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The IEEE 802.15.3 MAC enables high-rate communications between
devices in a wireless personal area network and has good support for
applications requiring quality of service (QoS). To meet
applications' QoS requirements, such as delay and jitter, the
channel time allocation (CTA) scheduler plays an important role in
sizing and positioning CTAs within each superframe. In this paper we
first present a novel CTA sharing protocol, called {\it VBR-MCTA},
that enables the sharing of CTAs belonging to streams with the same
group identity. This allows our protocol to exploit the statistical
characteristics of variable bit rate (VBR) streams by giving unused
time units to a flow that requires peak rate allocation. We then
present two optimizations to {\it VBR-MCTA}, namely {\it VBR-Blind}
and {\it VBR-TokenBus}. The former, by giving ownership of a CTA in
a round-robin manner without consideration to traffic profiles, does
not consume any valuable ``air-time'' with signaling overheads. The
latter allows a CTA to be shared by multiple devices that take turns
owning unused ``air-time'' from CTAs. We have simulated {\it
VBR-MCTA} and its optimizations in the {\it ns-2} simulator over an
implementation of the IEEE 802.15.3 MAC. Our results show that {\it
VBR-TokenBus} has the best delay and jitter as it provides a one to
six milliseconds reduction in both compared to standard CTA methods.
{\it VBR-Blind}, although having performing poorer than {\it
MCTA-Token} or {\it VBR-MCTA}, is still significantly better than
traditional CTA methods at a reduced overhead.
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