MECHANISM TO FACILITATE TIMING RECOVERY IN TIME DIVISION DUPLEX SYSTEMS
In general, the present invention relates to systems and methods to facilitate timing recovery and loop timing operations in a TDD communication system with significantly varying intervals of inactivity between periods of transmission. According to certain aspects, to facilitate timing recovery, embodiments of the invention define a maximum period of inactivity for each mode of transmission and associated “timing keep alive” signals during and/or between transmissions to assist the timing recovery function in the receiver. In embodiments, the receiver selects the desired format of the “timing keep alive” signal. According to further aspects, the timing recovery mechanisms of the invention maintain power saving objectives of G.fast, where power dissipation varies near linearly with traffic demand.
1 . A method to facilitate timing recovery at a receiver in a time division duplex (TDD) communication system, comprising:
defining a maximum period of inactivity of downstream transmissions in a TDD frame;
specifying timing keep alive signals; and
transmitting the specified timing keep alive signals downstream to the receiver during the downstream transmissions.
2 . A method according to claim 1 , wherein the timing keep alive signals comprise pilot tones.
3 . A method according to claim 2 , further comprising inserting the pilot tones in data bearing symbols.
4 . A method according to claim 2 , further comprising inserting the pilot tones in non-data bearing symbols.
5 . A method according to claim 1 , further comprising using the transmitted timing keep alive signals to update a receive clock at the receiver.
6 . A method according to claim 5 , wherein updating is performed using a phase lock loop.
7 . A method according to claim 1 , wherein the maximum period of inactivity is defined based on a maximum permitted phase drift at the receiver.
8 . A method according to claim 7 , wherein T inactive is the maximum period of inactivity and wherein φ drift is the maximum permitted phase drift, and wherein T inactive is determined according to
φ
drift
=
(
Δ
f
f
o
)
ppm
·
T
Inactive
where Δƒ is a root-mean-square frequency variation to reference frequency ƒ o .
9 . A method according to claim 1 , wherein the TDD communication system is in accordance with G.fast.
10 . A method according to claim 9 , wherein the TDD frame is in a L0 state.
11 . A method according to claim 9 , wherein the TDD frame is in a L.2.x state.
12 . A time division duplex (TDD) communication system, comprising:
an upstream transmitter; and
a downstream receiver,
wherein the downstream receiver is adapted to define a maximum period of inactivity of downstream transmissions in a TDD frame and to specify timing keep alive signals,
and wherein the transmitter is adapted to transmit the specified timing keep alive signals downstream to the receiver during the downstream transmissions.
13 . A TDD communication system according to claim 12 , wherein the timing keep alive signals comprise pilot tones.
14 . A TDD communication system according to claim 13 , wherein the transmitter is adapted to insert the pilot tones in data bearing symbols.
15 . A TDD communication system according to claim 13 , wherein the transmitter is adapted to insert the pilot tones in non-data bearing symbols.
16 . A TDD communication system according to claim 12 , wherein the receiver is adapted to use the transmitted timing keep alive signals to update a receive clock at the receiver.
17 . A TDD communication system according to claim 16 , wherein updating is performed using a phase lock loop at the receiver.
18 . A TDD communication system according to claim 12 , wherein the maximum period of inactivity is defined based on a maximum permitted phase drift at the receiver.
19 . A TDD communication system according to claim 18 , wherein T inactive is the maximum period of inactivity and wherein φ drift is the maximum permitted phase drift, and wherein T inactive is determined according to
φ
drift
=
(
Δ
f
f
o
)
ppm
·
T
Inactive
where Δƒ is a root-mean-square frequency variation to reference frequency ƒ o .
20 . A TDD communication system according to claim 12 , wherein the TDD communication system is in accordance with G.fast.