On the Effect of Radio Channel Propagation Models - Oulu

able to track two separate channels simultaneously. In our
spread spectrum approach, nodes are able to track transmissions
with two different spreading codes. One is a
common code channel (C-code), which is used for route
maintenance and broadcast communications. The other is a
receiver based code channel (R-code), which is unique for
each node, unlike the C-code. R-code is used for all directed
transmission (e.g., data) per hop basis. [6]
Since BCCA is a channel access method to enable distributed
CDMA-like (Code Division Multiple Access) communications
for ad hoc networks, a medium access control
(MAC) method is also needed. BC-MAC is designed to be
used with BCCA. It is a straightforward CSMA-based
(Carrier Sense Multiple Access) MAC solution with fast
ARQ (Automatic Repeat Request) implementation. BC-
MAC with BCCA has been shown to have very high performance
in ad hoc environment. [7]
With the use of BCCA, it is required that the R-type
spreading codes of the nodes are known, For the cases
where the codes are unknown, an efficient network layer
spreading code distribution method is being developed.
However, we will use the assumption of known codes in
this paper, since it is a fair assumption in military scenarios.
Hereafter, when we refer to BC-MAC, it is assumed
that BCCA functionality is also implemented to it.
B. IEEE 802.11
The basic IEEE 802.11 (later referred to as 802.11) is considered
as a point of comparison. The modeling of 802.11
is based on the basic OPNET model of 802.11 with some
modifications to enable the functionality with the used
routing protocol (as in our previous studies [6], [7]). Some
modifications were also needed to enable the possibility to
use different propagation models, but no functional
changes to the MAC have been made.
The DSSS (Direct Sequence Spread Spectrum) mode of
802.11 is used, since DS spreading is also used in BC-
MAC. Virtual carrier sensing is not used, because it produces
additional overhead [8], which is not desired (multiple
handshaking transmissions, latency, capacity decrement
under heavy traffic). Carrier sensing is done by
power level measurement with a threshold in both 802.11
and BC-MAC. This threshold is set according to the power
level, where the reception is just more likely to succeed
than fail in theory.
C. The Physical Layer
At the physical layer, channel data rate is set to 1 Mbit/s.
To allow potentiality for interference suppression and to
enhance LPD (Low Probability of Detection) and LPI
(Low Probability of Interception) properties, sufficient
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spreading factor is needed. Spreading code length of Nc
= 63 chip is used. With bit-wise spreading, processing
gain is directly 63 (- 18.0 dB). To make fair comparisons
between BC-MAC and 802.11, the same spreading
factor is also set to 802.11 instead of the standard value
of 11. The frame formats are set to be equal according to
- 802.11. The systems operate on 2.4 GHz ISM (Industrial,
Scientific and Medical) frequency band.
B. The Network Layer
The network layer functionality is kept simple enough so
that it does not provide any extra effects to the results.
The functionality is mainly under control of Ad hoc Ondemand
Distance Vector (AODV) routing protocol with
some basic characteristics of IP (Internet Protocol). But
for example address resolution protocol (ARP) is not
used, instead, it is assumed for simplicity, that the MAC
and network layer share the same address base.
AODV is a reactive pure on-demand type routing protocol,
where routes are formed and updated only when
needed [9]. The used OPNET model of AODV is the
same as in our previous work (e.g., [6], where it is also
described how to enable BCCA functionality with
AODV). AODV hello-messages and passive listening at
MAC layer are not used, because in military scenario,
we want to avoid extra control traffic
C. Transport & Application Layer
In order to keep the focus on lower layer functionalities,
it is not desired that the upper layers will add complexity
and more parameters to the currently already quite large
set of parameters. Thus, transport layer functionalities
are minimized and the layer above network layer is basically
the application layer. In a sense, the transport layer
can be assumed to exist as UDP-lite (Lightweight User
Datagram Protocol, [10]), if one assumes that the UDP
fields exist in the data packet.
Application layer is modeled statistically with traffic
models. VBR-M [6] model is used as a traffic model,
and the data packet length has a constant value of 4096
bits. Because the focus is on lower layer functionalities,
heavy tailed modern data traffic models are not used,
since they also introduce their own effects to the network
performance (e.g., [11]). As a session model, dynamic
connections are used [7].
E. Simulation Scenarios and performance metrics
This study is based on OPNET simulations. A basic ad
hoc scenario is chosen, where the operational area has a
size of 1500 m x 300 m and the nodes have effective
radio range of about 250 m. The area contains 50 nodes,
of which 20 run applications (i.e., generate traffic), but