SOFT HANDOFF FOR OFDM

• US 20110096751A1
• Filed: 01/03/2011
• Published: 04/28/2011
• Est. Priority Date: 03/22/2002
• Status: Active Grant

• Current Assignee: Microsoft Technology Licensing LLC (Microsoft Corporation)
• Original Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Inventors: Yu, Dong-Sheng, Tong, Wen, Ma, Jianglei, Zhu, Peiying, Jia, Ming
• Granted Patent: US 8,619,713 B2
• Time in Patent Office: Days
• US Class Current: 370/331
• CPC Class Codes: H04B 7/022 Site diversity; Macro-diversity using two or more spaced independent antennas H04B7/04
• CPC Class Codes: H04L 1/0618 Space-time coding
• CPC Class Codes: H04L 25/0232 by interpolation between sounding signals
• CPC Class Codes: H04L 27/2613 Structure of the reference signals
• CPC Class Codes: H04L 27/26134 Pilot insertion in the transmitter chain, e.g. pilot overlapping with data, insertion in time or frequency domain
• CPC Class Codes: H04L 27/2626 Arrangements specific to the transmitter only
• CPC Class Codes: H04L 27/2647 Arrangements specific to the receiver only equalisation H04L27/01
• CPC Class Codes: H04L 27/2655 Synchronisation arrangements
• CPC Class Codes: H04L 5/0007 the frequencies being orthogonal, e.g. OFDM(A), DMT
• CPC Class Codes: H04L 5/0023 Time-frequency-space
• CPC Class Codes: H04L 5/0025 Spatial division following the spatial signature of the channel
• CPC Class Codes: H04L 5/0037 Inter-user or inter-terminal allocation
• CPC Class Codes: H04L 5/0044 allocation of payload
• CPC Class Codes: H04L 5/0048 Allocation of pilot signals, i.e. of signals known to the receiver
• CPC Class Codes: H04W 36/18 for allowing seamless reselection, e.g. soft reselection
• CPC Class Codes: H04W 36/304 due to measured or perceived resources with higher communication quality

Abstract

The present invention relates to soft handoffs in an OFDM system. Each mobile terminal measures pilot signal strengths of transmissions from adjacent base stations. If the pilot signal strength for a base station exceeds the defined threshold, that base station is added to an active set list. Each mobile terminal notifies the base stations of their active set lists. By providing the set list to the base station controller and the servicing base station, the mobile terminal identifies the sole servicing base station or triggers a soft handoff mode when multiple base stations appear on the active set list. The soft handoff mode uses a combination of scheduling and space-time coding to affect efficient and reliable handoffs.

Claims

1. A system adapted to schedule data for a mobile terminal during a soft handoff mode and deliver at least a portion of scheduled data to the mobile terminal from at least one of a plurality of base stations comprising each base station participating in the soft handoff being adapted to:

a) process the scheduled data for delivery to the mobile terminal;

b) provide coding for the scheduled data to generate a plurality of spatially coded signals;

c) perform a transform on each of the plurality of spatially coded signals to generate a plurality of orthogonal frequency division multiplexed signals, each of the plurality of spatially coded signals configured to result in corresponding frequency division multiplexed orthogonal signals mapped into defined sub-bands in an orthogonal frequency division multiplexed spectrum of sub-bands, the defined sub-bands not used by other ones of the base stations participating in the soft handoff; and

d) transmit the plurality of orthogonal frequency division multiplexed signals with spatial diversity for reception by the mobile terminal.

2. The system of claim 1 wherein each base station is further adapted to provide the coding such that the plurality of coded signals are configured to result in a power boost associated with the defined sub-bands.

3. The system of claim 1 wherein each base station is further adapted to provide the coding such that the plurality of coded signals are configured to result in a null associated with sub-bands in the orthogonal frequency division multiplexed spectrum other than the defined sub-bands.

4. A base station in an orthogonal frequency division multiplexing system comprising:

a) encoding logic adapted to provide spatial coding of scheduled data to generate a plurality of spatially coded signals;

b) transform logic adapted to provide a transform on each of the plurality of spatially coded signals to generate a plurality of orthogonal frequency division multiplexing signals, wherein during a soft handoff, each of the plurality of spatially coded signals is configured to result in corresponding orthogonal frequency division multiplexing signals mapped into defined sub-bands in an orthogonal frequency division multiplexing spectrum of sub-bands, the defined sub-bands not used by other base stations participating in the soft handoff; and

c) transmit circuitry having a plurality of transmit paths to facilitate transmitting the plurality of orthogonal frequency division multiplexing signals with spatial diversity for reception by the mobile terminal.

5. A method for facilitating soft handoffs in an orthogonal frequency division multiplexing system comprising:

a) providing spatial coding of scheduled data to generate a plurality of spatially coded signals;

b) generating a transform on each of the plurality of spatially coded signals to generate a plurality of orthogonal frequency division multiplexing signals, wherein during a soft handoff, each of the plurality of space-time coded signals is configured to result in corresponding orthogonal frequency division multiplexing signals mapped into defined sub-bands in an orthogonal frequency division multiplexing spectrum of sub-bands, the defined sub-bands not used by other base stations participating in the soft handoff; and

c) transmitting the plurality of orthogonal frequency division multiplexing signals with spatial diversity for reception by the mobile terminal.

6. A mobile terminal for facilitating soft handoffs in an orthogonal frequency division multiplexing system comprising:

a) receive circuitry adapted to receive and downconvert a plurality of orthogonal frequency division multiplexing signals, which were encoded using spatial encoding and transmitted using spatial diversity;

b) transform logic adapted to provide a transform on each of the plurality of orthogonal frequency division multiplexing signals to generate a plurality of spatially coded signals;

c) decoder logic adapted to provide spatial decoding on the plurality of spatially coded signals to recover scheduled data from servicing base stations;

d) processing logic adapted to;

(i) monitor signal strength from a plurality of base stations;

(ii) identify ones of the plurality of base stations having signal strength over a defined threshold; and

(iii) enter a soft handoff mode when more than one of the plurality of base stations have a signal strength over the defined threshold; and

e) transmit circuitry adapted to transmit information identifying the ones of the plurality of base stations having signal strength over the defined threshold.