METHODS FOR MITIGATING THE CONTROL CHANNEL INTERFERENCE
BETWEEN A MACRO CELL AND A SMALL CELL
BACKGROUND OF THE INVENTION
Embodiments relate to small cell deployment within a macro cell in a
wireless network.
Mobile radio frequency band(s) are both scarce and precious
resources. After the inception of commercial mobile radio
communication in the 1980's the numbers of subscribers have been
growing exponentially. The underlying radio technology also has
grown at a fast pace. In addition to conventional voice
communication, data, video and real time gaming have been
introduced.
These new services require a relatively higher number of bits
transmitted in a unit time than conventional voice services. There are
two main ways to achieve larger bit rate demands, first, efficient use of
spectrum using advanced technology (based on, for example, multiple
transmit and receive antennas) and, second, the use of a larger
frequency band. As the frequency spectrum is already crowded the
latter is often not feasible.
Introduction of the cellular concept in the 1980's allowed efficient
reuse of frequency spectrums. A service area may be divided into
hexagonal grids of cells which are further grouped into clusters of
cells. The frequency band may be apportioned within and reused
between the clusters so as to intelligently keep the co-channel
interference low.
Next generation wireless technologies are based on code division
multiple access (CDMA) technologies that are more robust to
interference and thus universal frequency reuse or re-use of the same
frequencies across cells was introduced in 2nd and 3rd generation
CDMA networks.
Orthogonal frequency division multiplexing (OFDM) technology is the
technique used for future 4G or International Mobile
Telecommunications (IMT) -advanced networks. While OFDM is a
spectrally efficient scheme and is also more suitable for multiple
antenna techniques (MIMO), OFDM is more susceptible to
interference. Therefore, the efficient and intelligent use of the
frequency spectrum across cells is important for successful
deployment of the OFDM networks.
Substantial research effort has been devoted to improve spectral
efficiency, or in other words, frequency reuse of the OFDM system.
Several solutions have been proposed, e.g., fractional frequency reuse
(FFR) (dynamic and static), inter-cell interference coordination (ICIC)
and small cell deployment (heterogeneous networks).
FFR uses a portion of the spectrum for a certain area of the cell. The
portion of the spectrum is dynamically changed or allocated in a static
manner. If the spectrum is dynamically allocated the uplink control
signals from the surrounding cells may be used to make the allocation
decisions.
In ICIC the cells periodically share some metric, for example a channel
quality indicator (CQI), of a frequency band via the backhaul
communication interface. The cells make the decision to allocate a
frequency band from its own measurements and the information
received from the surrounding cells.
Small cell deployments within a larger macro cell efficiently use the
spectrum and deliver the demand for the higher bit rate in certain
areas of the cell. Generally the small cells use lower transmit power
to serve a small area where the demand for the service is high, or in
other words, they have cell radius' of a few meters to few hundred
meters. Small cells may use wireless or wired backhaul connections
to the back bone network.
Indoor and outdoor pico cells, femto cells and micro cells are the main
types of small cells. The categorization of the small cells are based on,
for example, their transmit power levels, deployment scenarios and/or
the ownership of the small cell network. If different types of small
cells are deployed within a macro cell the network is also called a
heterogeneous network.
FIG. 1 illustrates a conventional heterogeneous network 100. As
shown, a plurality of cells 105 are arranged in a hexagonal grid of
cells. Each cell may include one or more antennas 115 associated
with, for example, a base station (not shown). One or more of the cells
may include a plurality of small cells 115 to support services in a
localized area within a cell 105.
The widely used GSM, GPRS, UMTS, HSDPA and HSUPA wireless
macro cellular standards were created by the third generation
partnership project (3GPP). 3GPP recently finalized the LTE standard
(Release 8 ) and is working towards their new standards namely,
release 9 and 10. Release 10 is targeted to satisfy the IMT-advanced
specifications. Currently several operators around the world are
planning to deploy LTE as their future macro cellular network to
deliver the demand for the higher data rates.
The down link frame structure 200 of the current 3GPP LTE standard
(release 8 ) is illustrated in FIG. 2. As shown, the down link frame
structure 200 may include a physical down link control channel
(PDCCH) 205 which may be associated with the first 1-3 OFDM
symbols in each sub-frame. The down link frame structure 200 may
include a physical control format indicator channel (PCFICH) 2 10
associated with the first OFDM symbol in each sub-frame.
The down link frame structure 200 may further include a physical
broadcast channel (PBCH) 2 15 once every 10 ms and for 4 OFDM
symbols. The down link frame structure 200 may further include one
or more primary synchronization signals (PSS) 225 and one or more
secondary synchronization signals (SSS) 220 associated with an
OFDM symbol.
In the down link, the primary synchronization signal (PSS) 225,
secondary synchronization signal (SSS) 220 and physical broadcast
channel (PBCH) 2 15 may be transmitted centered on a center
frequency and they occupy 6 resource blocks or 72 sub carriers. In
the time domain, the PSS 225 and SSS 220 may occupy 1 OFDM
symbol each while the PBCH 2 15 may occupy 4 OFDM symbols.
The periodicity of the PSS 225, SSS 220 and PBCH 2 15 may be 5ms,
5ms and 10ms, respectively. The PSS 225 and SSS 220 may be used
for synchronization and they may carry some cell specific sequence for
cell identification as well. The PBCH 2 15 may carry some system
information common for all users in the cell including, for example,
the allocated bandwidth information. As will be recalled, the PCFICH
2 10 and the PDCCH 205 may occupy the entire system bandwidth
and they may be transmitted in the first 1-3 symbols in every
subframe.
The subframe duration may be 1 ms. The PCFICH 10 may carry the
control format indicator which indicates how many symbols are used
to control transmission in a subframe. The PDCCH 205 may carry the
user specific control information including, for example, resource
allocation information. The down link physical channels may be
knotted for interference rejection. The scrambling sequence generator
may be reinitialized (except for PBCH 2 15) every subframe based on,
for example, the cell id, subframe number and a mobile identity. This
may randomize the interference between cells and between mobiles.
The up link frame structure 300 of the current 3GPP LTE standard
(release 8 ) is illustrated in FIG. 3. As shown, the up link frame
structure 300 may include one or more physical uplink control
channels (PUCCH) 305. Although FIG. 3 shows several other channel
blocks, they are not described herein for the sake of brevity. One
skilled in the art will refer to 3GPP LTE standard (release 8 ) for a more
detailed description of the up link frame structure 300.
The LTE up link transmission (physical uplink shared channel
(PUSCH) and physical uplink control channel (PUCCH) 305) may use
cell specific hopping for interference averaging. The PUCCH 305 may
carry the uplink control information including, for example,
scheduling requests, CQI, preferred matrix index (PMI), rank
information (RI), and ACK/NACK information. Multiple user control
information may be code division multiplexed (CDM) and transmitted
in one PUCCH 305 region.
A PUCCH 305 region may consist of two blocks. 1 Resource Block
(RB) X 1 slot resource units on the both side of the system bandwidth
a s shown in FIG. 3. Depending on the system bandwidth, the number
of PUCCH 305 regions varies. For a 10 MHz bandwidth there may be 8
PUCCH 305 regions. The periodicity of the PUCCH may be
configurable by the base station via the down link control signals.
SUMMARY OF THE INVENTION
One embodiment includes a method that includes determining, by a
macro cell, a binary sequence based on a number of symbols in a first
frame associated with a macro cell. The method further includes
performing, by the macro cell, an autocorrelation calculation on the
binary sequence. The method further includes determining, by the
macro cell, a time offset based on minimum values of the
autocorrelation calculation. The method further includes
broadcasting, by the macro cell, a control signal including the time
offset to a plurality of small cells.
One embodiment includes a method that includes detecting, by the
small cell, a frame associated with a macro cell. The method further
includes receiving, by the small cell, a control signal including a time
offset associated with the frame associated with the macro cell. The
method further includes transmitting, by the small cell, a frame
associated with the small cell synchronized with the frame associated
with the macro cell and offset in time by the time offset.
One embodiment includes a method that includes detecting, by the
small cell, a frame associated with a macro cell. The method further
includes determining, by the small cell, a binary sequence based on a
number of symbols in the frame associated with the macro cell. The
method further includes performing, by the small cell, an
autocorrelation or crosscorrelation calculation on the binary
sequence.
The method further includes determining, by the small cell, a time
offset based on minimum values of the autocorrelation or
crosscorrelation calculation. The method further includes
determining, by the small cell, a start time of the frame associated
with the macro cell. The method further includes transmitting, by the
small cell, the frame associated with the small cell synchronized with
the frame associated with the macro cell and offset in time by the time
offset.
One embodiment includes a method that includes determining if a
small cell includes a closed subscriber group. The method further
includes time-blanking, by the macro cell, a portion of the first frame
associated with a synchronizing signal of the second frame if no closed
subscriber group is present. The method further includes timeblanking,
by the small cell, a portion of the frame associated with the
small cell associated with a synchronizing signal of the frame
associated with the macro cell if a closed subscriber group is present.
In one embodiment the determined time offset is associated with one
of the 11, 17, 25, 3 1, 39, 45, 53 and 59 orthogonal frequency division
multiplexing (OFDM) symbols if the physical control channel (PDCCH)
associated with both the first and second frames occupy 3 OFDM
symbols, if the binary sequence has a length of 140 symbols and if
each element of the binary sequence represents an OFDM symbol.
In one embodiment the determined time offset is associated with one
of the 11, 12, 16, 17, 25, 26, 30, 3 1, 39, 40, 44, 45, 53, 54, 58 and 59
orthogonal frequency division multiplexing (OFDM) symbols if the
physical control channel (PDCCH) associated with both the first and
second frames occupy 2 OFDM symbols, if the binary sequence has a
length of 140 symbols and if each element of the binary sequence
represents an OFDM symbol.
In one embodiment the determined time offset is associated with one
of the 12, 17, 26, 3 1, 40, 45, 54 and 59 orthogonal frequency division
multiplexing (OFDM) symbols if a physical control channel associated
with the first and second frames occupy 3 and 2 OFDM symbols,
respectively, if the binary sequence has a length of 140 symbols and if
each element of the binary sequence represents an OFDM symbol.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the
detailed description given herein below and the accompanying
drawings, wherein like elements are represented by like reference
numerals, which are given by way of illustration only and thus are not
limiting of the present invention and wherein:
FIG. 1 illustrates a prior art system layout of a small cell overlay;
FIG. 2 illustrates a prior art 3GPP LTE (release 8 ) down link frame
structure;
FIG. 3 illustrates a prior art 3GPP LTE (release 8 ) up link frame
structure;
FIG. 4 illustrates a down link frame structure with small cell overlay
according to an example embodiment;
FIG. 5 illustrates a down link frame structure timing sequence for
small cell overlay according to an example embodiment;
FIG. 6 illustrates a periodic autocorrelation versus offsets in OFDM
symbols when PDCCH occupies 3 OFDM symbols according to an
example embodiment;
FIG. 7 illustrates a periodic autocorrelation versus offsets in OFDM
symbols when PDCCH occupies 2 OFDM symbols according to an
example embodiment;
FIG. 8 illustrates a periodic autocorrelation versus offsets in OFDM
symbols when PDCCH occupies 3 and 2 OFDM symbols according to
an example embodiment;
FIG. 9 illustrates an up link frame structure with small cell overlay
according to an example embodiment.
It should be noted that these Figures are intended to illustrate the
general characteristics of methods, structure and / or materials utilized
in certain example embodiments and to supplement the written
description provided below. These drawings are not, however, to scale
and may not precisely reflect the precise structural or performance
characteristics of any given embodiment, and should not be
interpreted as defining or limiting the range of values or properties
encompassed by example embodiments. For example, the relative
thicknesses and positioning of molecules, layers, regions and/or
structural elements may be reduced or exaggerated for clarity. The use
of similar or identical reference numbers in the various drawings is
intended to indicate the presence of a similar or identical element or
feature.
DETAILED DESCRIPTION OF THE EMBODIMENTS
While example embodiments are capable of various modifications and
alternative forms, embodiments thereof are shown by way of example
in the drawings and will herein be described in detail. It should be
understood, however, that there is no intent to limit example
embodiments to the particular forms disclosed, but on the contrary,
example embodiments are to cover all modifications, equivalents, and
alternatives falling within the scope of the claims. Like numbers refer
to like elements throughout the description of the figures.
It will be understood that, although the terms first, second, etc. may
be used herein to describe various elements, these elements should
not be limited by these terms. These terms are only used to
distinguish one element from another. For example, a first element
could be termed a second element, and, similarly, a second element
could be termed a first element, without departing from the scope of
example embodiments. As used herein, the term "and/or" includes
any and all combinations of one or more of the associated listed items.
It will be understood that when an element is referred to as being
"connected" or "coupled" to another element, it can be directly
connected or coupled to the other element or intervening elements
may be present. In contrast, when an element is referred to as being
"directly connected" or "directly coupled" to another element, there are
no intervening elements present. Other words used to describe the
relationship between elements should be interpreted in a like fashion
{e.g., "between" versus "directly between," "adjacent" versus "directly
adjacent," etc.).
The terminology used herein is for the purpose of describing particular
embodiments only and is not intended to be limiting of example
embodiments. As used herein, the singular forms "a," "an" and "the"
are intended to include the plural forms as well, unless the context
clearly indicates otherwise. It will be further understood that the
terms "comprises," "comprising," "includes" and/ or "including," when
used herein, specify the presence of stated features, integers, steps,
operations, elements and/or components, but do not preclude the
presence or addition of one or more other features, integers, steps,
operations, elements, components and/ or groups thereof.
It should also be noted that in some alternative implementations, the
functions/ acts noted may occur out of the order noted in the figures.
For example, two figures shown in succession may in fact be executed
concurrently or may sometimes be executed in the reverse order,
depending upon the functionality/ acts involved.
Unless otherwise defined, all terms (including technical and scientific
terms) used herein have the same meaning as commonly understood
by one of ordinary skill in the art to which example embodiments
belong. It will be further understood that terms, e.g., those defined in
commonly used dictionaries, should be interpreted as having a
meaning that is consistent with their meaning in the context of the
relevant art and will not be interpreted in an idealized or overly formal
sense unless expressly so defined herein.
Portions of the example embodiments and corresponding detailed
description are presented in terms of software, or algorithms and
symbolic representations of operation on data bits within a computer
memory. These descriptions and representations are the ones by
which those of ordinary skill in the art effectively convey the
substance of their work to others of ordinary skill in the art. An
algorithm, as the term is used here, and as it is used generally, is
conceived to be a self-consistent sequence of steps leading to a desired
result. The steps are those requiring physical manipulations of
physical quantities. Usually, though not necessarily, these quantities
take the form of optical, electrical, or magnetic signals capable of
being stored, transferred, combined, compared, and otherwise
manipulated. It has proven convenient at times, principally for
reasons of common usage, to refer to these signals as bits, values,
elements, symbols, characters, terms, numbers, or the like.
In the following description, illustrative embodiments will be described
with reference to acts and symbolic representations of operations (e.g.,
in the form of flowcharts) that may be implemented as program
modules or functional processes include routines, programs, objects,
components, data structures, etc., that perform particular tasks or
implement particular abstract data types and may be implemented
using existing hardware at existing network elements. Such existing
hardware may include one or more Central Processing Units (CPUs),
digital signal processors (DSPs), application-specific-integratedcircuits,
field programmable gate arrays (FPGAs) computers or the
like.
It should be borne in mind, however, that all of these and similar
terms are to be associated with the appropriate physical quantities
and are merely convenient labels applied to these quantities. Unless
specifically stated otherwise, or as is apparent from the discussion,
terms such as "processing" or "computing" or "calculating" or
"determining" of "displaying" or the like, refer to the action and
processes of a computer system, or similar electronic computing
device, that manipulates and transforms data represented as physical,
electronic quantities within the computer system's registers and
memories into other data similarly represented as physical quantities
within the computer system memories or registers or other such
information storage, transmission or display devices.
Note also that the software implemented aspects of the example
embodiments are typically encoded on some form of program storage
medium or implemented over some type of transmission medium. The
program storage medium may be magnetic (e.g., a floppy disk or a
hard drive) or optical (e.g., a compact disk read only memory, or "CD
ROM"), and may be read only or random access. Similarly, the
transmission medium may be twisted wire pairs, coaxial cable, optical
fiber, or some other suitable transmission medium known to the art.
The example embodiments not limited by these aspects of any given
implementation .
As used herein, the term "user equipment" may be considered
synonymous to, and may hereafter be occasionally referred to, as a
mobile, mobile unit, mobile station, mobile user, subscriber, user,
remote station, access terminal, receiver, etc., and may describe a
remote user of wireless resources in a wireless communication
network. The term "base station" may be considered synonymous to
and/ or referred to as a base transceiver station (BTS), NodeB,
extended NodeB, evolved NodeB, femto cell, pico cell, access point, etc.
and may describe equipment that provides the radio baseband
functions for data and/ or voice connectivity between a network and
one or more users.
Example embodiments relate to a method to overlay small cells on a
same carrier in a 3GPP LTE (release 8 ) macro cell. The following goals
form the basis of the method: (1) There is no change in the 3GPP LTE
(release 8 ) air interface standard and the 3GPP LTE (release 8 )
frequency band where the macro cell is deployed; and (2) the small cell
deployment is fully compatible with the current 3GPP LTE (release 8 )
standard and it should be forward compatible to future releases (e.g.,
release 9 & 10).
Therefore, implementation of the following methods may be
accomplished in a heterogeneous network 100 including the macro
cells and small cells such as in FIG. 1 via programs/ firmware. One
skilled in the art will understand that although example embodiments
refer to macro cells, small cells, pico cells, femto cells, micro cells and
etc., each of these cells is implemented through hardware such as a
base station, a base transceiver station (BTS), NodeB, extended
NodeB, evolved NodeB, femto cell, pico cell, access point, etc. and may
describe equipment that provides the radio baseband functions for
data and/ or voice connectivity between a network and one or more
users.
In the 3GPP LTE (release 8 ) standard, interference cancellation (IC)
and inter-cell interference coordination (ICIC) for the control signals
are not supported. The transmit power of the small cells may be
relatively lower than that of the macro cell. If a small cell is overlaid
in an existing macro cell and if the control signals of the small cells
and the macro cells overlap, the small cell control signals may not be
decoded accurately.
For mobile units that see low CQI from both the small cell and the
macro cell a signal interference may be more detrimental. On the
other hand, if the control signals and the traffic signals overlap the
signals may be decoded with minimal performance loss. Using macro
cell ICIC for the traffic signal the transmit power may be lowered (or
turned off) when the small cell control and the macro traffic overlap.
In some types of small cells (e.g., femto cells) closed subscriber groups
(CSG) may be present. The users in the CSG may only access that
particular small cell and the other users, even if they are very close to
the center of the small cell, may not be allowed access. Performance of
those users who are not part of CSG may suffer.
FIG. 4 illustrates a down link frame structure 400 with small cell
overlay according to an example embodiment. As shown, the down
link frame structure 400 may include a macro cell physical down link
control channel (PDCCH) 405. The down link frame structure 400
may further include a macro cell physical control format indicator
channel (PCFICH) 4 10.
The down link frame structure 400 may further include a macro cell
physical broadcast channel (PBCH) 4 15. The down link frame
structure 400 may further include one or more macro cell primary
synchronization signals (PSS) 425, and one or more macro cell
secondary synchronization signals (SSS) 420.
As shown, the down link frame structure 400 may also include a
macro cell time blanking slot 430. The down link frame structure 400
may also include a small cell frame structure 440 with an associated
allocated band 445. The small cell frame structure 440 may include a
small cell PCFICH 450 and PDCCH 455. The down link frame
structure 400 may further include a time offset 435 (k OFDM symbols)
to prevent the collision of the macro cell PDCCH 405 and the small
cell PDCCH 455. The down link frame structure 400 may further
include a small cell PSS 460, SSS 465 and PBCH 470. The small cell
frame structure 440 may further include a macro cell time blanking
slot 475 (shown as the dashed line box).
To comply with the goal that the macro cell LTE band should not be
modified, example embodiments may transmit the small cell PSS 460,
SSS 465 and PBCH 470 on the same center frequency as that of the
macro cell. Example embodiments may have a time offset between a
starting point of the macro cell PSS 425 and the small cell PSS 460 to
avoid the control signal collisions as shown in FIGS. 4 and 5
(described below). Therefore, the small cell may be synchronized in
time with the macro cell.
FIG. 5 illustrates a down link frame structure timing sequence 500 for
small cell overlay according to an example embodiment. The down
link frame structure timing sequence 500 may include a macro cell
frame 505 and a small cell frame 5 10. The macro cell frame 505 may
include the macro cell PSS 425 and SSS 450 referenced as block 5 15
occupying 2 OFDM symbols. The macro cell frame 505 may include a
plurality of sub-frames 535. Each sub-frame 535 may be, for
example, 2 slots equaling 1ms having 14 OFDM symbols.
The macro cell frame 505 may include the macro cell PBCH 4 15
referenced as block 5 15 occupying 4 OFDM symbols. The macro cell
frame 505 may include the macro cell time blanking slot 430
referenced as block 525 occupying an entire subframe in the time
domain and the entire band may be allocated for the small cell in the
frequency domain. The macro cell frame 505 may include the macro
cell PDCCH 405 referenced as block 530 occupying maximum 3
OFDM symbols in every sub frames.
The small cell frame 5 10 is structured is the same the macro cell
frame 505 except small cell frame 5 10 applies to the small cell. The
small cell frame 5 10 structure will not be discussed in further detail
for the sake of brevity. If a CSG associated with the small cell is
present the small cell may time blank its synchronization signal
transmission referenced as blocks 545 also referenced as 475 during
the macro cell synchronization signal (PSS, SSS, PBCH) transmission.
The time blanking of the small cell synchronization signal may help
the non CSG users connect to the macro cell. The small cell frame
5 10 may include a time offset 540 in reference to the macro cell frame
505.
The time offset 540 may be equal to, for example, 11 OFDM symbols.
To determine the desired time offset 540 the following method may be
used. Determine a binary sequence based on a number of symbols in
a frame associated with a macro cell. This determination may be
performed by, for example, a base station of the macro cell or a base
station of the small cell. The time offset information may be shared
between the macro and the small cells via medium access control
layer (MAC) messages or the X2 interface. The offsets may be static or
semi static. For example, the offset may be saved at some entity
which is connected to both the macro and the small cell. During the
initial call setup or periodically the macro and the small cell may get
the offset information from that particular entity.
For example, a binary sequence of length 140 where each element
represents an OFDM symbol is formed corresponding to the 10 ms of
the 3GPP LTE (release 8 ) frame. If a particular OFDM symbol is used
for control or sync or PBCH the corresponding element is assigned "1"
otherwise "0" is assigned. When the macro cell PDCCH 405 and the
small cell PDCCH 455 occupies 3 OFDM symbols the sequence
becomes:
" 1 1 1 0 0 1 1 1 1 1 1 0 0 0
1 1 1 0 0 0 0 0 0 0 0 0 0 0
1 1 1 0 0 0 0 0 0 0 0 0 0 0
1 1 1 0 0 0 0 0 0 0 0 0 0 0
1 1 1 0 0 0 0 0 0 0 0 0 0 0
1 1 1 0 0 1 1 0 0 0 0 0 0 0
1 1 1 0 0 0 0 0 0 0 0 0 0 0
1 1 1 0 0 0 0 0 0 0 0 0 0 0
1 1 1 0 0 0 0 0 0 0 0 0 0 0
1 i 1 0 0 0 0 0 0 0 0 0 0 0"
A periodic autocorrelation calculation may be performed on the binary
sequence. An autocorrelation calculation is a cross-correlation of the
binary sequence with itself. This calculation may be performed by, for
example, a base station of the macro cell or a base station of the small
cell. If the periodic autocorrelation of this sequence is plotted, the
time offsets where, for example, valleys or minimums occur which
may be the best candidates for the time offsets between the macro cell
and the small cell frames.
FIGS. 6-8 illustrate a periodic autocorrelation versus offsets in OFDM
symbols when the macro cell PDCCH 405 occupies 3 and 2 OFDM
symbols respectively according to an example embodiment. For
example, referring to FIG. 5, the control block (PDCCH) of sub-frame 7
in the macro cell frame 505 is overlapped by the sync block (PSS, SSS)
of sub-frame 6 in the small cell frame 5 10.
As shown in FIG. 6, example offset values may be 11, 25, 39, 53, 17,
3 1, 45 and 59 OFDM symbols. If the time offset is 11 OFDM symbols
a s shown in FIG. 5 there is no control -control and sync -sync overlap
but there is a one symbol overlap of control - sync in the blanked
macro subframe.
Therefore, the correlation didn't go to zero in FIG. 6. If blanking is
used in a sub-frame the macro cell PDCCH 405 may not occupy 3
OFDM symbols and a maximum of two symbols may be enough (there
is no allocation info to carry). In the 2 OFDM symbols the first symbol
is for the macro cell PCFICH 4 10 + common reference signal (CRS)
and the next symbol for the allocation info. Therefore, by allocating
maximum 2 OFDM symbols for the macro cell PDCCH 405 in blanked
sub-frames the correlation value may be zero or very close to zero.
As shown in FIG. 7, if the macro cell PDCCH 405 and the small cell
PDCCH 455 occupies 2 OFDM symbols the possible offsets are 11, 12,
16, 17, 25, 26, 30, 3 1, 39, 40, 44, 45, 53, 54, 58 and 59 OFDM
symbols. In this case, the correlation is zero at all possible offset
values. Therefore, there is no overlap between control-control or syncsync
or control-sync at these offset values. Another observation is
that the offset values for PDCCH 405 occupies 3 OFDM symbols is a
subset of the offset values when the PDCCH 405 occupies 2 OFDM
symbols.
If the PDCCH of the macro 405 occupies 3 OFDM symbols while small
cell PDCCH 455 occupies 2 OFDM symbols, we construct 2 sequences
correspond to macro and small cell as follows:
" 1 1 0 0 1 1 1 1 1 1 0 0 0
1 1 1 0 0 0 0 0 0 0 0 0 0 0
1 1 1 0 0 0 0 0 0 0 0 0 0 0
1 1 1 0 0 0 0 0 0 0 0 0 0 0
1 1 1 0 0 0 0 0 0 0 0 0 0 0
1 1 1 0 0 1 1 0 0 0 0 0 0 0
1 1 1 0 0 0 0 0 0 0 0 0 0 0
1 1 1 0 0 0 0 0 0 0 0 0 0 0
1 1 1 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0" 1 1
and
" 1 0 0 0 1 1 1 1 1 1 0 0 0
1 1 0 0 0 0 0 0 0 0 0 0 0 0
1 1 0 0 0 0 0 0 0 0 0 0 0 0
1 1 0 0 0 0 0 0 0 0 0 0 0 0
1 1 0 0 0 0 0 0 0 0 0 0 0 0
1 1 0 0 0 1 1 0 0 0 0 0 0 0
1 1 0 0 0 0 0 0 0 0 0 0 0 0
1 1 0 0 0 0 0 0 0 0 0 0 0 0
1 1 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0" 1
If the periodic cross correlation between these two sequences are
calculated. The time offsets which give the lowest cross correlation
may be the best time offsets. As shown in FIG. 8, if the macro cell
PDCCH 405 and the small cell PDCCH 455 occupies 3 and 2 OFDM
symbols the possible offsets are 12, 17, 26, 3 1, 40, 45, 54 and 59
OFDM symbols.
If the macro determines the time offset, the macro cell may broadcast
a control signal including the time offset to the small cells in the
heterogeneous network. The small cell may detect a frame associated
with a macro cell. If the small cell is to make the time offset
calculation, the small cell uses the detected frame in that calculation.
Otherwise, the small cell may receive a control signal including a time
offset from the macro cell. The small cell transmits a frame associated
with the small cell synchronized with the frame associated with the
macro cell and offset in time by the time offset. Therefore, the small
cell frame may be overlaid, in-band, on the macro cell frame.
The small cell PBCH 470 of the small cell carries information on the
band allocated for the small cell. If the band allocated for the small
cell is smaller than the macro cell band the small cell band may hop
within the macro band for interference averaging. This hopping may
be enabled using the small cell PBCH 470.
As the transmit powers of the small cells are low, mobiles which
experience low CQI from small cells may not see the small cell sync
signals due to interference from the macro cell. To avoid this situation
in the down link, the macro cell time-blanks the traffic signal during
the small cell PSS 460, SSS 465 and PBCH 470 transmission as
shown in FIG. 4.
If there is a closed subscriber group (CSG) small cell present, the
mobiles which are located very close to the small cell site but not in
the CSG may have difficulty in decoding the synchronization signals
from the macro due to the interference from the small cell. Therefore,
the small cell time blanks its traffic transmission during macro cell
PSS 420, SSS 425 and PBCH 4 15 transmission.
FIG. 9 illustrates an up link frame structure 900 with small cell
overlay according to an example embodiment. As shown, the up link
frame structure 900 may include a small cell uplink frame 9 10. The
up link frame structure 900 may include one or more macro cell
physical uplink control channels (PUCCH) regions 905. The small cell
up link frame 9 10 may include one or more small cell physical uplink
control channels (PUCCH) regions 9 15. The small cell PUCCH may
include a time offset 920.
Although FIG. 9 shows several other channel blocks, they are not
described herein for the sake of brevity. One skilled in the art will
refer to 3GPP LTE standard (release 8 ) for a more detailed description
of the up link frame structure 900.
In the LTE up link the macro cell PUCCH 905 may occupy both edges
of the up link spectrum. The PUCCH 905 and Physical Uplink
Shared Channel (PUSCH) use cell specific frequency hopping and
CDM. This may provide some form of orthogonality in the up link. If a
CSG associated with a small cell is present, the mobiles which are not
in the CSG but closer to the small cell site may connect to the macro
cell for the up link. Therefore, they need to transmit the signals at the
larger power associated with the macro cell.
This will cause large interference to the CSG users of the small cell at
the small cell receiver. This may also be referred to as the near-far
problem. When the CSG users see the larger interference the small
cell may ask the CSG users to transmit at a larger power. This may
interfere with the signals of the mobiles connected to the macro cell.
Eventually all the mobiles end up transmitting at the maximum power
and cause interference to other co-channel users.
If the small cells are deployed in the macro cell and if their PUCCHs
905, 9 15 overlap each other, the macro cell may not be able to decode
the PUCCH 905 from non-CSG mobiles due to the near-far problem.
Example embodiments describe a method for determining a time offset
in the down link. The same time offset is applied in the up link
between the macro cell and the small cell PUCCHs 905, 9 15 to prevent
their collision. If a CSG associated with a small cell is present,
applying the time offset may also prevent collisions.
When multiple small cells are deployed on the same carrier in a macro
cell, the same band may be allocated or different non-overlapping
bands may be allocated for small cells. The small cells and the macro
cell transmit their PSS (460, 420), SSS (465, 425) and PBCH (470,
4 15) on the same frequency resources or carrier. If the small cells are
well separated in space they may not interfere with each. Therefore,
the same time offset may be used by all the small cells. Otherwise,
different time offsets should be used in small cells to minimize the
interference.
While example embodiments have been described in relation to 3GPP
LTE (release 8), example embodiments are not limited thereto. One
skilled in the art will understand how to apply the example
embodiments to other technologies be they standard technologies or
not.
While example embodiments have been particularly shown and
described, it will be understood by one of ordinary skill in the art that
variations in form and detail may be made therein without departing
from the spirit and scope of the claims.
WE CLAIM:
1. A method, comprising:
determining, by a macro cell (105), a binary sequence based on
a number of symbols in a first frame associated with the macro
cell;
performing, by the macro cell, an autocorrelation calculation on
the binary sequence;
determining, by the macro cell, a time offset (540) based on
minimum values of the autocorrelation calculation; and
broadcasting, by the macro cell, a control signal including the
time offset to a plurality of small cells.
2. The method of claim 1, further comprising:
receiving, by the macro cell, a second frame associated with one
of the small cells;
determining, by the macro cell, if the small cell associated with
the second frame includes a closed subscriber group; and
time-blanking, by the macro cell, a portion of the first frame
(420) associated with a synchronizing signal of the second frame if no
closed subscriber group is present.
3. The method of claim 1, wherein the autocorrelation calculation
is a cross-correlation of the binary sequence with itself.
4. The method of claim 3, wherein
the determined time offset is associated with one of the 11, 12,
16, 17, 25, 26, 30, 3 1, 39, 40, 44, 45, 53, 54, 58 and 59 orthogonal
frequency division multiplexing (OFDM) symbols if a physical control
channel associated with both the first and second frame occupy 2
OFDM symbols, if the binary sequence has a length of 140 symbols
and if each element of the binary sequence represents an OFDM
symbol,
the determined time offset is associated with one of the 11, 17,
25, 3 1, 39, 45, 53 and 59 orthogonal frequency division multiplexing
(OFDM) symbols if a physical control channel associated with both the
first and second frames occupy 3 OFDM symbols , if the binary
sequence has a length of 140 symbols and if each element of the
binary sequence represents an OFDM symbol, and
the determined time offset is associated with one of the 12, 17,
26, 3 1, 40, 45, 54 and 59 orthogonal frequency division multiplexing
(OFDM) symbols if a physical control channel associated with both the
first and second frame occupy 2 and 3 OFDM symbols, if the binary
sequence has a length of 140 symbols and if each element of the
binary sequence represents an OFDM symbol.
5. A method, comprising:
detecting, by a small cell (1 15), a frame associated with a macro
cell (505);
receiving, by the small cell, a control signal including a time
offset (540) associated with the frame associated with the macro cell;
and
transmitting, by the small cell, a frame (5 10) associated with the
small cell synchronized with the frame associated with the
macro cell and offset in time by the time offset.
6. The method of claim 5, further comprising:
determining, by the small cell, if a closed subscriber group is
present; and
time-blanking, by the small cell, a portion of the frame
associated with the small cell (420) associated with a synchronizing
signal of the frame associated with the macro cell if a closed
subscriber group is present.
7. A method, comprising:
detecting, by a small cell (1 15), a frame associated with a macro
cell (505);
determining, by the small cell, a binary sequence based on a
number of symbols in the frame associated with the macro cell;
performing, by the small cell, an autocorrelation calculation on
the binary sequence;
determining, by the small cell, a time offset (540) based on
minimum values of the autocorrelation calculation;
determining, by the small cell, a start time of the frame
associated with the macro cell; and
transmitting, by the small cell, a frame associated with the
small cell (5 10) synchronized with the frame associated with the
macro cell and offset in time by the time offset.
8. The method of claim 7, further comprising:
determining, by the small cell, if a closed subscriber group is
present; and
time-blanking, by the small cell, a portion of the frame
associated with the small cell (420) associated with a synchronizing
signal of the frame associated with the macro cell if a closed
subscriber group is present.
9. The method of claim 7, wherein the autocorrelation calculation
is a cross-correlation of the binary sequence with itself.
10. The method of claim 11, wherein
the determined time offset is associated with one of the
11, 12, 16, 17, 25, 26, 30, 3 1, 39, 40, 44, 45, 53, 54, 58 and 59
orthogonal frequency division multiplexing (OFDM) symbols if a
physical control channel associated with both the first and
second frame occupy 2 OFDM symbols, if the binary sequence
has a length of 140 symbols and if each element of the binary
sequence represents an OFDM symbol,
the determined time offset is associated with one of the
11, 17, 25, 3 1, 39, 45, 53 and 59 orthogonal frequency division
multiplexing (OFDM) symbols if a physical control channel
associated with both the first and second frame occupy 3 OFDM
symbols, if the binary sequence has a length of 140 symbols
and if each element of the binary sequence represents an OFDM
symbol, and
the determined time offset is associated with one of the
12, 17, 26, 3 1, 40, 45, 54 and 59 orthogonal frequency division
multiplexing (OFDM) symbols if a physical control channel
associated with both the first and second frame occupy 2 and 3
OFDM symbols, if the binary sequence has a length of 140
symbols and if each element of the binary sequence represents
an OFDM symbol.