An embedded CDMA-receiver A design example (original) (raw)
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Rapid acquisition techniques in CDMA spread-spectrum systems
IEE Proceedings F Communications, Radar and Signal Processing
A rapid acquisition scheme for code division multiple access (CDMA) spread-spectrum systems which utilises partial correlation of maximum length sequences is investigated. The method involves subsequence matched filtering (SMF) to acquire sync, and feedback control to verify sync, acquisition. Expressions of probabilities of sync, acquisition, false alarm, false dismissal and mean acquisition time are derived. Numerical results for a multiuser environment are presented. Acquisition performance for a single-user case is compared with those of the RASE and RARASE methods. The SMF method offers rapid sync, acquisition over a wide range of signal/noise ratios. 1 Introduction Multiple access communication is concerned with the situation in which users wish to communicate with each other simultaneously. Depending on the application, a number of different multiple access schemes have appeared in the literature. This paper considers the multiple access problem in a spread-spectrum communication system, the basic theory and application of which are well documented in Reference 1. Specifically, the paper is concerned with studying the initial acquisition aspect of a spread-spectrum communication system which employs code division multiple access (CDMA). The main focus is on the acquisition of a wanted signal in the presence of other unwanted signals, i.e. a multiuser environment. To this end, we consider the situation where a single reference frequency is used to convert the received RF signal to quadrature lowpass signals. In this case we expect that there is little Doppler shift in the multiuser communication environment. If the system were expected to cope with large Doppler shifts, the reference frequency should be tunable over the expected range of frequency shifts. By code division multiple access, it is meant that either a direct sequence (DS) or a frequency hopping (FH) or a hybrid FH/DS signalling scheme is used, in which signal separation is achieved by assigning different code or spreading sequences with desirable autocorrelation and crosscorrelation properties to each user. In spreadspectrum communication it is desirable that the spreading function be a member of an orthogonal set. Spreading functions used in practice, however, are time functions, which exhibit low crosscorrelation and low off-peak autocorrelation values. While there exists a variety of spreading funciions, this paper considers a binary-valued function of the form: a(t)= t ajP Tc (t-jT c) where {a,} is a binary (±1) sequence and 0<t<T < P M = j TcK ' [0 otherwise (1) is a rectangular pulse with duration or 'chip time' T c. To obtain a good signal processing gain the binary sequence {a,} needs to be fairly long and to have excellent correlation properties [1]. While sequences such as the Gold sequences [2] possess good correlation properties, Paper 2893F (E7, E8).
Implementation of a digital receiver for DS-CDMA communication systems using HW/SW codesign
2005
An optimized partitioning algorithm for HW/SW Codesign is described and applied to the implementation of a digital RAKE receiver for DS-CDMA on an heterogeneous (hardware/software) platform. The development of this partitioning algorithm aims to gather quality features that have already proved effective when applied separately, but had not been used together yet. The partitioning algorithm is applied to a flexible RAKE architecture able to adapt its number of demodulation fingers to each propagation environment. This RAKE detector uses a serial and iterative computation of the required processing in order to jointly optimize performance and computational power.
As k approaches infinity, the Rician distribution becomes a delta function, which matches the simple line-of-sight model. As k approaches zero, the Rician distribution transforms into a Rayleigh distribution. The AWGN, Rayleigh, and Rician channel models are simple, compact models for approximating the effects of radio propagation. An overview of more complicated models is available in [ 1 I]. 1.5 Wireless Standards It is vital to use the radio spectrum efficiently and to share the limited resource among multiple users. That requires multiple-access schemes that separate users by frequency, time, and/or orthogonal codes, as shown in Figure 1.15. Most systems divide the radio spectrum into frequency channels and strategically assign those channels, a practice known as frequency division multiple access (FDMA). The channel assignment strategies minimize interference between users in different cells. Interference is caused by transmitted signals that extend outside the intended coverage area into neighboring cells. To limit interference, frequency channels are generally assigned based on the The CDMA Concept CDMA is a multiple-access scheme based on spread-spectrum communication techniques [l-3]. It spreads the message signal to a relatively wide bandwidth by using a unique code that reduces interference, enhances system processing, and differentiates users. CDMA does not require frequency or time-division for multiple access; thus, it improves the capacity of the communication system. This chapter introduces spread-spectrum modulation and CDMA concepts. It presents several design considerations tied to those concepts, including the structure of the spreading signal, the method for timing synchronization, and the requirements for power control. This chapter also points out CDMA IS95 [4] details to illustrate practical solutions to these design issues. 2.1 Direct-Sequence Spread-Spectrum Communications Spread-spectrum communications is a secondary modulation technique. In a typical spread-spectrum communication system, the message signal is first modulated by traditional amplitude, frequency, or phase techniques. A pseudorandom noise (PN) signal is then applied to spread the modulated waveform over a relatively wide bandwidth. The PN signal can amplitude modulate the message waveform to generate direct-sequence spreading, or it can shift the carrier frequency of the message signal to produce frequency-hopped spreading, as shown in Figure 2.1. The direct-sequence spread-spectrum signal is generated by multiplying the message signal d(t) by a pseudorandom noise signal pn (t): g(t) = pn W&)
Hardware design issues for a mobile unit for next-generation CDMA systems
Advanced Signal Processing Algorithms, Architectures, and Implementations VIII, 1998
This paper addresses hardware design issues of a mobile receiver for future generation direct sequence CDMA wireless communication systems. In the design of a mobile unit, xed-point hardware is an attractive alternative because of increased speed, reduced power consumption, and reduced hardware costs. In this paper, we focus on the xed-point implementation of`blind' detection and channel estimation schemes that do not require knowledge of spreading codes of the other users. The error pattern of the`blind' algorithms, wordlength requirements, and the operation count required for implementation of such algorithms are evaluated. Our results show that the blind maximum likelihood channel estimation along with the blind MMSE detection algorithm can achieve approximately ve times improvement in performance over the conventional correlator based receivers. These newer algorithms require slightly higher wordlength but similar computational complexity.
A STUDY AND DESIGN OF DIRECT SEQUENCE CODE DIVISION MULTIPLE ACCESS (DS-CDMA) TRANSCEIVER
This paper addresses CDMA transceiver using direct sequence spread spectrum and includes real time signal processing for scrambling of spread spectrum signal, and descrambling the signal at the receiver using the same pseudo-random code. The simulation of the entire DS-CDMA transceiver system has been implemented using MATLAB, which provides real time audio input to the system.
Packet Random Access in CDMA Radio Networks 1
2005
A packet random access system is considered with transmitters located at random positions inducing random received (uncontrolled) power levels. Various joint detection methods at the central receiver, such as spread ALOHA representing the basic conventional system, a decorrelation receiver, a minimum-mean square error receiver, successive interference cancelation, and a novel parallel cancellation-type multistage receiver are examined for their throughput efficiency. The multistage receiver structure proposed operates on partitions of the original spreading sequences and can be implemented as an enhancement to a matched filter receiver, that is to spread ALOHA. It is shown that this receiver provides superior throughput performance to all other detectors. It is illustrated that system load translates directly into network connectivity, and that the extent of connectivity is related to the type of detector used at the central receiver. Throughput curves and conclusions for cellular and ad hoc networks are presented and discussed.
FPGA implementation of DS-CDMA Transmitter and Receiver
Direct sequence spread Spectrum (DSSS) is also known as direct sequence code division multiplexing. In direct sequence spread spectrum the stream of information to be transmitted is divided into small pieces each of which is allocated across to a frequency channel across the spectrum. Data signal at the point of transmission is collaborated with a higher data-rate bit sequence (also called chipping code) that divides the data according to a spreading ratio. A redundant chipping code helps the signal resist interference and also enables the original data to be recovered if data bits are damaged during the transmitting. In this project direct sequence spread spectrum principle based code division multiple access (CDMA) transmitter and receiver is implemented on SPARTAN 3E FPGA. The Xilinx synthesis technology (XST) of Xilinx ISE tool used for synthesis of transmitter and receiver on FPGA Spartan 3E.
Random Packet CDMA: Reducing Delay and Increasing Throughput of WLAN Systems
We revisit Random Packet Code Division Multiple Access (RP-CDMA), a recently proposed Physical/MAC layer scheme for wireless CDMA networks. We revise earlier results by adopting a more realistic Spread Aloha model for header transmission and packet sizes with distributions typical for Internet2 traffic. Thanks to timing recovery in the RP-CDMA header and greatly reduced packet collision probability, unlike Spread Aloha, RP-CDMA enables the use of multiuser receivers for data detection. We simulate the throughput characteristics of RP-CDMA with the matched filter, the decorrelator, the MMSE and partitioned spreading demodulation detection and compare performance to Spread Aloha in a base station centric network.
An Overview of Code-Spread CDMA
Code-division multiple-access (CDMA) has gained a lot of attention recently when the third generation mobile communication systems are developed. In this paper we discuss how using low-rate channel coding instead of direct sequence spreading can more efficiently use the available spectrum. This leads to large capacity improvements.
Low Complexity Rake Receiver and Channel Estimator Implementation for DSSS-CDMA Systems
2006
In this paper the implementation of a low complexity Rake Receiver (RR) and Channel Estimator (CE) for Direct Sequence Spread Spectrum (DSSS) systems is presented with main scope to provide adaptability in receiver parameters while keeping the complexity at low levels. The trend in communication systems is towards dynamically adjusting system implementations that can adapt their structure to the continuously changing transmission characteristics. Following this trend we propose an architecture that is based on the spreading factor of the system, which controls the number of the taps that the CE and the RR will use depending on the current channel conditions. Moreover, based on a resource sharing approach of the main system components, that yet meets the standard bit rates, low complexity is achieved avoiding otherwise necessary complex structures such as complex multipliers. Furthermore we implement the proposed system on FPGA that gives us the ability to measure performance in terms of Bit Error Rate (BER) in addition to that of power dissipation and area. Thus the performance of the system is compared with the performance of a high level system simulation of the same system, accomplishing the same BER levels. Finally the system implemented on a Xilinx Spartan-3 MB board occupies 56% of the total slices and consumes 103.45mW with a 5V supply.