Cell Radius in LTE
The cell radius in LTE is affected and/or determined by three factors:
a) The Preamble Format
b) The Cyclic Shift the corresponds to the ZeroCorrelationZoneConfig parameter and,
c) The Cell Radius Parameter
A) Preamble Format
LTE FDD supports four preamble formats (as of today, not all of them currently supported
by the equipment manufacturers). The preamble consists of a cyclic prefix (to handle
multipath interference) followed by an 800 s sequence. In preamble formats 2 and 3, the
sequence is repeated. The total length of the cyclic prefix and the sequence(s) determines
how long it takes to transmit the preamble. Since the actual physical transmission occurs in
units of sub-frames (1 ms), the remaining time determines how far away the UE can be
without overlapping another UE's access attempt (the guard time). For further details, refer
3GPP TS 36.211 - Physical Channels and Modulation.
The operator typically must pick a preamble format to determine the coverage area desired.
In the event of remote sites deployment, the length of the fiber to the remote cells must be
considered as part of the cell radius (this includes Distributed Antenna Systems -DAS- ).
Since the speed of electromagnetic waves over fiber is only two thirds of the speeds in free
space, the total cell radius reduces to the values shown in the table below.
In the previous part (1 out of 3) we discussed the relationship between the preamble format
and the cell radius. In this delivery, we will discuss how the ZeroCorrelationZoneConfig
parameter affects the cell radius.
The parameters ZeroCorrelationZoneConfig and RootSequenceIndex are used to generate
64 random access signatures in each cell (all these access signatures should be different in
each cell). Both, the ZeroCorrelationZoneConfig and the RootsequenceIndex paramaters
are broadcast in SIB2. The random access sequences are built via the selection of a Zadoff-
Chu sequence (one out of 839) given by RootSequenceSequence and a cyclic shift (used 64
times to generate the 64 random access signatures from the Zadoff-Chu sequence selected).
The cyclic shift is indirectly given to the UE by the parameter ZeroCorrelationZoneConfig,
as shown in the table below (see columns two and three and note that the cyclic shift has
limited values). The available cyclic shifts are listed in 3GPP TS 36.211 table 5.7.2.-2.
The cyclic shift is also related to the cell size. The relationship between the cyclic shift and
the cell size is given by equation (1):
(N
CS
- 1) * (800 s/839) RTD + Delay Spread (1)
In the equation, RTD stands for Round Trip Delay (twice the cell radius). Hence:
RTD = 2 R/c (2)
Then, the cell radius is given by:
R [c/2]*[(N
CS
- 1)*(800 s/839)-Delay spread] (3)
For instance, if we assume that ZeroCorrelationZoneConfig is 12, then from the table
above, Ncs = 119. Furthermore, if the delay spread = 6 sec, then the cell size will be
approximately 15.97km. Note that the smaller the cyclic shift, the smaller cell size.
The delay spread in the equation above should be calculated by the RF engineer after a
drive test is carried out in the areas of interest. The value of the delay spread is typically
different for rural, suburban, urban and dense urban environments.
The third factor that affects the cell radius in LTE is the parameter cell radius. Equipment
manufacturers typical offer a parameter called cellradius, that allows the modification of
the cell radius. The units of this parameter are typically Kilometers.
EXAMPLE:
Let's assume that the preamble format picked (or the only one currently available) is type 0
(which offers a maximum cell radius of approximately 14 km). The possible values of the
parameters PrachconfigurationIndex are, therefore, 0 to 15. A network operator may
decide to classify their cells into rural, suburban, urban and dense urban cells. Furthermore,
the operator may allocate a cell radius to different morphologies, say: Rural = 14 km,
Suburban = 8 km, urban = 5 km and dense urban = 2 km. In this case, the values of the
parameters associated with the cell radius could be:
PrachConfigurationIndex = Any number between 0 and 15 (Preamble Format 0).
Cellradius = 14 (rural), 8 (suburban), 5 (urban), 2 (Dense urban),
ZeroCorrelationZoneConfig = 12 (rural), 9 (suburban), 8 (urban) and 4 (dense urban).
Notes:
a) The value of PrachconfigurationIndex affects the RACH capacity (addressed in a future
blog).
b) The value of ZeroCorrelationZoneConfig affects the cell radius as explained in a
previous blog (See table below).
