Encoder and method for efficient synchronisation channel encoding in UTRA TDD mode

Description

FIELD OF THE INVENTION

This invention relates to wireless communication networks and particularly to UTRA (Universal Mobile Telephone System Terrestrial Radio Access) networks operating in TDD (Time Division Duplex) mode.

BACKGROUND OF THE INVENTION

In UTRA TDD mode the synchronisation channel (SCH) has two functions. The primary function is to provide a signal that enables a ‘UE’ (user equipment, such as a wireless terminal) to search for and identify a ‘Node B’ (i.e, a wireless Base Station of a UMTS system). The secondary function is to provide sufficient information to allow a UE to demodulate a P-CCPCH (Primary Common Control Physical CHannel) transmission and obtain the system information, sent on the BCH (Broadcast Channel) transport channel which is carried by the P-CCPCH, needed in order to be able to communicate with the network.

There are two cases of SCH and P-CCPCH allocation as follows:

• • Case 1) SCH and P-CCPCH allocated in timeslot #k, where k=0 . . . 14
  • Case 2) SCH allocated in two timeslots: timeslot #k and timeslot #k+8, where k=0 . . . 6; P-CCPCH allocated in timeslot #k
    where timeslot #k is the k^th timeslot. Due to this SCH scheme, the position of P-CCPCH is known from the SCH. The SCH consists of one real-valued primary synchronisation code (PSC) and three complex secondary synchronisation codes (SSCs), all of length 256 chips. The PSC is common for all Node Bs, but the SSCs are Node B specific. The PSC and SSC are transmitted simultaneously from a given Node B at a specific fixed time offset (t_offset) from the start of time slot 0. The time offset is included to prevent the possible capture effect that would otherwise occur as a consequence of all Node Bs transmitting the common primary code at the same time.

Considering a network where multi-chip rates are supported, in an initial start-up condition, the UE will not be aware of the chip rate that is available. To cope with this situation, it is known for the SCH to be always transmitted at a fixed chip rate (e.g., 3.84Mcps), and for the chip rate being used in the cell by the other transport channels to be signalled to the UEs by using the secondary synchronisation code, SSC (by modulating the secondary sequences).

Heretofore, this has been done by storing code group and frame position information defining the codes in tables in memory in the network and UE. However, the amount of information needing to be stored in this way may be considerable.

A need therefore exists for efficient synchronisation channel encoding in UTRA TDD mode wherein the abovementioned disadvantage(s) may be alleviated.

STATEMENT OF INVENTION

In accordance with a first aspect of the present invention there is provided a method for efficient synchronisation channel encoding in UTRA TDD mode, comprising:

producing a codeword a, where a=dG+z modulo-2, where d represents a predetermined code group to be encoded, G represents a predetermined generator matrix, and z represents a function of the code group number and a row of the generator matrix,

producing values s_k=2a_2k+1+a_2k; k=0, 1, 2, 3, and associated values b₀,b₁,b₂, and

producing a value S_ssc associated with the code group, where S_ssc=(b₀c_π(0),b₁c_π(1),b₂c_π(2)), c_π represents a code within the code group, and b₀,b₁,b₂ε(±1,±j).

In accordance with a second aspect of the present invention there is provided a method for efficient synchronisation channel encoding in UTRA TDD mode, comprising:

• • producing a codeword from a predetermined code group to be encoded, a predetermined generator matrix G, and a function of the code group number and a row of the generator matrix, and
  • producing a value S_ssc associated with the code group, where S_ssc=(b₀c_π(0),b₁c_π(1),b₂c_π(2)), c_π represents a code within the code group, and b₀,b₁,b₂ε(±1,±j),
  • wherein the generator matrix has values such that a parameter produced therefrom changes between a first value indicating a first chip rate and a second value indicating a second chip rate.

In accordance with a third aspect of the present invention there is provided an encoder for efficient synchronisation channel encoding in UTRA TDD mode, comprising:

means for producing a codeword a, where a=dG+z modulo-2, where d represents a predetermined code group to be encoded, G represents a predetermined generator matrix, and z represents a function of the code group number and a row of the generator matrix,

means for producing values s_k=2a_2k+1+a_2k; k=0, 1, 2, 3, and associated values b₀,b₁,b₂, and

means for producing a value S_ssc associated with the code group, where S_ssc=(b₀c_π(0),b₁c_π(1),b₂c_π(2)), c_π represents a code within the code group and b₀,b₁,b₂ε(±1,±j).

In accordance with a fourth aspect of the present invention there is provided an encoder for efficient synchronisation channel encoding in UTRA TDD mode, comprising:

• • means for producing a codeword from a predetermined code group to be encoded, a predetermined generator matrix G, and a function of the code group number and a row of the generator matrix, and
  • means for producing a value S_ssc associated with the code group, where S_ssc=(b₀c_π(0),b₁c_π(1),b₂c_π(2)), c_π represents a code within the code group, and b₀,b₁,b₂ε(±1,±j),
  • wherein the generator matrix has values such that a parameter produced therefrom changes between a first value indicating a first chip rate and a second value indicating a second chip rate.

BRIEF DESCRIPTION OF THE DRAWINGS

One encoder and method for efficient synchronisation channel encoding in UTRA TDD mode incorporating the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 shows in schematic form the format of SCH in UTRA TDD mode;

FIG. 2 shows in tabular form code allocation for CASE 1;

FIG. 3 shows in tabular form code allocation for CASE 2; and

FIG. 4 shows in block schematic form an encoder incorporating the invention.

DESCRIPTION OF PREFERRED EMBODIMENT(S)

The general format of the SCH is shown schematically in FIG. 1. As shown, the primary synchronisation code (PSC), C_p, is a real-valued sequence of length 256 chips, transmitted at power P_PSC. The secondary synchronisation code (SSC), C_s,i(i=1, 2, 3), of length 256 is transmitted simultaneously with the P_SSC; the total power of the SSC is set to P_ssc. In addition these codes are multiplied by a complex value, b_j(j=0, 1, 2). The subscript s in C_s,i refers to a code set, of which there 32, as specified in the technical specification ‘3GPP TS 25.223’ publicly available on the website (www.3gpp.org) of the 3^rd Generation Partnership Project. The code sets, s, in conjunction with the complex multiplier values, b_j, are used to transfer the information bits to the UE.

The location of the SCH relative to the beginning of the time slot is defined by t_offset,n. It is calculated as follows:

t offset , n = { n · T c ⁢ ⌊ 976 - 256 15 ⌋ n < 16 ( 976 + 512 + ( n - 16 ) ⁢ ⌊ 976 - 256 15 ⌋ ) ⁢ T c n ≥ 16
which can be simplified to:

t offset , n = { n · 48 · T c ⁢ ⁢ n < 16 ( 720 + n · 48 ) ⁢ T c n ≥ 16 ⁢
where T_c is the chip duration and n=0, 1, . . . , 31. The value of n is related to the code group and is obtained by demodulating the information on the SSC.
Encoding Information on SSC

The three codes that make up the SSC are QPSK (Quadrature Phase Shift Key) modulated and transmitted in parallel with the PSC. The QPSK modulation carries the following information:

• • the code group that the base station belongs to (32 code groups: 5 bits; Cases 1, 2);
  • the position of the frame within an interleaving period of 20 msec (2 frames: 1 bit, Cases 1, 2);
  • the position of the SCH slot(s) within the frame (2 SCH slots: 1 bit, Case 2).

The SSCs are partitioned into two code sets for Case 1 and four code sets for Case 2. The set is used to provide the following information:

Code Set Allocation for CASE 1

	Code Set	Code Group
	1	0-15
	2	16-31

The code group and frame position information is provided by modulating the secondary codes in the code set.

Code Set Allocation for CASE 2

	Code Set	Code Group
	1	0-7
	2	8-15
	3	16-23
	4	24-31

The following SCH codes are allocated for each code set:

• • Case 1
    • Code set 1: C₁, C₃, C₅.
    • Code set 2: C₁₀, C₁₃, C₁₄.
  • Case 2
    • Code set 1: C₁, C₃, C₅.
    • Code set 2: C₁₀, C₁₃, C₁₄.
    • Code set 3: C₀, C₆, C₁₂.
    • Code set 4: C₄, C₈, C₁₅.
      Code Allocation for CASE 1

FIG. 2 shows a table illustrating code allocation for CASE 1.

It may be noted that the code construction for code groups 0 to 15 using only the SCH codes from code set 1 is shown. It will be understood that the construction for code groups 16 to 31 using the SCH codes from code set 2 is done in the same way.

Code Allocation for CASE 2

FIG. 3 shows a table illustrating code allocation for CASE 2.

It may be noted that the code construction for code groups 0 to 15 using the SCH codes from code sets 1 and 2 is shown. The construction for code groups 16 to 31 using the SCH codes from code sets 3 and 4 is done in the same way.

It will be understood that the conventional approach is to store the information defined in the tables of FIG. 2 and FIG. 3 in memory in the network and UE. It will be appreciated that, using this conventional approach, the amount of information needing to be stored may be considerable.

A novel procedure, which simplifies the above mapping process, is now presented.

Efficient Encoder for CASE 1

For ease of explanation, the following notation is introduced. If C=(c_π(0),c_π(1),c_π(2)) represents the permutation of the codes in the code set, where π is the permutation, then the SSC associated with a code group is given by the component-wise product
S_ssc=bC
where S_ssc=(b₀c_π(0),b₁c_π(1),b₂c_π(2)) and b₀,b₁,b₂ε(±1,±j).

Let u=(u₀,u₁,u₂,u₃,u₄)^T—the superscript T indicating matrix transposition—be the binary representation of the code group number, and define the following generator matrix

G 1 = [ 0 0 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 0 1 0 0 0 ]
with rows labelled g₀¹,g₁¹,g₂¹,g₃¹. We define a binary codeword as
a=dG¹+z modulo-2
where d=(f,u₀,u₁,u₂)^T,

f = { 0 ⁢ Frame1 1 ⁢ Frame2
and z=u₃(u₂+1)g₃¹) reduced modulo-2. We map the elements of the codeword, a, pairwise to the set of integers (0, 1, 2, 3) using the expression
s_k=2a_2k+1+a_2k; k=0, 1, 2, 3
where the sequence s=(s₀,s₁,s₂) has an associated complex sequence b=(j^s0,j^s1,j^s2).

The choice of code set and permutation is given by

TABLE 1
	Code	Code Set
(u4, u3, u2)	Group	Permutation
000	0→3	c1c3c5
001	4→7	c1c3c5
010	8→11	c1c5c3
011	12→15	c3c5c1
100	16→19	c10c13c14
101	20→23	c10c13c14
110	24→27	c10c14c13
111	28→31	c13c14c10

Efficient Encoder for CASE 2

Define the following generator matrix

G 2 = [ 0 0 0 0 0 1 0 1 0 1 0 0 0 0 0 1 0 0 1 0 1 0 0 0 ]
with rows labelled g₀²,g₁².g₂²,g₃². We define a binary codeword as
a=dG²+z modulo-2
where d=(K,f,u₀,u₁)^T,

K = { 0 1 ⁢ slot ⁢ ⁢ k slot ⁢ ⁢ k + 1
and z=u₂(u₁+1)g₃² reduced modulo-2. The process follows in identical manner to case 1 with the exception that the permutation and code set map is changed to represent case 2, as shown below.

TABLE 2
	Code	Code Set
(u4, u3, u2, u1)	Group	Permutation
0000	0→1	c1c3c5
0001	2→3	c1c3c5
0010	4→5	c1c5c3
0011	6→7	c3c5c1
0100	8→9	c10c13c14
0101	10→11	c10c13c14
0110	12→13	c10c14c13
0111	14→15	c13c14c10
1000	16→17	c0c6c12
1001	18→19	c0c6c12
1010	20→21	c0c12c6
1011	22→23	c6c12c0
1100	24→25	c4c8c15
1101	26→27	c4c8c15
1110	28→29	c4c15c8
1111	30→31	c8c15c4

Thus, it will be understood that only the data in the above tables 1 and 2 needs to be stored in memory, in order to allow the necessary secondary synchronisation channel encoding to be effected, following the encoding process described above, rather than having to store the data in tables of FIG. 2 and FIG. 3 as in the prior art.

An efficient encoder 400 for carrying out the above processes for CASE 1 and CASE 2 is shown generally in FIG. 4. As can be seen in this figure, in block step 410 the binary codewords a=dG+z modulo-2 are calculated. Next, in block 420, the expressions s_k=2a_2k+1+a_2k; k=0, 1, 2, 3 are calculated. Finally, in block 430, the secondary synchronisation codes S_SSC=(b₀c_π(0),b₁c_π(1),b₂c_π(2)) are calculated.

Signalling Higher Chip Rate

As will be explained below, modification of the generator matrix allows encoding of the chip rate used in the cell.

Let C_r denote the chip rate, where

C r = { 0 ⁢ ⁢ 3.84 ⁢ ⁢ Mcps 1 ⁢ ⁢ 7.68 ⁢ ⁢ Mcps

We have the following:

CASE 1:

Define the following generator matrix

G 1 = [ 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 0 1 0 0 0 ]
and a binary codeword as
a=dG¹+z modulo-2
where d=(C_r,f,u₀,u₁,u₂)^T and z=u₃(u₂+1)g₄¹. We note that when C_r is 0 the codeword produced by the generator matrix is unchanged. When C_r is 1, denoting the higher chip rate, the third element of the sequence b=(j^s0,j^s1,j^s2) becomes imaginary rather than real.
CASE 2:

Define the following generator matrix

G 2 = [ 0 0 0 0 1 0 0 0 0 0 0 1 0 1 0 1 0 0 0 0 0 1 0 0 1 0 1 0 0 0 ]
and a binary codeword as
a=dG²+z modulo-2
where d=(C_r,K,f,u₀,u₁)^T and z=u₂(u₁+1)g₄². We note that when C_r is 0 the codeword produced by the generator matrix is unchanged. When C_r is 1, denoting the higher chip rate, the third element of the sequence b=(j^s0,j^s1,j^s2) becomes imaginary rather than real.

It will be appreciated that the process for encoding described above will typically be carried out in software running on a processor (not shown), and that the software may be provided as a computer program element carried on any suitable data carrier (not shown) such as a magnetic or optical computer disc.

It will be appreciated that the encoder described above will typically be incorporated in a base station (‘Node B’—not shown) and a mobile station (‘UE’—not shown) of a UMTS system, with complementary decoding being provided in the ‘UE’ and ‘Node B’ respectively.

It will be understood that the encoder and method for efficient synchronisation channel encoding in UTRA TDD mode described above provides the following advantages:

• • an efficient encoding architecture for the synchronisation channel in UTRA TDD mode
  • in addition, by simple manipulation of the generation matrix, a higher chip rate signal may be signalled whilst still preserving the signalling information for the lower chip rate.