On Spatial Reuse and Capture in Ad Hoc Networks
University of South Carolina
University of South Carolina
Romit Roy Choudhury
Abstract—Neighbors of both the transmitter and the receiver
must keep quiet in a 802.11 wireless network as it requires
bidirectional exchange, i.e., nodes reverse their roles as trans-
mitters and receivers, for transmitting a single DATA frame.
To reduce role reversals and to improve spatial reuse, a pig-
gybacked acknowledgment based approach has been proposed
to enable concurrent transmissions. Recent findings on physical
layer capture show that it is possible to capture a frame of
interest in the presence of concurrent interference and that the
SINR threshold is dependent on the relative order in which
the frame and the interference arrive at the receiver. In this
paper, we show that it is possible to exploit capture and increase
concurrent transmissions in wireless adhoc networks. We develop
a distributed channel access scheme and demonstrate that it
offers significant throughput gain particularly at lower data rates.
Achieving optimal network capacity for wireless networks
in the presence of interference is a challenging task and it is
fundamentally related to spatial reuse. Efficient spatial reuse
is inhibited by interference, limitations of MAC protocols,
external noise and many other physical factors. The 802.11
protocol with its virtual carrier sensing has role reversals which
reduce the hidden node problem but introduce the exposed
node problem, further restricting spatial reuse.
Multiple packets arriving at a receiver are generally consid-
ered to cause packet loss due to the collision at the receiver.
For this reason, nodes in a wireless network avoid transmitting
concurrently to mitigate interference at the expense of spatial
reuse. However, there have been several studies that have
shown that a sufficiently stronger frame can still be success-
fully received by the receiver in spite of a collision  .
This phenomenon is called physical layer capture (PLC).
If we approach the concurrent transmission problem with
the knowledge of this interesting effect, there is scope for
improvement although the role reversals are still a hurdle. In
this paper, we propose a MAC protocol which reduces role re-
versals and takes advantage of the PLC to improve the number
of concurrent transmissions in wireless adhoc networks. Our
MAC protocol makes use of the channel condition information
obtained by the physical layer in making a good assessment of
the channel and staggers transmissions to achieve concurrency.
The rest of the paper is organized as follows. Section II
provides the background, details of the capture model and the
method for reducing role reversals. Section III describes the
proposed capture-aware MAC protocol in detail. Section IV
presents the results of simulations in QualNet evaluating the
performance of our protocol. We compare our work with other
related works in section V before concluding in section VI.
Role Reversals: 802.11 networks counter the ill affects
of the hidden terminal problem by using physical carrier
sensing and the 4-phase (RTS-CTS-DATA-ACK) MAC pro-
tocol. In this protocol, each node reverses its role (transmitter
to receiver and vice versa) twice for delivering one DATA
frame. With role reversals, all nodes around the transmitter
and receiver will cautiously remain silent even if they do not
affect the reception. This is called the exposed node problem
which greatly reduces the spatial reuse in 802.11 networks.
Several schemes like  have been suggested to alleviate
this problem by making optimizations to the MAC protocols.
These protocols address the exposed node problem to some
extent, but the primary condition that the SINR value be
above a high threshold seriously limits the possibilities. Two
neighbors cannot transmit simultaneously unless the SINR
value at each of their receivers is greater than a high threshold.
Alleviating Role Reversals If there was no ACK phase
in the protocol, two nodes can transmit DATA simultaneously
without worrying about the reception of ACK and the exposed
sender problem can be solved partially. But, the ACK phase
is the only way a node can know about the success/failure of
a transmitted DATA packet. We proposed a remedy for this
problem using a piggybacked ACK mechanism in
we use the same mechanism in this work. The piggybacked
ACK mechanism encapsulates acknowledgements for multiple
neighbors in each of the packets (RTS, CTS, DATA) transmit-
ted by a node. Eliminating the ACK phase reduces one role
reversal and removes one hurdle for concurrency.
Fig. 1. Concurrent transmissions possible due to physical layer capture effect
even when all nodes are within the range (357m for 12Mbps) of each other.
Capture Effect: The phenomenon of physical layer capture
was characterized in by experimentation. The authors
demonstrated that a stronger frame can be received correctly
even if it starts after the beginning of an interfering frame.
An example of capture corresponding to Fig. 1 is shown in
Fig. 2. In this sample topology, R1 can receive a packet from
S1 in spite of the interference from S3, even if the packet from
80 100 120 140 160
Aggregate Throughput in Mbps
Max. Hop Distance
80 100 120 140 160
Percentage improvement over 802.11
Max Hop Distance
Fig. 8. Random topology with 1-hop flows with varying max hop distance: (a) aggregate throughput for 12Mbps; (b) percentage gain for all data rates.
the authors allow a secondary DATA-only transmission to
take place if it is smaller than the primary DATA. In ,
nodes distributedly decide when to transmit simultaneously
by making use of the received signal strength metric and the
RTSS/CTSS messages. This approach is interesting but it does
not take capture into account. In  the authors propose a
centralized power and rate control algorithm to improve spatial
reuse. In  the authors study the effect of carrier sensing
and power control and conclude that a product of both should
be a predetermined constant to achieve optimal spatial reuse.
The use of piggybacked ACK instead of the explicit 802.11
ACK phase was proposed for reducing role reversals  and
for improving throughput .
Many theoretical models like  have been proposed to
explain physical layer capture. The first empirical evidence of
capture we know of is  which defined the packet timing
conditions for capture. The recent study in  quantifies
the SINR threshold requirements for 802.11a networks under
different packet arrival timings and gives a clear picture of
this phenomenon. A similar work for low power wireless
networks was done in . Capture awareness has been used
for collision resolution in . In , the authors propose
tuning the carrier sense threshold and show that there is scope
for improvement if nodes are capture aware. The unfairness
caused by capture is discussed in  and BER models for
capture were proposed in . In , a scheme is proposed
to perform suitable beam forming and avoid ’capture’ of
packets by directional antennas in their idle state. This capture
refers to locking on to an arriving signal and is different from
the capture effect discussed in our current work. An O(n2)
algorithm for estimating link state interference in multihop
wireless networks was proposed in  and a linear order
algorithm that takes capture into account was presented in .
VI. CONCLUSIONS AND FUTURE WORK
Spatial reuse in wireless networks is limited by the SINR
threshold requirements. This problem is amplified because of
role reversals in wireless networks. Physical layer capture can
improve the spatial reuse by staggering the transmissions. In
this work we explored the possibilities by combining reduced
role reversals with capture. Our simulation results show that
the number of concurrent transmissions can be improved
significantly though the scope for improvement reduces with
the higher data rates for which the SINR requirements are
very high. Our ongoing work includes further evaluation of the
protocol and to develop distributed and centralized protocols
for improving the performance of fixed wireless networks.
 A. Kochut, A. Vasan, A. U. Shankar, and A. Agrawala, “Sniffing out the
correct physical layer capture model in 802.11b,” in ICNP, Oct. 2004.
 J. Lee et al, “An experimental study on the capture effect in 802.11a
networks,” in WinTECH, Sept. 2007.
 A. Acharya et al, “MACA-P: A MAC protocol to improve parallelism
in multi-hop wireless networks,” in PERCOM, 2003.
 N. Santhapuri, J. Wang, Z. Zhong, and S. Nelakuditi, “Piggybacked-
Ack-aided Concurrent Transmissions,” in ICNP Poster Session, 2005.
 J. Padhye et al, “Estimation of link interference in static multi-hop
wireless networks,” in IMC, 2005.
 J. Lee et al, “Rss-based carrier sensing and interference estimation in
802.11 wireless networks,” in SECON, 2007.
 “Qualnet Network Simulator,” http://www.scalable-networks.com/.
 K. Mittal and E. M. Belding, “Rtss/ctss: Mitigation of exposed terminals
in static 802.11-based mesh networks,” in WiMesh, Sept. 2006.
 D. Shukla, L. Chandran-Wadia, and S. Iyer, “Mitigating the exposed
node problem in ieee 802.11 ad hoc networks,” in ICCCN, Oct 2003.
 T-S Kim, H. Lim, and J. Hou,
tuning transmit power, carrier sense threshold, and data rate in multihop
wireless networks,” in Proc. ACM Mobicom, 2006.
 J. Fuemmeler et al,“Selecting transmit powers and carrier sense
thresholds for csma protocols,” in UIUC TechReport, Oct. 2004.
 R. R. Choudhury, A. Chakravarty, and T. Ueda,
knowledgment: An improvement to 802.11,” in 4th IEEE/ACM Wireless
Telecommunications Symposium, Apr. 2005.
 O. Dousse M. Durvy and P. Thiran, “Modeling the 802.11 protocol under
different capture and sensing capabilities,” in Proc. IEEE Infocom, 2007.
 D. Son, B. Krishnamachari, and J. Heidemann,
transmissions in low-power wireless networks,” in SENSYS, 2006.
 K. Whitehouse et al, “Exploiting the capture effect for collision detection
and recovery,” in Emnets, May 2005.
 K. Jamieson et al, “Understanding the real world performance of carrier
sense,” in ACM SIGCOMM E-WIND Workshop, 2005.
 C. Ware, J. Chicharo, and T. Wysocki, “Unfainess and capture behavior
in 802.11 adhoc networks,” in ICC, June 2000.
 H. Chang et al, “A general model and analysis of physical layer capture
in 802.11 networks,” in Proc. IEEE Infocom, 2006.
 R. R. Choudhury and N. Vaidya, “Mac-layer capture: A problem in
wireless mesh networks using beamforming antennas,” in SECON, 2007.
“Improving spatial reuse through
“Implicit mac ac-