Note: Descriptions are shown in the official language in which they were submitted.
CA 02357198 2001-09-07
AN ADAPTIVE RECEIVER FOR INTERFERENCE SUPPRESSION AND
MULTIPATH RECEPTION IN LONG-CODE CODE-DIVISION
MULTIPLE-ACCESS WIRELESS COMMUNICATION SYSTEMS
Field of the Invention
The present invention relates to an adaptive receiver for interference
suppression and multipath reception in long-code code-division multiple-access
(CDMA) wireless communication systems. The present invention is applicable to
current and future wireless communication systems that are based on code-
division multiple-access (CDMA) technology.
ro Background and Advantages of the Invention
At the present time, wireless communication systems in Americas and far-
east Asia are based on two types of CDMA systems as follows: long-code (or
Random) and short-code (or Deterministic). In long-code CDMA, long aperiodic
codes are used for signature sequences of active users. In short-code CDMA,
r5 short periodic codes are used for the signature sequences. The invention is
optimized for long-code CDMA systems, which are currently deployed in
Americas and considered as strong candidates for third-generation (3G)
systems. However, it can be applied to short-code CDMA but will result in sub-
optimum performance.
:>_o There are two major sources of signal distortion in CDMA wireless
communication systems: 1 ) interference, whose cause can be either other
active
users in the system (also referred to as multiple-access interference or MAI)
or
the signal of the desired user from adjacent time intervals that spills over
the
time interval under attention (also referred to as intersymbol interference or
ISI),
:>_s and 2) multipath channel conditions which appear in environments where,
due to
reflection, refraction, and scattering of radio waves by buildings and other
man-
made obstacles, the transmitted signal most often reaches the receiver by more
than one path. Hence, the transmitted power is diversified to many paths,
which
arrive at the receiver front end with different delays and powers.
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The invention is a filter for the signal processing unit of the receiver. It
makes the receiver capable of suppressing MAI and ISI. Also, the filter is
able to
detect, track, and acquire the energy in all paths of the desired user that
fall
within its time-span.
The industry solution for multipath diversity combining is the RAKE
receiver. In wide bandwidth communications systems (e.g, 3G CDMA), it is
possible to identify, isolate, and harness the energy of each path of the
desired
user. The technique is employed in the well-known RAKE receiver. Similar to
the
way a rake, with teeth in one end of it, gathers together loose leaves, hay,
or
~ o straw, the RAKE receiver gathers the energies in scattered paths of the
desired
user. A RAKE receiver consists of a limited number of fingers (just like the
teeth
in the rake) where each finger takes care of one path. It performs fairly well
in
channels where the number of paths with significant amounts of energy is close
to the number of fingers. The RAKE receiver does not perform any kind of MAI-
suppression and treats interference from other users as background thermal
noise.
Exploiting spatial diversity through adaptive antenna arrays is the way
industry is approaching the problem of interference suppression. Multiple
sensors, directed to different positions with various angles, gather the
signal. It
ao is then processed by the signal-processing unit to retrieve the signals) of
interest. Antenna array, though expensive, is a useful way of interference
suppression for base stations but appears impractical in mobile terminals
where
the unit size is small.
Many other schemes have been proposed in the literature for interference
2.5 suppression but most of them seem to be too complex to have a practical
value.
Current RAKE receivers that are employed in industry have a selective
nature. They have L fingers in their structure and process L strongest paths
associated with the desired user and ignore the rest. Field experiments show
that in dense multipath environments, the desired signal may arrive via 10
paths
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3
or more. Furthermore, next generation systems that are based on a wider
bandwidth cause this number to grow even more. One trivial solution to this
problem is adding more fingers to the structure of the receiver. This,
however,
causes the complexity and power consumption of the receiver to grow linearly.
s Thus, the number of fingers is limited by power consumption and complexity.
Though some companies in Europe and Japan have already started adding to
the number of fingers (Current RAKE receivers have 3 fingers), it is believed
that
this is a short-term solution and the existence of a design like ours will
become
essential in the long run.
~io The receiver according to the present invention considers all paths of the
desired user, no matter how many. The interesting feature is the fact that its
complexity is independent of multipath density which is defined as the number
of
desired paths per time period.
The inventors have achieved this through transforming the conventional
~i 5 decentralized structure of the RAKE receiver to a centralized
architecture that
eliminates a significant amount of hardware and makes the overall complexity
constant and independent of the number of fingers.
More specifically, the inventors have developed an architecture that does
not require a code-tracking unit for each finger. In RAKE receivers, each
finger
.>_o relies on a separate code-tracking unit to identify and acquire a
specific path.
Consequently, a RAKE receiver with 10 fingers will need 10 code-tracking units
resulting in an undesirable amount of complexity.
Furthermore, the architecture of the invention performs interference
suppression as well unlike the RAKE receiver. This is achieved at no extra
:?5 computational complexity and yields additional gain.
The architecture of the receiver of the invention relies on an adaptive filter
that automatically performs the function of code-tracking units. The
implementation of the adaptive filter is quite feasible with state-of-the-art
VLSI
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technology that has greatly enhanced the capabilities of digital signal
processing
(DSP) chips.
Summary of the Invention
According to one aspect of the present invention, an adaptive receiver for
interference suppression and multipath reception in long-code code-division
multiple-access (CDMA) wireless communication systems is provided, which
comprises a FSE block and an adaptive algorithm for updating FSE weights.
The FSE block and the adaptive algorithm are responsible for equalization of
the
distorted signal and are the core of the new signal-processing unit. They
no distinguish this invention from the RAKE receiver. The RAKE has one weight
for
each finger, which is updated regularly by a simple averaging technique. This
single weight is responsible for equalizing the signal in one path of the
desired
user. The system of the invention assigns more than one weight to a path and
they are updated using a recursive adaptive algorithm. With the existence of
the
ns FSE, only one code synchronizer and tracking unit is required whereas in
the
RAKE structure, a separate code-tracking unit is required for each finger.
The existence of the code synchronizer and tracker is important as well
although this is a standard block in CDMA receivers. Depending on the type of
adaptive algorithm that is employed, the PLL and AGC blocks play an important
:?o role as well.
A further understanding of the other features, aspects, and advantages of
the present invention will be realized by reference to the following
description,
appended claims, and accompanying drawings.
Brief Description of the Drawings
:?s Embodiments of the invention will now be described with reference to the
accompanying drawings, in which:
Figure 1 shows a block diagram of an adaptive receiver according to one
CA 02357198 2001-09-07
embodiment of the present invention; and
Figure 2 shows a block diagram of another embodiment of the present
invention.
Detailed Description of the Preferred Embodiments
5 Figure 1 shows a block diagram of an adaptive receiver according to one
embodiment of the present invention. As illustrated in Figure 1, the structure
of
the adaptive receiver comprises:
1. A chip-matched filter (CMF) labeled Q(~;.
2. A sampler operating at a rate of T~, = NS,IT~ where T~ is the chip period;
no 3. A fractionally-spaced equalizer (FSE) whose structure is given in Fig. 2
(figures.pdf). The FSE has 2M+7 complex weights and 2M delay elements of
duration TS,;
4. A sampler operating at T~;
5. A multiplier that multiplies the sampled output of the FSE with the
ns locally generated signature sequence;
6. A signature sequence generator, which generates the sequence of the
desired user and synchronizes it with the received signal;
7. A summer, which sums the outputs of FSE and normalizes them;
8. A sampler that operates at the bit rate Tb and samples the output of the
:?o summer to form a bit estimate:
9. A decision device that first takes the real part of the bit estimate and
then compares it with two thresholds (+1 and -1 ) to decide on the bit
estimate;
and
10. An adaptive algorithm that updates the weights of the FSE every bit
:>_5 period based on the previous information of bit estimates and the outputs
of the
decision device.
The operation of the present invention will be described.
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The received signal is assumed to have been brought down to the base
band by common means of frequency conversion that are addressed in the
literature. The base band received signal is called r(t) and is fed to the
CMF. The
output of this filter is sampled NS times per each chip period. The quantized
samples are serially input to the FSE and delayed TS, seconds. The FSE then
multiplies the delayed samples by its 2M+1 complex weights and sums the
result. The result is sampled every chip period and multiplied by the locally
generated code. The output is one chip estimate. N chip estimates, where N is
called the spreading factor, are summed and normalized and sampled at the bit
ro rate to form the bit estimate. The real part of the bit estimate is
compared with a
hard decision device that performs a "sign(.)" operation on it. (It outputs
the sign
of the bit estimate.)
Every bit period, the weights of FSE (labeled w; in Figure 2) are updated
by an iterative adaptive algorithm. The algorithm can be either supervised
(with
~~ 5 a known pre-determined training sequence) or unsupervised. An example of
the
supervised algorithm can be the least-mean-square (LMS) algorithm that uses
the known training sequence and the bit estimate to form an error term and
corrects the weights. An example of an unsupervised algorithm is the leaky
constant modulus algorithm (LCMA) that uses only the bit estimates to correct
the weights.
The rates of the three samplers and the value of M are all design
parameters and can vary. Usually NS is 2 or 4. The value of M is determined by
the time-window that the FSE filter is supposed to support and the chip rate.
The present invention can be embodied or implemented in various ways
25 as follows: For example, the architecture in Figure 1 can be easily
generalized to
quadriphase spreading when there are the in-phase and quadrature branches.
Also, the CMF can be replaced with another filter (e.g., noise-whitening
matched-filter) in cases where the noise level is high. The FSE can be
replaced
with a baud-spaced equalizer (fewer number of taps with larger delays between
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them). The normalization can be performed before the FSE by means of an
automatic gain control (AGC) unit.
Also, the adaptive algorithm can have many variations (e.g., recursive
least-square or CMA) depending on the desired level of performance and the
availability of training sequences. If CMA or any other of its variation is
chosen
as the adaptive algorithm, a separate phase-locked loop (PLL) unit is also
required to track the phase shift of the desired user's signal since CMA is
phase
invariant and can equalize the distorted signal only up to a phase ambiguity.
The hard decision device (or the sign operator) can be replaced by soft
~ o decision methods and coding schemes can also be incorporated to improve
the
performance. The use of adaptive antenna arrays can also compliment the
above system.
The present invention will be further understood by the appendixes A to F
attached hereto.
While the present invention has been described with reference to specific
embodiments, the description is illustrative of the invention and is not to be
construed as limiting the invention. Various modifications may occur to those
skilled in the art without departing from the true spirit and scope of the
invention
as defined by the appended claims.
a.o
