Note: Descriptions are shown in the official language in which they were submitted.
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TRANSMITTERS, RECEIVERS AND METHODS OF TRANSMITTING AND
RECEIVING WITH SCATTERED AND CONTINUOUS PILOTS IN AN OFDM
SYSTEM
Field of the Disclosure
The present disclosure relates to transmitters, receivers and methods of
transmitting and
receiving in an OFDM communications system.
Background of the Disclosure
There are many examples of radio communications systems in which data is
communicated
using Orthogonal Frequency Division Multiplexing (OFDM). Systems which have
been arranged to
operate in accordance with Digital Video Broadcasting (DVB) standards for
example, utilise OFDM.
OFDM can be generally described as providing K narrow band sub-carriers (where
K is an integer)
which are modulated in parallel, each sub-carrier communicating a modulated
data symbol such as
Quadrature Amplitude Modulated (QAM) symbol or Quadrature Phase-shift Keying
(QPSK) symbol.
The modulation of the sub-carriers is formed in the frequency domain and
transformed into the time
domain for transmission. Since the data symbols are communicated in parallel
on the sub-carriers, the
same modulated symbols may be communicated on each sub-carrier for an extended
period, which
can be longer than a coherence time of the radio channel. The sub-carriers are
modulated in parallel
contemporaneously, so that in combination the modulated carriers form an OFDM
symbol. The
OFDM symbol therefore comprises a plurality of sub-carriers each of which has
been modulated
contemporaneously with different modulation symbols.
To facilitate detection and recovery of the data at the receiver, the OFDM
symbol can include
pilot sub-carriers, which communicate data-symbols known to the receiver. The
pilot sub-carriers
provide a phase and timing reference, which can be used to estimate an impulse
response of the
channel through which the OFDM symbol has passed and perform tasks such as
channel estimation
and correction, frequency offset estimation etc. These estimations facilitate
detection and recovery of
the data symbols at the receiver. In some examples, the OFDM symbols include
both Continuous
Pilot (CP) carriers which remain at the same relative frequency position in
the OFDM symbol and
Scattered Pilots (SP). The SPs change their relative position in the OFDM
symbol between
successive symbols, providing a facility for estimating the impulse response
of the channel more
accurately with reduced redundancy. However, the location of the pilots is
required to be known at the
receiver so the receiver can extract the pilot symbols from the correct
locations across the OFDM sub-
carriers.
The development of communications system which utilise OFDM symbols to
communicate
data can represent a significant and complex task. In particular, the
optimisation of communications
parameters particular in respect of frequency planning and network deployment
can present a
significant technical problem requiring considerable effort to identify the
communications parameters
which are suitable for a communications system which utilises OFDM. As will be
appreciated much
work has been performed to optimise the parameters of DVB standards and in
particular DVB T2.
Summary of the Disclosure
A receiver recovers data from Orthogonal Frequency Division Multiplexed (OFDM)
symbols,
the OFDM symbols including a plurality of sub-carrier signals. Some of the sub-
carrier signals
carrying data symbols and some of the sub-carrier signals carrying pilot
symbols, the pilot symbols
comprising scattered pilots symbols and continuous pilot symbols. The
continuous pilot symbols are
distributed across the sub-carrier signals in accordance with a continuous
pilot symbol pattern and the
scattered pilot symbols are distributed across the sub-carrier signals in
accordance with a scattered
pilot signal pattern. The receiver comprises a demodulator configured to
detect a signal representing
the OFDM symbols and to generate a sampled digital version of the OFDM symbols
in the time
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domain. A Fourier transform processor is configured to receive the time domain
digital version of the
OFDM symbols and to form a frequency domain version of the OFDM symbols, from
which the pilot
symbol bearing sub-carriers and the data symbol bearing sub-carriers can be
recovered. A detector is
configured to recover the data symbols from the data bearing sub-carrier
signals of the OFDM
symbols and to recover the pilot symbols from the pilot bearing sub-carrier
signals of the OFDM
symbols in accordance with the scattered pilot symbol pattern and the
continuous pilot symbol
pattern. The scattered pilot symbol pattern is one of a plurality of scattered
pilot symbol patterns and
the continuous pilot pattern is independent of the scattered pilot symbol
pattern. The detector
comprises a memory configured to store a master continuous pilot pattern and a
processor configured
to detect the number of sub-carrier signals in the plurality of sub-carrier
signals and to derive the
continuous pilot pattern from a master pilot pattern based on the number of
sub-carrier signals.
The provision of continuous pilot patterns that are independent of scattered
pilot patterns
means that fewer continuous pilot patterns have to be stored in memory when
there is a plurality of
scattered pilot patterns. Furthermore, the ability to derive continuous pilot
patterns from a master pilot
pattern dependent on the number of sub-carriers may allow fewer continuous
plot patterns to be stored
in memory when the number of sub-carriers varies from symbol to symbol.
In some examples the number of sub-carrier signals in the plurality of sub-
carrier signals is
one of a set of sub-carrier signal numbers and the master pilot symbol pattern
is the pilot symbol
pattern for the continuous pilot symbols for OFDM symbols which include the
highest number of sub-
carrier signals from the set of sub-carrier signal numbers.
The provision of a master pilot pattern which is for the highest order sub-
carrier mode means
that the pilot sub-carrier patterns for modes with fewer subcarriers can be
derived without storing
separate pilot patterns. This therefore may allow a single pilot pattern to be
stored that covers all
possible sub-carrier numbers, thus saving memory anywhere a continuous pilot
pattern is required to
be stored for each mode.
In some examples the set of sub-carrier numbers includes approximately 8k,
16k, and 32k
sub-caniers, the master pilot pattern being provided for the 32k sub-carriers,
and the continuous pilot
pattern for the 8k and 16k sub-carriers being derived from the 32k sub-carrier
continuous pilot
pattern.
Various further aspects and features of the present technique are defined in
the appended
claims and include a transmitter for transmitting OFDM symbols, a method for
transmitting OFDM
symbols and a method for receiving OFDM symbols.
=
Brief Description of the Drawings
Embodiments of the present invention will be now be described by way of
example only with
reference to the accompanying drawings where like parts are provided with
corresponding reference
numerals:
Figure 1 provides a schematic diagram of an example OFDM transmitter;
Figure 2 provides an example OFDM super frame;
Figure 3 provides a schematic diagram of an example OFDM receiver,
Figure 4 provides a diagram of part of an example OFDM frame;
Figure 5 provides a graph illustrating the distribution of continuous pilot
locations in a DVB-
T2 system that do not coincide with scattered pilot positions.
Figure 6 provides a table of continuous pilot symbol sub-carrier locations for
an 8k mode in
accordance with an example of the present disclosure;
Figure 7 provides an illustration of continuous pilot symbol sub-carrier
locations for an 8k
mode in accordance with an example of the present disclosure;
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Figure 8 provides a histogram of the spacing of continuous pilot symbol sub-
carrier locations
for an 8k mode in accordance with an example of the present disclosure;
Figure 9 provides a histogram of dither applied to the continuous pilot symbol
sub-carrier
locations in accordance with an example of the present disclosure;
Figure 10 provides a table of continuous pilot symbol sub-carrier locations
for a 16k mode in
accordance with an example of the present disclosure;
Figure 11 provides an illustration of continuous pilot symbol sub-carrier
locations for a 16k
mode in accordance with an example of the present disclosure;
Figure 12 provides a histogram of the spacing of continuous pilot symbol sub-
carrier
locations for a 16k mode in accordance with an example of the present
disclosure;
Figure 13 provides a table of continuous pilot symbol sub-carrier locations
for a 32k mode in
accordance with an example of the present disclosure;
Figure 14 provides an illustration of continuous pilot symbol sub-carrier
locations for a 32k
mode in accordance with an example of the present disclosure;
Figure 15 provides a histogram of the spacing of continuous pilot symbol sub-
carrier
locations for a 32k mode in accordance with an example of the present
disclosure;
Figure 16 provides a flow diagram of the operation of a transmitter in
accordance with an
example of the present disclosure; and
Figure 17 provides a flow diagram of the operation of a receiver in accordance
with an
example of the present disclosure.
Description of Example Embodiments
Figure 1 provides an example block diagram of an OFDM transmitter which may be
used for
example to transmit video images and audio signals in accordance with the
proposed ATSC 3
standard or DVB-T, DVB-T2 or DVB-C2 standards. In Figure I a program source
generates
data to be transmitted by the OFDM transmitter. A video coder 2, and audio
coder 4 and a data coder
6 generate video, audio and other data to be transmitted which are fed to a
program multiplexer 10.
The output of the program multiplexer 10 forms a multiplexed stream with other
information required
to communicate the video, audio and other data. The multiplexer 10 provides a
stream on a
connecting channel 12. There may be many such multiplexed streams which are
fed into different
branches A, B etc. For simplicity, only branch A will be described.
As shown in Figure 1, an OFDM transmitter 20 receives the stream at a
multiplexer
adaptation and energy dispersal block 22. The multiplexer adaptation and
energy dispersal block 22
randomises the data and feeds the appropriate data to a forward error
correction encoder 24 which
performs error correction encoding of the stream. A bit interleaver 26 is
provided to interleave the
encoded data bits which for the example in a DVB-T2 system is the LDCP/BCH
encoder output. The
output from the bit interleaver 26 is fed to a bit into constellation mapper
28, which maps groups of
bits onto a constellation point of a modulation scheme, which is to be used
for conveying the encoded
data bits. The outputs from the bit into constellation mapper 28 are
constellation point labels that
represent real and imaginary components. The constellation point labels
represent data symbols
formed from two or more bits depending on the modulation scheme used. These
can be referred to as
data cells. These data cells are passed through a time-interleaver 30 whose
effect is to interleave data
cells resulting from multiple LDPC code words.
The data cells are received by a frame builder 32, with data cells produced by
branch B etc. in
Figure 1, via other channels 31. The frame builder 32 then forms many data
cells into sequences to be
conveyed on OFDM symbols, where an OFDM symbol comprises a number of data
cells, each data
cell being mapped onto one of a plurality of sub-carriers. The number of sub-
carriers will depend on
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the mode of operation of the system, which may include one or more of 8k, 16k
or 32k, each of which
provides a different number of sub-carriers and therefore fact Fourier
transform (F.t,T) sizes.
The sequence of data cells to be carried in each OFDM symbol is then passed to
the symbol
interleaver 33. The OFDM symbol is then generated by an OFDM symbol builder
block 37 which
introduces pilot and synchronising signals generated by and fed from a pilot
and embedded signal
former 36 according to pilot symbol pattern(s). An OFDM modulator 38 then
forms the OFDM
symbol in the time domain which is fed to a guard insertion processor 40 for
generating a guard
interval between symbols, and then to a digital to analogue convertor 42 and
finally to an RF
amplifier within an RF front end 44 for eventual broadcast by the COFDM
transmitter from an
antenna 46.
Frame Format
For the system of Figure 1, the number of sub-carriers per OFDM symbol can
vary depending
upon the number of pilot and other reserved carriers. An example illustration
of a "super frame" is
shown in Figure 2.
For example, in DVB-12, unlike in DVB-T, the number of sub-carriers for
carrying data is
not fixed. Broadcasters can select one of the operating modes from lk, 2k, 4k,
8k, 16k, 32k each
providing a range of sub-carriers for data per OFDM symbol, the maximum
available for each of
these modes being 1024, 2048, 4096, 8192, 16384, 32768 respectively. In DVB-T2
a physical layer
frame is composed of many OFDM symbols. Typically the frame starts with a
preamble or P1 symbol
as shown in Figure 2, which provides signalling information relating to the
configuration of the DVB-
T2 deployment, including an indication of the mode. The P1 symbol is followed
by one or more P2
OFDM symbols 64, which are then followed by a number of payload carrying OFDM
symbols 66.
The end of the physical layer frame is marked by a frame closing symbols (FCS)
68. For each
operating mode, the number of sub-carriers may be different for each type of
symbol. Furthermore,
the number of sub-carriers may vary for each according to whether bandwidth
extension is selected,
whether tone reservation is enabled and according to which pilot sub-carrier
pattern has been
selected.
Receiver
Figure 3 provides an example illustration of an OFDM receiver which may be
used to receive
signals transmitted from the transmitter illustrated in Figure 1. As shown in
Figure 3, an OFDM
signal is received by an antenna 100 and detected by a tuner 102 and converted
into digital form by an
analogue-to-digital converter 104. A guard interval removal processor 106
removes the guard interval
from a received OFDM symbol, before the payload data and pilot data is
recovered from the OFDM
symbol using a Fast Fourier Transform (FFT) processor 108 in combination with
a channel estimator
and corrector 110, an embedded-signalling decoding unit 111 and pilot symbol
pattern(s). The
demodulated data is recovered from a de-mapper 112 and fed to a symbol de-
interleaver 114, which
operates to effect a reverse mapping of the received data symbol to re-
generate an output data stream
with the data de-interleaved. Similarly, the bit de-interleaver 116 reverses
the bit interleaving
performed by the bit interleaver 26. The remaining parts of the OFDM receiver
shown in Figure 3 are
provided to effect error correction decoding 118 to correct errors and recover
an estimate of the
source data.
Embodiments of the present technique provide a communication system which
utilises
OFDM to transmit data and reuses much of the system design and configuration
parameters which
have been adopted for the DVB-T2 standard. However the communication system is
adapted to
transmit OFDM symbols within channels of 6 MHz rather than the 8 MHz which is
used for the DVB
T2 standard and utilise 8k, 16k and 32k modes. Accordingly, the present
disclosure presents an
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adaptation of the parameters for an OFDM system for 6 MHz but rationalising
where possible the
parameters that were developed for the DVB T2 standard to simplify
architecture and implementation
of a communications system.
Pilot Symbols
In addition to signalling data and a payload data, OFDM frames and the cells
they include
may also comprise pilot symbols which have been inserted at the transmitter.
These pilot symbols
may for instance have been generated by the pilot and embedded signal former
36 and inserted by the
symbol builder 37. Pilot symbols are transmitted with a known amplitude and
phase and the sub-
carriers upon which they are transmitted may be termed pilot sub-carriers.
Pilot symbols may be
required for a range of different purposes at the receiver, for example,
channel estimation,
synchronisation, coarse frequency offset estimation and fine frequency offset
estimation. Due to the a
priori knowledge of the pilot symbols' amplitude and phase, the channel
impulse response may be
estimated based on the received pilot symbols, with the estimated channel then
being used for
=
purposes such as equalisation.
In order for the receiver to receive the pilot symbols and differentiate the
pilots signals from
other signalling symbols and data symbols, the pilot symbols may be
distributed across the subcarriers
and symbols of an OFDM frame according to a sub-carrier pilot symbol pattern.
Consequently, if the
receiver has knowledge of pilot symbol pattern and is synchronised with the
OFDM frame, it will be
able to extract the received pilot symbols from the appropriate locations or
sub-carriers in the OFDM
symbols and frame.
The distribution of pilots with respect to OFDM sub-carriers may fall into two
categories:
continuous pilots and scattered pilots. Continuous pilots are formed from
pilot symbols whose
location relative to the sub-carriers does not change from symbol to symbol
with the result that they
are transmitted on a same sub-carrier each time. Scattered pilots broadly
describe pilot symbols whose
location changes from symbol to symbol, possibly according to some repeating
pattern.
Figure 4 illustrates a series of OFDM symbols where the circles represent OFDM
cells and
shaded circles represent pilot symbols. In Figure 4 the horizontal direction
represents frequency or the
sub-carrier number, and the vertical direction represents time or the symbol
number. Continuous pilot
symbols 120 are located on the same subcarrier (CP) each time whereas
scattered pilots 122 are
located on different sub-carriers from symbol to symbol. The repetition of the
scattered pilots can be
represented by variables Dx and Dy. Dx represents a separation between
scattered pilots in the
frequency domain from one OFDM symbol to another, so that the scattered pilot
symbols on a first
OFDM symbol is displaced by a number of sub-carriers equal to Dx in the
frequency domain on a
subcanier in the next OFDM symbol. Dy represents a parameter indicating a
number of OFDM
symbols before the same subcanier is used again to carry a pilot symbol on the
next occasion. For
instance, in Figure 4 the location of the scattered pilots symbols may be
represented by Dy = 8, and
Dx = 10. Scattered pilots are an efficient way of providing pilot symbols
because channel estimates
for sub-carriers and symbols in between scattered pilot symbols can be
estimated by interpolation in
both time and frequency from the known pilot symbols or channel estimates.
Consequently, pilot
symbols may not be required to be present on all sub carriers in order to
obtain channel estimates for
each sub-carrier and cell within an OFDM frame.
Pilot symbols occupy sub-carriers and cells which may otherwise be carrying
data, therefore
pilot symbols adversely affect the capacity of a system and it may be
advantageous to minimise the
number of pilot symbols. Consequently, a well-designed pilot pattern that
enables channel estimates
etc. to be obtained across the entire OFDM frame whilst using a small number
of pilot symbols is
desirable.
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The scattered pilot pattern chosen for an OFDM signal may be dependent upon a
number of
factors, such as the rate of channel variation with respect to time and
frequency. For instance, the
density of the pilots must fulfil the sampling theorem in both time and
frequency if accurate channel
estimates are to be obtained i.e. the maximum channel impulses response length
determines the pilot
symbol repetition in the frequency direction, and the maximum Doppler
frequency of the channels
determines the pilot symbol repetition in the time domain. In some example
OFDM systems the guard
interval is determined by the length of the channel impulse response and
therefore the pilot symbol
repetition in the frequency direction may also be dependent upon the guard
interval duration.
It may be beneficial if the location of continual pilot symbols and scattered
pilot symbols do
not overlap or coincide so that there is an approximately constant number of
pilot symbols per frame
and there are no significant "blind spots". In OFDM frames where there is
large number of
neighbouring cells which do not include a pilot symbol, this area may be
referred to as a blind spot. It
is generally desirable to avoid such situations because they may lead to
reduced accuracy channel
estimation and interpolation as well as a possible inability to detect and
compensate for coloured noise
such as analogue TV or other narrow band interference. Figure 5 provides a
graph of continuous pilot
locations which do not coincide with scattered pilot positions in a DVB-T2
system and illustrates the
aforementioned problems, where blind spots 124 are shown as regions where
there is a lack of pilot
symbols. Also shown in Figure 5 are the edges of the frequency band 126 where
measurements taken
via pilot symbols on these regions may be subject to increased noise and
attenuation and should
therefore be avoided if possible.
A measure of the extent which continual pilot symbols and scattered pilots
symbol coincide
may be referred to as a utilisation ratio, and can be calculated using the
formula below
Number of CPnSP
Utilisation Ratio = _________________________
x 100%
Total Number of CP
where CPnSP represents the number of continual pilot symbols which do not
coincide with scattered
pilots sub-carriers during an OFDM frame. Consequently, due to the reasons
given above, it may be
beneficial to try and maxim' ise the utilisation ratio. There are also a
number of other factors which
may have to be taken into account when determining the scattered pilot and
continual pilot patterns,
for instance, it may not be useful to have pilot symbols close to the outer
sub-carriers of an OFDM
signal because it is likely that these sub-carriers may be within the
transition band of tuner filters and
be subject to extra noise as mentioned above. It may also be beneficial to
randomise the location of
pilot symbols to some extent in order to ensure that interference is
adequately modelled and reliable
channel estimates obtained. Furthermore, due to the dependence of the
scattered pilot patterns on
factors such as the guard interval duration and Doppler spread, an OFDM system
may have a plurality
of scattered pilot patterns available to use, each specified by the repetition
rates DX and Dy.
Due to the possible variation of scattered pilot patterns, in order to
maximise the utilisation
ratio, minimise blind spots and avoid pilots symbols being located close to
the outer sub-carriers,
different continuous pilot patterns may be required for one or more of the
scattered pilots patterns. For
instance, in DVB-T2 in some modes there are eight scattered pilot patterns and
eight corresponding
continuous pilot patterns. In some OFDM systems there may be more than one
pattern per mode and
different patterns across different modes so that in total there may be a sign
i Remit number of pilot
patterns.
The pilot signal embedder 36 which embeds the pilot symbols at the transmitter
and the pilot
signal extractor 111 which extracts the pilot symbols at the receiver require
knowledge of the pilot
patterns. Consequently, it is Rely that all the pilot patterns which may be
used in a system will have
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to be stored in ROM at both the transmitter and the receiver, thus requiring a
significant amount of
memory if there are multiple modes and multiple pilot patterns per mode. This
memory requirement is
particularly relevant to the receiver in a broadcast system because there is
likely to be a large number
of receivers compared to transmitters and the cost of the receivers is likely
to be lower than that of the
transmitters. Consequently, reducing memory requirements will likely be
beneficial, especially in the
receiver side of a system.
In addition to memory requirements, utilising a large number of different
scattered and
continuous pilot patterns in a system also makes the system more complex
because the transmitter has
to select which pilot pattern is most appropriate for the current channel
conditions and signal
properties, and the receiver needs to identify the pilot pattern which is
being used. The receiver may
do this via the signalling information which specifies the pilot pattern(s)
and mode of operation, or the
receiver may detect the mode and pilot patterns via charaetelistics of the
signal. However, both of
these approaches become more complex and have larger overheads when more pilot
patterns are
available in a system. Therefore, it would be desirable to reduce the number
of pilot patterns which
are used in a system whilst maximising the utilisation ratio, avoiding blinds
spots and minimising the
number of pilots near to the outer sub-carriers.
In accordance with an example of the present technique, an OFDM system with a
6MHz
bandwidth and 8k, 16k, and 32k modes has a single continuous pilot sub-carrier
pattern for each
mode, which is suitable for use with a plurality of different scattered pilot
symbol patterns within each
mode. In one example, there is a continuous pilot pattern which is suitable
for use with one or more of
the scattered pilot patterns given in Table 2 the below
Scattered
Dx DY
Pilot Pattern
P4,2 4 2
P4,4 4 4
P8,2 8 2
P16,2 16 2
P32,2 32 2
Table 2: Scattered Pilot Patterns
In an 8k mode (normal or extended) of an OFDM system that utilises the
scattered pilots
sequence given in Table 2 above, the distribution of the continuous pilots may
be given by the table in
Figure 6. The same locations as given in Figure 6 are also given by 41, 173,
357, 505, 645, 805, 941,
1098,1225, 1397, 1514, 1669, 1822, 1961, 2119,/245, 2423, 2587, 2709,2861,
3026, 3189,3318,
3510, 3683, 3861, 4045, 4163, 4297, 4457, 4598, 4769, 4942, 5113, 5289, 5413,
5585, 5755, 5873,
6045, 6207, 6379, 6525, 6675, 6862 in terms of sub-carrier locations in the
extended bandwidth
mode. For operation in normal 8k mode the pilot pattern may be derived by
discarding the final sub-
carrier location. The location of the continuous pilot symbols relative to the
sub-carriers given in
Figure 6 do not coincide with the location of the scattered pilots given in
Table 2 above and therefore
the continuous pilot pattern obtains a utilisation ratio of 100%. Figure 7
graphically illustrates the
location of the continuous pilots of Figure 6 for the extended 8k mode and
shows that there is a
substantially uniform distribution of continuous pilots across the subcarriers
of the extended 8k mode
without any substantial blind spots. Figure 8 provides a histogram of the
continuous pilot symbol
spacing with respect to sub-carriers. The histogram once again shows that
there is a substantially
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consistent distribution of continuous pilot symbols across the sub-carriers,
thus reinforcing the
absence of blind spots. Although the distribution of pilot symbols across the
sub-carriers is
substantially uniform, their location has been randomised to some extent by
the introduction of dither.
Figure 9 illustrdtes the dither which has been applied to the placement of the
continuous pilot symbols
in Figure 6.
In a 16k mode (normal or extended) of an OFDM system that utilises the
scattered pilots
sequence given in Table 2 above, the distribution of the continuous pilots may
be given by the table in
Figure 10. The same locations as given in Figure 10 are also given by 82, 243,
346, 517, 714, 861,
1010, 1157, 1290, 1429, 1610, 1753, 1881, 2061, 2197, 2301, 2450, 2647,
2794,2899, 3027, 3159,
3338, 3497, 3645, 3793,3923, 4059,4239, 4409, 4490, 4647, 4847, 5013, 5175,
5277, 5419, 5577,
5723, 5895, 6051, 6222, 6378, 6497, 6637, 6818, 7021, 7201, 7366, 7525, 7721,
7895, 8090, 8199,
8325, 8449, 8593, 8743, 8915, 9055, 9197, 9367, 9539, 9723, 9885, 10058,
10226, 10391, 10578,
10703, 10825, 10959, 11169, 11326, 11510, 11629, 11747, 11941, 12089, 12243,
12414, 12598,
12758, 12881, 13050, 13195, 13349, 13517, 13725, 13821in terms of sub-carrier
locations in the
extended bandwidth mode. For operation in normal 16k mode the pilot pattern
may be derived by
discarding the final two sub-carrier locations. The location of the continuous
pilot symbols relative to
the sub-carriers given in Figure 10 do not coincide with the location. of the
scattered pilots given in
Table 2 above and therefore the continuous pilot pattern obtains a utilisation
ratio of 100%. Figure 11
graphically illustrates the location of the continuous pilots of Figure 10 for
the extended 16k mode
and shows that there is a substantially -uniform distribution of continuous
pilots across the subcarriers
of the extended 16k mode without any substantial blind spots. Figure 12
provides a histogram of the
continuous pilot symbol spacing with respect to sub-carriers. The histogram
once again shows that
there is a substantially consistent distribution of continuous pilot symbols
across the sub-carriers, thus
reinforcing the absence of blind spots. As for the 8k mode, although the
distribution of pilot symbols
across the sub-carriers is substantially uniform, their location has been
randomised to some extent by
the introduction of dither. The same dither as applied to the 8k continuous
pilot symbol placement has
also been applied to the 16k continuous pilot symbol placement and therefore
Figure 9 illustrates the
dither which has been applied the placement of the continuous pilot symbols in
Figure 10.
In a 32k mode of an OFDM system that utilises the scattered pilots sequence
given in Table 2
above, the distribution of the continuous pilots may for example be given by
the table in Figure 13.
The same locations as given in Figure 13 are also given by 163, 290, 486, 605,
691, 858, 1033,
1187, 1427, 1582, 1721, 1881, 2019, 2217, 2314, 2425, 2579, 2709, 2857, 3009,
3219, 3399,
3506, 3621, 3762, 3997, 4122, 4257, 4393, 4539, 4601, 4786, 4899, 5095, 5293,
5378,5587,
5693, 5797, 5937, 6054, 6139, 6317, 6501, 6675, 6807, 6994, 7163, 7289, 7467,
7586, 7689,
7845, 8011, 8117, 8337, 8477, 8665, 8817, 8893, 8979, 9177, 9293, 9539, 9693,
9885, 10026,
10151, 10349, 10471, 10553, 10646, 10837, 10977, 11153, 11325, 11445, 11605,
11789, 11939,
12102, 12253, 12443, 12557, 12755, 12866, 12993, 13150, 13273, 13445,
13635, 13846, 14041, 14225, 14402, 14571, 14731, 14917, 15050, 15209, 15442,
15622, 15790,
15953, 16179, 16239, 16397, 16533, 16650, 16750, 16897, 17045, 17186, 17351,
17485, 17637,
17829, 17939, 18109, 18246, 18393, 18566, 18733, 18901, 19077, 19253,
19445, 19589, 19769, 19989, 20115, 20275, 20451, 20675, 20781, 20989, 21155,
21279, 21405,
21537, 21650, 21789, 21917, 22133, 22338, 22489, 22651, 22823, 23019, 23205,
23258, 23361,
23493, 23685, 23881, 24007, 24178, 24317, 24486, 24689, 24827, 25061,
25195, 25331, 25515, 25649, 25761, 25894, 26099, 26246, 26390, 26569, 26698,
26910, 27033,
27241, 27449, 27511, 27642, 27801 in terms of sub-carrier locations in the
extended bandwidth
mode. For operation in normal 32k mode the pilot pattern may be derived by
discarding the final four
sub-carrier locations. The location of the continuous pilot symbols relative
to the sub-carriers given in
Figure 14 do not coincide with the location of the scattered pilots given in
Table 2 above and
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therefore the continuous pilot pattern obtains a utilisation ratio of 100%.
Figure 14 graphically
illustrates the location of the continuous pilots of Figure 13for the extended
32k mode and shows that
there is a substantially uniform distribution of continuous pilots across the
subcarriers of the extended
8k mode without any substantial blind spots. Figure 15 provides a histogram of
the continuous pilot
symbol spacing with respect to sub-carriers. The histogram once again shows
that there is a
substantially consistent distribution of continuous pilot symbols across the
sub-carriers, thus
reinforcing the absence of blind spots. As for the 8k and 16k modes, although
the distribution of pilot
symbols across the sub-carriers is substantially uniform, their location has
been randomised to some
extent by the introduction of dither. The same dither as applied to the 8k and
16k continuous pilot
symbol placement has also been applied to the 32k continuous pilot symbol
placement and therefore
Figure 9 illustrates the dither which has been applied the placement of the
continuous pilot symbols in
Figure 13.
As previously mentioned, the proposed continuous pilot patterns described
above may also
achieve substantially a 100% utilisation ratio, however, they also achieve a
capacity loss which is
approximately 0.65% in a system such as a proposed ATSC 3 system as previously
described.
The continuous pilot patterns specified above may provide advantages over
existing
continuous pilot patterns because only a single continuous pilot pattern is
required to operate with all
five of the scattered pilots patterns specified in Table 2. Furthermore, these
pilot patterns also reduce
the number of blind spots in comparison to continuous pilot patterns such as
those specified in DVB-
T2. Since only one continuous pilot pattern is required to be stored at both
the transmitter and the
receiver compared to five if conventional continuous pilot patterns were used,
memory requirements
have been reduced by approximately 80%. However, memory for multiple
continuous pilot patterns
may still be required when there is more than one mode of operation e.g. 8k,
16k, 32k, and both
normal and extended modes are available. Consequently, in a system such as a
proposed ATSC 3
system where there are three modes, it is likely that three continuous pilot
patterns are still required to
be stored.
In accordance with another example of the present technique, the continuous
pilots patterns
illustrated in Figures 6, 10, and 13 are related such that the continuous
pilot patterns of the 8k and the
16k modes are derivable from the 32k mode continuous pilot symbol pattern.
This therefore allows a
transmitter and a receiver to store only a single master continuous pilot
pattern for the highest order
mode then derive the continuous pilot patterns for lower order modes when they
are required.
For instance, at the transmitter the pilot and embedded signal former 36 may
comprise a
processor which is operable to detect or receive data which conveys the
operating mode of the OFDM
system and then derive the appropriate continuous pilot pattern from a master
pilot pattern based on
the number of sub-carriers, where the master pilot pattern is stored in a
memory at the pilot and
embedded signal former 36. In the case of the continuous pilot patterns
discussed above, the master
continuous pilot pattern would be the 32k pilot pattern and the 16k continuous
pilot pattern and the 8k
continuous pilot pattern would be derived from the 32k pilot pattern by the
processor according to the
following equations below where the master pilot pattern is given by the
following sub-carrier
locations for the extended bandwidth mode 163, 290, 486, 605, 691, 858, 1033,
1187, 1427,
1582, 1721, 1881, 2019, 2217, 2314, 2425, 2579, 2709, 2857, 3009, 3219, 3399,
3506, 3621,
3762, 3997, 4122, 4257, 4393, 4539, 4601, 4786, 4899, 5095, 5293, 5378,5587,
5693, 5797,
5937, 6054, 6139, 6317, 6501, 6675, 6807, 6994, 7163, 7289, 7467, 7586, 7689,
7845, 8011,
8117, 8337, 8477, 8665, 8817, 8893, 8979, 9177, 9293, 9539, 9693, 9885, 10026,
10151,
10349, 10471, 10553, 10646, 10837, 10977, 11153, 11325, 11445, 11605, 11789,
11939, 12102,
12253, 12443, 12557, 12755, 12866, 12993, 13150, 13273, 13445, 13635, 13846,
14041, 14225,
14402, 14571, 14731, 14917, 15050, 15209, 15442, 15622, 15790, 15953, 16179,
16239, 16397,
16533, 16650, 16750, 16897, 17045, 17186, 17351, 17485, 17637, 17829, 17939,
18109, 18246,
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18393, 18566, 18733, 18901, 19077, 19253, 19445, 19589, 19769, 19989, 20115,
20275, 20451,
20675, 20781, 20989, 21155, 21279, 21405, 21537, 21650, 21789, 21917, 22133,
22338, 22489,
22651, 22823, 23019, 23205, 23258, 23361, 23493, 23685, 23881, 24007, 24178,
24317, 24486,
24689, 24827, 25061, 25195, 25331, 25515, 25649, 25761, 25894, 26099, 26246,
26390, 26569,
26698, 26910, 27033, 27241, 27449, 27511, 27642, 27801.
In order to derive the 16k continuous pilot locations from the 32k pilot
positions given in
Figure 13 and above, every other 32k continuous pilot position is taken, the
position divided by two
and the result rounded up. In terms of a computer implementable equation, this
is given by
CP16K_pos = round(CP_32K__pos(1:2:last 32k cp_pos)/2).
In order to derive the 8k continuous pilot locations from the 32k pilot
positions given in Figure 13
every four of the 32k continuous pilot positions is taken, the taken position
divided by four and the
result rounded up. In terms of a computer implementable equation, this is
given by
CP_8K_pos = round(CP32K_pos(1:4:last_32k cp_pos)/4).
Using the equations above it is possible that the 8k, 16k and 32k continuous
pilot patterns may be
derived from a single master set and therefore an OFDM system is effectively
able to operate with a
single continuous pilot pattern across all modes and all scattered pilot
patterns. This may therefore
simplify the operation of an OFDM system in terms of memory requirements but
also the processing
required because it is no longer necessary to switch between independent
continuous pilot patterns
which are unrelated.
Although in the preceding paragraphs the derivation of the continuous pilot
patterns takes
place at the transmitter, a similar process may also be performed at the
receiver. For instance, the
embedded signal decoding unit 111 may also comprises a processor which is
substantially similar the
processor described with reference to the pilot and embedded signal former 36.
The processor would
be operable to detect or receive data which conveys the operating mode of the
OFDM system i.e.
number of sub-carriers per OFDM symbol, and then derive the appropriate
continuous pilot pattern
from a master pilot pattern as previously described.
Due to the computational simplicity of the derivation processes described
above, a decrease in
ROM memory requirements i.e. the memory required to store the 8k and 16k
continuous pilot
patterns, may be achieved with only a small increase computational complexity.
In some examples in
accorrinnce with the present technique, the derivation in the transmitter and
the receiver may be
performed by existing computational elements within the pilot related elements
and therefore no
additional components would be required in these cases.
In other examples in accordance with the present technique, the continuous
pilot patterns for
8k, 16k and 32k modes may be used in an OFDM system, such as an ATSC 3.0
system for example,
in order to exploit the intrinsic advantages of the continuous pilot symbol
patterns. For instance, the
advantages relating to the regular distribution of the pilot locations and the
reduction in pilot locations
near the outer sub-carriers can be achieved by one of the continuous pilot sub-
carrier patterns with the
following indices:
41, 173, 357, 505, 645, 805, 941, 1098, 1225, 1397, 1514, 1669, 1822, 1961,
2119, 2245, 2423, 2587,
2709, 2861, 3026, 3189, 3318, 3510, 3683, 3861, 4045, 4163, 4297, 4457, 4598,
4769, 4942, 5113,
5289, 5413, 5585, 5755, 5873, 6045, 6207, 6379, 6525, 6675,(6862)
for the 8k mode;
82, 243, 346, 517, 714, 861, 1010, 1157, 1290, 1429, 1610, 1753, 1881, 2061,
2197, 2301, 2450,
2647, 2794, 2899, 3027, 3159, 3338, 3497, 3645, 3793, 3923, 4059, 4239, 4409,
4490, 4647, 4847,
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5013, 5175, 5277, 5419, 5577, 5723, 5895, 6051, 6222, 6378, 6497, 6637, 6818,
7021, 7201, 7366,
7525, 7721, 7895, 8090, 8199, 8325, 8449, 8593, 8743, 8915, 9055, 9197, 9367,
9539, 9723, 9885,
10058, 10226, 10391, 10578, 10703, 10825, 10959, 11169, 11326, 11510, 11629,
11747, 11941,
12089, 12243, 12414, 12598, 12758, 12881, 13050, 13195, 13349, 13517, (13725,
13821)
for the 16k mode; and
163, 290, 486, 605, 691, 858, 1033, 1187, 1427, 1582, 1721, 1881, 2019, 2217,
2314,
2425, 2579, 2709, 2857, 3009, 3219, 3399, 3506, 3621, 3762, 3997, 4122, 4257,
4393, 4539,
4601, 4786, 4899, 5095, 5293, 5378,5587, 5693, 5797, 5937, 6054, 6139, 6317,
6501, 6675,
6807, 6994, 7163, 7289, 7467, 7586, 7689, 7845, 8011, 8117, 8337, 8477, 8665,
8817, 8893,
8979, 9177, 9293, 9539, 9693, 9885, 10026, 10151, 10349, 10471, 10553, 10646,
10837, 10977,
11153, 11325, 11445, 11605, 11789, 11939, 12102, 12253, 12443, 12557, 12755,
12866, 12993,
13150, 13273, 13445,
13635, 13846, 14041, 14225, 14402, 14571, 14731, 14917, 15050, 15209, 15442,
15622, 15790,
15953, 16179, 16239, 16397, 16533, 16650, 16750, 16897, 17045, 17186, 17351,
17485, 17637,
17829, 17939, 18109, 18246, 18393, 18566, 18733, 18901, 19077, 19253,
19445, 19589, 19769, 19989, 20115,20275, 20451, 20675, 20781, 20989, 21155,
21279, 21405,
21537, 21650, 21789, 21917, 22133, 22338, 22489, 22651, 22823, 23019, 23205,
23258, 23361,
23493, 23685, 23881, 24007, 24178, 24317, 24486, 24689, 24827,25061,
25195, 25331, 25515, 25649, 25761, 25894, 26099, 26246, 26390, 26569, 26698,
26910, 27033,
27241, (27449, 27511, 27642, 27801) for the 32k mode, where the values in
brackets relate to the
extended bandwidth modes.
Summary of Operation
An example flow diagram illustrating the operation of a transmitter according
to the present
technique is shown in Figure 16, an operation of a receiver to detect and
recover data from a received
OFDM symbol is provided in Figure 17. The process steps illustrated in Figure
15 are summarised as
follows:
Si: As a first step to transmitting data using OFDM symbols a data formatter
receives the
data for transmission and forms the data into sets of data symbols for each of
the OFDM symbols for
transmission. Thus the data symbols are formed into the sets each have a
number of data symbols
corresponding to an amount of data which can be carried by an OFDM symbol.
S2: An OFDM symbol builder then receives each of the sets of data symbols from
the data
formatter and combines the data symbols with pilot symbols according to
predetermined scattered and
continuous pilot patterns. In accordance with the present technique, the pilot
patterns are given by
Table 2 for scatted pilots and Figures 6, 10, and 13 for continuous pilots,
where the sub-carrier
locations in Figure 6 and Figure 10may be derived from the locations given in
Figure 13. The
predetermined pattern sets out the subcarriers _of the_ OFDM symbol which are
to carry the pilot
symbols. The remaining subcarriers of the OFDM symbol carry the data symbols.
The OFDM
symbols therefore each include a plurality of subcarrier symbols, some of the
subcarrier symbols
carrying data symbols and some of the subcarrier symbols carrying pilot
symbols.
S4: A modulator maps the data symbols and the pilot symbols onto modulation
symbols in
accordance with the value of the data symbols and the pilot symbols. With the
modulation symbols
each of the subcarriers is then modulated to form the OFDM symbols in the
frequency domain.
S6: An inverse Fourier transformer then converts the OFDM symbols in the
frequency
domain into the time domain within a bandwidth of the communication system
which is 6 MHz or
approximately 6 MHz.
S8: A guard interval inserter adds a guard interval to each of the time domain
OFDM symbols
by copying a part of the OFDM symbols which is a useful part containing data
symbols or pilot
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CA 2904775 2017-04-20
symbols and appending the copied part sequentially in the time domain to the
OFDM symbols. The part
which is copied has a length which corresponds to a guard interval which is a
predetermined guard
interval duration.
S10: A radio frequency transmission unit then modulates a radio frequency
carrier with the time
domain OFDM symbols and transmits the OFDM symbols via an antenna of the
transmitter.
The operation of a receiver to detect and recover data from the OFDM symbols
transmitted by the
method of transmission is presented in Figure 17 which are summarised as
follows:
S12: A demodulator receives a signal from an antenna and a radio frequency
down converter and
detects a signal representing the OFDM symbols. The demodulator generates a
sampled digital version of
the OFDM symbols in the time domain. A bandwidth of the OFDM symbols in the
frequency domain in
accordance with the present technique is substantially 6 MHz, that is
approximately 6MHz.
S14: A guard interval correlator correlates the set of samples corresponding
to the guard
interval of the OFDM symbols to detect a timing of a useful part of the OFDM
symbols. A section of the
received signal samples corresponding to the guard interval are copied and
stored and then correlated
with respect to the -same received signal samples in order to detect a
correlation peak identifying where
the repeated guard intervals are present in the useful part of the OFDM
symbols.
S16: A Fourier transform processor then transforms a section of the time
domain samples of the
received signal for a useful part of the OFDM symbols identified by the timing
detected by the guard
interval correlator into the frequency domain using a Fourier transform. From
the OFDM symbols in the
frequency domain the pilot symbols can be recovered from the pilot symbol
bearing subcarriers and data
symbols can be recovered from data bearing subcarriers. In accordance with the
present technique, the pilot
sub-carrier locations are given by Table 2 for scatted pilots and Figures 6,
10, and 13 for continuous pilots,
where the sub-carrier locations in Figure 6 and Figure 10 may be derived from
the locations given in
Figure 13.
S18: A channel estimation and correction unit estimates an impulse response of
a channel
through which the OFDM symbols have passed from the recovered pilot symbols
and corrects the received
data symbols bearing subcarriers using the estimated channel impulse response.
Typically this is in
accordance with the equalisation technique where the received signal in the
frequency domain is divided by
a frequency domain representation of the channel impulse response.
S20: A de-mapper recovers the data symbols from the data bearing subcarriers
of the OFDM
symbols by performing a reverse mapping to that which was performed at the
transmitter.
As will be appreciated the transmitter and receiver shown in Figures 1 and 3
respectively are
provided as illustrations only and are not intended to be limiting. For
example, it will be appreciated that
the present technique can be applied to a different transmitter and receiver
architecture.
As mentioned above, embodiments of the present invention find application with
an ATSC
standard such as ATSC 3Ø For example embodiments of the present invention
may be used in a transmitter
or receiver operating in accordance with hand-held mobile terminals. Services
that may be provided may
include voice, messaging, internct browsing, radio, still and/or moving video
images, television services,
interactive services, video or near-video on demand and option. The services
might operate in combination
with one another.
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