and PSCH time slot index based on which SSCs were received, step 52.

For a TDD system using 32 code groups and two possible PSCH time slots 20 per frame, such as time slots K and K+8, one approach to identify the code group and PSCH time slot index is to send a signal having one of 64 SSCs. Each of the synchronization codes corresponds to one of the 32 code groups and two possible .21-0572001.. י.' , . US 00001433£ φ . PSCH time slots. This approach adds complexity at theUE 32! requiring at least 64 φ watched filters and extensive processing. Toidentify thecodc group andPSCH time slot index 17 344 real additions and 128 real multiplications are required in each .V׳ .N-^:. :NyiU. . . ’ . י ' . . ; <!. .

I

PSCH time slot and 64 real additions are required for tiie decision.

An alternative approach for stop-2 of cell search procedure uses 17 SSCs.־

These 17 SSCs are used to index the 32 code groups and two possible PSCH time slots per frame. To implement this approach, at least 17 matched filters are require!

To identify the code group and time slot, 1,361 real additions and. 34 real multiplications are required for each PSCH time. slot. Additionally, 512 real additions are required for the decision.

WO 99/12273 discloses a system for synchronizing to a base station. A base ' station transmission is divided into time slots. Each time slot includes a primary synchronization code and a secondary synchronization code including'both framing synchronization and scrambling or long code information.

TR 101 146 Universal Mobile Telecommunications System. 30.06 version !£!. ?«.יי:; ;.;iן j pii;. : ' . 1 ..'.»ן..' ' 3.0.0 discloses a base station synchronization system. A primary synchronization code is transmitted for a frame and a phase reference. Each of 16 possible base station code groups are assigned to a unique secondary synchronization code. The secondary synchronization code transmitted by the base station identifies the code ί7.!ί.^:;.??:׳. : ;;i ! ,. I יי . . . , . ' . .

!‘׳ ־ .י. group of the base station. ‘

Higuchi et al., ^,'Fast Cell Search AlgorithminDS-CDMA Mobile Radio using

Long Spreading Codes, discloses a system for assigning long spreading codes to a

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, ' AMENDED SHEET

15:22• 6s1 eit 21Mai ח Empfa .' ._; ., ' י ־ .: ._. . U0 υυυυ14־ΰ39 *

cell A control channel is spread by a combination of cell site - unique long code and a short code common to all cell sites. Each cell’s transmitted short code has a long ;h? ... יד ’ ' י י: ־. ^; :

code group identifier code to identify the long code.

ז

It would be desirable to reduce the complexity required by a UE 32! to perform cell search procedure.

SUMMARY

Abase station sends a synchronizationsignalin an assigned time slot to auser equipment in a time division duplex code division multiple access communication system. The base station has an assigned code group out of a predetermined number ip? ,. ; י : .;ן; . .. . ׳

Of code groups. The base station'transmits selected secondary synchronization code signals out of a set of secondary synchronization code signals. The plurality of secondary synchronization code signals numbers less than half of the predetermined number of code groups.’The user equipment identifies the transmitted selected

Enpfangszei t 21.Mai. 15:22

AMENDED SHEET asecondary code signals. Based on in part the identified secondary synchronization code signals, the assigned code group is determined.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a prior art TDD/CDMA system.

Figure 2 illustrates time slots in repeating frames of a TDD/CDMA system.

Figure 3 is a flow chart of cell search.

Figure 4 illustrates time offsets used by differing base stations sending primary synchronization code signals.

Figure 5 is a diagram of the simplified components of a user equipment and a base station using binary phase shift keying modulation for cell search.

Figure 6 is a flow chart of secondary synchronization code assignment.

Figure 7 illustrates the simplified components of a user equipment and a base station using quadrature phase shift keying modulation for cell search.

Figure 8 illustrates the simplified components of a user equipment and a base station reducing the maximum number of transmitted secondary synchronization codes using quadrature phase shift keying modulation.

Figures 9 to 17 are graphs depicting the performance of various synchronization systems under varying simulated channel conditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments will be described with reference to the drawing figures where like numerals represent like elements throughout. Figure 5 shows the simplified circuitry of a base station 30! and a UE 32! for use in cell search. During step-1 of the cell search, the base station 30! generates a PSC signal using a PSC spread spectrum signal generator 66 having the time offset in the time slot 42 associated with the base station 30!. The PSC signal is combined by a combiner 63 with M SSC signals. The combined signal is modulated by a modulator 62 to carrier frequency. The modulated signal passes through an isolator 60 and is radiated by an 10 antenna 58 or, alternately, an antenna array.

The UE 32! receives signals using an antenna 70 or, alternately, an antenna array. The received signals are passed through an isolator 72 where they are demodulated by a demodulator 74 to baseband frequency. During step-1 of the cell search, the PSC matched filter 76 is used by the processor 80 to search through all 15 the chips of a frame 38 to identify the PSC signal of the base station 30! having the strongest signal.

One approach for detection of a PSC signal location in a frame is as follows. A selected number of positions in the received signal frame, such as forty, having the highest number of accumulated chip matches (i.e. maximum signal strength), are 20 repeatedly correlated at the same positions in subsequent frames 38. Out of the selected locations, the one having the highest number of cumulative matches (i.e. the maximum signal strength) is identified as the location of the PSC signal.

For step-2 of the cell search procedure, the base station 30! generates SSC signals, SSC! to SSC_M, using SSC spread spectrum signal generators 68! to 68_M. To reduce the complexity at the UE 32!, a reduced number of SSCs are used. By reducing the SSCs, the number of matched filters required at the UE 32! is reduced.

Additionally, the reduced SSCs decreases the processing resources required to distinguish the different codes. The reduced SSCs also reduces the probability of י incorrect detection of a code group number and PS CH time slot index (see Figures

9-15). _ :

One approach to reduce the SSCs is shown in the flow chart of Figure 6. The 10 number of SSCs used, M, is based on the number of code groups and PSCH time slots used per frame, step 54. The number of SSCs, M, is the log base two of the : maximum combination number rounded up to the next higher integer, step 56, as in

Equation?.

M = 10g₂ of Code Groups x# of PSCH Time Slots per frame)

Equation ר

The base station 30! generates, using SSC signal generators 68! to 68_M, the SSC signals associated with the base station's code group and the number of PSCH time 20 slots per frame. The SSC signals are combined with each other as well as the PSC signal by combiner 63. Subsequently, the combined signal is modulated by the modulator 62, passed through the isolator 60 and radiated by the antenna 58. The

UE 32_ג receives the transmitted signal, passes it through the isolator 72 and demodulates the received signal using the demodulator 74. Using corresponding SSC! to SSC_M matched filters 78! to 78_M, the processor 80 determines the binary code that SSCs are modulated. Based on the determined binary code, the base station's code group and PSCH time slot index in the frame is determined. To illustrate for a system using 32 code groups and two possible time slots per frame, ' such as slots K and K+8, the number of binary bits needed to modulate SSCs, M, is six (10g₂ 64). In such a system, the six SSCs are modulated with six bits using binary phase shift keying (BPSK) modulation. The six SSCs are chosen among the 256 rows of Hadamarak matrix, H₈. The Hadamarak matrix is generated sequentially, such as by Equations 8 and 9.

Ho = (1)

Equation 8

Equation 9

A particular code, C_{k n}(i), where n is the code group number associated with a SSC is produced using Equation 10. The six rows of Hadamarak matrix; H₈, are r(k) = [24,40, 56,104, 120, 136].

C_kn(i) = b_k,n x h_r(k)(i) x y(i), where i = 0,1,.., 255 and k = 1,.., 6

Equation 10

The value of b₂ to b₆ are depicted in Table 1.

<td> Code Group (n)</td><td>^b6,״</td><td></td><td>^b4,n</td><td>^b3,n</td><td> .Art</td>

<td> 1</td><td> +1</td><td> +1</td><td> +1</td><td> +1</td><td> +1</td>

<td> 2</td><td> .+1</td><td> +1</td><td> +1</td><td> +1</td><td></td>

<td> 3</td><td> +1</td><td> +1</td><td> +1</td><td></td><td> +1</td>

<td></td><td></td><td> ..</td><td> ..</td><td> ..</td><td> .״</td>

<td> 32</td><td> _L_</td><td></td><td></td><td></td><td> . -1</td>

Table 1

The value of b_u is depicted in Table 2.

<td> PSCH time slot order in the frame</td><td>^bl,n</td>

<td> K, where K = 0,-,7</td><td> +1 .</td>

<td> K + 8__</td><td></td>

Table 2

Each code corresponds to one SSC, SSC, to SSC₆. To distinguish the differing base stations' SSC signals from one another, eachof the base stations' SSC signals has the same offset as its PSC signal. At the UE 32״ the step-2 of the cell search procedure (i.e. code group number and PSCH slot order detection) is performed as follows, The received baseband signal is first correlated with C_p as per Equation 4 to obtain phase reference. This correlation is performed by PSC matched filter 76 m Figure

5. The phase reference is obtained by normalizing the correlation value obtained at the output of the PSC matched filter 76. The received baseband signal is also correlated with Cl,..,C6 as per Equation 10 to obtain binary data that represent the code group of the base station 30! and PSCH slot, order in the frame. This correlation is performed by SSC matched filters 78!-78_M in Figure 5. These matched filter outputs are derotated before BPSK demodulation. The derotation is 5 performed by complex multiplication of the complex conjugate’of the phase reference. The derptated SSC matched filter outputs are BPSK demodulated. The BPSK demodulation is performed by a hard limiter on the real part of the derotated SSC matched filter outputs. As a result, if the real part of the derotated SSC matched filter output is greater than zero, it is demodulated as +1. Otherwise, it is 10 demodulated as -1. The demodulated binary data represents the code group of the base station 30! and the PSCH time slot order in. the frame as depicted in Table 1 and Table 2, respectively. To ease detection of the six SSCs, the UE 32_taccumulates the derotated outputs of the SSC matched filters 78!78־_M over anumber of the PSCH time slots, such as four or eight.

Using six SSCs, for 32 code groups and two possible PSCH time slots, requires 653 real additions and 28 real multiplications at the UE 32! to identify the code group/PSCH time slot index. For the decision, no additions or multiplications are required. Accordingly, reducing the number of transmitted SSCs in the PSCH time slot reduces the processing at the UE 32!.

Alternately, to reduce the number of SSCs even further quadrature phase shift keying (QPSK) modulation is used. To reduce the SSC number, each SSC signal is sent on either an In-phase (I) or Quadrature (Q) component of the PSCH. One extra

I bit of data associated with either using the I or Q carrier is used to distinguish the code group/PSCH time slots. As a result, the number of SSCs, M, required by Equation 6 is reduced by one.

For instance, to distinguish 32 code groups and two possible PSCH time slots, five SSCs (M = 5) are required. The code groups are divided in half (code groups

1-16 and code groups 17-32). When the SSCs are transmitted on the I carrier, it restricts the code groups to the lower half (code groups 1-16) and when the SSCs are transmitted on the Q carrier, it restricts the code groups to the upper half (code groups 17-32). The five SSCs distinguish between the remaining sixteen possible code groups and two possible PSCH time slots.

A simplified base station 30! and UE 32! using QPSK modulation are shown in Figure 7. The base station 30! generates the appropriate SSC signals for its code group and PSCH time slot using the SSC spread spectrum signal generators 68! to 68_M. Also based on the base station's code group/PSCH time slot index, switches 90!

to 90_M either switch the outputs of the generators 68! to 68_M to anj combiner 86 or . a Q combiner 88. The combined I signal which includes the PSC signal is modulated by ah I modulator 82 prior to transmission. The combined Q signal is modulated by a Q modulator 84 prior to transmission. One approach to produce the Q carrier for modulating the signal is to delay the I carrier by ninety degrees by a . delay device 98. The UE 32! demodulates the received signals with both an I demodulator 92 and a Q demodulator 94. Similar to the base station 30_υ the UE 32! may produce a Q carrier for demodulation using a delay device 96. Obtaining binary data representing the lower or higher half of the 16 code groups and PSCH time slot index is the same as applying BPSK demodulation on the I and Q components of the received signal respectively. The I matched filters 100! to 100_M are used by the processor 80 to determine whether any SSC. signals were sent on the I component of the PSCH. A decision variable, I_dvar, is obtained such as by using Equation 11.

־ I_dvar = |rX1l + |rX2l + - ⁺ W Equation 11 |rXi| is the magnitude of the real component (I component) of the 1^th SSC matched filter output. Likewise, the Q matched filters 102! to 102_M are used by the processor 80 to determine whether any SSC signals were sent on the Q component ; of the PSCH. A decision variable, Q_dvaf, is obtained such as by using Equation 12.

Q_dvar = |ix₁| + N + - + W Equation 12

- _ |ix!| is the magnitude of the imaginary (Q component) of the 1^th SSC matched filter outputs.

If I_dvar is greater than Q_dvar, the SSC .signals were transmitted on the I component. Otherwise, the SSC signals were transmitted on the Q component.

Another approach using QPSK modulation to reduce the number of SSC signals transmitted is depicted in Figure 8. Instead of transmitting the number of SSCs of Figure 7, the number of SSCs, M, representing the code group number and

PSCH time slot index is reduced by one. To regain the one lost bit of information by reducing the SSCs, two sets of M SSCs are used. For instance using 32 code groups and two possible PSCH time slots* one set, SSC״ to SSC₁₄, is assigned to the lower code groups, such as code groups 1 to 16, and the second set, SSC₂₁ to SSC₂₄, 5 is assigned to the upper code groups, such as code groups 17 to 32.־ For the lower code group, sending SSC״ to SSC₁₄ on the I carrier restricts the code groups to 1 to

8. The Q carrier restricts the code groups to 9 to 16. Likewise, for the upper code group, in phase SSC₂₁ to SSC₂₄ restricts the code groups to 17 to 24 and Q,SSC_2l to SSC₂₄ restricts the code groups to 25 to 32. As a result, the maximum number of 10 SSCs transmitted at one time is reduced by one. By reducing the number of SSCs, the interference between SSC signals is reduced. Reduced interference between SSCs allows higher transmission power levels for each SSC signal easing detection at the UE 32_v

A simplified base station 30_x and UE 32_x implementing the reduced SSC 15 approach is shown in Figure 8. At the base station 30_1י two sets of M SSC spread spectrum signal.generators 104״ to 104_2M generate the SSC signals corresponding to the base station's code group and PSC time slot. The corresponding SSC signals are switched using switches 106״ to 106_2M to either an 182 or Q modulator 84 as appropriate for that base station's code group and PSCH time slot. At the UE 32_1י20 an I set of matched filters 108״ to 108_2Q is used to determine if any of the SSCs were sent on the I carrier. A Q set of matched filters 110_״ to 110_2M is used to determine if any of the SSCs were sent on the Q carrier. By detecting the transmitted I and Q

SSCs, the processor 80 determines the base station's code group and PSCH time slot.

One approach to determining which of 32 code groups and two possible PSCH time slots is used by the base station 32! follows. After the processor 80 5 accumulates data from matched filters 110_u to 110_M, the code group set, either SSC!! to SSC_I4 or SSC₂! to SSC₂₄, is determined using Equations 13 and 14.

var_setl = |rx״| + |i x״| +..+ |r x״| + |i x״| Equation 13 var_set 2 = |r x״| + |i x2־| +..+ |r x״|. + |i X24| _ , Equation 14

The values, rx_״ to rx₂₄, are the number of accumulated matches for a respective SSC, SSC_U to SSC₂₄, received in the I channel. Similarly, ix_n to ix₂₄ are the number of accumulated matches for the Q channel for SSC_״ to SSC₂₄. Equations 13 and 14 15 require a total of 16 real additions. var_set 1 represents the total accumulations of the first SSC set, SSC!! to SSC₁₄i var_set 2 represents the total accumulations of the second SSC set, SSC₂₁ to SSC₂₄. The processor 80 compares var_set 1 to var_set 2 and the larger of the two variables is presumed to be the SSC set transmitted by the base station 32!.

To determine whether the SSCs were transmitted on the I or Q channel,

Equations 15 and 16 are used.

var_ Γ = |r x_P1|+..+ |r x_P4| Equation 15 var_Q = |i x_P!|+..+|i x_P&lt;| Equation 16

If var_set 1 is selected as being larger than var_set 2, the value of p is one.

. Conversely, if var_set 2 is larger, the value of p is.two. var_I is the accumulated values for the selected set on the I carrier and var_Q is the accumulated values on the Q carrier. The larger of the two variables, var_I and var_Q, is presumed to be the' channel that the selected set was transmitted over. By ordering the additions in Equations 13 and 14, the values of var_I and var_Q can be determined simultaneously with var_set 1 and var_set 2. Accordingly; determining whether the l or Q carrier was Used requires no additional additions. As a result, using QPSK modulation and two SSC sets requires 803 real additions and 36 real multiplications in each time slot and 16 real additions for the decision.

Figures 9 to 15 are graphs illustrating the performance for distinguishing 32 code groups/two PSCH time slots of systems using 32 SSCs 128,17 SSCs 124 and

SSCs 126. The graphs show the performance for various simulated channel conditions. The simulations accumulated the SSC matches at the UE 32! over four or eight PSCH time slots and compared the probability of an incoaect synchronization to the channel's signal to noise ratio (SNR) in decibels.

The Figure 9 simulation uses an additive white gaussian noise (AWGN) channel and accumulation over eight PSCH time slots. The Figure 10 simulation uses a single path Rayleigh fading channel with a six kilohertz (kHz) frequency offset and accumulation over four PSCH time slots. The Figure 11 simulation is the same as the Figure 10 simulation except the accumulation was over eight PSCH time slots. The Figure 12 simulation uses an ITU channel with three multipaths with a UE 32_x moving at 100 kilometers per hour (km/h) and accumulation over eight PSCH time slots. The Figure 13 simulation uses an ITU channel with three multipaths having six kilohertz (kHz) frequency offset and a UE 32! moving at 500 km/h with_accumulation over eight PSCH time slots. The Figure 14 simulation uses a single path Rayleigh channel having 10 kHz frequency offset with accumulation over eight PSCH time slots. The Figure 15 simulation uses an ITU channel with three multipaths having 10 kHz frequency offset and the UE 32! moving at 500 km/h i with accumulation over eight PSCH time slots.

Under the simulated conditions of Figures 14 and 15, 6 SSCs 128 outperforms the other techniques 124,126. As shown in Figures 9 to 13, 6 SSCs 15 128 performs favorably in comparison to the other techniques 124,126.

Figure 16 is a graph of the simulated performance of 6 SSCs 114 using BPSK and the two sets of 4 SSCs 112 using QPSK modulation. The simulation used an eight PSCH time slot accumulation of the matches for each SSC and transmission over an AWGN channel. As shown, two set QPSK modulation 112 outperformed 20 6 SSC BPSK modulation 114.

Figure 17 illustrates the performance ofBPSK and two set QPSK modulation accumulating matches over four and eight PSCH time slots. The SSCs were simulated as being transmitted over a single path Rayleigh channel. Performance for both modulation schemes improves with additional time slot correlations. Two set QPSK modulation for four PSCH time slots 116 and eight PSCH time slots 120 outperforms BPSK modulation for four PSCH time slots 118 and eight PSCH time 5' ״ ־ slots 122, respectively.

All passages of the description, which are not within the scope of the claims, do not form part of the invention.