FIG. 1 depicts a wireless spread spectrum TDD/CDMA communication system. The system has a plurality of base stations 301 to 307. Each base station 301 has an associated cell 341 to 347 and communicates with user equipments (UEs) 321 to 323 in its cell 341.
In addition to communicating over different frequency spectrums, TDD/CDMA systems carry 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 FIG. 2 use repeating frames 38 divided into a number of time slots 361 to 36n, such as sixteen time slots 0 to 15. In such systems, a communication is sent in selected time slots 361 to 36n using selected codes. Accordingly, one frame 38 is capable of carrying multiple communications distinguished by both time slot 361 to 36n and code.
For a UE 321 to communicate with a base station 301, time and code synchronization is required. FIG. 3 is a flow chart of the cell search and synchronization process. Initially, the UE 321 must determine which base station 301 to 307 and cell 341 to 347 to communicate with. In a TDD/CDMA system, all the base stations 301 to 307 are time synchronized within a base station cluster. For synchronization with UEs 321 to 327, each base station 301 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 301 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 X1 and X2 in Equations 1 and 2.X1=[1, 1, −1, −1, 1, −1, 1, −1, −1, −1, −1, −1, 1, 1, 1, −1]  Equation 1X2=[1, 1, −1, −1, −1, −1, 1, −1, 1, 1, −1, 1, 1, 1, −1, 1]  Equation 2Equation 3 illustrates one approach to generate a 256 hierarchal code, y(i), using X1 and X2.y(i)=X1(i mod 16)×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 Hadamard matrix, ho, to produce Cp(i) as in Equation 4.Cp(i)=y(i)×h0(i), where i=0, . . . , 255   Equation 4
Since the first row of the Hadamard matrix is an all one sequence, Equation 4 reduces to Equation 5.Cp(i)=y(i), where i=0, . . . , 255   Equation 5
The Cp(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 301 to 307 sends its PSC signal with a unique time offset, toffset, from the time slot boundary 40. Differing time offsets are shown for time slot 42 in FIG. 4. To illustrate, a first base station 301 has a first time offset 441, toffset,1 for the PSC signal, and a second base station 302, has a second time offset 442, toffset,2.
To differentiate the different base stations 301 to 307 and cells 341 to 347, each base station 301 to 307 within the cluster is assigned a different group of codes (code group). One approach for assigning a toffset for a base station using an nth code group 44n, toffset,n is Equation 6.toffset,n=n≅71Tc   Equation 6
Tc is the chip duration and each slot has a duration of 2560 chips. As a result, the offset 42n for each sequential code group is spaced 71 chips.
Since initially the UE 321 and the base stations 301 to 307 are not time synchronized, the UE 321 searches through every chip in the frame 38 for PSC 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 301 having the strongest signal. This process is referred to as step-1 of cell search procedure.
After the UE 321 identifies the PSC signal of the strongest base station 301, the UE 321 needs to determine the time slot 361 to 36n in which the 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 301. This process is referred to as step-2 of cell search procedure. To indicate the code group assigned to the base station 301 and the PSCH time slot index, the base station 301 transmits signals having selected secondary synchronization codes (SSCs), step 48. The UE 321 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 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 PSCH time slots. This approach adds complexity at the UE 321 requiring at least 64 matched filters and extensive processing. To identify the code group and PSCH time slot index, 17,344 real additions and 128 real multiplications are required in each PSCH time slot and 64 real additions are required for the decision.
An alternative approach for step-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 required. 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.
It would be desirable to reduce the complexity required by a UE 321 to perform cell search procedure.