CA2357198A1 - An adaptive receiver for interference suppression and multipath reception in long-code code-division multiple-access wireless communication systems - Google Patents

Abstract

An adaptive receiver for interference suppression and multipath reception in long-code code-division multiple-access (CDMA) wireless communication systems is disclosed. The adaptive receiver 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. With the existence of the FSE, only one code synchronizer and tracking unit is required.

Description

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.

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 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

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 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.

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 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

Claims (3)

What is claimed is:

1. An adaptive receiver for interference suppression and multipath reception in long-code code-division multiple-access (CDMA) wireless communication systems, said adaptive receiver comprising a FSE block and an adaptive algorithm for updating FSE weights, wherein 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.

2. An adaptive receiver of claim 1, wherein only one code synchronizer and tracking unit is required.

3. An adaptive receiver of claim 1, further comprising a PLL and AGC
blocks.