User equipment and base station for frame timing and cell group synchronization

Abstract

Methods and systems for receiving a primary and secondary synchronization signal from a cell are provided. The secondary synchronization signal may be derived from at least one of a set of sequences and received based on the timing of the received primary synchronization signal. Determining a time slot within a frame of the received secondary synchronization signal and a group of the cell is provided, wherein the time slot and the group are determined by at least one sequence of the secondary synchronization signal and at least one carrier out of a plurality of carriers used for one sequence out of the at least one sequence of the secondary synchronization signal. Determining a frame timing and a sequence used by the cell, based on the received primary synchronization signal and the secondary synchronization signal is further provided.

Claims

1. A user equipment (UE) comprising: an antenna; a receiver, operatively coupled to the antenna, configured to receive a primary synchronization signal from a cell; wherein the receiver is further configured, based on a timing of the received primary synchronization signal, to receive a secondary synchronization signal; wherein the secondary synchronization signal is derived from at least one of a set of sequences; and circuitry configured to determine a time slot within a frame of the received secondary synchronization signal and a group of the cell; wherein the time slot and the group are determined by at least one sequence of the secondary synchronization signal and at least one carrier out of a plurality of carriers used for one sequence out of the at least one sequence of the secondary synchronization signal; wherein the circuitry is further configured to determine a frame timing and a sequence used by the cell based on the received primary synchronization signal and the secondary synchronization signal; wherein the receiver is further configured to receive a downlink signal from the cell having the determined sequence used by the cell.

2. The UE of claim 1, wherein the time slot within the frame is determined from the carrier out of the plurality of carriers used for the one sequence.

3. The UE of claim 1, wherein the secondary synchronization signal is derived from at least two sequences and a first sequence and a second sequence of the at least two sequences are received using different carriers.

4. The UE of claim 3, wherein the group is determined from the at least two sequences of the secondary synchronization signal.

5. The UE of claim 1, wherein the UE utilizes time division duplex for communication.

6. The UE of claim 1, wherein the UE utilizes code division multiple access for communication.

7. The UE of claim 1, further comprising: an isolator operatively coupled between the antenna and the receiver.

8. The UE of claim 1, further comprising: a demodulator operatively coupled between the antenna and the circuitry; and wherein the demodulator is configured to demodulate signals received by the antenna from radio frequency to baseband.

9. A base station comprising: an antenna; and a transmitter operatively coupled to the antenna, wherein the transmitter is configured to transmit a primary synchronization signal for a cell; wherein the transmitter is further configured to transmit a secondary synchronization signal having a timing synchronized with the primary synchronization signal; wherein the secondary synchronization signal is derived from a plurality of sequences chosen among a set of sequences; wherein the plurality of sequences are transmitted using different carriers; wherein a time slot within a frame and a group of the transmitted secondary synchronization signal is for determination by a user equipment (UE) based on the plurality of sequences and at least one carrier used to transmit the sequences; wherein the transmitter is further configured to transmit a downlink signal having a sequence determinable from the transmitted primary synchronization signal and the secondary synchronization signal.

10. The base station of claim 9, wherein the time slot within the frame is determined from the plurality of sequences transmitted using different carriers.

11. The base station of claim 9, wherein the secondary synchronization signal is derived from at least two sequences and a first sequence and a second sequence of the at least two sequences are transmitted using different carriers.

12. The base station of claim 11, wherein the group is determined from the at least two sequences of the secondary synchronization signal.

13. The base station of claim 9, wherein the base station utilizes time division duplex for communication.

14. The base station of claim 9, wherein the base station utilizes code division multiple access for communication.

15. The base station of claim 9, further comprising: an isolator operatively coupled between the antenna and the transmitter.

16. The base station of claim 9, further comprising: a modulator operatively coupled between the antenna and the circuitry; wherein the modulator is configured to modulate the primary synchronization signal and secondary synchronization signal to a carrier frequency.

17. A method for operation in a user equipment (UE), the method comprising: receiving a primary synchronization signal from a cell; receiving a secondary synchronization signal, based on a timing of the received primary synchronization signal, wherein the secondary synchronization signal is derived from at least one of a set of sequences; determining a time slot within a frame of the received secondary synchronization signal and a group of the cell; wherein the time slot and the group are determined by at least one sequence of the secondary synchronization signal and at least one carrier out of a plurality of carriers used for one sequence out of the at least one sequence of the secondary synchronization signal; determining a frame timing and a sequence used by the cell based on the received primary synchronization signal and the secondary synchronization signal; and receiving a downlink signal from the cell having the determined sequence used by the cell.

18. The method of claim 17, wherein the secondary synchronization signal is derived from at least two sequences and a first sequence and a second sequence of the at least two sequences are received using different carriers.

19. The method of claim 17, wherein the secondary synchronization signal is derived from at least two sequences and a first sequence and a second sequence of the at least two sequences are received using different carriers.

20. The method of claim 17, wherein the group is determined from the at least two sequences of the secondary synchronization signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates a prior art TDD/CDMA system.

(2) FIG. 2 illustrates time slots in repeating frames of a TDD/CDMA system.

(3) FIG. 3 is a flow chart of cell search.

(4) FIG. 4 illustrates time offsets used by differing base stations sending primary synchronization code signals.

(5) FIG. 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.

(6) FIG. 6 is a flow chart of secondary synchronization code assignment.

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

(8) FIG. 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.

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

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) The preferred embodiments will be described with reference to the drawing figures where like numerals represent like elements throughout. FIG. 5 shows the simplified circuitry of a base station 30.sub.1 and a UE 32.sub.1 for use in cell search. During step-1 of the cell search, the base station 30.sub.1 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.sub.1. 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 antenna 58 or, alternately, an antenna array. The UE 32.sub.1 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 the chips of a frame 38 to identify the PSC signal of the base station 30.sub.1 having the strongest signal.

(11) 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 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.

(12) For step-2 of the cell search procedure, the base station 30.sub.1 generates SSC signals, SSC.sub.1 to SSC.sub.M, using SSC spread spectrum signal generators 68.sub.1 to 68.sub.M. To reduce the complexity at the UE 32.sub.1, a reduced number of SSCs are used. By reducing the SSCs, the number of matched filters required at the UE 32.sub.1 is reduced. Additionally, the reduced SSCs decrease the processing resources required to distinguish the different codes. The reduced SSCs also reduce the probability of incorrect detection of a code group number and PSCH time slot index (see FIGS. 9-15).

(13) One approach to reduce the SSCs is shown in the flow chart of FIG. 6. The 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 7.
M=log.sub.2 (#of Code Groups#of PSCH Time Slots per frame)Equation 7

(14) The base station 30.sub.1 generates, using SSC signal generators 68.sub.1 to 68.sub.M, the SSC signals associated with the base station's code group and the number of PSCH time 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.sub.1 receives the transmitted signal, passes it through the isolator 72 and demodulates the received signal using the demodulator 74. Using corresponding SSC.sub.1 to SSC.sub.M matched filters 78.sub.1 to 78.sub.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 (log.sub.2 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 Hadamard matrix, H.sub.8. The Hadamard matrix is generated sequentially, such as by Equations 8 and 9.

(15) $\begin{array}{cc}{H}_0=\left(1\right)& \mathrm{Equation}8\\ H_t=\left[\begin{array}{cc}{H}_{t-1}& H_{t-1}\\ H_{t-1}& H_{t-1}\end{array}\right],t=1,\mathrm{.Math.},8& \mathrm{Equation}9\end{array}$

(16) A particular code, C.sub.k,n(i), where n is the code group number associated with an SSC is produced using Equation 10. The six rows of Hadamard matrix, H.sub.8, are r(k)=[24, 40, 56, 104, 120, 136].
C.sub.k,n(i)=b.sub.k,nh.sub.r(k)(i)y(i), where i=0, 1, . . . , 255 and k=1, . . . , 6Equation 10
The value of b.sub.2 to b.sub.6 are depicted in Table 1.

(17) TABLE-US-00001 TABLE 1 Code Group (n) b.sub.6, n b.sub.5, n b.sub.4, n b.sub.3, n b.sub.2, n 1 +1 +1 +1 +1 +1 2 +1 +1 +1 +1 1 3 +1 +1 +1 1 +1 . . . . . . . . . . . . . . . . . . 32 1 1 1 1 1
The value of b.sub.1,n is depicted in Table 2.

(18) TABLE-US-00002 TABLE 2 PSCH time slot order in the frame b.sub.1, n K, where K = 0, . . . , 7 +1 K + 8 1

(19) Each code corresponds to one SSC, SSC.sub.1 to SSC.sub.6. To distinguish the differing base stations' SSC signals from one another, each of the base stations' SSC signals has the same offset as its PSC signal. At the UE 32.sub.1, 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.sub.p as per Equation 4 to obtain phase reference. This correlation is performed by PSC matched filter 76 in FIG. 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 C1, . . . , C6 as per Equation 10 to obtain binary data that represent the code group of the base station 30.sub.1 and PSCH slot order in the frame. This correlation is performed by SSC matched filters 78.sub.1-78.sub.M in FIG. 5. These matched filter outputs are derotated before BPSK demodulation. The derotation is performed by complex multiplication of the complex conjugate of the phase reference. The derotated 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 demodulated as 1. The demodulated binary data represents the code group of the base station 30.sub.1 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.sub.1 accumulates the derotated outputs of the SSC matched filters 78.sub.1-78.sub.M over a number of the PSCH time slots, such as four or eight.

(20) 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.sub.1 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.sub.1.

(21) 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 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.

(22) 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.

(23) A simplified base station 30.sub.1 and UE 32.sub.1 using QPSK modulation are shown in FIG. 7. The base station 30.sub.1 generates the appropriate SSC signals for its code group and PSCH time slot using the SSC spread spectrum signal generators 68.sub.1 to 68.sub.M. Also based on the base station's code group/PSCH time slot index, switches 90.sub.1 to 90.sub.M either switch the outputs of the generators 68.sub.1 to 68.sub.M to an I combiner 86 or a Q combiner 88. The combined I signal which includes the PSC signal is modulated by an 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.sub.1 demodulates the received signals with both an I demodulator 92 and a Q demodulator 94. Similar to the base station 30.sub.1, the UE 32.sub.1 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.sub.1 to 100.sub.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.sub.dvar, is obtained such as by using Equation 11.
I.sub.dvar=|rx.sub.1|+|rx.sub.2|+ . . . +|rx.sub.m|Equation 11

(24) |rx.sub.i| is the magnitude of the real component (I component) of the i.sup.th SSC matched filter output. Likewise, the Q matched filters 102.sub.1 to 102.sub.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.sub.dvar, is obtained such as by using Equation 12.
Q.sub.dvar=|ix.sub.1|+|ix.sub.2|+ . . . +|ix.sub.M|Equation 12

(25) |ix.sub.i| is the magnitude of the imaginary (Q component) of the i.sup.th SSC matched filter outputs.

(26) If I.sub.dvar is greater than Q.sub.dvar, the SSC signals were transmitted on the I component. Otherwise, the SSC signals were transmitted on the Q component.

(27) Another approach using QPSK modulation to reduce the number of SSC signals transmitted is depicted in FIG. 8. Instead of transmitting the number of SSCs of FIG. 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.sub.11 to SSC.sub.14, is assigned to the lower code groups, such as code groups 1 to 16, and the second set, SSC.sub.21 to SSC.sub.24, is assigned to the upper code groups, such as code groups 17 to 32. For the lower code group, sending SSC.sub.11 to SSC.sub.14 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 SSC.sub.21 to SSC.sub.24 restricts the code groups to 17 to 24 and Q SSC.sub.21 to SSC.sub.24 restricts the code groups to 25 to 32. As a result, the maximum number of 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.sub.1.

(28) A simplified base station 30.sub.1 and UE 32.sub.1 implementing the reduced SSC approach is shown in FIG. 8. At the base station 30.sub.1, two sets of M SSC spread spectrum signal generators 104.sub.11 to 104.sub.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.sub.11 to 106.sub.2M to either an I 82 or Q modulator 84 as appropriate for that base station's code group and PSCH time slot. At the UE 32.sub.1, an I set of matched filters 108.sub.11 to 108.sub.2M is used to determine if any of the SSCs were sent on the I carrier. A Q set of matched filters 110.sub.11 to 110.sub.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.

(29) One approach to determining which of 32 code groups and two possible PSCH time slots is used by the base station 32.sub.1 follows. After the processor 80 accumulates data from matched filters 110.sub.11 to 110.sub.24, the code group set, either SSC.sub.11 to SSC.sub.14 or SSC.sub.21 to SSC.sub.24, is determined using Equations 13 and 14.
var_set 1=|rx.sub.11|+|ix.sub.12|+ . . . +|rx.sub.14|+|ix.sub.14|Equation 13
var_set 2=|rx.sub.21|+|ix.sub.22|+ . . . +|rx.sub.24|+|ix.sub.24|Equation 14

(30) The values, rx.sub.11 to rx.sub.24, are the number of accumulated matches for a respective SSC, SSC.sub.11 to SSC.sub.24, received in the I channel. Similarly, ix.sub.11 to ix.sub.24 are the number of accumulated matches for the Q channel for SSC.sub.11 to SSC.sub.24. Equations 13 and 14 require a total of 16 real additions. var_set 1 represents the total accumulations of the first SSC set, SSC .sub.11 to SSC.sub.14. var_set 2 represents the total accumulations of the second SSC set, SSC.sub.21 to SSC.sub.24. 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.sub.1.

(31) To determine whether the SSCs were transmitted on the I or Q channel, Equations 15 and 16 are used.
var_I=|rx.sub.p1|+ . . . +|rx.sub.p4|Equation 15
var_Q=|ix.sub.p1|+ . . . +|ix.sub.p4|Equation 16

(32) 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 I 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.

(33) FIGS. 9 to 15 are graphs illustrating the performance for distinguishing 32 code groups/two PSCH time slots of systems using 32 SSCs 126, 17 SSCs 124 and 6 SSCs 128. The graphs show the performance for various simulated channel conditions. The simulations accumulated the SSC matches at the UE 32.sub.1 over four or eight PSCH time slots and compared the probability of an incorrect synchronization to the channel's signal to noise ratio (SNR) in decibels.

(34) The FIG. 9 simulation uses an additive white Gaussian noise (AWGN) channel and accumulation over eight PSCH time slots. The FIG. 10 simulation uses a single path Rayleigh fading channel with a six kilohertz (kHz) frequency offset and accumulation over four PSCH time slots. The FIG. 11 simulation is the same as the FIG. 10 simulation except the accumulation was over eight PSCH time slots. The FIG. 12 simulation uses an ITU channel with three multipaths with a UE 32.sub.1 moving at 100 kilometers per hour (km/h) and accumulation over eight PSCH time slots. The FIG. 13 simulation uses an ITU channel with three multipaths having six kilohertz (kHz) frequency offset and a UE 32.sub.1 moving at 500 km/h with accumulation over eight PSCH time slots. The FIG. 14 simulation uses a single path Rayleigh channel having 10 kHz frequency offset with accumulation over eight PSCH time slots. The FIG. 15 simulation uses an ITU channel with three multipaths having 10 kHz frequency offset and the UE 32.sub.1 moving at 500 km/h with accumulation over eight PSCH time slots.

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

(36) FIG. 16 is a graph of the simulated performance of 6 SSCs 112 using BPSK and the two sets of 4 SSCs 114 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 114 outperformed 6 SSC BPSK modulation 112.

(37) FIG. 17 illustrates the performance of BPSK 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 slots 120, respectively.