An adaptive Software Radio receiver architecture for linear bidimensional modulations Pedro C. Pinto, S´ ergio B. Silva, Henrique C. Miranda FEUP / INESC Porto, Rua Dr. Roberto Frias, s/n 4200-465 Porto (Portugal) Phone: +351 22 5081474, Fax: +351 22 2094250, E-mail: hmiranda@fe.up.pt Abstract – This paper proposes a new, self-reconfigurable architecture which can be used to perform Physical Layer functions in Software Radio receivers. The receiver is capa- ble of handling any digital modulation, as long as its sym- bols belong to a linear, two-dimensional signal space (e.g., M-PSK and M-QAM). It operates by identifying key pa- rameters of the incoming signal (such as symbol rate and constellation type), which are then used to dynamically adjust several receiver blocks. Five main aspects of self- reconfiguration are analyzed: baseband filtering, symbol rate estimation, symbol synchronization, carrier synchro- nization and constellation recognition. The performance of the receiver is evaluated based on simulation results and several usage scenarios for the proposed architecture are presented. Keywords – software radio, self-reconfiguration, synchro- nization, constellation recognition, fuzzy clustering. 1 Introduction Intelligence and self-reconfigurability are very desirable features of communication receivers. An intelligent re- ceiver would be able to perform blind recognition of many properties of the incoming signal (such as bandwidth and modulation scheme), and reconfigure itself in order to de- modulate the received signal. Several authors [1][2] have tried to develop such architectures, but these support only a very limited number of constellations, lacking both gen- erality and performance. In this paper, an adaptive digital receiver architec- ture suitable for Software Defined Radio systems is pre- sented. This receiver is prepared to handle not only M-PSK and M-QAM schemes, but also other modulation schemes whose symbols belong to a linear, two-dimensional signal space (e.g., non-uniform or fractal constellations). Thus, the proposed architecture achieves high generality and acceptable performance, being able to identify key sig- nal parameters such as symbol rate and constellation, and self-reconfigure accordingly. This architecture has a wide range of applications. In the civilian context, it can be used for surveillance and management of the electromagnetic spectrum, since these operations often need to be carried out automatically and independently of signal properties. In electronic warfare scenarios, the recognition of the target signal character- istics enables a more efficient application of electronic counter measures such as jamming. In commercial applica- tions (e.g., mobile communications), mobile terminals us- ing different air interfaces can be served by a single multi- standard adaptive base station, which is able to identify the modulation properties of the received signals. This en- ables roaming of clients through geographical zones with different adopted standards, without modification of the mobile terminal. Section 2 presents the proposed receiver architecture, focusing on the five main aspects of self-reconfiguration: baseband filtering, symbol rate estimation, symbol syn- chronization, carrier synchronization and constellation recognition. The receiver filter response is configured based on parameters fed by a symbol rate estimator. The symbol synchronizer is used to sample the baseband sig- nal at the optimal instants, while the carrier synchronizer prevents the constellation from rotating due to a carrier frequency error. The symbol decision block is aided by a constellation identifier, which estimates the optimal deci- sion regions for the received signal modulation. All these blocks are able to operate independently of the modulation scheme. Section 3 presents simulation results for several blocks of the proposed receiver, in order to evaluate their perfor- mance, advantages and limitations. Section 4 deals with the hardware and software imple- mentation of the receiver. Section 5 exemplifies where the proposed receiver can be located in the usual communications protocol stack, and how it can interact with the other protocol layers. Section 6 provides some important conclusions and pos- sibilities for future development. 2 Proposed receiver architecture The proposed receiver architecture is illustrated in the di- agram of Fig. 1, revealing the information flow among its blocks. 2.1 Downconversion and filtering The DDC (Digital Down Converter) is responsible for the filtering, decimation and IF-to-BB frequency translation of the received signal. The resulting complex baseband samples are applied to a FIR filter with a dynamically adjusted bandwidth. The filter coefficients are calculated using the standard formulae for a root raised-cosine filter with parameters ( ˆ R, α), where ˆ R is the estimated symbol rate provided by the rate estimator block, and α is a fixed roll-off factor. On a multi-mode adaptive receiver, the use of a dynamic filter is of utmost importance, in order to