We propose a MAC protocol for very low radiated power (1 micro-watt)
ultra-wide band (UWB) mobile networks. Some specifics of UWB, compared
to narrowband are that it is optimal to send short pulses and that
carrier sensing is impossible. First, we exploit this former property
and propose an interference mitigation scheme, at the physical layer,
which greatly reduces, but does not entirely cancel, the impact of
interference. Second, we analyze how the optimal MAC protocol should
be designed, and find that it should not use mutual exclusion (as is
commonly done by random access or TDMA protocols) but, in contrast,
should allow interference to occur and adapt to it. With the optimal
MAC protocol, competing sources are able to send concurrently, causing
rate reductions instead of collisions. Third, we design a MAC protocol
accordingly. It is made of two components: ``Dynamic Channel Coding"
and ``Private MAC". Through the use of the former component, we can
allow sources to send simultaneously at the maximum power permitted by
hardware and regulation constraints. Sources then adapt to
interference by dynamically adjusting their channel codes (thus their
bit rates). Such an approach is suggested by information theoretical
results; it sharply contrasts with the traditional, alternative
interference management method that would control transmission power
instead of channel code. We show by numerical analysis and detailed
simulations in ns-2 that our approach is indeed superior. The latter
component (private MAC) solves the contention between sources
competing for the same destination; it is required because nodes are
assumed to be able to receive from only one source at a time. We solve
the problem of absence of carrier sensing by a combination of
invitation and request, and the use of predictable time hopping
sequences. No common channel is used; this avoids the issues of hidden
and exposed terminals altogether. Our MAC protocol integrates both
components in a single design, and is entirely distributed. It is
fully implemented in ns-2. We show by simulation that we achieve a
significant increase in network throughput, compared to MAC protocols
for UWB that use the traditional approaches of power management or
mutual exclusion. Our work shows that it is not optimal to simply
port to UWB the design of existing MAC protocols, which were invented
for narrowband systems. Our distributed MAC protocol also appears to
be the first of its kind to apply channel code adaptation as an
alternative to mutual exclusion and power management.
People
Jean-Yves Le Boudec
Ruben Merz
Bozidar Radunovic
Jörg Widmer
Papers
- R. Merz, J. Widmer, J.-Y. Le Boudec, and B. Radunovic. A joint
PHY/MAC architecture for low-radiated power TH-UWB wireless ad-hoc
networks. Special Issue on UWB Communications in Wireless
Communications and Mobile Computing Journal, 2005.
- J.-Y. Le Boudec, R. Merz, B. Radunovic, and J. Widmer. DCC-MAC:
A decentralized MAC protocol for 802.15.4a-like UWB mobile ad-hoc
networks based on dynamic channel coding. In First International
Conference on Broadband Networks (BroadNets 2004), San Jose, CA,
October 2004.
- R. Merz, J.-Y. Le Boudec, J. Widmer, and B. Radunovic. A
rate-adaptive MAC protocol for low-power ultra-wide band ad-hoc
networks. In 3rd International Conference on AD-HOC Networks and
Wireless (Ad-Hoc Now), Vancouver, British Columbia, Canada, July
2004.
Errata: The version in the proceedings contains a small typo. On page
4 it should read "When S with codeIndex=i
in the cache receives an ACK with codeIndex=i ',
if i '<i then S sets codeIndex to
i-1, else it sets codeIndex to i '."
(instead of i+1).
- Older technical report:
J. Y. Le Boudec, R. Merz,
B. Radunovic, J. Widmer "A
MAC protocol for UWB Very Low Power Mobile Ad-hoc Networks based on
Dynamic Channel Coding with Interference Mitigation", Technical
report No. IC/2004/02, January 2004.
- Poster
presented at the NCCR-MICS Scientific Conference, Monte Verita Ascona,
October 13 to 15, 2003
Available code
- The most up-to-date ns-2 code of the UWB MAC and PHY layers
can be found here
- Example how to use the UWB PHY and
MAC
- An old version of the code that was used for the technical
report below is still available but we discourage its use since it contains some minor bugs.
Modulation
BER tables derived from Matlab simulations.
Physical Layer
Moved the scheduling of packet reception from the MAC layer to the
PHY. Also keeps a list of concurrent packets for the interference
calculation.
MAC Layer
TBD
Propagation Model
We implemented the UWB path loss model described in S.S. Ghassemzadeh
and V. Tarokh, "UWB path loss characterization in residential
environments", IEEE Radio Frequency Integrated Circuits (RFIC)
Symposium, June 2003. It is derived from UWB indoor measurements. With
the default values, different random numbers according to the
distribution given in the paper are drawn for each new packet. To have
deterministic path loss set:
Propagation/Tarokh set sigma_g_ 0.0
Propagation/Tarokh set mi_s_ 0.0
Propagation/Tarokh set sigma_s_ 0.0