\iffalse
%%% A brain dump of findings from literature. Will need to be
%%% organized later.

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IEEE Communications May 08 issue
David Matolak
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Channel characteristics, such as severe channel fading, etc cause
packet losses and hence retransmissions.  This impacts higher level
protocol designs.  Mitigation techniques exist at the physical layer
through schemes such as FEC, antenna diversity, rapid power control
and equalization.  At higher layers, one could think of network coding
and cooperative retransmissions.  Nevertheless, one always needs good
models of the wireless channel to devise any such schemes.

Wireless channels can be dispersive or non-dispersive; the former is
one where the phase velocity is a function of frequency.  Wideband
signals encounter dispersion more than narrowband.  

To model a wireless channel, a most effective means is to develop a
statistical model the channel impulse response (CIR).  Note that due
to reflection and other factors, multiple replicas of the original
signal travel with potentially some phase lag among each other.

Another term used heavily in channel models is the Wide Sense
Stationary (WSS) in time => uncorrelated Doppler shifts at different
frequencies within the channel band.  Moreover, scattering occurring
at different delays are uncorrelated (US) giving rise to a combined
model called WSSUS.

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IEEE Communications May 08 issue
Lin Cheng et. al
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Need for widely used, inexpensive onboard devices is very high.  This
is possible using commodity 802.11 devices (DSRC).  DSRC uses a 75MHz
spectrum at 5.9 GHz for 802.11p (DSRC/WAVE).  802.11p is a scaled down
version of 802.11a from 20MHz channel bandwidth to 10 MHz.  The reason
for this scaling is to increase the tolerance to delay spread.

The paper illustrates properties such as coherence time, Doppler
spread, coherence bandwidth, and delay spread.

A V2V channel is a multipath channel, i.e., a signal arrives at a
receiver via various paths with different delays.  Thus, if a short
pulse is transmitted, several echoes with different amplitudes and
spacing will show up at the receiver.  The ``maximum excess delay'' is
the difference between the first response and the last echo whose
signal exceeds some preset threshold.

In 802.11a which uses the orthogonal frequency division multiplexing
(OFDM) is a multicarrier transmission scheme.  It spreads data over
the multiple carriers.  The spacing between carriers is maintained
such that the frequency intervals are smallest possible and that the
frequencies are othogonal to each other over the duration of the
symbol. 

Echoes of one symbol may very well overlap with echoes of a subsequent
echo.  To distinguish between symbols, each signal for a symbol is
prepended with a cyclic prefix called a Guard Interval (GI).
Naturally, the length of a GI chosen represents the bandwidth
available for real data versus reliability.  

Coherence time of a channel is that time when the packet remains
unchanged despite fluctuations in amplitude and fading,
etc. Naturally, the duration of a packet therefore must be smaller
than the coherence time.

In general, the GI must be larger than the max excess delay.  The
carrier spacing must be smaller than the coherence bandwidth but much
larger than the Doppler spread.  Finally, the intervals between
channel estimation, i.e., duration of a packet, must be smaller than
the coherence time of a channel.


%% Ideas on what to write. 
%% Talk about us using commodity wireless networks based on 802.11x
%% and the upcoming 802.11p standard.
%% We say that the combined effect of the
%% above makes it hard to develop applications. Now demonstrate a
%% series of such challenges. First we outline the V2I challenges.
% (1) time window to talk to an AP: depends on speed, distance and
% radio power.   Show graphs of speed versus time window, power versus
% time window.  Show ARP issues.
% (2) radio range and handoff: AP re-association, ARP refresh, routing
% table changes, potential TCP retransmissions due to network
% connection loss. Highlight the Mobile IP vs DHCP soln.
%
% Next, showcase V2V challenges - ad hoc nature, tcp behavior.

%%%%% Arrangement%%%%%
%
% (1) Scanning for AP with and without active scanning for different
% speeds and radio powers. Impact with increasing number of vehicles.
% (2) Use non association but now demonstrate data transfer using
%     TCP. SYN packet itself leads to ARP requests. There might be
%     many retransmissions until you arrive in range. So appln must
%     somehow be restricted.
%