Echo profile probe
View Patent ↗In an orthogonal frequency division multiplex (OFDM) communication network, a node transmits an echo profile probe to other nodes in the network. The echo profile probe is a message that allows characterization of the unique echo profile through the communication channel between each node pair. The echo profile is used to calculate the cyclic prefix length needed for optimum communication from one node to the other.
1. A network node within a network, the node comprising:
a) a receiver that receives an echo profile probe having a sequence of bits from a second node of the network, the echo profile being received over a channel;
b) a cross-correlator that cross correlates the received sequence to a local copy of the sequence;
c) a processor that determines the time distance between the earliest significant correlation peak and the latest significant correlation peak for the purpose of determining the delay spread in the response of the channel, and using the delay spread to determine an appropriate cyclic prefix; and
d) a transmitter for returning the cyclic prefix to the second node.
2. The network node of claim 1 , wherein the processor determines the time distance between peaks by:
a) setting a threshold as a function of an expected echo amplitude in the channel;
b) looking for the first peak that exceeds the threshold and recording the time of the peak as “start profile time”;
c) looking for the last instance of a peak that exceeds the threshold and recording the time of the peak as “end profile time”;
d) determining the difference between start profile time and end profile time; and
e) if the difference between the start profile time and end profile time is greater than a first predetermined value, then reducing the threshold by a second predetermined value and repeating steps b-e to determine new values for the start profile time and the end profile time until the difference between the start profile time and the end profile time is not greater than the first predetermined value.
3. The network node of claim 2 , wherein the processor also:
a) determines when the maximum peak occurs and recording this time as “peak time”; and
b) computing the system bias time by subtracting the peak time from the most recent value of start profile time.
4. A network node within a network, the node comprising:
a) a processor for generating an echo profile probe composed of a sequence of 1024 pseudo-random BPSK-modulated bits generated using an m-sequence of degree 10, given by the generator polynomial 22018 with an initial seed of 3FF16; and
b) a transmitter for transmitting a message including a preamble and a time-domain data payload, the payload including the echo profile probe.