Processing of random access preamble sequences

1. A method in a network node of a network, the method comprising:

a preamble receiver of the network node receiving via a physical random access channel (PRACH) a radio signal transmitted by a wireless communication device (WCD) seeking random access to the network, the radio signal comprising a random-access preamble sequence, wherein the random-access preamble sequence comprises a plurality of identical short sequences and at least one offset indicator sequence, the at least one offset indicator sequence being different from each of the short sequences and located at a respective pre-determined location in the preamble sequence; and

after receiving the random-access preamble sequence:

(i) determining, by the preamble receiver, a first estimate of the actual arrival time of the preamble sequence based on Fast Fourier Transform (FFT) processing of the received radio signal, wherein a size of the FFT equals a size of an FFT that is used for detecting an orthogonal frequency-division multiplexing (OFDM) symbol carrying data traffic in a radio access network of the preamble receiver;

(ii) determining, by the preamble receiver, an arrival time component based on the at least one offset indicator sequence;

(iii) determining, by the preamble receiver, a final estimate of the actual arrival time of the preamble sequence based on a sum of the first estimate of the actual arrival time of the preamble sequence and the arrival time component; and

(iv) facilitating wireless communication between the WCD and the network node by the network node using the final estimate of the actual arrival time to estimate a timing offset between the network node and the WCD.

2. The method according to claim 1 , further comprising detecting the preamble sequence in the received radio signal based on FFT processing of the received radio signal.

3. A computer program product comprising a non-transitory computer readable medium storing computer program code which, when executed in a preamble receiver, causes the preamble receiver to execute a method according to claim 1 .

4. The method of claim 1 , wherein

determining the arrival time component comprises choosing a value for the arrival time component, which choosing comprises choosing one of: (1) a value of zero as the value for the arrival time component and (2) a value of T as the value for the arrival time component,

T is the time duration of each of the identical short sequences, and

the choice between (1) and (2) is based on a value of a metric and a detection threshold value.

5. The method of claim 4 , wherein selecting one of: (1) zero and (2) T as the value for the arrival time component based on the value of the metric and the detection threshold value comprises:

comparing the value of the metric to the detection threshold value;

determining, based on the comparison, whether the value of the metric is less than the detection threshold value; and

choosing zero as the value for the arrival time component as a result of determining that the value of the metric is less than the detection threshold value and otherwise selecting T as the value for the arrival time component.

6. A preamble receiving device, the preamble receiving device comprising:

a receiver for receiving, via a physical random access channel (PRACH), a preamble sequence transmitted by a wireless communication device (WCD) seeking random access to a network, wherein the preamble sequence comprises a plurality of identical short sequences and at least one offset indicator sequence, the at least one offset indicator sequence being different from each of the short sequences and having a respective pre-determined location in the preamble sequence, wherein each of the identical short sequences has a time duration of T; and

a processing system, comprising a processor, coupled to the receiver, wherein the processing system is configured such that after receiving the preamble sequence, the processing system:

determines a first estimate of the actual arrival time of the preamble sequence based on Fast Fourier Transform (FFT) processing of the plurality of identical short sequences, wherein a size of the FFT equals a size of an FFT that is used for detecting an orthogonal frequency-division multiplexing (OFDM) symbol carrying data traffic in a radio access network of the preamble receiver,

determines an arrival time component based on processing of the at least one offset indicator sequence,

calculates a final estimate the actual arrival time of the preamble sequence based on a sum of the first estimate of the actual arrival time of the preamble sequence and the arrival time component, and

uses the final estimate of the actual arrival time to estimate a timing offset between the preamble receiving device and the WCD.

7. The preamble receiving device according to claim 6 , wherein the processing system is configured to determine the first arrival time estimate using a number F of FFT windows arranged sequentially in time over a receiver time span and indexed by a variable f from 0 through F−1, wherein

a first offset indicator sequence has zero amplitude and a respective pre-determined location at an end of the preamble sequence,

the processing system is arranged to determine a first value of a first metric indicating the presence of a short sequence at an end of said receiver time span by an FFT window having index f=F−1, and

the processing system is further arranged to determine the arrival time component by selecting one of: (1) zero and (2) T as the value for the arrival time component based on the value of the first metric and a detection threshold value, wherein

the processing system is configured to select zero as the value for the arrival time component when the value of the first metric is lower than the detection threshold value, and

the processing system is configured to select T as the value for the arrival time component when the value of the first metric is higher than the detection threshold value.

8. The preamble receiving device according to claim 7 , wherein

a second offset indicator sequence has zero amplitude and a respective pre-determined location substantially in the middle of the preamble sequence, and

the processing system is further arranged to determine the value of the first metric based on FFT processing of an FFT window corresponding to the second location.

9. The preamble receiving device according to claim 7 , wherein the processing system is further arranged to determine a value of a second metric indicating the presence of a short sequence at a beginning of the receiver time span by FFT processing of an FFT window having index f=0, wherein the detection threshold is set to equal the value of the second metric.

10. The preamble receiving device according to claim 9 , wherein the processing system is further arranged to determine the value of the second metric based on FFT processing of an FFT window after the FFT window corresponding to the second location.

11. The preamble receiving device according to claim 7 , wherein, for each value of d from 1 through D, the processing system is arranged to determine a respective metric value indicating a presence of the short sequence at a respective FFT window with an index f=F−d, the preamble sequence offset detector being arranged to determine the arrival time of the preamble sequence based on comparison between the respective metric values determined by the plurality of late arrival detectors with respect to the detection threshold.

12. A network node comprising the preamble receiving device according to claim 6 .