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The temporal behavior of the optical frequency emitted by several DFB lasers under direct square wave modulation was measured using an all-fiber implementation of a Mach-Zender interferometer with an imbalance of 30 ps. The impulse response of the optical frequency to injection current modulation was found to contain a time constant as short as 10-20 ns, together with a few longer ones. The existence of such a short time constant is consistent with a thermal analysis of a laser structure with finite thermal impedance of the active region and should be taken into consideration in various wide bandwidth applications of direct modulated semiconductor lasers.

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In this paper, a strategy for exploiting the flow control signaling between the RNC and Node B for UMTS (Universal Mobile Telecommunications System) HSDPA (High Speed Downlink Packet Access) is presented. The flow control manages the transfer of data to the scheduler, that is located at the Node B. Simulation of a TDD mode cell demonstrates that the strategy achieves near minimum latency whilst at the same time limiting the data loss on inter-Node B handover. The loading of the flow control signaling on the Iub interface is acceptable.