EE382C: EMBEDDED SOFTWARE SYSTEMS – FINAL REPORT DRAFT May 13, 2004 1 Modeling a Multicarrier Wireless Communication Transceiver Hunaid Lotia Abstract The objective of this work is to develop a model for the simulation of a multicarrier wireless communication transceiver and channel. Multicarrier wireless transmission techniques are widely deployed in today’s Wireless Local Area Networks(WLANs), Asymmetric Digital Subscriber Line (ADSL), very high bit-rate Digital Subscriber Line (VDSL) modems, etc. The explosive growth of wireless communication has created a demand for high speed and reliable quality of service over the wireless communication medium. When it comes to cellular and mobile communication systems, single carrier techniques as opposed to multicarrier are the main candidates. In this project, I model and simulate an OFDM (Orthogonal Frequency Division Multiplexing) transceiver and the wireless channel in National Instrument’s LabVIEW, which is a graphical programming tool based on the dataflow language G. The simulation would enable designers to explore wide deployment of a multicarrier transmission schemes for cellular or mobile communications. The project evaluates communication performance, transceiver complexity and implementation cost of cellular OFDM systems.EE382C: EMBEDDED SOFTWARE SYSTEMS – FINAL REPORT DRAFT May 13, 2004 2 I. INTRODUCTION Multicarrier modulation, in the most general sense, can refer to any modulation scheme that uses multiple carrier frequencies to transmit data. Multicarrier modulation finds its application in recently standardized high rate data transmission systems such as Digital Audio and Video Broadcasting, Wireless LAN, Asymmetric Digital Subscriber Lines (ADSL) and Very High Rate Digital Subscriber Lines (VDSL) [10]. The explosive growth of wireless communication has created a demand for high speed and reliable quality of service over the wireless communication medium. Multicarrier techniques have been developed since as early as the sixties. Initially, the underlying operations were too computationally intensive for cost-effective implementation. For about a decade, increasing computational power of Digital Signal Processors (DSPs) has been enabling cost-effective real-time implementations. For real-time operations to ensure reliability and efficiency, these complex operations must be done fast and hence requiring more powerful DSPs. Early commercial multicarrier modems used guard intervals in the time and frequency domains to reduce the effects of intersymbol interference (ISI) and interchannel interference (ICI). Each subcarrier was modulated using the same power and data rate. Towards the end of the sixties, a number of authors, notably Chang [1], used overlapping orthogonal spectra to increase the efficiency of multicarrier systems. II. TRANSMISSION ALGORITHMS The two primary candidates for a broadband mobile transmission technique are the Direct Sequence (DS) CDMA (Code Division Multiple Access) [3], [4], [5] and FrequencyEE382C: EMBEDDED SOFTWARE SYSTEMS – FINAL REPORT DRAFT May 13, 2004 3Hopping (FH)-CDMA [6] with powerful channel coding for both deploying a single carrier transmission scheme. The capacity of a DS-CDMA system is limited by multi-user interference. A channel estimation, equalization and power-control procedures are necessary in a DS-CDMA system, which makes the receiver more complex to design. On the other hand, in FH-CDMA systems, a narrowband receiver technique is used, but the carrier hops in a large bandwidth using frequency allocation techniques with orthogonal codes. In this case the receiver must be synchronized to the hopping carrier frequency using a precise synchronization technique. However, no channel estimation and equalization techniques have too be performed compared with DS-CDMA systems, but the impact of occurring multipath leads to fading effects in the received signals and hence powerful channel coding is essential [6]. The solution to the problems posed by the two CMDA proposals is to use a multicarrier transmission technique for broadband communications. No channel estimation or equalization as DS-CDMA is necessary nor a powerful synchronization as in FH-CDMA is required. The solution is basically is to use an OFDM (Orthogonal Frequency Division Multiplexing) transmission system [6]. III. THE OFDM TRANSMISSION SYSTEM Orthogonal Frequency Division Multiplexing (OFDM) is a special form of multicarrier modulation, which was patented in 1970. It is well suited for transmission over a dispersive channel [7]. Basically, in an OFDM system, the total bandwidth B is divided into K subbands with orthogonal subcarriers; that is a block of M symbols is transmitted.EE382C: EMBEDDED SOFTWARE SYSTEMS – FINAL REPORT DRAFT May 13, 2004 4Each block has 1/M of the available bandwidth instead of just transmitting one symbol as in single-carrier data transmission [8]. OFDM transmission techniques are widely deployed for WLANs, ADSL, VDSL modems, etc., but a standard using this technique for cellular or mobile communications still does not exist. Hence the motivation for this project. IV. MODELING AND SIMULATION A basic OFDM modem transceiver is illustrated below. Fig. 1. An OFDM modem transceiver Dataflow is a natural representation for signal processing algorithms. Applications are specified by a dataflow graph in which the nodes represent computations, and data tokens flow between them along the arcs of the graph. A. OFDM Transmitter and Receiver The transmitter and receiver of the OFDM modem described in the Fig. 1 contains the original data stream node, S/P (spreading node), IFFT/FFT, etc. All of these nodes consume and produce a constant number of tokens. For instance, the original data streamEE382C: EMBEDDED SOFTWARE SYSTEMS – FINAL REPORT DRAFT May 13, 2004 5node produces one token for each firing. The spreading node consumes one token and produces N tokens for each firing (where N is the number of symbols to be transmitted). The IFFT node consumes and produces N tokens for each firing where N is the N point IFFT and is also the same as the length of the spreading code. Since the number of tokens produced and consumed by the nodes in the transmitter and receiver are fixed in one firing, they can be perfectly modeled in
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