Note: Descriptions are shown in the official language in which they were submitted.
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FORWARD LINK CARRIER RECOVERY IN AN OCDMA SPREAD SPECTRUM
COM.MUNICATION SYSTEM WITHOUT A PILOT TONE
BACKGROUND AND BRIEF DESCRIPTION OF PRIOR ART
Spread spectrum (SS) communications is presently being
used for a nuinber of cornmercial applications and is expected
to proliferate as the demand for untethered communications
increases.
One example of commercial application of spread spectrum
techniques is disclosed in U.S. Patent No. 5,375,140 assigned
to Bustanlante, Magill, and Natali, titled "Wireless Direct
Sequence Spread Spectrum Digital Cellular Telephone System';
In this case, the base
station of the star-configured network transmits a set of
orthogonal Walsh functions which are overlaid with a pseudo-
noise (PN) sequence. Each orthogonal function carries voice or
data for a single user. See M.J.E. Golay, IDA Report 108, pg.
110(1965) which discloses this basic signal format. This type
of signaling is referred to as Orthogonal CDMA (OCDMA) in this
disclosure. The system incorporates a"Sound Burst" PN
sequence on the base station-to-handset link, as part of each
data frame, for time and frequency synchronization purposes.
Data demodulation is done with differentially coherent
detection.
Another example of commercial application of SS is the
IS-95 standard for cellular telephones. This system uses
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Orthogonal CDMA (OCDMA) on the forward (cell-to-mobile) links
and nonsynchronous CDMA on the return links. Coherent
demodulation of the cell-to-mobile link is facilitated by
incorporating a continuous "pilot signal" which contains 10 to
20 percent of the total signal power.
A number of consortiums have been formed to develop
satellite based Personal Communications Systems (PCS) with
global coverage. Some examples of these systems include
Globalstar (Globalstar System Application before the FCC by
Loral Cellular Systems, Corp., June 3, 1991) and Odyssey
(Application of TRW Inc. before the FCC to Construct a New
Communications Satellite System"Odyssey," May 31, 1991), among
others. The intent of these systems is that a subscriber can
place telephone calls directly through the satellite network
from almost anywhere on the Earth, using a portable handset
much like the present cellular telephones. Both of the
systems mentioned intend to use spread spectrum CDMA
techniques for a number of reasons.
The Globalstar application discloses a signal which is
essentially the same as the IS-95 standard. A continuous
pilot signal, which uses a significant amount of power, is
incorporated in the forward (earth station-to-mobile) link to
be used as a reference for coherent demodulation.
Signal power is at a premium in satellite systems. The
user capacity of satellite PCS systems is, in many scenarios,
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limited by the satellite forward link power. As a consequence, it is desirable
to use
as little satellite output power as possible for synchronization purposes.
Coherent demodulation is often performed in the absence of a reference
signal by using a x2 (or Costas) phase-locked loop (PLL) or a x4 PLL with BPSK
and QPSK signals respectively (reference J.J. Spilker, Jr., Digital
Communications
Prentice-Hall, Inc., New Jersey, 1977). However, these loops require
narrow loop bandwidths at low signal levels and may not perform adequately for
the
mobile channel when the coherence time is short.
SUMMARY OF THE INVENTION
This invention resides in a spread spectrum CDMA communication system
in which a base station communicates with a multiplicity of subscriber
terminals
over a common carrier frequency signal and the base signal transmitted by the
base
station is comprised of a set of substantially orthogonal functions which are
overlaid with a pseudo-noise (PN) sequence forming a coded spreading sequence
for an information signal. Each orthogonal function of the set carries data
for a
single user sharing the common carrier frequency, and there is provided a
source of
the common carrier frequency signal and means are provided to modulate the
information signal onto the common carrier frequency signal to form a transmit
signal and the coded spreading sequence on the transmit signal for
broadcasting.
Each subscriber terminal has a receiver with means to coherently demodulate
the
base station signal.
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According to one form of the invention each receiver has a plurality of
correlators, tuned to different functions of the orthogonal signal set,
respectively,
each followed by a selected nonlinearity for removing the data modulation and
provide correlator output signals, and means to sum the correlator output
signals
such that the summed output improves the estimate of carrier phase over that
obtained with a single correlator.
In another form of the invention, each receiver has a plurality of
correlators,
with each of the plurality of correlators being tuned to a different one of
the
orthogonal functions, respectively, of the orthogonal signal set, and wherein
there is
provided summer means, connected to sum the outputs of the correlators, phase
estimator means connected to the summer means for determining the phase of the
carrier, and means for synchronizing operation of the subscriber terminal with
the
base signal.
The invention also resides in a method in such a system of providing a
plurality of correlators for each receiver, tuning each correlator to a
different
function of the orthogonal signal set, respectively, removing any data
modulation,
and summing the outputs of the correlators, determining the phase of the
carrier
signals as a function of the summed outputs of the correlators and producing a
carrier phase signal, and synchronizing operation of each subscriber terminal
in
accordance with carrier phase signal.
Thus, it can be seen that an object of one aspect of this invention is to
provide a system for deriving an accurate and robust carrier phase estimate
for an
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OCDMA forward link without a pilot signal. This can greatly reduce the amount
of
signal power allotted to synchronization since the power required for time and
frequency synchronization is much smaller than that required for a continuous
phase estimate.
An object of another aspect of this invention is to improve bandwidth
efficiency. The pilot channel as described by IS-95 utilizes one of the
orthogonal
functions which would otherwise be available for transmission of control or
user,
data. Elimination of the pilot channel frees the channel for other uses.
Another object of this invention is to reduce interference due to access noise
by eliminating the need for a continuous carrier reference (pilot channel).
OCDMA
systems are subject to in-cell access noise - although much less than
asynchronous
CDMA -due to non-ideal conditions such as filtering, and receiver reference
time
and frequency error. Each of these conditions results in the users' signals no
longer
being exactly orthogonal to the desired signal and some access noise is
experienced. A continuous pilot channel contributes to this access noise.
Further,
the pilot channel contributes to the access noise in adjacent cells,
especially if 1:1
frequency reuse is employed.
The forward link signal, as received at a subscriber terminal in the
current invention may contain the data signals intended for all users in that
beam (or cell). User signals that share a common carrier frequency are
isolated
from each other by assigning each user a different member of an orthogonal
code set (whose length is equal to a channel data symbol length). There are
typically from 32 to 256 users on a single carrier. All users signals
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are synchronized, and share a common PN overlay code whose
transitions are synchronized to those of the orthogonal codes.
Individual users generally operate with an energy-to-
noise power density ratio (Eb/No) in the range of 3 to 9 dB,
depending on the type of FEC coding and the desired bit error
rate (BER). Accurate phase-lock loop (PLL) tracking of an
individual signal is often impossible for the mobile user due
to the short coherence time of the mobile channel.
In the present invention, the user receiver correlates
the incoming signal with its assigned orthogonal function as
well as some number of orthogonal functions assigned to other
users. The output of each of these correlators is then passed
through a suitable nonlinearity (for example, a squaring
device for BPSK or a x4 device for QPSK) and the outputs
summed. The signal-to-noise ratio (SNR) of the summed output
increases approximately linearly with the number of
correlators, i.e. all of the forward link signal power can be
made available for carrier phase estimation, if desired, with
no power wasted in a pilot signal.
Note that the invention disclosed here can also be used
to improve the carrier recovery performance of systems that do
include a continuous pilot tone, such as IS-95.
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DESCRIPTION OF THE DRAWINGS
The above and other objects, advantages and features of
the invention will become more apparent when considered with
the following specification and accompanying drawings,
wherein:
Figure 1 is a block diagram of a terrestrial network
configuration incorporating the invention,
Figure 2 is a block diagram of a satellite network
configuration incorporating the invention,
Figure 3 is a functional block diagram of the base
station modulator incorporated in the invention,
Figure 4 is a functional block diagram of a subscriber
receiver, and
Figure 5 is a functional block diagram of the carrier
recovery features of this invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present embodiment applies to the invention as used
in a star network. In this case, the hub base station
transmits and OCDMA signal to be received by a number of user
terminals that may include portable handsets as well as
vehicular mobile and fixed units. The invention is
particularly useful in satellite systems (due to the
importance of minimizing required satellite output power) but
is not limited to them. Typical terrestrial and satellite
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network configurations are shown in Figures 1 and 2
respectively.
The signal, as described in this embodiment, employs BPSK
PN modulation and BPSK data modulation. The orthogonal
functions are a set of Radamacher-Walsh(R-W functions. The
R-W and PN chips are aligned in time on a one-for-one basis.
The R-W function period is equal to one data symbol length,
while the PN overlay code may be of the same length but may
also be longer. Each R-W function addresses a single user.
In addition, a PN modulated carrier burst is periodically
inserted into the signal in a time division multiplex (TDM)
fashion for time and frequency synchronization purposes. The
R-W functions are turned off during this burst. This
synchronization PN code is typically a short code that can be
received with a matched filter,although this is not necessary.
This signal burst has the purpose of allowing rapid subscriber
terminal acquisition.
The functional block diagram of a modulator is shown in
Figure 3. Digitized voice signals or data signals that are to
be transmitted to various users are received at the hub
station. These binary signals are buffered 10, encoded 11,
interleaved 12 and then Mod-2 13 added to the assigned R-W
function for each user and summed 14. A common PN code 15 is
then Mod-2 added 16 to each of the signals which are then BPSK
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modulated on a common carrier. The resulting signals are
summed together before up-conversion and amplification.
The subscriber terminal receiver functions are shown in
Figure 4. Received signals are downconverted 40 and mixed 41
to produce the I and Q components which are digitized 42 and
further digitally downconverted 43. The timing loop extracts
code sync 44, 45 and timing logic 46. The derived timing
signals are applied to RW generator 47 which generates a
plurality of RW functions (see Fig. 5), PN generator 48 and
station RW generator 49. The plurality of RW functions (RWm,
RWn.... RW1) are used in the carrier phase recovery loop CPL,
one for each respective channel processor 53-01, 53-2...53-n.
The carrier recovery functions are detailed in Figure 5
(The elements common to Fig. 4 are shown parenthetically). In
this preferred embodiment, the received signal is down-
converted 50 to baseband where the in-phase (I) and quadrature
(Q) signals are converted to binary numbers. The I and Q
signals are multiplied 51 by the time synchronized PN code (
1) 52 and then correlated 53 against a subset of those R-W
functions 54 that are present. Note that code timing 55 is
derived independent of the data signals by tracking the
synchronization burst which is common for all users. The
correlator complex outputs are then squared 56, and the
imaginary parts taken. The imaginary outputs are summed 57 to
form an unmodulated carrier signal with enhanced SNR over that
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of a single user. This signal is input to a phase lock loop
(PLL) 58DPE, 58F which tracks the carrier phase. 'I'he PLL is
configured to remove residual frequency errors before code
correlation. The code correlator output signal corresponding
to the desired user is then coherently demodulated in the
usual way.
The operation of the invention can be further understood
by considering the following: The baseband digital input can
be represented in signal space as:
v1(tklt + A)k = ?1PA ((/1IllVr........ I-rI~IZi1n - F..,.,.... 144
KLVh.)exp(j(-vok'1.)+ oa~
wirere A = 51g(lElt E1rllE)IIIUdP,
P4 = ktl) sanrf>lo of PN cor.le
d~= riih data symbol
1MIn = nth RtZdcr acht:r-Walsh furiction
,,, = frequency effset
'l.' = sartiple period
c(~, - ir~l)ul pha; e
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The Number Controlled Oscillator (NCO) 50 output is
expressed as
''l.~rA ~ " (",?Cl)~- f(I1~nk.T) i t6A ) (2)
where estirnr.ite of the inj)ut phase
.zõ(k) = I1p,P, R IYõ(c1jR1Vi+....,.. ~ rfõ1tll;, r........ +=c!õ(k))-) (3)
wl'+cre Tk -- reference PN co(ju
Snf(~)= ~- -~J~a
ancl pkT)k =1 wlren the PN code yerreratar is properly synchronized.
The oulput of the rith r:on,clator is:
?'n(<<) r,cl[ =, Adõ(A-) (4)
so
anci ttie summecj ; ignal in;o ttie loop filter is:
NA7 (5)
Note that while the correlated signals add coherently, the
noise adds noncoherently (it is decorrelated by the different
RW functions) and so the SNR increases linearly with N.
The advantage of the multiple correlator approach to
carrier recovery as disclosed herein is that any user can take
advantage of the full signal power even though it is intended
for a multiplicity of users. However, if some of the RW codes
are not in use, the corresponding correlators would only add
noise to the phase error estimate and should not be used. Most
systems will have one or more RW channels which carry control
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data and are always present. However, additional RW functions
are usually dynamically assigned as traffic channels. These
channels are sometimes interrupted based on data activity
detection. A signal presence detector 58 based on the real
portion 60 will inhibit 59 the correlator output when no
signal is present.
In order to make efficient use of the available
correlators, the system can assign control and traffic
channels starting from the lowest number. Further, one may
choose to transmit some minimum number of channels under all
situations. For example, suppose only a few users are active
on a carrier capable of supporting 128 users. Assume RW
channels #1 and #2 are used for control functions, and the
next 3 channels are assigned for active traffic. The base
station could transmit the next five RW functions whether or
not they are required to carry traffic, thus ensuring a
minimum often RW functions being present out of the 128 total.
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