Soft handoff for OFDM

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.

Description

FIELD OF THE INVENTION [0001] The present invention relates to wireless communications, and in particular to facilitating soft handoffs in orthogonal frequency division multiplexing (OFDM) systems. BACKGROUND OF THE INVENTION [0002] Wireless communication systems divide areas of coverage into cells, each of which is served by a base station. A mobile terminal will continuously monitor the signal strengths of the servicing base station for the current cell as well as for adjacent cells. The mobile terminal will send the signal strength information to the network. As the mobile terminal moves toward the edge of the current cell, the servicing base station will determine that the mobile terminal's signal strength is diminishing, while an adjacent base station will determine the signal strength is increasing. The two base stations coordinate with each other through the network, and when the signal strength of the adjacent base station surpasses that of the current base station, control ...

AI Technical Summary

Problems solved by technology

Hard handoffs have proven problematic and often result in dropped calls.

Unfortunately, these systems generate strong co-channel interference, especially at cell borders.

Basic frequency reuse-one planning will lead to very low data rates and a poor quality of service for mobile terminals at cell borders.

Even though data repetition, re-transmission techniques, and fairness scheduling for data transmission can be employed, it is difficult to equalize data rate distribution across the cell.

Accordingly, high-speed data service is severely limited.

However, a straightforward extension of soft handoff techniques developed for CDMA systems is not applicable to the MIMO-OFDM systems, since the separation of the interference for the OFDM waveform is virtually impossible.

Because different spreading code masking is not available in OFDM transmission, the destructive interferences between base stations transmitting the same signal can cause significant degradation of performance.

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