Table Of ContentFrequency Reuse, Cell Separation, and Capacity
Analysis of VHF Digital Link Mode 3 TDMA
Mohammed A. Sham', Thanh C. Nguyenl, and Rafael D. Apaza2
1A nalex Corporatiod'FAA Aviation Research Office
NASA Glenn Research Center
21000 Brookpark Road
Cleveland, Ohio 44135
Abstract: have a sufficient Signal to Co-Channel
Interference Ratio. Several different values
The most recent studies by the Federal for the Signal to Co-Channel Interference
Aviation Administration (FAA) and the Ratio were utilized corresponding to the
aviation industry have indicated that it has current analog VHF DSB-AM systems, and
become increasingly difficult to make new the future digital VDL Mode 3. The
VHF frequency or channel assignments to required separation of Co-Channel cells is
meet the aviation needs for air-ground computed for most of the Frequency
communications. FAA has planned for Protected Service Volumes (FPSV's)
several aggressive improvement measures to currently in use by the FAA. Additionally,
the existing systems, but these measures the ideal cell capacity for each FPSV is
would not meet the projected voice presented. Also, using actual traffic for the
communications needs beyond 2009. FAA Detroit air space, a FPSV traffic distribution
found that since 1974 there has been, on the model is used to generate a typical cell for
average, a 4 percent annual increase in the channel capacity prediction. Such
number of channel assignments needed to prediction is useful for evaluating the
satisfy the air-ground communication traffic improvement of future VDL Mode 3
(approximately 300 new channel deployment and capacity planning.
assignments per year). With the planned
improvement measures, the channel Introduction:
assignments are expected to reach a
maximum number of 16615 channels by The aeronautical frequency spectrum
about 2010. Hence, the FAA proposed the assignment in the CS is operating at near
use of VDL Mode 3 as a new integrated capacity and expecting to reach its
digital voice and data communications limitations by the year 2009. The growth
systems to meet the future air traffic rate of 4 percent annually in spectrum
demand. utilization, which corresponds to 300 new
channel assignments yearly, suggests a new
This paper presents analytical results of approach to information transmission and
frequency reuse; cell separation and capacity frequency management is needed [l], [2].
estimation of VDL Mode 3 TDMA systems The use of VDL-3 in the NEXCOM
that FAA has planned to implement the transceiver replacement program is designed
future VHF air-ground communications to provide the needed channel capacity and
system by the year 2010. For TDMA, it is to meet aviation growing demands for the
well understood that the frequency reuse near future. Very High Frequency Digital
factor is a crucial parameter for capacity Link (VDL) Mode 3 provides both
estimation. Formulation of this frequency Aeronautical Telecommunication Network
reuse factor is shown, taking into account (ATN) data and digital voice services. It
the limitation imposed by the requirement to was proposed to the International Civil
This is a preprint or reprint of a paper intended for presentation at a
conference. Because changes may be made before formal
publication, this is made available with the understanding that it will
not be cited or reproduced without the permission of the author.
Aviation Organization (ICAO) in 1994 by The next few sections cover, formulation of
the United States Federal Aviation the cell capacity, frequency reuse, and cell
Administration (FAA) as an alternative to separation showing the mathematical
using 8.33 KHz channel spacing to relieve equations utilized in the following section
VHF congestion. VDL Mode 3 works by on Results for different FF'SV using FAA
providing four logical independent channels and actual air traffic data. The last section is
in a 25 kHz frequency assignment. Each the Conclusion.
channel can be used for voice or data
transfer. The appealing capability of VDL Formulation of Cell Capacity,
Mode 3 is that it uses a frequency channel Frequency Reuse and Cell Separation:
that can carry one analog voice transmission
and turns it into three or four simultaneous
We start by following a mathematical
transmissions using Time Division Multiple
formulation based on work by [5-71 for the
Access (TDMA). There are seven
capacity of a single cell of a TDMA system:
configurations defined for VDL Mode 3 in
the ICAO VDL Mode 3 Standards and
Recommended Practices (SAFWs). VDL
Mode 3 uses the same modulation scheme as
VDL Mode 2, which is Differential 8 Phase
Shift Keying (DSPSK) at a data rate of 3 1.5
With the following parameters
kbps [3].
f,, ConesPonding
BWrotol? Nslors 7 BWchnnel,
to the total available bandwidth in the VHF
Considering the existing system will reach
band, the number of time slots of the TDMA
its limitations by year 2009, this paper
channel, the bandwidth of each VHF
presents an analysis of frequency reuse, cell
channel, and the Frequency Reuse factor.
separation, and capacity using VDL Mode 3
The frequency reuse factor is defined as the
TDMA. The analysis was performed
number of times the total bandwidth has to
utilizing existing FAA Air Traffic Control
be divided to use in each cell. Note that
airspace sector boundaries and VHF
other definitions of frequency factor
frequency allocation [4]. The service
correspond to the inverse of that defined
volume included En-route such as Super
here, meaning the number of times a
high (SE) altitude, High altitude (HE),
frequency can be reused in a network of
Intermediate altitude (E)an d Low altitude
cells.
En-route (LE). In addition, terminal service
volumes such as Approach control,
Here we rely mainly on [4], [5-71 for the
Departure control, and Local control among
derivation of the frequency factor to be used
others were utilized. The frequency
in the capacity formulas. The derivation is
allocations were obtained via two methods;
summarized for convenience. In Figure 1,
one utilized frequency allocations specified
the distance from the undesired user to the
by FAA [4] to obtain maximum ideal limits
on frequency reuse, cell separation and desired user is shown by the parameter D,
capacity for each of the service volumes while the distance from the center of the cell
using the 524 Air Traffic Control (ATC) to the desired user is approximated by the
channels presently available. The second radius of that cell Rd. On the other end, the
method attempted to get a more realistic radius of the cell serving the undesired user
distribution of the frequency allocations per is assumed to have a radius R,. The intent is
service volume based on actual air traffic to allocate frequencies to cells in a most
data. The air traffic data used was obtained efficient manner. That in turn implies
for the terminal area over the Detroit having the maximum possible reuse of
Airspace and en-route data for sectors frequencies with minimum possible distance
serving the southeast part of Michigan. such that there would not be an unacceptable
performance level at the desired user. Two provide a given level of performance with
factors come into play, one is the radio line respect to Bit Error Rates Bit Error Rate
of sight between the two users (desired and (BER) for the digital VDL-3 link, assuming
undesired), and the second is the given all measures that can tolerate better levels
requirements on signal to interference ratios are taken into account such as the Forward
(for co-channel interference only for this Error Correction (FER) and the modulation
study). The signal to co-channel tY Pe.
interference ratio is specified in order to
Undesired Distance D,,
Service Volume A
Service Volume B
t
hd
Hexag
- approximation
of a circle
I Reuse D is,tan,ce D
c c
Figure 1 Illustration of two co-channel cells (hexagon approximated), service volume height and
desired and undesired users.
Frequency Reuse based on Line of (assume it is for the desired user) is given
Sight: by:
In this case the radio line of sight RLOS RLOS, (nmi)= 1. 23Jh, (2)
is defined as the distance from the radio
transceiver to the horizon point, of
Now looking at the RLOS for the second
“effective radio horizon” (see Figure 2). For
transceiver (assume it is for the undesired
large bodies such as the earth, the radio
user):
signals tend to bend toward the body
causing a large decrease in the power levels
that is assumed significant enough to RLOS, (nrni) = 1.23Jhy (3)
eliminate the signal strength. This assumes
no anomalous propagation conditions along
We want to make the distance between the
the signal path and a spherical earth. The
desired and undesired user sufficiently
formula for the RLOS of a single transceiver
enough such that their radio signal do not
overlap. That means that distance has to be the two cells servicing the desired and
the sum of the two RLOSs shown above or: undesired users would have to be then
sufficiently large such that the radius of the
+ Jh,) two cells are taken into account or:
D , 2 1.23(.& (4)
Doc = 1.23 * (&+ &) + R, + Ru
Using the above formulation, we also note
that the distance between the two centers of
* I
I 4 Ddcuc
Figure 2 Radio Line of Sight Distance
Next a formulation for the theoretical Positioning the desired and undesired
distance of the centers of the two cells using users at the edge of their corresponding
the same frequencies where our desired and cells for a worst-case interference
undesired users are located is given. This
scenarios, is equivalent to subtracting 2R
assumes an inscribed hexagon type cells
from formula (6). Substituting the result
with all of them having the same radius R
into (7) produces the formula for the
and a frequency reuse factor f,, [5], [7]:
frequency reuse factor [5] in terms of the
RLOS (or height h) and the cell radius R.
That need to be satisfied so that the center
of the desired and undesired cells are
sufficiently separated to not allow any two
Finally assuming the same heights for now
aircrafts (one in each cell) to see each other
on the desired and undesired users, we get
in terms of radio signals or RLOS:
that the distance Ddub ased on (4)s hould be
greater than two RLOS.
+
4((1.23&/ R) 1)2
2 (8)
D , 2 2 * 1.23(&) = 2 * RLOS (7) fru 3
Note that although the actual formulation for
Again with the radius of the desired and
the sufficient distances should be based on
undesired cells assumed equal (R) for the
(7), the assumptions and formulation done to
moment, and using same heights for desired
obtain (8) in [5] is to produce a formula for
and undesired users, the RLOS of the two
the frequency re-use factor, and specifically
users will then be the same. Also assume
to be able to use the formula (6) for the
desired and undesired users are R distance
center distances for a generic hexagon cell
from their respective centers at the closest
arrangement.
two points possible (see Figure 1).
Frequency Reuse based on Signal to Similarly looking at the power received due
total co-channel interference: to an interfering user, or from the interfering
tower, we have:
Next we formulate the frequency reuse
factor with the limitation imposed by the
requirement to have a sufficient Signal to
Co-channel Interference Ratio. What this
basically gives us is the possibility of having
closer distances of the centers of the desired Where P, ,Gtu is the power transmitted
and undesired cells than that given by (6) or from undisired user in the undesired cell,
(7). In other words it allows the two the gain of the antenna of the undesired user.
aircrafts to be within line of sight of each The distance in the above formula is from
other (or 2.RLOS) as long as the signal level desired user to undesired user, the gain of
of the undesired signal I,, is 1olog(sd/Iu) dbs the desired aircraft antenna, and the
lower than the desired one since the wavelength of the frequency of the channel
sd,
effects on the performance (i.e. BER) will respectively. Note the wavelength is same
be within specification. as clf where c is speed of light and f is the
frequency of the channel.
Using Figures 1 and 2, define the distances
to the desired aircraft from the center of its Assuming similar power setting for user and
own cell by Dd, which will be approximated tower Effective Radiated Power above an
by the desired cell radius Rd. Also define the Isotropic Antenna (EIRP) (a reasonable
distance of the undesired user to its own cell assumption given the tower power and VHF
by D, with again approximating it with its aircraft transmit powers are currently 10 W,
cell radius R,. with some aircraft transmitter being as large
Then again define the distance between the as 20 W for high altitude certification as
desired user and undesired user as before by shown in Table 1 in next section). Also
Ddu,e xcept this time that distance is limited assume setting for both user and undesired
by the and do not have to satisfy (7). user (i.e. antenna gains the aircrafts
sd/I,,
Hence assuming a distance square involved=[)%I , we get:
approximation for the free path loss, we 2
have that the signal received at the desired
SdIIu 3
user from its own cell tower given by:
2 (9) Finally, in [5]a more accurate assessment of
the interference source is discussed. There
the assumption is made that we can have up
Where p, ,G**<,G,~A is the power to 6 interfering users from cells in the first
transmitted from the desired cell tower, the tier in a hexagon type setting, and possibly
gain of the desired cell antenna tower, the more in a non-uniform type of setting.
gain of the desired aircraft antenna, and the Hence keeping that in mind the formulation
wavelength of the frequency of the channel for the signal to interference of co-channels
respectively. Note the wavelength is same should account for six such sources at least
as clf where c is speed of light and f is the assuming all aircrafts are transmitting at the
frequency of the channel. same time, which is a worst case. With that
we would then have:
Using (7),(8),(1 I ),( 12), and assuming for
[&-
2 this case only that the radius of the
d' "total -int erf = Sd 161, =. s undesired user and the desired users are the
same (R), we have:
[for total interference from 6 sources:
The above formula we feel is more
appropriate given that the specification for
signal to interference ratios do not specify
that it is from a single user. In [8] Lab
testing was done to show that a 20-db signal
to total co-channel interference ratio
satisfied the specification of BER for
VDL Mode 3.. Hence if the interference Results for different Frequency
assumed there is from all total sources, it Protected Service Volume (FPSV)
would then have to be distributed over all
the interfering users which justifies the Results using FAA channel allocations
favorable worst case formula rather than that
used in FAA manual [4]. We will use both Using the equations shown in the
of those for the tables to be shown in the previous section, along with the parameters
next section. for the VLD-3 system shown in Table 1
below, we can tabulate some of the results
Finally again we utilize the formulations in desired. We compare with some of the
[5] to relate the frequency reuse with the results in [4] for a more realistic approach to
signal to interference ratio so that we can the problem.
relate to earlier formulation of frequency
reuse factor with respect to RLOS and R.
Frequency Reuse Factor From previous section
fRU
VHF band for Aeronautical use 760*25 Mzt otal
BWtotal
524*25 khz ATC only
Number of VDL-3 TDMA slots 3 max data channels
Nslots
4 max voice
2 v,2 data
I
Each VHF channel bandwidth 1 BW.-h,.n,l 25 lcHz
Channel Data Rate
Tower Transmitter power
Airplane Transmitter power
I Rineteqrufierreedn cSeig rnaatilo t o Co-channel I= lo!dsd ztoral - mt erf ) required
lo requiied
Table 1: VDL 3 System parameters
A generic plot of the frequency reuse factor Co-Channel Interference requirements, we
vs. the RLOSR as in [5] is shown using the used a 20 db requirement [8] for the Signal
formula (8) for the linear part. For the to Co-channel Interference ratio (total or
minimum required levels based on signal to single), or:
number was chosen. Also in [4] a 14 db is
'OlOg('d 'total -int e$ )required = specified for VHF analog, hence it is felt
(14) that the 20 db number ismore appropriate
'OlOg(Sd '1, = 20"
)required due to testing done in [8] for VDL-3. Table
Note a maximum of 26 db is actually 2 below shows the required minimum
specified and is also found in [8] although frequency reuse factors for those three
testing was done for 20 db hence why that levels:
Required Signal to co channel Minimum frequency reuse factor Minimum frequency reuse
Interference Ratio (total or single) required based on equation (13) factor required based on
first part for Total Interference equation (13) second part for
assumption Single Interferer assumption
14 db min (analog system [4]) 67.9 (plotted in Figure 3) 16.4 9 (plotted in Figure 3)
20 db FAA tested VDL3 181 234 48.9 (plotted in Figure 3)
26 db a maximum specifi.edA [ 81 I 862.7 160.6
Table 2: Minimum Frequency Reuse Factor based on Signal to Interference Ratio
Minimum Frequency Reuse factor vs. RLOS/R
.......... .. ...............
RLOS/R=1.23*sqrt(h)/R
Figure 3: Minimum Frequency Reuse factor based on RLOS/R and S/I (Equations 8 and 13)
Given the values shown in the Table 3, and the interference limited case for at least up
at the same time comparing to plot of Figure to RLOS=4 with the exception of the 16.4
3 for minimum required frequency reuse single user case with 14 db which would be
factor for a 2RLOS distance between user reached at approximately an RLOSR or 2.5.
and interferer, it is evident that the numbers Since the more appropriate case of 20 db is
in the plot are less than the limits shown for
recommended, that limit is not within the divided into sectors. Sectors have been
plots range shown. designed to increase efficiency, safety, and
to make the air space more manageable;
To effectively control the large volume of sectors vary in the airspace size they cover.
air traffic in the US and meet the growing Furthermore, ARTCC’s divide their airspace
demands aviation places on the system, the by altitude to maximize air traffic system
National Airspace has been sectioned into efficiency creating different types of FPSVs.
smaller more manageable areas. The Air To meet the communication requirements of
Route Traffic Control Center (ARTCC) is the existing system, careful reuse of existing
responsible for controlling aircraft that are in frequencies by spectrum engineering has
the en-route stage of flight; there are 21 produced a system that uses almost all
ARTCC’s in the United States. Terminal channel allocations available to Air Traffic
Radar Approach Control (TRACON) has Control.
been developed to handle aircraft in the
approacWdeparture stage of flight and other For this study, we start by utilizing the
type of flight procedures that are not in the results of Figure 3, to list a range of values
en-route domain. Airspace controlled by of RLOS/R for different FPSVs dimensions
both the ARTCC and TRACON is further as per Table 3 shown below:
Service Type . Altitudes h (ft) Service Radius R 1 RLOS (nmi) from 1 RLOS/R
(nmi) formula (2)
Super High ~45000 150 ~260.9 ~1.74
Altitude En Route
(SE)
High Altitude En 45000 260.9
Route
Intermediate 25000 194.5
Altitude En Route
(E)
Low Altitude En 18000 60 165 2.75
Route (LE)
Approach Control 25000
(AC), Departure
Control (DC),
Arrival Automated
Terminal
Information
Service (ATIS)
Local Control 25000 30 194.48 6.5
(LC)
Weather 10000 25 123 4.92
(AWOS/ASOS)
Ground Control 100 2-5 12.3 6.15-2.46
(GC) , Clearance
Delivery (CD),
Departure ATIS
Table 3: RLOS and Radius of different FPSV
Looking at the Plot of Figure 3 and the or Interference Limited case based on the
RLOSR values of Table 3 we can obtain particular assumption made on the
some values for the required minimum interference limiting case. Obviously for
frequency reuse factor. Note that we choose most of those cases we do not reach the
the minimum of the values of from RLOS limits shown in Table 3. Again this is
fm
mainly due to the more stringent ratios involved. Table 3 below shows some
requirements on the signal to interference of those results:
Service Type RLOS/ fru from 5"f rom fru from fru from
R minimum minimum minimum minimum
value shown value shown value shown value shown
on Plot in on Plot in on Plot in on Plot in
Figure 3 Figure 3 with Figure 3 Figure 3 with
with 14 db 14 db signal to with 20 db 20 db signal to
signal to Interference signal to Interference
Interference ratio total Interference ratio (total
ratio (single Interferers) ratio (single Interferers)
Interferer) Interferer)
Super High >1.74 >10 >10 >10 >10
Altitude En Route
(SEI
High Altitude En 1.74 10 10
Route
Intermediate 3.24 23.98 23.98
Altitude En Route (interference
(E)
Low Altitude En 2.75 18.76 18.76
Route (LE) (interference
limited)
Approach Control 3.24 23.98 23.98
I
(AC), Departure (interference
Control (DC), limited)
Arrival Automated
Terminal
Information
Service (ATIS)
Local Control (LC) 6.5 48.9 75
(interference
limited)
Weather 4.92 46.74 46.74
(AWOS/ASOS) (interference
limited
Ground Control 6.15- 16.49 I 67.9 48.9 68.18-15.97
(GC),C learance 2.46 (interference
Delivery (CD), limited)- limited)- limited)-
Departure ATIS 15.97
Table 4: Frequency Reuse Factors based on Figure 3
Having the information in Table 4 on the user, and the recommended case of 20 db
frequency reuse factors enable us to with total users (or 6 in our case). The
compute capacity per cell (or total for any results are shown next in Table 5 for a total
geographic area) using the formula (1) number of 524 channel (out of the grand
initially shown. total of 760), which is documented in [4] as
In addition to that we list the corresponding the allocated channels for Air Traffic
distances that would be required between Control. Note, the remaining frequencies
center of desired cell to the center or a co- are used for other services such as
channel cell. All of the numbers are show emergency crews, flight training, guard
for the least stringent case based on a 14 db bands, and more [4]. Furthermore, in Table
signal to interference ratio with a single 5 below, notice that giving all 524 ATC
channels for each FPSV is unrealistic, but is in the next example. Nonetheless those
very useful in determining maximum maximum values shown in Table 5 serve as
possible values on cell capacities. In reality a guideline for initial planning and for each
all FPSV types will share the 524 channels type FPSV evaluation. Cell Separation is not
(as oppose to 100% to each), hence the dependent on the number of ATC channel
capacity values will be much lower as seen and its allocation.
Service Type Required Single Cell Required Single Cell
Separation Capacity using Separation Capacity using
(nmi) to a frequency reuse (nmi) to a frequency reuse
similar cell, factors of 14 db similar cell, factors of 20 db
using single interferer using total interferers
frequency (Table 4) and frequency (Table 4) and
reuse factors formula (1) reuse factors formula (1)
of 14 db single of 20 db total Cell - Copuciry = BW,,,N,,,
interferer interferers B W,,.,,fau
with Nslots=a3n d with Nslots=3a nd
(Table 4) and (Table 4) and
BWtoollmWchannel= BWtotalmWchannel=
formulRa (6) a52 4 with all 100 % formulRa (6) z524 with all 100 %
'douc = distribution to each 'dpC = distribution to each
Fpsv Fpsv
Super High S21.58 457 S21.58 457
Altitude En Route
(SE)
High Altitude En 821.58 157 821.58 157
Route
Intermediate 422.01 95 508.90 65
Altitude En Route
(E)
Low Altitude En 422.01 95 450 83
Route (LE)
Approach Control 422.01 95 508.90 65
(AC), Departure
Control (DC),
Arrival
Automated
Terminal
Information
Service (ATIS)
Local Control 211.00 95 450 20
(LC)
Weather 175.84 95 296.04 33
(AWOS/ASOS)
Ground Control 14.07- 95- 28.60- 23-
(GC), Clearance 34.61 98 34.61 98
Delivery (CD),
Departure ATIS
Table 5: Cell Separation and Capacity for two S/I levels and all FPSVs with all 524 ATC per FPSV
