Channel estimation in a radio receiver
A channel estimation method suitable for use in a CDMA communications system employs a high order interpolation using four interpolation points per slot. Four FIR interpolation filters ( 18 - 21 ) produce a channel estimate for each quarter of a slot by weighting the summed pilots of four slots by amounts related to pre-computed polynomial co-efficients. The invention has been shown wot mobiles and has the advantage of low computional complexity.
1. Receiver apparatus for computing channel estimates from a received signal having a slotted structure, each slot comprising a sequence of data symbols and a sequence of pilot symbols, the receiving apparatus including:
M summers for respectively summing the pilot symbols of each of M slots, where M is greater than 1 and the M summers produce respective M outputs x(i);
J interpolation filters each arranged to receive the outputs x(i) as inputs, where J specifies a chosen number of time instances of one of the M slots;
a computation module connected to each of the J interpolation filters for computing respective filter coefficients, Fj,i, therefor depending upon the chosen number of time instances Tj relating to said one of the M slots, time instances of the pilot symbols and a chosen polynomial interpolation order N greater than 2;
whereby each of the J interpolation filters is configured to compute a channel estimate,
ChanEst
j
=
∑
i
=
0
M
-
1
F
j
,
i
x
(
i
)
for one of said chosen number of time instances Tj, where j takes a value 0, . . . , J−1.
2. The apparatus according to claim 1 , in which the filter co-efficients F j,i are computed in accordance with the relationship;
F
j
,
i
=
∑
k
=
0
N
-
1
b
k
.
i
(
T
)
k
where b k,i is a (k, i) th element of a matrix B, which relates to time instances of the pilot symbols;
where P=BX; and
where P is an interpolation polynomial of an order, N, and X is a vector representing the M outputs, x(i).
3. The apparatus according to claim 1 in which M=4 and j=4.
4. The apparatus according to claim 1 , wherein the chosen number of time intervals of the M slots is greater than 1.
5. A method of computing channel estimates from a received signal having a slotted structure, each slot comprising a sequence of data symbols and a sequence of pilot symbols, the method including:
respectively summing the pilot symbols of each of the M slots using M summers to produce respective M outputs x(i), where M is greater than 1;
choosing a number of time instances J of one of the M slots;
providing J interpolation filters, each of the J interpolation filters receiving the M outputs x(i) as inputs;
computing respective filter coefficients, Fj,i, for each of the J interpolation filters depending upon the chosen number of time instances Tj relating to said one of the M slots, time instances of the pilot symbols and a chosen polynomial interpolation order N greater than 2; and
computing, within each of the J interpolation filters, a channel estimate,
ChanEst
j
=
∑
i
=
0
M
-
1
F
j
,
i
x
(
i
)
for one of said chosen number of time instances Tj, where j takes a value 0, . . . , J−1.
6. The apparatus according to claim 1 , further arranged to combine the computed channel estimates with a Common Pilot Channel (CPICH) estimation to produce a refined channel estimation.
7. The apparatus according to claim 6 , wherein the chosen number of time intervals of the M slots is greater than 1.
8. The apparatus according to claim 1 , further arranged to use the computed channel estimates to provide a quasi-pilot for a second iteration to produce a refined channel estimation.
9. The apparatus according to claim 8 , wherein the chosen number of time intervals of the M slots is greater than 1.
10. The apparatus according to claim 1 , further arranged to combine the computed channel estimates with an inter-path cancellation scheme.
11. The apparatus according to claim 10 , wherein the chosen number of time intervals of the M slots is greater than 1.
12. The apparatus according to claim 1 , wherein the J interpolation filters are arranged to generate J corresponding channel estimates respectively relating to the time instances Tj chosen.
13. The apparatus according to claim 12 , wherein the chosen number of time intervals of the M slots is greater than 1.
14. The method according to claim 5 , wherein the chosen number of time intervals of the M slots is greater than 1.
15. The method according to claim 5 , further comprising:
combining the computed channel estimates with a Common Pilot Channel (CPICH) estimation to produce a refined channel estimation.
16. The method according to claim 15 , wherein the chosen number of time intervals of the M slots is greater than 1.
17. The method according to claim 5 , further comprising: using the computed channel estimates to provide a quasi-pilot for a second iteration to produce a refined channel estimation.
18. The method according to claim 17 , wherein the chosen number of time intervals of the M slots is greater than 1.
19. The method according to claim 5 , further comprising:
combining the computed channel estimates with an inter-path cancellation scheme.
20. The method according to claim 5 , further comprising:
the J interpolation filters generate J corresponding channel estimates respectively relating to the time instances Tj chosen.