  A time projection chamber (TPC) with micropattern gaseous detector (MPGD)
readout is investigated as main tracking device of the International Large
Detector (ILD) concept at the planned International Linear Collider (ILC). A
prototype TPC equipped with a triple gas electron multiplier (GEM) readout has
been built and operated in an electron test beam. The TPC was placed in a 1 T
solenoidal field at the DESY II Test Beam Facility, which provides an electron
beam up to 6 GeV/c. The performance of the readout modules, in particular the
spatial point resolution, is determined and compared to earlier tests. New
studies are presented with first results on the separation of close-by tracks
and the capability of the system to measure the specific energy loss dE/dx.
This is complemented by a simulation study on the optimization of the readout
granularity to improve particle identification by dE/dx.
