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Claims  |
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We claim:
1. Apparatus for receiving an orthogonal frequency division multiplex
(OFDM) signal transmitted at low power in a frequency band subject to
interference from other transmissions, said OFDM signal comprising a
plurality of OFDM carriers modulated by a block of samples, comprising:
means for demodulating the received OFDM carriers so as to produce a block
of values representing the block of samples modulating said OFDM carriers;
means for decoding information contained in said sample values, the
decoding means excluding from the decoding process sample values
demodulated from OFDM carriers at frequencies likely to experience
interference from said other transmissions; and
means for outputting said decoded information.
2. Apparatus according to claim 1, wherein the decoding means excludes from
the decoding sample values demodulated from OFDM carriers at one or more
frequencies likely to experience co-channel interference from one or more
respective carriers at a first frequency or group of frequencies likely to
experience co-channel interference from the carrier of one of said other
transmissions.
3. Apparatus according to claim 2, wherein said other transmissions
comprise a television signal and said one or more carriers at one of said
other transmissions comprise a vision carrier of the television signal.
4. Apparatus according to claim 3, said one or more carriers of one of said
other transmissions comprise a second carrier of the television signal.
5. Apparatus according to claim 1, wherein the decoding means excludes from
the decoding sample values demodulated from OFDM carriers at one or more
frequencies likely to be affected by adjacent channel interference from
said other transmissions.
6. Apparatus according to claim 1, wherein the decoding means excludes from
the decoding sample values demodulated from OFDM carriers at one or more
frequencies likely to be affected by third order intermodulation products
attributable to said other transmissions.
7. Apparatus according to claim 1 and further comprising means for
demodulating the modulated OFDM carriers from a further carrier signal by
a heterodyne process.
8. Apparatus according to claim 7, wherein an intermediate frequency (IF)
used in the heterodyne process is selected so as to ensure that an image
channel interferer due to the carrier of one of said other transmissions
affects OFDM carrier or carriers at a frequency or group of frequencies at
one or more OFDM carriers modulated by sample values already being
excluded from the decoding by the decoding means.
9. Apparatus according to claim 1, wherein the decoding means translates at
least one demodulated sample value to a location within the block of a
demodulated sample values which corresponds to demodulated sample excluded
from the decoding.
10. Apparatus according to claim 3, further comprising means for
demodulating the modulated OFDM carriers from a further carrier signal by
a heterodyne process, an intermediate frequency (IF) used in the
heterodyne process being selected so as to insure that an image channel
interferer due to a further carrier of one of said other transmissions
affects OFDM carriers at said one or more frequencies likely to experience
co-channel interference from the vision carrier of the television signal.
11. Apparatus according to claim 4, further comprising means for
demodulating the modulated ODFM carriers from a further carrier signal by
a heterodyne process, an intermediate frequency (IF) used in the
heterodyne process being selected so as to ensure that an image channel
interferer due to a further carrier of one of said other transmissions
affects OFDM carriers at said one or more frequencies likely to experience
co-channel interference from the sound carrier of the television signal.
12. Apparatus according to claim 3, wherein said one or more carriers of
one of said other transmissions comprises a vision carrier and a sound
carrier of the television signal and further including means for
demodulating the modulated OFDM signal from a further carrier signal by a
heterodyne process, as intermediate frequency used in the heterodyne
process being selected so as to ensure that image channel inteferers due
to further carriers of one of said other transmissions affect OFDM
carriers at one or more frequencies likely to experience co-channel
interference from vision and sound carriers of the television signal.
13. Apparatus according to claim 1, wherein the decoding means comprises
means for processing decoded sample values to derive data corresponding to
sample values excluded from the decoding.
14. Apparatus according to claim 2, wherein the decoding means comprises
means for processing decoded sample values to derive data corresponding to
sample values excluded from the decoding.
15. Apparatus according to claim 5, wherein the decoding means comprises
means for processing decoded sample values to derive data corresponding to
sample values excluded from the decoding.
16. Apparatus according to claim 6, wherein the decoding means comprises
means for processing decoded sample values to derive data corresponding to
sample values excluded from the decoding.
17. Apparatus according to claim 2, wherein the decoding means translates
at least one demodulated sample value to a location within the block of
demodulated sample values which corresponds to a demodulated sample
excluded from the decoding.
18. Apparatus according to claim 5, wherein the decoding means translates
at least one demodulated sample value to a location within the block of
demodulated sample values which corresponds to a demodulated sample
excluded from the decoding.
19. Apparatus according to claim 6, wherein the decoding means translates
at least one demodulated sample value to a location within the block of
demodulated sample values which corresponds to a demodulated sample
excluded from the decoding.
20. Apparatus for transmitting and receiving information in a frequency
band subject to interference from other transmissions, comprising:
a transmitter, comprising:
means for inputting, in the form of blocks of digital data, the information
to be transmitted;
means for coding each of the data samples in a block into one of a
plurality of allowed values;
means for modulating a set of orthogonal frequency division multiplex
(OFDM) carriers with the coded data sample values such that a data sample
located in the block at a position corresponding to an OFDM carrier having
a frequency identified as likely to experience interference is at least
one of omitted and translated and duplicated to another location in the
block, whereby another OFDM carrier having a frequency which is not
identified as likely to experience is modulated; and
means for transmitting a plurality of OFDM carriers modulated with the
block of samples at a power which is low compared with the power of said
other transmissions; and
a receiver, comprising:
means for receiving the modulated OFDM carriers;
means for demodulating the received OFDM carriers so as to produce a block
of values representing the block of samples modulating said OFDM carriers;
means for decoding information contained in said sample values, the
decoding means excluding from the decoding process sample values
demodulated from OFDM carriers at frequencies likely to experience
interference from said other transmissions; and
means for outputting said decoded information.
21. Apparatus for transmitting and receiving information according to claim
20, wherein said means for coding further includes means for storing and
means for writing said coded data samples into said means for storing, and
means for reading out said coded data samples of said means for storing to
said modulating means, said means for writing including means for
addressing coded data sample values so that no coded sample is written
into a location in said means for storing that would be transmitted to
said modulating means for modulating an OFDM carrier at a frequency
identified as likely to experience co-channel interference from the
carrier of one of said other transmissions; and
wherein said decoding means excludes from the decoding sample values
demodulated from OFDM carriers at one or more frequencies likely to
experience co-channel interference from one or more respective carriers at
a first frequency or group of frequencies likely to experience co-channel
interference from the carrier of one of said other transmissions.
22. Apparatus for transmitting and receiving information according to claim
21, wherein said modulating means modulates the OFDM carriers using an
array of real and array of imaginary values, the real array being even
symmetrical about its center and the imaginary being skew symmetrical its
center, for producing a real baseband signal, and said transmitter further
comprising means for mixing the modulated OFDM carriers up to a further
frequency for transmission.
23. Apparatus for transmitting information in a frequency band subject to
interference from other transmissions, comprising:
a transmitter, comprising:
means for inputting, in the form of blocks of digital data, the information
to be transmitted;
means for coding each of the data samples in a block into one of a
plurality of allowed values;
means for modulating a set of orthogonal frequency division multiplex
(OFDM) carriers with the coded data sample values such that a data sample
located in the block at a position corresponding to an OFDM carrier having
a frequency identified as likely to experience interference is at least
one of omitted and translated and duplicated to another location in the
block, whereby another OFDM carrier having a frequency which is not
identified as likely to experience is modulated; and
means for transmitting a plurality of OFDM carriers modulated with the
block of samples at a power which is low compared with the power of said
other transmissions.
24. Apparatus for transmitting information according to claim 23, wherein
said means for coding further includes means for storing and means for
writing said coded data samples into said means for storing, and means for
reading out said coded data samples of said means for storing to said
modulating means, said means for writing including means for addressing
coded data sample values so that no coded sample is written into a
location in said means for storing that would be transmitted to said
modulating means for modulating an OFDM carrier at a frequency identified
as likely to experience co-channel interference from the carrier of one of
said other transmissions.
25. Apparatus for transmitting information according to claim 24, wherein
said modulating means modulates the OFDM carriers using an array of real
and array of imaginary values, the real array being even symmetrical about
its center and the imaginary being skew symmetrical its center, for
producing a real baseband signal, and said transmitter further comprising
means for mixing the modulated OFDM carriers up to a further frequency for
transmission. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the transmission and reception of information and
particularly to the transmission and reception of information in digital
form at frequencies which are hostile from the point of view of
interference from other signals. More particularly, the invention is
described in relation to transmitting in or close to the frequency bands
of existing ultra-high frequency (UHF) TV signals.
2. Related Art
There exist channels in the UHF TV Spectrum which are not used as part of
the frequency planning rules--these are known as the "taboo" channels. To
understand these taboo channels it is necessary to have a brief
understanding of the way in which the UHF TV band is planned.
The following description is given in the context of the frequency plan
adopted in the United Kingdom. It will be understood by a person skilled
in the art that, for similar reasons to those discussed below, taboo
channels exist in the frequency plans of other countries and that the
techniques described below for avoiding interference in a new low power
service in a taboo channel may be applied in those countries also (with
suitable alterations taking into account the different respective channel
bandwidths/channel spacings and sub-carrier frequencies in those
countries).
The frequency plan in the United Kingdom consists of 51 main transmitter
stations covering some 90% of the population using horizontal
polarisation. There then exists 950 small low power relay stations filling
the main coverage gaps these use vertical polarisation. Each main
transmitter station has a certain coverage area and needs perhaps 20 relay
stations for gap filling.
The relay stations in a given coverage area of a single main station have
restrictions on the frequencies to which they may be assigned because of
the frequency planning taboos. Some of the taboos came about as the result
of limited technology when the original UHF plan was designed back in
1961.
In the United Kingdom television channels are assigned 8 MHz segments of
the frequency spectrum. If it is desired to broadcast television signals
in channel number N, then a first pair of taboo channels (adjacent) arise
at channel numbers N.+-.1 because, with receiver technology as it was in
1961, the receiver filters accepting channel N could not reject
frequencies used by channels N.+-.1. Two other pairs of taboo channels
(local oscillator and image channel) also arise at channels numbers N.+-.5
and N.+-.9 respectively because of the heterodyning process used to
demodulate received television signals. If a first receiver were to
receive a broadcast signal at one of channels numbers N.+-.5 then during
the demodulation process frequencies would be generated at the receiver
which would propagate and could interfere with operation of a nearby
receiver attempting to demodulate a broadcast signal at channel number N.
There is now interest in exploiting these taboo channels in a way which
does no cause interference to the existing television service. The present
invention may be utilized for this purpose. More generally, the invention
may be applied to enable the transmission of relatively low power signals
in frequency bands subject to interference from other transmissions.
Transmissions in the taboo channels may take place without causing
interference to existing relay stations in surrounding coverage areas
which use the same frequencies providing very low power transmissions are
used in the taboo channels. This criterion can be met by using digital
modulation which enables transmitter power to be very much reduced without
significantly reducing the coverage area. Typically, a digital signal may
be transmitted using the methods of the present invention with 30 dB less
power for approximately the same coverage as analogue amplitude modulation
(AM). However, when sharing the UHF band at such low levels of transmitted
power the digital signal is very vulnerable to interference from the much
higher power levels of the existing services.
A proposal has been made in European patent application EP-A-0278192 to
transmit digital data in the same channel as a conventional television
signal. In this proposal the data to be transmitted is used to modulate
the carriers of an orthogonal frequency division multiplex OFDM signal.
Interference of the television signal into the OFDM signal is reduced by
using a frequency offset technique. This technique relies on the fact that
the energy in the frequency spectrum of a conventional television signal
is centered around multiples of the line frequency 15625 Hz. The carriers
of the OFDM signal are conditioned to exist only at frequencies which are
offset from the line repetition "harmonic" frequencies of the existing
television signal.
There is a finer repetitive structure to the conventional television signal
spectrum arising because of the frame repetition rate 25 Hz. EP-A-0278192
also proposes a precision offset technique in which the carriers of the
OFDM signal are conditioned to exist only at frequencies which are offset
from these frame repetition "harmonic" frequencies.
Offset and precision offset techniques are well-known for use in reducing
interference between broadcast television signals. For example, television
transmitters broadcasting the same channel are arranged to broadcast their
signals at frequencies offset from one another so that the line structure
of one spectrum interleaves with that of the other. See EBU technical
document 3254. However when contemplating applying an offset technique to
an OFDM signal there is a difficulty.
When reference is made to an OFDM signal the image generally brought to
mind is of a signal including orthogonal carriers overlapping by 50%, such
as that having a power spectrum as illustrated in FIG. 1a. With such a
signal the overall data transmission rate for the full channel bandwidth
almost reaches the ideal Nyquist rate (see U.S. Pat. No. 3,488,445 in the
name of Chang). It may be seen from FIG. 1b that such a signal containing
overlapping carriers cannot be interleaved with a conventional television
signal.
In order to implement an offset or precision offset technique using an OFDM
signal it is proposed in EP-A-0278192 to dispense with overlapping OFDM
carriers and instead to use a set of carriers spaced apart from one
another and each having a narrower width of the carrier peak. Such an OFDM
signal may be used in an offset or precision offset technique as
illustrated by FIG. 1c.
The above system has the disadvantage that the overall data transmission
rate of the OFDM signal is drastically reduced compared with the
theoretical maximum. Furthermore, if a precision offset technique is used
then the frequencies of the OFDM carriers must be very precisely locked to
the carrier frequency of the interfering television signal.
SUMMARY OF THE INVENTION
The present invention is based on a different principle from that described
above. The present invention seeks to identify particular individual
frequencies of the interfering signal which cause the worst interference
problems for the proposed new signal and to either prevent this
interference by cutting out of the new signal the frequencies that would
be affected and/or to reduce the effect of the interference by
conditioning a receiver to reject data transmitted at affected
frequencies.
Where the interferer is a conventional television signal there are two main
components that present continuous high power interfering elements which
would affect the proposed transmissions taking place, for example, in the
taboo channels, these are the vision carrier and the sound carrier.
Although the colour sub-carrier and the digital sound sub-carrier are also
present, these are reduced in level by the dispersal effect of their
modulating signals. Hence, these sub-carriers have a similar energy level
to the vision modulation which has much less peak power than the vision
and sound carrier levels and thus does not present such an interference
problem. The techniques of the invention may nevertheless be applied to
reduce the effects of interference from the colour and digital sound
sub-carriers if it is desired.
It follows that a conventional broadcast television signal may be
approximated to a spectrum consisting of two continuous wave tones (CW),
with the vision carrier at 0 MHz in the baseband and the sound carrier at
6 MHz in the baseband in the United Kingdom. This is shown in FIG. 2.
Transmissions from a given transmission site in the U.K. will resemble four
pairs of CW signals as shown in FIG. 3. The pairs of CW signals will
always be spaced apart by an integer multiple of 8 MHz in the United
Kingdom since successive channel numbers are spaced apart by 8 MHz. Given
this property of the interferer it is possible to design the wanted
digital channel, which is the interference victim, to resist the
interference tones. This will enable the digital signal to be capable of
transmission at a level of 30 dB less than the existing TV service.
Principal modes of interference are as follows:
(i) co-channel interference
(ii) adjacent channel interference
(iii) image channel interference
(iv) third order intermodulation products.
In order to be able to reject interfering tones it is useful to have a
wanted signal spectrum of a type such that pieces can be cut out at
frequencies where the interferers fall.
A likely candidate is the usual orthogonal frequency division multiplexed
signal spectrum (OFDM) which may be made up of a large number of
overlapping modulated carriers as shown in FIG. 1a. Typically 512
overlapping carriers might be transmitted each modulated with a low data
rate signal using say quadrature phase shift keying (QPSK). The total bit
rate of the signal is the number of carriers times the bit rate per
carrier. The resulting OFDM spectrum is rectangular and is an excellent
approximation to a noise signal.
If an interfering tone falls on a few OFDM carriers, these carriers may be
arranged to be ignored by the receiver, provided the interferer is in a
known position in the spectrum. Hence, the receiver cuts out a small
portion of the received spectrum by eliminating the information from the
affected carriers. Since the carriers suffering interference are not to be
processed by the receiver, it is not necessary to transmit them--hence the
spectrum may be transmitted with cut out portions if desired. The
advantage in providing the cut outs on transmission is that a very small
power saving occurs and interference of the OFDM signal into the other
existing transmission, e.g. a television signal, is slightly reduced.
Preferably, therefore, no useful information is broadcast on the carriers
which will be affected by interference. The relevant data which would
normally have been transmitted on the OFDM carriers affected by
interference either omitted or is simply translated so as to modulate OFDM
carriers at other frequencies.
However, it is also possible to use the OFDM spectrum in this environment
by duplicating the "lost" data at one or more other frequency locations in
the OFDM signal. Alternatively, if useful information is modulated onto
all of the OFDM carrier frequencies without translation or duplication,
known methods of data reconstruction may be employed at the receiver to
regenerate that data which is lost by the ignoring of specific frequencies
from the received data.
BRIEF DESCRIPTION OF THE DRAWINGS
Example embodiments of the invention will be described with reference to
the accompanying drawings in which:
FIG. 1a illustrates the power spectrum of an Orthogonal Frequency Division
Multiplex (OFDM) signal;
FIG. 1b compares the spectrum of an OFDM signal with the spectrum of a
conventional television signal;
FIG. 1c compares the spectrum of a specially conditioned OFDM signal with
the spectrum of a conventional television signal;
FIG. 2 shows the frequency spectrum of a typical television channel;
FIG. 3 shows an approximation to the frequency spectrum broadcast at a
typical UK transmitter site;
FIG. 4 is illustrative of co-channel interference;
FIG. 5 is illustrative of image channel interference;
FIG. 6 shows the spectrum of FIG. 3 with an OFDM spectrum added;
FIG. 7 shows a possible final OFDM spectrum according to the present
invention;
FIGS. 8a and 8b illustrate how 2-bit digital data samples may be
differentially QPSK coded;
FIGS. 9a and 9b show in block diagrammatic form examples of coders
according to the present invention;
FIG. 10a illustrates the allowed values of a signal which is QPSK
modulated;
FIG. 10b illustrates the allowed values of a signal which is 8 PSK
modulated;
FIG. 10c illustrates the allowed values of a signal which is 16 QAM
modulated;
FIG. 11 illustrates two possible approaches to the modulation of an OFDM
signal onto a carrier;
FIG. 12 is a simplified block diagram showing the transmission side of an
embodiment of the invention;
FIG. 13 shows a simplified block diagram of a receiver compatible with the
transmitter of FIG. 12;
FIG. 14 shows in block diagrammatic form a decoder in one embodiment of a
receiver according to the invention; and
FIG. 15 shows in block diagrammatic form a decoder in another embodiment of
a receiver according to the invention.
DETAILED DESCRIPTION
As mentioned above there are four principal types of interference likely to
affect a low power transmission:
i) co-channel interference,
ii) adjacent channel interference,
iii) image channel interference, and
iv) third order intermodulation products.
The description below of methods for handling these types of interference
is given in terms of the TV transmission frequency plan in the UK. For TV
transmission elsewhere there will be different channel bandwidths and
vision and sound carrier frequencies so the values in the calculations
will differ. For other types of interfering signal, e.g. radio
transmissions, appropriate changes will be needed in the calculations so
as to take into account the different carrier frequencies etc.
Also, particular frequencies within a broadcast band are often referred to
below as e.g. 6 MHz. In relation to existing television services it will
be understood that these numerical values are referenced to the channel
carrier frequency, F.sub.o, and so 6 MHz really indicates F.sub.o +6 MHz
in the broadcast signal. The actual numerical values will be correct when
considering the baseband signal before its modulation up to the desired
broadcast channel frequency. In relation to the OFDM signal "6 MHz"
indicates the frequency which if a TV signal occupied the channel would be
F.sub.o +6 MHz. Since TV signals in the UK are transmitted in vestigial
sideband form this frequency will be more than 6 MHz above the lowest
frequency present in the OFDM signal.
Straight co-channel interference where a signal according to the invention
is transmitted in the same channel, for example, as a conventional TV
signal is shown in FIG. 4. If two slots are cut out in the OFDM spectrum,
one at the vision carrier position (0 MHz) and one at the sound carrier
position (6 MHz), the level of interfering TV signal that may be tolerated
may be increased by some 30 dB, compared to the case of not having slots
in the OFDM spectrum. Hence, a very large improvement in the level of
co-channel interference from a TV signal is gained by slots in the OFDM
signal at 0 and 6 MHz.
For other co-channel interferers of the type where the modulation is
substantially continuous and causes a much less significant interference
problem than does the carrier, slots may be created in the OFDM spectrum
at locations corresponding to the or those carrier frequencies.
In relation to adjacent channel interference, often a large signal in the
adjacent channel will have spectral components that spill over into the
next channel. The OFDM receiver may be arranged to ignore a small number
of carriers at the edges of the spectrum in order to eliminate the effect
of adjacent channel interference caused by partial blocking. The
transmitted OFDM spectrum may be trimmed at the edges in order to omit
carriers at the frequencies likely to experience adjacent channel
interference and be ignored by the receiver.
Image channel interference arises as explained below.
A superheterodyne receiver tunes to a particular UHF channel by means of a
local oscillator (LO) and mixes the signal down to a fixed intermediate
frequency (IF). As a consequence of the first mixing stage an image
channel is also mixed down into the IF band. The image channel can be
thought of as a channel which folds into the wanted channel at I.F. with
its spectrum reversed. For conventional television transmissions in the UK
the wanted channel is at the UHF frequency which is the LO-IF and the
image channel is at the UHF frequency which is LO+IF. Normally an image
rejection filter is used at the front end of the receiver which has to be
tuned with the LO. However, the rejection offered by a typical, low-cost
image filter is insufficient to remove a TV signal interferer which is 30
dB above the OFDM signal. However, if the vision and sound carriers of the
TV signal can be made to fall into the OFDM slots already created or
envisaged at 0 and 6 HMz then the rejection of the unwanted image will be
vastly improved. This can be achieved by suitable choice of IF frequency
for the OFDM receiver and will cause the sound carrier image to fold back
into the 0 MHz slot and the vision carrier notch to fold back into the 6
MHz notch. This is shown in FIG. 5.
From FIG. 5, the image channel is n channels above the wanted channel and
each channel is 8 MHz wide in the UHF band.
Hence F'.sub.v =F.sub.v +8 n (MHz) and F.sub.s =F.sub.v +8 n+6 (MHz)
The condition for the image channel to fold back into the OFDM holes at
F.sub.v and F.sub.s is when
##EQU1##
the IF frequency can be any value which satisfies the above equation with
n an integer. The most convenient value of n=9 gives:
##EQU2##
An IF frequency of 39 MHz is very close to the standard values of 38.9 and
39.5 MHz currently used for television in the UK and a 38.9 MHz SAW IF
filter may be used for OFDM without modification.
Although the discussion of how to tackle image channel interference has
been cast in terms of selecting a local oscillator frequency for a first
mixing stage in the receiver such that the image channel carrier or
carriers is/are folded into notches that have been provided in the OFDM
spectrum to reduce co-channel interference it will be understood that the
important factor is to cause the image channel carrier(s) to affect
portions of the OFDM signal that are already affected by other types of
interference and that the receiver should already be disregarding. Thus it
does not matter if the image channel interferers are folded into locations
where no notch as such is actually provided (i.e. useful data has in fact
been modulated onto OFDM carriers at the frequencies which the receiver
will ignore).
When the new OFDM service is transmitted in the adjacent channels at 30 dB
less power than the existing TV service, problems may arise because
intermodulation products (IP) of the existing services may fall in the
OFDM band at similar levels to the OFDM signal. This section analyses
where the intermodulation products will fall in the OFDM spectrum when
there are any number of television channels spaced multiples of 8 MHz
apart, and gives the number of possible intermodulation products.
Intermodulation products might be caused in the receiver by the low noise
front end amplifier non-linearities--these cause principally 3rd order
intermodulation products. FIG. 6b shows a typical arrangement of four
incoming UHF TV channels with the new OFDM signal transmitted from the
same mast at a power level 30 db lower than the existing services. The new
signal may even be transmitted from the same mast.
If the transfer function of a non-linear device is given by the following
equation:
Y=K.sub.1 F(t)+K.sub.2 (F(t)).sup.2 +K.sub.3 (F(t)).sup.3 + . . .
then the third order intermodulation products are generated by the term
K.sub.3 (F(t)).sup.3
when F(t)=A sin 2 .pi.F.sub.1 t+B sin 2 .pi.F.sub.2 t+ . . . N sin 2
.pi.F.sub.n t the input signal has n carriers which can be written in a
shorthand form as
##EQU3##
(i) the first line has IPs of the form x.sup.3 which correspond to out of
band IPs given by 3F.sub.x. These can be ignored.
(ii) the second line has IPs of the form x.sup.2 y which give rise to IPs
generated from two of the n input frequencies F.sub.x and F.sub.y and IPs
at
IP=2F.sub.x -F.sub.y
(iii) the third line has IPs of the form xyz which give rise to IPs
generated from three of the n input frequencies F.sub.x, F.sub.y, F.sub.z
and generate IPs at:
IP=F.sub.x +F.sub.y -F.sub.z
(iv) the first set of IPs of form (2F.sub.x -F.sub.y) have a multiplying
coefficient of 3 whereas the second set of IPs have a multiplying
coefficient of 6 which causes the (F.sub.x +F.sub.y -F.sub.z) IPs to be
twice as large as the (2F.sub.x -F.sub.y) IPs. Hence for equal magnitude
input carries the (F.sub.x +F.sub.y -F.sub.z) IPs are 6 dB greater than
the (2F.sub.x +F.sub.y) IPs.
This is an important point to note as IPs caused by three frequencies are
more significant than those caused by only two frequencies, they are of
course only present when three or more input frequencies are present, i.e.
n.gtoreq.3.
The number of IPs generated from n input carriers can be found from the
theory of permutations.
Consider a set of n objects (a,b,c,d, . . . n) the number of different ways
of choosing r objects from the set of n objects is given by:
##EQU4##
Hence IPs given by two frequencies chosen from a set of n input frequencies
generate
##EQU5##
intermod products.
Consider now the equation (F.sub.x +F.sub.y -F.sub.z), the number of IPs
generated is
##EQU6##
because there are 3 frequencies chosen from the n frequency input set.
However, since the permutation (F.sub.x +F.sub.y -F.sub.z) and the
permutation (F.sub.y +F.sub.x -F.sub.z) generate the same IP frequency the
number of IPs actually generated is halved, and is:
______________________________________
##STR1##
IP Frequency
Number of IPs
Magnitude of each IP
______________________________________
2F.sub.x - F.sub.y
##STR2## KX.sup.2 Y
F.sub.x + F.sub.y - F.sub.z
##STR3## 2KXYZ
______________________________________
TABLE 1 INTERMODULATION PRODUCTS GENERATED FROM N INPUT CARRIERS
Where K is some constant caused by the non-linearity and can be determined
from the 3rd Order Intercept point of the device. X is the magnitude of
F.sub.x, Y is the magnitude of F.sub.y and Z is the magnitude of F.sub.z.
F.sub.x, F.sub.y and F.sub.z are any three frequencies chosen from an n
frequency input set.
FIG. 6 shows a typical spectrum of incoming signals to an OFDM receiver
front end. Four TV channels consisting of a vision and sound carrier are
shown which are spaced integer multiples of 8 MHz apart. Since the vision
and sound carrier of a channel are spaced 6 MHz apart it is convenient to
show the carrier sitting on a 2 MHz grid, which is the lowest common
multiple of 8 MHz and 6 MHz. The OFDM signal is 30 dB down on the vision
carrier and for the purpose of the analysis sits in one of the possible
channels--in this case an adjacent channel. However, the analysis is not
dependent in which channel the OFDM signal sits provided F.sub.o, the
nominal position of the vision carrier in that channel, is an integer of 8
MHz away from each vision carrier of the other TV signals.
Intermodulation products will occur at:
(i) IP=2F.sub.x -F.sub.y
(ii) IP=F.sub.x +F.sub.y -F.sub.z
where x, y and z are any of the carriers shown in FIG. 6. Since all
carriers are on a 2 MHz spacing and referring all frequencies to the datum
frequency F.sub.3 :
F.sub.x =F.sub.o +21 (MHz)
F.sub.y +F.sub.o +2 m (MHz)
F.sub.z =F.sub.o +2 n (MHz)
The 2 is the 2 MHz spacing and l, m and n are integers that give the
distance of F.sub.x, F.sub.y and F.sub.z from the datum frequency F.sub.o
in 2 HMz steps. The integers may be positive or negative.
Hence IPs are generated at the following frequencies:
##EQU7##
since (21-m) is just another integer, say K, then IP=F.sub.o +2K (MHz)
Furthermore:
##EQU8##
since (l+m-n) is just another integer, say K, then: IP=F.sub.o +2K (MHz)
Hence all IPs generated from any of the input carriers can only fall at
integer multiples of 2 MHz. Therefore, the position of the IPs in the OFDM
spectrum can only be at the following frequencies:
F.sub.o (MHz) position of vision carrier
F.sub.o +2 (MHz)
F.sub.o +4 (MHz)
F.sub.o +6 (MHz) position of sound carrier
In practice the actual number of IPs falling at these four frequencies will
vary depending on the position of the TV channels relative to the OFDM
spectrum. However, this analysis shows the upper limit to the position of
all the possible 3rd order intermodulation product from any number of
incoming TV channels. Hence, from the above theory placing cut-outs in the
OFDM spectrum at 0, 2, 4 and 6 MHz will prevent interference from 3rd
order IPs.
The OFDM spectrum with information eliminated at the nominal position of
the vision carrier 0 MHz, at 2 MHz and 4 MHz and the nominal position of
the sound carrier 6 MHz is able to resist a variety of interferers. By
means of this strategy, interference from co-channel, image channel and
3rd order intermodulation product may be rejected. The image channel
rejection requires a correct choice to be made for the IF frequency.
Adjacent channel overlapping interferers may be rejected by removing
information at the edges of the OFDM spectrum.
There is an additional advantage to removing information at the low
frequency (d.c.) edge of the OFDM baseband spectrum. Having no energy at
d.c. allows a.c. coupled amplifiers to be used in the signal processing.
Hence the OFDM spectrum with novel conditioning lends itself to
broadcasting in a very hostile interference environment. The resulting
OFDM spectrum is shown in FIG. 7.
As shown in FIG. 1a, an orthogonal frequency division multiplex (OFDM)
signal consists of a large number of carriers each of which is modulated
by a signal whose level varies discretely rather than continuously and
thus the power spectrum of each modulated carrier follows a (sin/x).sup.2
curve. The symbol rate of the modulating signals, and the carrier
frequencies, are such that the peak of each modulated carrier occurs at a
frequency corresponding to nulls for all of the other modulated carriers.
The carrier spacing is equal to the reciprocal of the symbol rate of each
modulating signal (assuming that all of the modulating signals have the
same symbol rate).
The overall spectrum of the OFDM signal is very close to rectangular when a
large number of carriers are contained in the OFDM signal.
During a time period, T, the OFDM signal may be represented by a block of N
time domain samples. The value of the kth sample is, as follows:
##EQU9##
The N values X(n) represent the respective values, during period T, of the
discretely-varying signals which modulate the OFDM carriers e.sup.2jnk/N.
It may be seen from the above equation that the OFDM signal corresponds to
the inverse Discrete Fourier Transform of a set of data samples, X(n)
Thus, a stream of data may be converted into an OFDM signal by splitting
the data stream up into blocks of N samples X(n) and subjecting each block
of data samples to an inverse Discrete Fourier Transform.
The succession of data samples, X(n.sub.i), which appear at a particular
sample position n.sub.i over time constitute a discretely-varying signal
which modulates a carrier at a frequency, f.sub.n.
According to the present invention it is preferred to have only a
restricted set of values which the samples X(n) may take, the set of
values representing a set of phase states and amplitudes to be imparted to
carriers, fn. In particularly preferred embodiments of the invention the
set values to which the samples X(n) are restricted comprises values +1+j,
+1-j, -1+j and -1-j. This set of values corresponds to four allowable
equally spaced phase states for the modulated carriers f.sub.n, and the
same amplitude. Thus, the modulation of each carrier, f.sub.n, in these
embodiments amounts to quadrature phase shift keying (QPSK). QPSK has the
advantage of simplicity and good performance. Further advantages may be
gained by differentially coding the data (this avoids the need for carrier
references). An OFDM signal produced in this way will also tolerate
non-phase-equalised channels much better than would conventional signals.
If the data to be QPSK modulated on to the OFDM carriers consists of data
samples, each data sample taking one of the four possible levels, then it
is relatively simple to code the input data into one of the four allowed
modulating values .+-.1.+-.j. However, where this is not the case (for
example, where the data consists of 3 (or more) bit data samples) then it
is necessary to use an indirect process to code the input data into the
four allowed sample values .+-.1.+-.j. One way of doing this is to first
convert the input data into a binary | | |
