1. travel
2. transports
3. air travel
4. helicopters

PN OFFSET PLANNING STRATEGIES FOR
NON-UNIFORM CDMA NETWORKS
Chu Rui Chang, Jane Zhen Wan and Meng F. Yee
NORTEL Wireless Engineering Services
Richardson, TX 75083-3805
Abstract - This paper presents a novel
methodology in planning the PN-offsets of a highly
non-uniform CDMA network and discusses the
major related issues. The strategy is applicable to
any real life PCS and cellular networks. It is shown
that by optimally choosing/ arranging parameters, it
is not only possible to mitigate the CO- and adjacent
PN-offset interferences, sufficient PN-offset values
can also be reserved for new cells to be added into
existing clusters for future network development.
I. INTRODUCTION
In a CDMA system, pilots from all sectors are
spreaded by the same PN Short Code, and a mobile
distinguishes the pilots via their distinct time shifts
of the basic sequence (PN-offsets). Therefore, a
careful PN-offset planning should be designed to
avoid pilot confusion for a CDMA network. In order
to distinguish a pilot from a remote BTS from a
multipath component of the home pilot, enough
separation between the adjacent E"-offsets must be
provided to avoid adjacent PN-offset confusion. In
cases where PN-offset values must be re-used, the
re-used distance must be large enough to avoid
PN-offset confusion.
For a uniform network where all cells having similar
radii, the PN-offset planning is relatively straight
forward since the Short Code should provide
sufficient number of distinct PN-offsets to
accommodate a large PN re-use pattern, while
maintaining a reasonably large adjacent PN
separations.
However, the virtual PCS/Cellular network is
seldom uniform. A typical situation is that the cells
in rural and highway areas are much larger than
cells in urban areas, as shown in Fig. 1. For
extremely non-uniform networks, the task of I"offset planning becomes much more challenging. In
the real network, the co-offset confusion is the major
concern for small cell clusters, but the adjacent offset
0-7803-3659-3/97
$ 1 0.000 1 997 IEEE
confusion is more likely to happen in large cells. If
the small cell clusters and large cell clusters are
forced to use the same re-use pattern, then it is
possible that a sufficient reuse distance cannot be
maintained, particularly in between of small cell
clusters and large cell clusters, as shown in Fig. 2.
Therefore, a more sophisticated planning strategy is
needed.
In this paper we will derive the criterion for
avoiding the CO- and adjacent PN confusions, exam
several possible PN-related problems that can
happen in a real system and present a practical
planning strategy for a non-uniform CDMA
network.
11. CRITERION FOR AVOIDING
ADJACENT AND CO-PN CONFUSIONS
The major task of PN-offset planning is to avoid the
PN confusion problem. There is a significant
difference between a "I" confusion" and an
ordinary "interference".The interference caused by a
PN confusion is 19.3 dB worse than an ordinary
interference, assuming a 13.3 kbits vocoder is used.
A CDMA signal that is not inside the Active Set
Search Window (SRCH-WIN-A)
cannot be
despreaded by the fingers of the RAKE receiver, and
will only contribute to the background noise. On the
other hand, a CDMA signal in the search window
despreaded by a finger will obtain a 19.3dB
processing gain. If a remote CDMA signal which
does not belong to the home cell but falls into the
SRCH-WIN-A and becomes one of the three
strongest components, the mobile will treat this
remote signal as one of the multipath components of
the home signal, and will perform coherent
combining on the two unrelated signals, resulting in
strong interference (Fig. 3). It is the task of the PN
offset planning to avoid the I" confusion. However,
the ordinary interference can only be controlled, but
not avoided.
1543
the system, the large cells are more vulnerable to
adjacent PN confusion than small cells.
The adjacent PN-offset confusion can happen due to
large differences in the propagation delay.
Assuming two pilots with adjacent PN-offsets, when
reaching the mobile, the pilot with an earlier phase
propagates an extra distance that is large enough so
that its phase shifts behind and it falls into the search
window of the pilot with a later phase, the mobile
will confuse these two pilots as two multipath
components of the same pilot. Therefore a necessary
condition for the adjacent PN confusion is a
sufficient large difference in propagation delay
between two signals.
IS95 specifies that the minimum separation between
two adjacent valid I"-offsets is 64 chips, which is
larger than most of multipath spread. To further
reduce the chance of the adjacent PN confusion, the
minimum separation can be increased by assigning a
global parameter PILOT-INC > 1, so that the
minimum adjacent PN separation becomes
64*PILOT_INC [chips]. Since the RF wave
propagates 244 meters per chip, if two pilots with
adjacent PN-offset separation of 64*PILOT_INC
chips are to have confusion, the pilot with the earlier
phase will have to travel an extra distance of
[
"I
-
AD[m]= PILOT INCX 64 -- x 244 (I)
2
where AD is the difference in propagation distance
"1
[
half
(2)
separation must be:
- 11
(4)
If Cell A and Cell B have different radii, then R
should be the maximum cell radius.
To avoid the PN confusion, it is sufficient to require
that either the PN separation S is bigger than the
difference in propagation distance; or the remote
signal is at least 21 dB weaker than the home signal.
It can be proven that the minimum adjacent PN
L
the minimum required physical separation between
two BTS that reuse the same PN-offsets to be:
D>%+2R
2
Note that
of the window size, W A / ~ ,is used
since the mobile always centers its Active Set Search
Window to the earliest usable multipath component
of the arriving signal.
S >Rx
( D - d , ) - d , >-WA
2
Since d, I R , from the above formula we obtain
rrr
between the home pilot and the remote pilot, and
WA is SRCH-WIN-A of the mobile. In (l), the
effective PN-offset separation S is expressed as:
S[chip]= PILOT- INC x 64 - 2
Next we derive the criterion to avoid the CO-PN
confusion. In Fig. 4, Cell A and Cell B both uses the
same PN-offset (omni-cells are the worst case) and
CO-I"
confusion problem will happen if the pilots
from two different cells fall into the same
SRCH-WIN-A of the mobile, and both become one
of the three strongest signal components. Note that if
one pilot signal propagates much longer than the
other, so that the difference in propagation delay
causes the remote pilot to "fall out" of the mobile's
SRCH-WIN-A, the mobile's search finger will never
find the remote pilot and the CO-E"
confusion can be
avoided. In Fig. 4, assuming the distance between
Cell A and Cell B is D. Further assume the worst
case scenario that a mobile is located between these
two cells and its distance to the home Cell A (the cell
in which the mobile establishes its time reference
from) is d b and its distance to the remote Cell B is
(D - dH). To guarantee the remote pilot "fall out" of
the Active Set Search Window, one needs
(3)
2
Where, cx is the propagation path loss exponent.
From (3) it is clear that the minimum required PN
separation is proportional to the cell radius R. If
using the same adjacent PN separation through out
Assuming the number of cells in a PN reuse cluster
is K, then the reuse distance is approximately
D=R&
(5)
which reduces proportional to the cell radius R.
Since the search window WA should be independent
of R, the required CO-PNdistance,
WA
D =+2R,
2
does not decrease proportionally with R. Therefore
small cells will be much more vulnerable to CO-PN
confusion since they tend to have a very small reuse
distance.
A third type of PN-confusion is the so-called
confused handoff. Assume two sectors A and B
having the same PN-offsets, Sector A is close to the
mobile and is in the Neighbor List, and Sector B is
not. However, due to different antenna orientations
1544
or terrain conditions, the arriving signal from Sector
B is strong (>T-add), but the arriving signal from
Sector A is weak. If the Pilot B falls into the mobile's
Neighbor Set Search Window W,, the mobile will
confuse Pilot B as Pilot A and will handoff to Sector
A, while despreading signal from Sector B. This
results in strong forward link interference (Fig. 5).
Proper antenna down tilting which reduces the
spilled RF energy to remote cells, is an effective way
to reduce the likelihood of confused handoff.
Increasing the reuse distance, and proper allocation
of the PN-offsets also helps to mitigate the problem.
111. I"-OFFSET ALLOCATION SCHEMES
In l" planning, the first parameter to be determined
is PILOT-INC. The setting tradeoff is that a large
PILOT-INC will increase the adjacent PN-offset
separation, but will reduce the number of valid PNoffsets, which in turn will reduce the reuse distance.
A low setting will do just the opposite. Also there is
a lower bound on PILOT-INC, that is
PILOT- INC x 64[chips] > max(W,, W,>
Where W, ,W, are the search window sizes for the
Remaining and Neighbor Set. If this condition is
violated, the two adjacent search windows will
overlap and the measured value of the PN-offset of a
pilot found in the overlapping region will have an
ambiguity, as it belongs to both search windows.
In the following, we present the I"-planning
strategy via an example. Assume the maximum cell
radius 5 15 km (= 61 chips), and further assume
that for the worst case the path loss exponent a = 3.2,
and (3)gives S = 216 chips. If the search window WA
= 28 chips, from (2) the PILOT-INC = 4, since
(4 x 64 - 14) = 242 > 216. The total number of
valid PN-offsets is then 512/PILOT_INC = 128.
Assuming 3-sectors/cell, 128 valid PN-offsets will
yield a total of 42 cells with distinct PN-offsets.
The maximum search window size that can be
accommodated for PILOT-INC = 4 is 226 chips,
which is more than sufficient for most cases.
For uniform networks (ideal case), we may deploy
42 cells/cluster, or 37 cells/cluster plus some
reserved PN-offsets for future use. It is desirable to
reserve certain number of PN-offsets so that a new
cell can be inserted into the existing cluster without
disturbing the others.
As mentioned previously (Fig. 1, 2), for a highly
non-uniform network, it is often not desirable to
force large cell clusters and small cell clusters into
the same reuse pattern. Typically the shape of the
small cell clusters and large cell clusters are
different: small cells clusters usually cover an area
(2-D), but large cells often cover the highways (1-D),
as shown in Fig. 1.
We propose to divide the total available PN-offsets
into two disjoint sets, one for small cells, and one for
large cells. In this way, the small cells and large cells
no longer share the PNs from the same set, the reuse
distance from large cells to small cells is no longer a
concern. Also the small cell clusters can have a
different reuse pattem from the large cell clusters.
The number of cells per reuse cluster for small cells
must be much higher than that for large cells, since
small cells are more vulnerable to CO-PNconfusion.
Depending on the actual cell configuration, we
recommend K, = 27 - 32 cells per reuse cluster
for small cells. For large cells, the number of cells per
clusterKL can be much smaller, specially if the
large cells are used only to cover the highways.
Usually K , = 7 - 12 is sufficient. The resulting reused distance must satisfy (4), also K , K L I 42.
+
In frequency planning, the adjacent channels should
not be allocated to the adjacent sectors. The situation
is just the opposite for I" planning: the adjacent
sectors are _least vulnerable to adjacent PN
confusion. A large path difference is a necessary
condition for adjacent pilot confusion and it is least
likely for two pilots from the adjacent sectors to
produce a large path difference. Since the two
sectors are facing different directions and in order
for two adjacent pilots starting from the same point
(BTS tower), and ending at the same point (mobile),
one pilot must go through at least one reflection and
become much weaker, as shown in Fig. 6. Also the
pilot with earlier phase has to travel an extra
distance of about 59 km [(l) gives U =
(4 x 64 - 14) x 244 = 59km1, which is highly
unlikely. Therefore, one should allocate adjacent PNoffsets to adjacent sectors, and allocate non-adjacent
PNs to remote sectors.
1545
Fig. 2 shows that it is more likely for large cells with
high antennas to cause interference to small cells,
since signals from high antenna can propagate much
farther. Therefore it is best to allocate the E" with
later phases to large cells and those with earlier
phases to small cells. This further reduces the
likelihood of confusion because the propagation
delay will tend to shift the phase behind, so a pilot
from a large cell with later phase, after propagation
delay, will appear with an even later phase and will
be even less likely get confused with the pilot from
small cells with early phases. The small cell pilots,
with lower antennas, cannot propagate very far. The
only exception is that because the Short Code is
periodic with the period = 512*64 [chips], so a PNoffset of 512*64 is the same as I"-offset of 0. Thus it
is not desirable to use PN offset of 0. We recommend
the first I"-offset value starts at 4 [ x 64 chips],
and the last one be 508 [ x 64 chips 1.
The assignment of PILOTJ'Ns
to each cell within a
small cell reuse cluster or within a large cell cluster,
should follow a consistent fashion. For example, if
values are assigned to each cell within a cluster in a
spiral fashion (Fig. 2), then the assignment for other
clusters should follow the same fashion as well. This
produces an approximately equal reuse distance for
every cell in the cluster. However, the assignment
fashion for large cell clusters and for small cell
clusters do not need to be identical, since they use
PNs from disjoint sets.
IV. SUMMARY OF PN-OFFSET
PLANNING STRATEGIES
There are three types of I"-offset confusion
problem: CO-PN, adjacent I" and confused
handoff. I" confusion is more harmful than
conventional interference.
Propagation delay, search window sizes and
path loss exponent are the three key factors in
PN-offset planning.
Sufficient reuse distance and I" separation,
proper antenna down tilt, together with proper
allocation of I"-offset values, reduces the
likelihood of PN confusion.
For non-uniform networks, reuse clusters for
large cells and small cells may have different
shapes and may use I"-offsets from two
disjoint sets; small cell clusters using I"-offsets
with smaller values and large cell clusters using
larger offset values.
The general expression for allocating PILOT-PN to
each sector is:
{a,p, y } - Sector's Offset
(6)
= ( P N , ) j + 12k - (8, 4, O}
Where (PN, ) is the first I"-offset value from the
sets, and k = l , 2,
..., Kj, K j is the number of
cells in a reuse cluster. Equation (6) is applicable for
both the small cell and large cell clusters. For small
cell clusters, the subscript j = S in (6), and for large
cell clusters, j = L .
Larae Cells in RurallSub
A
n
Small Cells
Near the
Center
Figure 1 A typical situation where the urban areas are
covered by large number of small cells and rural/Hwy
are covered by much larger cells. Also large cell clusters
have different shapes from small cell clusters.
F i m e 2 An example of forcing cells with different
sizes into the same reuse pattern. In this example K=7
and R=2r.The reuse distance within small cluster is
4.6r, and within large cluster is 4.6R.But from large
cell cluster to small cell cluster is only 2.3R. The
situation is much worse if R >> r.
1546
Short Code Length = 2
Cell A
= 32768 Chips
. . .. ... .. . , , , , .. , . ... . .... . . . . .. ,
2!5473
Mobile
Remote Pilot (phwe advanced)
I
'. .._
. .
'. ..
.._
I
I
I
I
Cell B
...
I
I
I
i Home Pilot (phase hehind)
I
~-
1
I
I
*
Pilot A
Pilot B
T h [ehlp]
4 -
Active Set Scar ch Window
e
Delayed
Remote
'lot
Home Pilot
Active S e t k i n d o w
1
Figure 3 Illustration of adjacent PN confusion, where
a pilot with an earlier phase propagated an extra distance
and its phase falls into the home pilot's SRCH-WIN-A.
F i m e 4 Illustration of CO-PNconfusion: both cells
use the same PN-offset and their pilots fall into the
same SRCH-WIN-A. Note that the CO-PNconfusion
will not happen if the remote pilot travels an extra
distance and falls out of SRCH-WIN-A.
0
0
r*.
I
I
I
Figure 5 Illustration of confused handoff. Assume the
a-sector of both Cell A and Cell B use the same PNoffset. A mobile located in Cell A's y-sector will have
Cell A's a-sector as its neighbor. However, if the signal
from Cell A's a-sector is weak but the signal from Cell B's
a-sector is strong, and if the a-Pilot B falls into mobile's
SRCH-WIN-N, the mobile will confuse a-Pilot B as
a-Pilot A and will handoff to a-sector of Cell A.
Figure 6 Illustration of allocating adjacent PNs to
adjacent sectors. If a mobile receives two pilots from
two adjacent sectors via a direct path, then there will
be no path difference between them. For there to be an
adjacent PN confusion, the signal with earlier phase
has to go through at least one reflection and becomes
much weaker; it also has to travel an extra 59 km.
1547