Downlink network synchronization mechanism for femtocell in cellular OFDM systems
A method of downlink synchronization for a femto base station in a cellular orthogonal frequency division multiplexing (OFDM) system is provided. The femto base station first scans one or more received reference signals transmitted from a plurality of neighboring macro base stations. The femto base station then determines a desired reference signal from the received one or more reference signals based on the scanning result. Finally, the femto base station configures its downlink radio signal transmission time based on the desired reference signal such that the femto base station is synchronized with the plurality of neighboring macro base stations.
1. A method for downlink transmission synchronization in a cellular orthogonal frequency division multiplexing (OFDM) system, the method comprising:
(a) scanning and receiving a plurality of reference signals transmitted from a plurality of neighboring macro base stations by a femto base station;
(b) determining a desired reference signal by selecting from the received plurality of reference signals during a predefined observation window, wherein the observation window length is smaller than half of a frame length but substantially larger than propagation delay among the plurality of neighboring macro base stations; and
(c) configuring downlink radio signal transmission of the femto base station based on the desired reference signal such that the femto base station is synchronized with the plurality of neighboring macro base stations.
2. The method of claim 1 , wherein downlink radio signals transmitted by the plurality of neighboring macro base stations are synchronized before the scanning the plurality of reference signals by the femto base station.
3. The method of claim 1 , wherein two base stations are synchronized when arrival time difference for radio signals transmitted from the two base stations is smaller than a guard interval of an OFDM symbol.
4. The method of claim 1 , wherein the desired reference signal is a selected reference signal first received by the femto base station during the observation window.
5. The method of claim 1 , wherein the desired reference signal is a downlink preamble transmitted at a starting boundary of a downlink frame, wherein the preamble is received by the femto base station at a certain timing, and wherein the femto base station transmits a downlink frame at the same certain timing.
6. The method of claim 1 , wherein the desired reference signal is transmitted during a downlink frame with an offset, wherein the desired reference signal is received by the femto base station at a certain timing, and wherein the femto base station transmits a downlink frame at the same certain timing with the same offset.
7. The method of claim 1 , wherein the plurality of reference signals comprise a reference signal transmitted by a base station with multipath effect.
8. The method of claim 1 , wherein the femto base station is an access-point base station used for indoor coverage in the cellular OFDM system, and wherein the femto base station is connected to a backhaul server through a normal broadband physical link.
9. The method of claim 1 , wherein the femto base station and each of the plurality of neighboring macro base stations have overlapping cell coverage.
10. A femto base station in a cellular orthogonal frequency division multiplexing (OFDM) system, the femto base station comprising:
a radio frequency (RF) module that receives a plurality of reference signals transmitted by a plurality of neighboring macro base stations;
a timing detector that detects a corresponding arrival time of each of the reference signals;
a timing abstractor that determines a desired timing reference based on the detected arrival time of a desired reference signal determined by selecting from the plurality of reference signals during a predefined observation window, wherein the observation window length is smaller than an OFDM frame length but substantially larger than propagation delay among the plurality of neighboring macro base stations; and
a timing configuration module that configures downlink transmission timing of the femto base station based on the desired timing reference such that the femto base station is synchronized with the plurality of neighboring macro base stations.
11. The femto base station of claim 10 , wherein two base stations are synchronized when arrival time difference for radio signals transmitted from the two base stations is smaller than a guard interval of an OFDM symbol.
12. The femto base station of claim 10 , wherein the desired reference signal is a reference signal first received by the femto base station during the observation window.
13. The femto base station of claim 10 , wherein the desired reference signal is a downlink preamble transmitted at a starting boundary of a downlink frame, wherein the preamble is received by the femto base station at the reference timing, and wherein the femto base station transmits a downlink frame at the same reference timing.
14. The femto base station of claim 10 , wherein the desired reference signal is transmitted during a downlink frame with an offset, wherein the desired reference signal is received by the femto base station at the reference timing, and wherein the femto base station transmits a downlink frame at the same reference timing with the same offset.
15. The femto base station of claim 10 , wherein the plurality of reference signals comprise a reference signal transmitted by a base station with multipath effect.
16. The femto base station of claim 10 , further comprising:
a correlation module that correlates one or more received sequences in frequency domain such that the timing detector detects the corresponding arrival times of the plurality of received reference signals in time domain.
17. The femto base station of claim 10 , wherein the femto base station is an access-point base station used for indoor coverage in the cellular OFDM system, and wherein the femto base station is connected to a backhaul server through a normal broadband physical link.
18. A femto base station in a cellular orthogonal frequency division multiplexing (OFDM) system, the femto base station comprising:
a radio frequency module that receives a plurality of reference signals transmitted by a plurality of neighboring macro base stations; and
means for detecting a corresponding arrival time of each of the plurality of reference signals and thereby determining a desired timing reference of a desired reference signal by selecting from the plurality of reference signals during a predefined observation window, wherein the observation window length is smaller than half of a frame length but substantially larger than propagation delay among the plurality of neighboring macro base stations, and wherein the femto base station configures downlink transmission timing based on the desired timing reference such that the femto base station is synchronized with the plurality of neighboring macro base stations.
19. The femto base station of claim 18 , wherein two base stations are synchronized when arrival time difference for radio signals transmitted from the two base stations is smaller than a predefined value, and wherein the predefine value is smaller than a guard interval of an OFDM symbol.
20. The femto base station of claim 18 , wherein the desired reference signal is a reference signal first received by the femto base station during the observation window.
21. The femto base station of claim 18 , wherein the means comprises a timing detector and a timing abstractor that detects and determines the desired timing reference.