Link level performance comparison between LTE V2X and DSRC 103
2.1 DSRC
The US FCC (Federal Communication Commission)
allocated 75 MHz of the spectrum for V2V and
V2I communications. The main purpose is to en-
able safety-related applications in vehicular networks
to improve traffic conditions and prevent accidents
(traffic safety). There are two types of channels in
DSRC, each of them with a 10-MHz bandwidth: the
SCH (Service Channel) and the CCH (Control Chan-
nel). SCHs are available both for safety and non-
safety use, and CCHs are restricted to safety com-
munications only. Applications for vehicular com-
munications can be placed in three main categories:
traffic safety, traffic efficiency and value-added ser-
vices (e.g., infotainment/business)
[15-17]
.
The DSRC band is a free yet licensed spectrum.
Because the FCC does not charge a fee for spectrum
usage, it is free. However, we should not confuse this
with the unlicensed bands at 900 MHz, 2.4 GHz, and
5 GHz, which are also free for use. These unlicensed
bands place no restrictions on the technologies other
than some emission and co-existence rules. On the
other hand, usage of the DSRC band is more re-
stricted. FCC rulings regulate usage within certain
channels and limit all radios to be compliant with a
standard. In other words, although DSRC is limited
in transmission power with regard to the unlicensed
band, one cannot develop a different radio technol-
ogy (e.g., one that uses all 75 MHz of the spectrum)
in the DSRC band. These DSRC usage rules are
referred as “license by rule.”
2.1.1 Physical layer architecture
The physical layer standard is made up of two sub-
layers: the PMD (Physical Medium Dependent) sub-
layer and PLCP (Physical Layer Convergence Proto-
col) sublayer. The PMD sublayer defines the param-
eters to establish the signal, such as channel coding,
modulation, and demodulation. On the other hand,
the PLCP sublayer deals with interference between
different PHY layers and makes sure that the MAC
layer receives the data in a common format, inde-
pendent of the particular PMD sublayer. Through
the correspondent SAPs (Service Access Points), the
PLCP communicates with the PMD sublayer and
MAC layer.
2.1.2 Coding scheme: convolutional code
In DSRC, convolutional code is used. This is also
called NSC (Nonsystematic Convolutional) codes.
2.1.3 DSRC frequency offset estimation al-
gorithm
For the DSRC receiver, there are two steps to es-
timate and correct frequency errors. The detailed
steps can be seen in Algorithm 1.
Algorithm 1 DSRC frequency offset estimation
1: The short training sequences (for coarse frequency off-
set estimation) and the long training sequence (for fine
frequency offset estimation) are utilized in the PLCP
preamble to correct the frequency error, and the inte-
ger and non-integer parts of the frequency error can
be corrected at the same time;
2: Four pilot subcarriers of every OFDM symbol are used
for carrier phase tracking to alleviate the residual fre-
quency error and phase noise.
The short training sequence is defined as 1.6 us,
and the FFT/IFFT period for the 10-MHz band-
width is 6.4 us in IEEE 802.11p protocols. Because
the phase offset of the neighboring short training se-
quence is limited to π, the maximum frequency error
estimation of the 10-MHz DSRC can be two times
the subcarrier spacing, theoretically. Though the
fixed frequency error X = 40 ppm (236 kHz) for
DSRC is higher than the subcarrier spacing of 156
kHz, the fixed frequency error of 236 kHz is lower
than the theoretical maximum frequency error esti-
mation of 312 kHz of the 10-MHz DSRC. Then, the
frequency compensation can be based on the estima-
tion of the frequency error. The fixed frequency error
of 40 ppm (236 kHz) can be compensated with short
and long training sequences simultaneously for the
integer and non-integer parts of the frequency error.