IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 55, NO. 9, SEPTEMBER 2007 1783 Interference-Resilient Block-Spreading CDMA With Minimum-MAI Sequence Design Stefano Tomasin, Member, IEEE, and Filippo Tosato, Member, IEEE Abstract—Code-division multiple-access (CDMA) schemes based on block spreading implement the spreading of entire data blocks rather than single symbols, thus achieving a higher robust- ness against the frequency selectivity of the channel and allowing the use of efficient modulation/equalization schemes operating in the frequency domain (FD). In this paper, we present a new block CDMA (B-CDMA) system where a single cyclic prefix (CP) is used at the end of each spread block. This provides a higher spectral efficiency with respect to existing schemes. By observing that com- plete orthogonality among users is achievable only for half-loaded systems on dispersive channels, we introduce new criteria for the design of spreading and despreading sequences, which aim at min- imizing the mean-square error at the output of the despreader. For the equalization of the received signal, we propose an iterative block decision feedback equalizer, which iterates between equal- ization and decoding. Equalization filters are designed to minimize the mean-square error and take into account the residual inter- ference due to the nonorthogonality of the spreading sequences. The performance of B-CDMA is evaluated in an uplink wireless scenario and compared to existing CDMA schemes. Index Terms—Block spreading, chip-interleaving, frequency- domain (FD) equalization, spread spectrum. I. INTRODUCTION T HE combination of frequency-domain (FD) equalization techniques with code-division multiple-access (CDMA) schemes has recently proven to be an interesting solution for multiuser communication on dispersive channels. Among FD- CDMA techniques, we mention the various schemes combining CDMA and multicarrier (MC) transmission, in the form of or- thogonal frequency-division multiplexing (OFDM) [1], which perform the spreading either in the time domain MC direct- sequence CDMA (MC-DS-CDMA) or in the FD (MC-CDMA) [2]. In order to reduce peak to average power ratio (PAPR) and sensitivity to frequency-offsets [3], [4], single carrier (SC) transmission with FD equalization has been proposed [5], [6] and applied to CDMA systems [7], [8]. Cyclic prefix (CP), zero-padding (ZP) or pseudonoise (PN) extension, required by both SC-FD and MC transmissions, have some drawbacks such as: 1) reduction of the available Paper approved by S. Ulukus, the Editor for Spread Spectrum of the IEEE Communications Society. Manuscript received April 10, 2005; revised July 9, 2006. This work was supported by the Fund for Investments in Basic Re- search (FIRB) within the framework of “Reconfigurable Platforms for Wide- band Wireless Communications.” This paper was presented in part at the IEEE GLOBECOM 2005, St. Louis, MO, November 2005. S. Tomasin is with the Department of Information Engineering, University of Padova, Padova 35131, Italy (e-mail: tomasin@dei.unipd.it). F. Tosato was with the Department of Information Engineering, University of Padova, Padova 35131, Italy. He is now with Philips Research Laboratories, Redhill RH1 5HA, U.K. (e-mail: filippo.tosato@philips.com). Digital Object Identifier 10.1109/TCOMM.2007.904398 bandwidth; 2) inefficient use of the transmit power; 3) increased processing delay; and 4) reduced robustness against channel variations. Overlap-and-cut techniques can be applied to increase bandwidth efficiency [9], [10] at the expense of interference between adjacent blocks. A set of shift-orthogonal sequences can also be used to enforce orthogonality without CP [11], but the number of orthogonal spreading sequences is limited to half the spreading factor. In this paper, we propose a new CDMA scheme for commu- nications over frequency dispersive channels that significantly reduces the overhead of the extensions, by exploiting the signal structure of the spread signal. In the proposed architecture, de- noted block CDMA (B-CDMA), the data signal is divided into blocks that are repeated and multiplied by a spreading sequence before transmission. At the end of each spread block, a CP is in- serted. The spreading process is similar to MC-DS-CDMA [2], [12], [13] or other extended block-spreading schemes [14], [15], known also as chip-interleaved block spread CDMA (CIBS- CDMA). However, while in existing systems, a CP is inserted before block spreading, in B-CDMA, the CP is inserted after the block spreading, thus, increasing the bandwidth efficiency. In our B-CDMA, the absence of CP before spreading results, in general, in both multiple-access interference (MAI) and in- tersymbol interference, which are mitigated at the receiver by a purposely-built despreader and an equalizer. Due to the dif- ferent effects of the transmission channel on the various parts of the received block, two despreading sequences are used on two subblocks of the received blocks. As spreading and de- spreading sequences, we consider: 1) orthogonal sequences that completely eliminate MAI but allow only a limited number of users and 2) minimum MAI sequences that allow any number of active users. This is a distinctive feature with respect to [11], where despreading was performed using sequences that were matched to the spreading ones. After despreading, we propose a nonlinear iterative equalizer, efficiently implemented in the FD [16] that takes into account the residual MAI. The key ad- vantages of B-CDMA over existing schemes are both a reduced MAI and a reduced overhead (with respect to MC-CDMA with CP). Moreover, the transmission of SC signal yields a lower peak-to-average power ratio (PAPR) [5] and moves the bulk of signal processing from the transmitter to the receiver, which is particularly useful for uplink mobile communications. Finally, the nonlinear equalizer has a better performance than the lin- ear equalization of [14], [15], yielding a coded bit-error rate comparable to OFDM [5]. The paper outline is as follows. In Section II, we describe the general architecture of a B-CDMA transceiver. Section III details the characteristics of block despreading. In Section IV, 0090-6778/$25.00 © 2007 IEEE