METHOD AND APPARATUS FOR REFERENCE SYMBOL RECEPTION
BACKGROUND OF THE INVENTION
The present invention relates to telecommunications apparatus, methods, systems and apparatus
for transmitting data to and/or receiving data from mobile terminals in a wireless communications system.
Example embodiments of the present technique can provide a facility for varying the level of power in
selected transmissions.
There is an approximate relationship between coverage within a wireless communications system
and the power with which signals are transmitted from infrastructure equipment such as base stations (e.g.
eNodeBs) and/or network controllers (e.g. RNCs, eNodeBs) to user equipment (UE - i.e. wireless
communications devices).
A distance between infrastructure equipment and user equipment (UE) is the main factor in
determining the power in signals received from the infrastructure equipment at the UE. The further apart
the UE and infrastructure equipment lie, the greater the attenuation a signal will experience until the point
when the attenuated signal has a power level of the same order as the ambient noise.
A location of a UE can also determine whether the transmission power is adequate. UEs
positioned indoors or underground experience significant attenuation: being within a predetermined radial
distance of infrastructure equipment may be a necessary condition but is not a sufficient one.
An anticipated widespread deployment of third and fourth generation cellular networks has led to
the parallel development of a class of devices and applications which, rather than taking advantage of the
high data rates available, instead take advantage of the robust radio interface and increasing ubiquity of
the coverage area. This parallel class of devices and applications includes MTC devices and so-called
machine to machine (M2M) applications, wherein semi-autonomous or autonomous wireless
communication devices typically communicate small amounts of data on a relatively infrequent basis.
Unlike a conventional third or fourth generation terminal device such as a smartphone, an MTCtype
terminal is preferably relatively simple and inexpensive: in addition MTC-devices are often deployed
in situations that do not afford easy access for direct maintenance or replacement - reliable and efficient
operation can be crucial. Furthermore, while the type of functions performed by the MTC-type terminal
(e.g. collecting and reporting back data) do not require particularly complex processing to perform, third
and fourth generation mobile telecommunication networks typically employ advanced data modulation
techniques (such as 16QAM or 64QAM) on the radio interface which can require more complex and
expensive radio transceivers to implement.
A "virtual carrier" tailored to low capability terminals such as MTC devices is thus provided
within the transmission resources of a conventional OFDM type downlink carrier (i.e. a "host carrier").
Unlike data transmitted on a conventional OFDM type downlink carrier, data transmitted on the virtual
carrier can be received and decoded without needing to process the full bandwidth of the downlink host
OFDM carrier, for at least some part of a subframe. Accordingly, data transmitted on the virtual carrier
can be received and decoded using a reduced complexity receiver unit.
As noted above, the nature of MTC devices can lead to their deployment in locations where radial
distance to the infrastructure equipment is not the only significant factor in attenuation of signals. To
improve coverage for such devices, it would be desirable to provide signalling at a higher transmission
power. Coverage can then be extended by ensuring that data is transmitted at a sufficiently high power
that the MTC device can receive the signal.
To permit channel estimation LTE however relies upon the transmission power of certain
symbols, known as reference or pilot symbols. Channel estimation refers to the facility for measuring
channel characteristics (such as the complex gain) at certain, predetermined, positions in a radio frame
(i.e. times and/or frequencies) so that the approximate channel characteristics at all positions in the frame
can be deduced. Channel estimates are in turn used to equalize the effects of noise on all channels.
As the transmission power of the reference signals is one of the channel characteristics measured
in channel estimation, these reference signals cannot be transmitted at different powers without disrupting
the channel estimation and subsequent equalization functions. For those UEs where greater attenuation is
experienced (such as MTC devices installed in cellars), this leads to a situation where the reference
symbols are attenuated to such an extent they are inadequate for channel estimation.
SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided a method of receiving data at a
communications device from a wireless communications network, the method comprising: receiving data
symbols transmitted from the wireless communications network at the communications device via a
wireless access interface, the wireless access interface providing a plurality of communications resource
elements across a system bandwidth, which are divided in time into a plurality of time divided radio
frames, the wireless access interface providing within the system bandwidth, a first section of
communications resource elements within a first frequency bandwidth for allocation preferably to
reduced capability communications devices to receive signals representing the data transmitted by the
infrastructure equipment within the first frequency bandwidth forming a virtual carrier, the reduced
capability communications devices each having a receiver bandwidth which is greater than or equal to the
first frequency bandwidth but less than the system bandwidth, wherein receiving the data symbols
comprises receiving the data symbols from a first subset of resource elements in one or more of the radio
frames, and receiving common reference symbols from a second subset of resource elements in each
radio frame, the common reference symbols having been transmitted with a first transmission power and
the data symbols having been transmitted via the virtual carrier with a second transmission power, and
wherein the method further comprises receiving specific reference symbols which have been transmitted
via the resource elements of said virtual carrier at the second transmission power; determining a
difference in power between the second transmission power and the first transmission power, and if the
difference in power substantially exceeds a threshold, generating channel estimates using only the
specific reference symbols received from the virtual carrier.
The wireless communications network preferably (i.e. preferentially) allocates the
communications resources to the reduced capability devices in the sense that the reduced capability
devices are given priority to the communications resources of the first second section of communications
resources over the allocation of the communications resources to communications devices which are able
to communicate using the full bandwidth of the host carrier of the mobile communications network. In
one example, the first section of the communications resources forming the first virtual carrier is reserved
for allocation to the reduced capability devices only, but in other examples, some of the communications
resources of the first section of the first virtual carrier may be allocated to the full capability
communications devices, if a demand for the communications resources from the reduced capability
devices leaves some of the communications resources un-allocated.
Various further aspects and embodiments of the invention are provided in the accompanying
independent and dependent claims.
It will be appreciated that features and aspects of the invention described above in relation to the
first and other aspects of the invention are equally applicable to, and may be combined with,
embodiments of the invention according to the different aspects of the invention as appropriate, and not
just in the specific combinations described above. Furthermore features of the dependent claims may be
combined with features of the independent claims in combinations other than those explicitly set out in
the claims.
BRIEF DESCRIPTION OF DRAWINGS
Embodiments of the present invention will now be described by way of example only with
reference to the accompanying drawings, where like parts are provided with corresponding reference
numerals and in which:
Figures 1A, IB and 1C illustrate schematically certain functional elements of a conventional
mobile telecommunications network;
Figure 2 provides a schematic diagram illustrating a conventional LTE radio frame;
Figure 3 provides a schematic diagram illustrating an example of a conventional LTE downlink
radio sub-frame;
Figure 4 provides a schematic diagram illustrating an example of a LTE downlink radio subframe
in which a narrow band virtual carrier has been inserted;
Figures 5A and 5B illustrate a subframe in which a narrow band virtual carrier has been inserted
at a boosted transmission power;
Figure 6 provides a schematic diagram illustrating the resource elements in a pair of resource
blocks in a downlink radio sub-frame;
Figures 7A and 7B contrast the transmission powers of a plurality of subcarriers including
subcarriers providing a narrow band virtual carrier with and without, respectively, power boosting of data
symbols within the virtual carrier;
Figure 8 schematically illustrates the relative transmission powers of a plurality of subcarriers
including subcarriers providing a narrow band virtual carrier with power boosting of data symbols within
the virtual carrier and the insertion of user specific reference symbols;
Figure 9 shows schematically the operation of infrastructure equipment in determining whether to
insert user specific reference symbols; and
Figure 10 shows schematically the operation of user equipment, there being user specific
reference symbols in addition to common reference symbols, in determining whether to use only the user
specific reference symbols or a combination of common and user specific reference symbols to generate
channel estimates.
DETAILED DESCRIPTION
Figure 1A provides a schematic diagram illustrating some basic functionality of a conventional
mobile telecommunications network, using for example Long Term Evolution (LTE) architecture.
The network includes a plurality of base stations 104 (only one is shown for simplicity)
connected to a core network 110 (in dotted box). Each base station 104 provides a coverage area (i.e. a
cell) within which data can be communicated to and from terminal devices (also referred to as mobile
terminals, MT or User equipment, UE) 102. Data is transmitted from base stations 104 to terminal
devices 102 within their respective coverage areas via a radio downlink 124. Data is transmitted from
terminal devices 102 to the base stations 104 via a radio uplink 122.
The core network 110 routes data to and from the terminal devices 102 via the respective base
stations 104 and provides functions such as authentication, mobility management, charging and so on.
Typical entities in a core network include a Mobility Management Entity, MME, 106 and a subscriber
database (HSS) 108: these entities facilitate the provision of communications services to UEs wherever
they are located within the coverage of the network. Access to data services is provided by a serving
gateway 112 and a packet data network, PDN, gateway 114.
Figure 1A also shows elements which extend the network to allow efficient management of
machine type communication (MTC) devices. The illustrated core network 110 incorporates an MTC
server 116. An optional MTC gateway 120 is also shown in Figure 1A: such a gateway may provide a
hub terminal device which is in communication with one or more MTC devices and in turn establishes
uplink and/or downlink communication paths with the base stations 104 on behalf of the connected MTC
devices.
The UE 102 has certain functional blocks which provide a receive path, as illustrated in Figure
IB. Signals in the radio downlink 124 are received at an antenna arrangement 142 and sent to a
synchronisation and down-conversion block 132 via a radio frequency receiver unit 130. The radio
frequency receiver unit 130 typically includes a Low Noise Amplifier (LNA), which amplifies the
received signal from the antenna arrangement 142. The synchronisation and down -conversion block 132
converts RF signals to baseband (BB) signals. A variety of receiver architectures may be adopted to
provide suitable down-conversion (for example direct conversion, super heterodyne etc.). The
synchronisation and down-conversion block 132 also typically includes a local oscillator (LO), which re
generates a clock for demodulation, and an Analog to Digital converter (A/D) 1506, which converts
analogue signals to digital signals for processing in baseband circuitry.
Baseband processing functions are controlled at a controller 140. Synchronised and downconverted
digital signals are demodulated in a demodulator unit 134 (for example an inverse fast Fourier
transform unit) and passed to an equaliser unit 136. The controller unit 140 takes as input the
synchronised and down-converted digital signals as well as the equalised signals output by the equaliser
unit 136. The controller controls a reference signal processing unit 146 instructing the identification of
suitable reference signals and, if necessary, the generation of virtual reference signals, from the
demodulated signals output by the demodulator unit 134. From the radio characteristics of the reference
signals identified in the reference signal processing unit 146, a channel estimator unit 148 generates
estimated characteristics for all channels in the received signals. The equalizer unit 136 then uses the
channel estimates to generate equalised signals. The equalised signals are then passed to protocol
circuitry for channel decoding.
The infrastructure equipment, such as the base station 104, has certain functional blocks which
are required in the preparation of a signal for downlink 124 transmission, as illustrated in Figure 1C. In
particular, the base station includes a radio antenna arrangement 144, which transmits radio signals.
Typically more than one antenna element is provided for diversity/MIMO transmission. A radio
frequency block 154 provides the signal for transmission by the antenna arrangement 144. Typically it
will include an amplifier which applies a gain to the signal for transmission and an RF transceiver which
up converts from BB to RF, as required. Baseband circuitry which provides functionality such as channel
coding/decoding, modulation/demodulation, channel estimation, equalization etc. includes a scheduler
152 for scheduling the downlink data for a UE based on measured or predetermined radio characteristics
and a controller unit 150, which determines the operation of the scheduler 152.
In mobile telecommunications systems such as those arranged in accordance with the 3GPP
defined Long Term Evolution (LTE) architecture, communication between base stations (e.g. eNodeB
104) and communications terminals (e.g. UE 102, MTC gateway 120) is conducted over a wireless airinterface,
Uu. Downlink 124 on the Uu interface uses an orthogonal frequency division multiple access
(OFDMA) technology, while uplink 122 uses single carrier frequency division multiple access (SCFDMA)
technology. In both cases, the system bandwidth is divided into a plurality of "subcarriers" (each
occupying 15 kHz).
The downlink Uu interface organises resources in time using a "frame" structure. A downlink
radio frame is transmitted from an eNode B and lasts 10 ms. As shown in Figure 2, the downlink radio
frame 204 comprises ten subframes 202, each subframe lasting 1 ms. The subframe in turn comprises a
predetermined number of "symbols", which are each transmitted over a respective 1/14 ms period. Each
symbol comprises a predetermined number of orthogonal subcarriers distributed across the bandwidth of
the downlink radio carrier. Here, the horizontal axis represents time while the vertical represents
frequency.
The smallest allocation of user data for transmission in LTE is a "resource block" comprising
twelve sub-carriers transmitted over one slot (0.5 sub-frame). Each individual box in the sub-frame grid
in Figure 3 corresponds to twelve sub-carriers transmitted on one symbol.
Figure 3 shows, in hatching, resource allocations for four LTE terminals 340, 341, 342, 343. For
example, the resource allocation 342 for a first LTE terminal (UE 1) extends over five blocks of twelve
sub-carriers (i.e. 60 sub-carriers), the resource allocation 343 for a second LTE terminal (UE2) extends
over six blocks of twelve sub-carriers and so on.
Control channel data is transmitted in a control region 300 (indicated by dotted-shading in Figure
3) of the sub-frame comprising the first n symbols of the sub-frame where n can vary between one and
three symbols for channel bandwidths of 3MHz or greater and where n can vary between two and four
symbols for channel bandwidths of 1.4MHz. For the sake of providing a concrete example, the following
description relates to host carriers with a channel bandwidth of 3MHz or greater so the maximum value of
n will be 3. The data transmitted in the control region 300 includes data transmitted on the physical
downlink control channel (PDCCH), the physical control format indicator channel (PCFICH) and the
physical HARQ indicator channel (PHICH).
PDCCH contains control data indicating which sub-carriers on which symbols of the sub-frame
have been allocated to specific LTE terminals. Thus, the PDCCH data transmitted in the control region
300 of the sub-frame shown in Figure 3 would indicate that UE1 has been allocated the block of resources
identified by reference numeral 342, that UE2 has been allocated the block of resources identified by
reference numeral 343, and so on.
PCFICH contains control data indicating the size of the control region (typically between one and
three symbols, but four symbols being contemplated to support 1.4 MHz channel bandwidth).
PHICH contains HARQ (Hybrid Automatic Request) data indicating whether or not previously
transmitted uplink data has been successfully received by the network.
Symbols in the central band 310 of the time-frequency resource grid are used for the transmission
of information including the primary synchronisation signal (PSS), the secondary synchronisation signal
(SSS) and the physical broadcast channel (PBCH). This central band 310 is typically 72 sub-carriers wide
(corresponding to a transmission bandwidth of 1.08 MHz). The PSS and SSS are synchronisation signals
that once detected allow an LTE terminal device to achieve frame synchronisation and determine the cell
identity of the enhanced Node B transmitting the downlink signal. The PBCH carries information about
the cell, comprising a master information block (MIB) that includes parameters that LTE terminals use to
properly access the cell. Data transmitted to individual LTE terminals on the physical downlink shared
channel (PDSCH) can be transmitted in other resource elements of the sub -frame. Further explanation of
these channels is provided below.
Figure 3 also shows a region of PDSCH 344 containing system information and extending over a
bandwidth of R344. A conventional LTE frame will also include reference signals which are discussed
further below but not shown in Figure 3 in the interests of clarity.
The number of sub-carriers in an LTE channel can vary depending on the configuration of the
transmission network. Typically this variation is from 72 sub carriers contained within a 1.4MHz channel
bandwidth to 1200 sub-carriers contained within a 20MHz channel bandwidth (as schematically shown in
Figure 3). As is known in the art, data transmitted on the PDCCH, PCFICH and PHICH is typically
distributed on the sub-carriers across the entire bandwidth of the sub-frame to provide for frequency
diversity. Therefore a conventional LTE terminal must be able to receive the entire channel bandwidth in
order to receive and decode the control region.
As mentioned above, the anticipated widespread deployment of third and fourth generation
networks has led to the parallel development of a class of devices and applications which, rather than
taking advantage of the high data rates available, instead take advantage of the robust radio interface and
increasing ubiquity of the coverage area. This parallel class of devices and applications includes MTC
devices and so-called machine to machine (M2M) applications, wherein semi-autonomous or autonomous
wireless communication devices typically communicate small amounts of data on a relatively infrequent
basis.
Examples of MTC (and M2M) devices include: so-called smart meters which, for example, are
located in a customer's house and periodically transmit information back to a central MTC server data
relating to the customers consumption of a utility such as gas, water, electricity and so on; "track and
trace" applications such as transportation and logistics tracking, road tolling and monitoring systems;
remote maintenance and control systems with MTC-enabled sensors, lighting, diagnostics etc.;
environment monitoring; point of sales payment systems and vending machines; security systems, etc.
Further information on characteristics of MTC-type devices and further examples of the
applications to which MTC devices may be applied can be found, for example, in the corresponding
standards, such as ETSI TS 122 368 V10.530 (2011-07) / 3GPP TS 22.368 version 10.5.0 Release 10) [1].
Whilst it can be convenient for a terminal such as an MTC type terminal to take advantage of the
wide coverage area provided by a third or fourth generation mobile telecommunication network, there are
at present disadvantages and challenges to successful deployment. Unlike a conventional third or fourth
generation terminal device such as a smartphone, an MTC-type terminal is preferably relatively simple
and inexpensive: in addition MTC-devices are often deployed in situations that do not afford easy access
for direct maintenance or replacement - reliable and efficient operation can be crucial. Furthermore,
while the type of functions performed by the MTC-type terminal (e.g. collecting and reporting back data)
do not require particularly complex processing to perform, third and fourth generation mobile
telecommunication networks typically employ advanced data modulation techniques (such as 16QAM or
64QAM) on the radio interface which can require more complex and expensive radio transceivers to
implement.
It is usually justified to include such complex transceivers in a smartphone as a smartphone will
typically require a powerful processor to perform typical smartphone type functions. However, as
indicated above, there is now a desire to use relatively inexpensive and less complex devices to
communicate using LTE type networks. In parallel with this drive to provide network accessibility to
devices having different operational functionality, e.g. reduced bandwidth operation, there is a desire to
optimise the use of the available bandwidth in a telecommunications system supporting such devices.
In many scenarios, providing low capability terminals such as those with a conventional highperformance
LTE receiver unit capable of receiving and processing (control) data from an LTE downlink
frame across the full carrier bandwidth can be overly complex for a device which only needs to
communicate small amounts of data. This may therefore limit the practicality of a widespread
deployment of low capability MTC type devices in an LTE network. It is preferable instead to provide
low capability terminals such as MTC devices with a simpler receiver unit which is more proportionate
with the amount of data likely to be transmitted to the terminal.
A "virtual carrier" tailored to low capability terminals such as MTC devices is thus provided
within the transmission resources of a conventional OFDM type downlink carrier (i.e. a "host carrier").
Unlike data transmitted on a conventional OFDM type downlink carrier, data transmitted on the virtual
carrier can be received and decoded without needing to process the full bandwidth of the downlink host
OFDM carrier, for at least some part of a sub-frame. Accordingly, data transmitted on the virtual carrier
can be received and decoded using a reduced complexity receiver unit.
The term "virtual carrier" corresponds in essence to a narrowband carrier for MTC-type devices
within a host carrier for an OFDM-based radio access technology (such as WiMAX or LTE).
The virtual carrier concept is described in a number of co-pending patent applications (including
GB 1101970.0 [2], GB 1101981.7 [3] , GB 1101966.8 [4], GB 1101983.3 [5], GB 1101 853.8 [6], GB
1101982.5 [7], GB 1101980.9 [8] and GB 1101972.6 [9]), the contents of which are incorporated herein
by reference. For ease of reference, however, an overview of certain aspects of the concept of virtual
carriers is set out in Annex 1.
Figure 4 schematically represents an arbitrary downlink subframe according to the established
LTE standards as discussed above into which an instance of a virtual carrier 406 has been introduced. The
subframe is in essence a simplified version of what is represented in Figure 3. Thus, the subframe
comprises a control region 400 supporting the PCFICH, PHICH and PDCCH channels as discussed above
and a PDSCH region 402 for communicating higher -layer data (for example user -plane data and nonphysical
layer control-plane signalling) to respective terminal devices, as well as system information,
again as discussed above. For the sake of giving a concrete example, the frequency bandwidth (BW) of
the carrier with which the subframe is associated is taken to be 20 MHz. Also schematically shown in
Figure 4 by black shading is an example PDSCH downlink allocation 404. In accordance with the defined
standards, and as discussed above, individual terminal devices derive their specific downlink allocations
404 for a subframe from PDCCH transmitted in the control region 400 of the subframe.
By contrast with the conventional LTE arrangement, where a subset of the available PDSCH
resources anywhere across the full PDSCH bandwidth could be allocated to a UE in any given subframe,
in the T-shaped arrangement illustrated in Figure 4, MTC devices maybe allocated PDSCH resources
only within a pre-established restricted frequency band 406 corresponding to a virtual carrier.
Accordingly, MTC devices each need only buffer and process a small fraction of the total
PDSCH resources contained in the subframe to identify and extract their own data from that subframe.
The pre-established restricted frequency band used to communicate, e.g. on PDSCH in LTE, from
a base station to a terminal device, is thus narrower than the overall system frequency band (carrier
bandwidth) used for communicating physical-layer control information, e.g. on PDCCH in LTE. As a
result, base stations may be configured to allocate downlink resources for the terminal device on PDSCH
only within the restricted frequency band. As the terminal device knows in advance that it will only be
allocated PDSCH resources within the restricted frequency band, the terminal device does not need to
buffer and process any PDSCH resources from outside the pre -determined restricted frequency band.
In this example it is assumed the base station and the MTC device have both pre-established that
data is to be communicated from the base station to the MTC device only within the restricted frequency
band defined by upper and lower frequencies f l and f2 (having a bandwidth Af). In this example the
restricted frequency band encompasses the central part of the overall system (carrier) frequency band
BW. For the sake of a concrete example, the restricted frequency band is assumed here to have a
bandwidth (Af) of 1.4 MHz and to be centred on the overall system bandwidth (i.e. f1 = fc - Af/2 and 2 =
fc + Af72, where fc is the central frequency of the system frequency band). There are various mechanisms
by which the frequency band can be established / shared between a base station and terminal device and
some of these are discussed further below.
Figure 4 represents in shading the portions of each subframe for which the MTC device is
arranged to buffer resource elements ready for processing. The buffered part of each subframe comprises
a control region 400 supporting conventional physical-layer control information, such as the PCFICH,
PHICH and PDCCH channels as discussed above, and a restricted PDSCH region 406. The physical-layer
control regions 400 that are buffered are in the same resources as the physical-layer control regions
buffered by any conventional UE. However, the PDSCH regions 406 which are buffered by the MTC
device are smaller than the PDSCH regions buffered by conventional UEs. This is possible because, as
noted above, the MTC devices are allocated PDSCH resources only within a restricted frequency band
that occupies a small fraction of the total PDSCH resources contained in the subframe.
Accordingly, the MTC device will in the first instance receive and buffer the entire control region
400 and the entire restricted frequency band 406 in a subframe. The MTC device will then process the
control region 400 to decode PDCCH to determine what resources are allocated on PDSCH within the
restricted frequency band, and then process the data buffered during PDSCH symbols within the restricted
frequency band and extract the relevant higher-layer data therefrom.
In one example LTE-based implementation, each subframe is taken to comprise 14 symbols
(timeslots) with PDCCH transmitted on the first three symbols and PDSCH transmitted on the remaining
11 symbols. Furthermore, the wireless telecommunications system is taken in this example to operate
over a system frequency band of 20 MHz (100 resource blocks) with a pre-established restricted
frequency band of 1.4 MHz (six resource blocks) defined for communicating with the terminal devices
supporting virtual carrier operation.
As explained above, in OFDM-based mobile communication systems such as LTE, downlink
data is dynamically assigned to be transmitted on different sub-carriers on a sub-frame by sub-frame
basis. Accordingly, in every sub-frame, the network signals which sub-carriers on which symbols contain
data relevant to which terminals (i.e. downlink allocation signalling).
As can be seen from Figure 3, in a conventional downlink LTE sub-frame information regarding
which symbols contain data relevant to which terminals is transmitted on the PDCCH during the first
symbol or symbols of the sub-frame.
It has been proposed to allow the transmission power to be boosted in the virtual carrier to
improve MTC coverage. This is a practical suggestion where a considerable proportion of the MTC
devices can be assumed to be installed at locations with sub-optimal coverage.
Figure 5A illustrates a subframe in similar to that in Figure 4 in which a narrow band virtual
carrier 503 has been inserted at a boosted transmission power. As in Figure 4, the subframe comprises a
control region 502 supporting the PCFICH, PHICH and PDCCH channels as discussed above and a
PDSCH region 510,512 for communicating higher-layer data (for example user-plane data and nonphysical
layer control-plane signalling) to respective terminal devices.
Figure 5B shows the relative transmission power in the "normal" PDSCH region 510,512 outside
the virtual carrier at a given time, represented as the cut A—A in Figure 5A. The difference in
transmission power between the boosted virtual carrier 503 and the surrounding PDSCH region 510,512,
X, allows coverage to be increased for MTC devices.
Figure 6 provides a schematic diagram illustrating the resource elements in a pair of resource
blocks in a downlink radio sub-frame. As noted above, a sub-frame typically includes three subsets of
resource elements; Data Elements (corresponding to PDSCH) which are the resource elements containing
user data, Control Elements (corresponding to PDCCH, PHICH, and/or PFICH) containing control
information, and Reference Elements (Common Reference Signal - CRS) which are used for channel
estimation. Data elements are indicated as rectangles containing the word "Data"; Control elements, with
the letters "Cntrl"; and Reference Elements with the letters "CRS". As can be seen, the common
reference symbols are inserted at predetermined, periodic, positions in time and frequency within each
resource block of the sub-frame.
The two resource blocks in Figure 6 represent resource block in the virtual carrier bandwidth,
however resource blocks in other regions of the sub-frame have similar constituent resource elements.
Figure 7A illustrates a section through a subframe at a given time and shows the transmission
power as the vertical axis and frequency as the horizontal axis. Normal PDSCH regions 710,712
surround a narrow band virtual carrier 703, which (for simplicity) occupies only 12 subcarriers, 1RB.
The data resource elements 702 and reference signal resource elements 704 in PDSCH regions 710,712
and the virtual carrier region 703 are all shown at the same transmission power.
Figure 7B illustrates a similar section where the data resource elements 706 in the virtual carrier
region 705 are of higher transmission power than data resource elements 702 of the PDSCH regions
710,712. Common reference signals 704,704' remain at the same transmission power whether they fall
within the PDSCH regions 710,712 or the virtual carrier region 705.
In the non-power boosted case illustrated in Figure 7A, the resource elements carrying data and
CRS are transmitted at substantially the same power.
In the power boosted case, Figure 7B, only the data resource elements 706 can be power boosted.
The CRS resource elements 704' in the virtual carrier region 705 cannot be power boosted because they
will be received and used by other UEs (whether they are MTC devices or generic UEs with full receiver
capabilities) within the cell to estimate the channel (hence the name "Common" Reference Signal), not
just those that need the power boosted signal.
As noted above, acquiring the known common reference signal and conducting channel
estimation is an important part of demodulation of the data signal. If the received power of the CRS is
too low, the UE may not be able to correctly receive the data correctly, regardless of whether the data
signals are power boosted (as they are in Figure 7B).
The restriction on varying the transmission power of pilot or reference signals is known in the
field of beamforming. Common Reference Signals can not be beamformed for much the same reason
they cannot have different powers at certain frequencies.
In beamforming, a known approach is to insert UE specific reference signals (illustrated as
rectangles with the letters "DMRS" - Demodulation Reference Signal in Figure 8). These specific
reference signals can be power boosted along with the data signal. As these reference signals are used
only by the UE that is receiving the power boosted data signal, their presence will not affect other UEs in
the cell.
Figure 8 schematically illustrates the relative transmission powers of a plurality of subcarriers
including subcarriers providing a narrow band virtual carrier 805 with power boosting of data symbols
806 within the virtual carrier and the insertion of user specific reference symbols 808. As for Figures 7A
and 7B, Figure 8 illustrates a section through a subframe at a given time and shows the transmission
power as the vertical axis and frequency as the horizontal axis. Normal PDSCH regions 810,812
surround a narrow band virtual carrier 805, which again occupies only 12 subcarriers, IRB. The data 802
and reference signal resource elements 804 in PDSCH regions 810,812 are all shown at the same
transmission power, the data resource elements 806 in the virtual carrier region 805 are of higher
transmission power that those of the PDSCH regions 810,812 while common reference signals 804'
remain at the same transmission power as they do in the PDSCH regions 810,812. In place of selected
data resource elements, the common reference signals 804' are augmented by specific reference signals
808. The specific reference signals are raised to the same (higher) transmission power as the data
resource elements 806.
There is a cost to the insertion of specific reference symbols as they replace data resource
elements. Furthermore, they provide no additional benefit to any UE other than the UEs for which a
powerboosted reference signal is required.
Finally, it has been realised that (non4 oosted) common reference signals are adequate for
channel estimation purposes alongside transmission power boosted data resource elements up to a certain
threshold power level. Below this threshold level, data can be transmitted at higher power (for increased
coverage) without the insertion of specific reference signals being needed for channel estimation.
This threshold power level differs depending upon the precise modulation and coding scheme
(MCS) applied to the transmitted resource elements. The MCS has a bearing on the tolerance of error in
channel estimation. Thus an MCS such as QPSK leads to a channel estimation process that is more
tolerant of error that 64QAM or 16QAM, say. Table 1 below illustrates the differences in the threshold
power levels for selected MCS reflecting the different degrees of error tolerance in channel estimation
using the respective MCS..
Table 1
Consequently, it is beneficial to determine whether insertion of specific reference signals is likely
to be of significant advantage. This determination is made depending on the level of the power boosting
required and type of modulation used for the data signals.
Figure 9 shows an example of the operation of infrastructure equipment in determining whether
to insert user specific reference symbols.
In cases where the required level of power boosting is low (i.e. the UE is nearby and the
transmission power of the common reference signals and data resource elements are sufficiently close)
and when the modulation used for data is more tolerant towards channel estimation errors, the eNB may
choose not to insert specific reference symbols. By contrast, in cases where the required level of power
boost is high and when modulation used for data is less tolerant towards channel estimation errors, the
infrastructure equipment (e.g. an eNB) should insert specific reference signals to exploit fully the gains of
power boosting.
The presence of specific reference symbols (DMRS) needs to be signalled to the UE for which
the power boost is required, so that the UE knows which reference signals are available for use in
demodulating the data signal. Signalling of the level of power boost may be achieved in PDCCH
signalling.
The infrastructure equipment firstly determines the level of power boost required for
transmissions to the UE. In addition, the modulation and coding scheme for the data is determined (step
S905).
A table such as Table 1 is consulted to extract a threshold power level value that corresponds to
the MCS determined in step S905. Next, it is determined whether the required level of power boost
exceeds the threshold power level for the determined MCS (step S910).
Where the power level does exceed the respective threshold value, the infrastructure equipment
inserts specific reference signals at higher transmission power (S915): this higher transmission power
may conveniently be substantially the same power as the power boosted data resource elements.
Where the power level does not exceed the respective threshold value, the infrastructure
equipment transmits only data resource elements at the boosted transmission power (S920).
Finally, in this example, the infrastructure equipment signals the fact that specific reference
signals have been inserted (S930). The same signalling conveniently indicates where in the sub-frame
grid they are inserted and/or what their transmission power level is. The boosted data channel PDSCH is
then transmitted alongside (boosted specific reference signals) and non-boosted common reference
signals.
The reader will readily appreciate that in an alternative arrangement, the infrastructure equipment
may elect to insert specific reference signals regardless of any perceived benefit at modest power boost
levels.
The guaranteed presence of specific reference signals in addition to common reference signals
leads to a further consideration: if the reference signals (of either type) are reliable, the UE should in
principle be able to conduct a more accurate channel estimation by virtue of the presence of additional
reference points (compared to common reference signals alone). However, if some of the reference
points are too weak, then combining these unreliable reference points may degrade the total channel
estimation accuracy to a level below what can be achieved using the specific reference signals alone.
In Figure 10, a method is illustrated whereby a user equipment (UE) decides whether to use
specific reference signals alone or a combination of specific reference signals and common reference
signals to conduct channel estimation, depending on the power boosting level.
Figure 10 shows schematically the operation of the UE in determining whether to use only the
user specific reference symbols or a combination of common and user specific reference symbols to
generate channel estimates.
Here, the UE receives signalling from the infrastructure equipment indicating the level of power
boosting in data resource element transmissions (step S1010).
The UE then compares the level of power boosting to a predetermined threshold value (step
S1020). Where the level of power boosting lies below (or on) the predetermined threshold value, it is
assumed that the common reference signals (CRS) represent suitably reliable reference points and the
channel estimation process uses both common and specific reference signals in the generation of channel
estimates (step SI022).
Where the level of power boosting exceeds the predetermined threshold value, it is assumed that
the common reference signals (CRS) are unreliable as reference points and the channel estimation process
discards common reference signals, using specific reference signals alone in the generation of channel
estimates (step 1024).
Finally, whether common reference signals are used or discarded, the resulting channel estimates
are used by the UE in demodulating the received data symbols (PDSCH) - step 1030.
In alternative implementations, the UE may determine the power boosting level independently by
measuring the power levels of the received common reference signals and specific reference signals.
Thus step S1010, may be replaced by an alternative step of determining the level of power boosting by
determining the relative difference between the received power levels and extrapolating from knowledge
of the common reference signal transmission power.
In further alternative implementations, the relative contribution of common reference signals and
specific reference signals is not determined in a binary manner. Instead the respective reference signals
are weighed differently to one another when calculating the channel estimation, the weights applied
depending on the relative power levels.
The skilled reader will appreciate that while much of the preceding discussion is cast in terms of
power boosting the various embodiments of the invention apply equally to situations where transmission
power is continuously adjusted to ensure constant receive power levels at the receiving UE. Power
boosting strictly speaking refers to the instantaneous increase of transmission power to transmit signal
(typically in an attempt to send data to "hard to reach" UEs; the continuous adjustment of transmission
power may be considered a form of power control.
The following numbered clauses define further example aspects and features of the present
technique:
1. A method of receiving data at a communications device from a wireless communications
network, the method comprising:
receiving data symbols transmitted from the wireless communications network at the
communications device via a wireless access interface, the wireless access interface providing a plurality
of communications resource elements across a system bandwidth, which are divided in time into a
plurality of time divided radio frames,
the wireless access interface providing within the system bandwidth, a first section of
communications resource elements within a first frequency bandwidth for allocation preferably to
reduced capability communications devices to receive signals representing the data transmitted by the
infrastructure equipment within the first frequency bandwidth forming a virtual carrier, the reduced
capability communications devices each having a receiver bandwidth which is greater than or equal to the
first frequency bandwidth but less than the system bandwidth,
wherein receiving the data symbols comprises
receiving the data symbols from a first subset of resource elements in one or more of the radio
frames, and
receiving common reference symbols from a second subset of resource elements in each radio
frame, the common reference symbols having been transmitted with a first transmission power and the
data symbols having been transmitted via the virtual carrier with a second transmission power, and
wherein the method further comprises
receiving specific reference symbols which have been transmitted via the resource elements of
said virtual carrier at the second transmission power;
determining a difference in power between the second transmission power and the first
transmission power, and
if the difference in power substantially exceeds a threshold, generating channel estimates using
only the specific reference symbols received from the virtual carrier.
2. A method according to clause 1, wherein data symbols outside the virtual carrier are
transmitted at the first transmission power.
3. A method according to clause 1 or 2, wherein if the difference in power does not exceed
the threshold, generating channel estimates using both the common reference symbols and the specific
reference symbols, thereby improving the channel estimates by comparison with the channel estimation
procedure using common reference symbols alone.
4. A method according to clause 3, wherein the step of generating channel estimates
includes weighting the common reference symbols and the specific reference symbols differently
depending upon the relative transmission powers in each symbol.
5. A method according to any preceding clause, wherein the difference in transmission
power results from an instantaneous increases of transmission power within the virtual carrier, thereby the
transmission power being boosted for specific communications devices.
6. A communications device for generating channel estimates from data received from an
infrastructure equipment forming part of a wireless communications network, the device comprising:
a receiver unit which operates to receive data via a wireless access interface, the wireless access
interface providing a plurality of communications resource elements across a system bandwidth, which
are divided in time to form a plurality of time divided radio frames, and
a controller configured to control the receiver unit to receive, within the system bandwidth, a first
section of communications resource elements within a first frequency bandwidth for allocation preferably
to reduced capability communications devices to receive signals representing the data transmitted by the
infrastructure equipment within the first frequency bandwidth forming a virtual carrier, the reduced
capability communications devices each having a receiver bandwidth which is greater than or equal to the
first frequency bandwidth but less than the system bandwidth,
wherein the controller in combination with the receiver unit are configured
to receive data symbols in a first subset of the resource elements in one or more of the radio
frames, and
to receive common reference symbols in a second subset of the resource elements in each radio
frame, said second subset having a first transmission power; and
wherein the data symbols in the virtual carrier have been transmitted at a second transmission
power,
wherein the controller in combination with the receiver unit are further configured to receive
specific reference symbols that are transmitted at the second transmission power in the resource elements
of the virtual carrier; and
wherein the controller is further configured to determine a difference in power between the
second transmission power and the first transmission power, and, if the difference in power substantially
exceeds a threshold, to generate channel estimates for frequencies within the first section using only the
specific reference symbols.
REFERENCES
[1] ETSI TS 122 368 V10.530 (201 1-07) / 3GPP TS 22.368 version 10.5.0 Release 10)
[2] UK patent application GB 1101970.0
[3] UK patent application GB 1101981.7
[4] UK patent application GB 1101966.8
[5] UK patent application GB 1101983.3
[6] UK patent application GB 1101853.8
[7] UK patent application GB 1101982.5
[8] UK patent application GB 1101980.9
[9] UK patent application GB 1101972.6

CLAIMS
1. A method of receiving data at a communications device from a wireless communications
network, the method comprising:
receiving data symbols transmitted from the wireless communications network at the
communications device via a wireless access interface, the wireless access interface providing a plurality
of communications resource elements across a system bandwidth, which are divided in time into a
plurality of time divided radio frames,
the wireless access interface providing within the system bandwidth, a first section of
communications resource elements within a first frequency bandwidth for allocation preferably to
reduced capability communications devices to receive signals representing the data transmitted by the
infrastructure equipment within the first frequency bandwidth forming a virtual carrier, the reduced
capability communications devices each having a receiver bandwidth which is greater than or equal to the
first frequency bandwidth but less than the system bandwidth,
wherein receiving the data symbols comprises
receiving the data symbols from a first subset of resource elements in one or more of the radio
frames, and
receiving common reference symbols from a second subset of resource elements in each radio
frame, the common reference symbols having been transmitted with a first transmission power and the
data symbols having been transmitted via the virtual carrier with a second transmission power, and
wherein the method further comprises
receiving specific reference symbols which have been transmitted via the resource elements of
said virtual carrier at the second transmission power;
determining a difference in power between the second transmission power and the first
transmission power, and
if the difference in power substantially exceeds a threshold, generating channel estimates using
only the specific reference symbols received from the virtual carrier.
2. A method as claimed in claim 1, wherein data symbols outside the virtual carrier are
transmitted at the first transmission power.
3. A method as claimed in claim 1 or claim 2, wherein if the difference in power does not
exceed the threshold, generating channel estimates using both the common reference symbols and the
specific reference symbols, thereby improving the channel estimates by comparison with the channel
estimation procedure using common reference symbols alone.
4. A method as claimed in claim 3, wherein the step of generating channel estimates
includes weighting the common reference symbols and the specific reference symbols differently
depending upon the relative transmission powers in each symbol.
5. A method as claimed in any one the preceding claims, wherein the difference in
transmission power results from an instantaneous increases of transmission power within the virtual
carrier, thereby the transmission power being boosted for specific communications devices.
6. A communications device for generating channel estimates from data received from an
infrastructure equipment forming part of a wireless communications network, the device comprising:
a receiver unit which operates to receive data via a wireless access interface, the wireless access
interface providing a plurality of communications resource elements across a system bandwidth, which
are divided in time to form a plurality of time divided radio frames, and
a controller configured to control the receiver unit to receive, within the system bandwidth, a first
section of communications resource elements within a first frequency bandwidth for allocation preferably
to reduced capability communications devices to receive signals representing the data transmitted by the
infrastructure equipment within the first frequency bandwidth forming a virtual carrier, the reduced
capability communications devices each having a receiver bandwidth which is greater than or equal to the
first frequency bandwidth but less than the system bandwidth,
wherein the controller in combination with the receiver unit are configured
to receive data symbols in a first subset of the resource elements in one or more of the radio
frames, and
to receive common reference symbols in a second subset of the resource elements in each radio
frame, said second subset having a first transmission power; and
wherein the data symbols in the virtual carrier have been transmitted at a second transmission
power,
wherein the controller in combination with the receiver unit are further configured to receive
specific reference symbols that are transmitted at the second transmission power in the resource elements
of the virtual carrier; and
wherein the controller is further configured to determine a difference in power between the
second transmission power and the first transmission power, and, if the difference in power substantially
exceeds a threshold, to generate channel estimates for frequencies within the first section using only the
specific reference symbols.
7. A method of receiving data at a communications device from a wireless communications
network substantially as hereinbefore described with reference to the accompanying drawings.
8. A communications device for receiving data at a communications device from a wireless
communications network substantially as hereinbefore described with reference to the accompanying
drawings.