A New Adjustable Hybrid Spread OFDM Modulator M´ erouane Debbah , Marc de Courville , Markus Muck and Philippe Loubaton Centre de Recherche Motorola Paris, ´ Espace Technologique Saint-Aubin 91193 Gif-sur-Yvette, France Universit´ e de Marne la Vall´ ee, Cit´ e Descartes, 5 Boulevard Descartes Champs sur Marne 77454 Marne la Vall´ ee, France E-mail: Marc.de.Courville@crm.mot.com Abstract— The improvement brought by Successive Decoding algo- rithms is marginal when applied to uniformly spread OFDM-CDMA systems, because of the harmonization of the Signal to Noise Ratio across the subbands at the receiver. Thus this paper proposes a new Hybrid Spread OFDM (SOFDM) transmission scheme in which the spreading of the information is adjustable and not uniform along the carrier (fre- quency selective). Moreover a MMSE version of the V-BLAST Succes- sive Interference Cancellation algorithm suited for this hybrid modula- tor is derived. The performance of the combination of SHOFDM and MMSE V-BLAST is shown to outperform classical iterative detection algorithms for SOFDM in the realistic scenario of the 5GHz HIPER- LAN/2 system. Keywords—Spread OFDM, OFDM-CDMA, BLAST, HIPERLAN/2, Successive Decoding I. I NTRODUCTION M ulti-carrier OFDM system [1] using a Cyclic Prefix (CP) for preventing inter-block interference is known to be equivalent to multiple flat fading parallel transmission channels in the Frequency Domain (FD). In such a system, the information sent on some carriers might be subject to strong attenuations and could be unrecoverable at the re- ceiver. This has motivated the proposal of more robust trans- mission schemes combining the advantages of CDMA with the strength of OFDM known as OFDM-CDMA [2], in which the information is spread across all the carriers by a pre- coding unitary matrix (e.g. the Walsh-Hadamard: WH trans- form). This combination increases the overall frequency diversity of the modulator, so that unreliable carriers can still be re- covered by taking advantage of the subbands enjoying a high Signal to Noise Ratio (SNR). Although originally proposed for a multiuser access scheme, this concept is extended to all single user OFDM systems and is referred in the sequel as Spread OFDM (SOFDM). Due to the inter-carrier interference generated by the spreading, the frequency domain channel transfer function of a single antenna SOFDM system can be modeled using a full MIMO flat fading (scalar) matrix. Actually this is an assump- tion often made in multiple emitting and receiving antenna communications and already exploited in V-BLAST. Here, the advantage of OFDM systems with CP is that it validates the above assumption even for channels with memory. Thus this paper presents both an extension of the Succes- sive Interference Cancellation (SIC) algorithm V-BLAST [3] in a spirit similar to that for CDMA multiuser detection [4] and a new spreading method that combined with this new al- gorithm, provides an additional performance gain. SIC algorithms relies on a sequential detection of the re- ceived block. At each step, one symbol is detected before its contribution is subtracted from the received block. This introduces successively additional degrees of freedom which enable the reduction of noise/interference influence for the next users to be detected and therefore increases the overall reliability of the decision process. But for performing a good interference cancellation, due to the underlying feedback mechanism involved in the succes- sive detection mechanism, one should decode first the reliable carriers enjoying a greater SNR and then the most corrupted ones. Unfortunately with a WH spreading, all the carriers share the same SNR resulting in practice to marginal perfor- mance gain when applying SIC approaches. In order to overcome this problem, and achieve higher gains, we propose in this paper a new adjustable hybrid modulator scheme adopting a non uniform spreading along the carriers (frequency selective) instead of the classical uni- form one, achieving a tradeoff somewhere between flat WH- OFDM and plain OFDM. The purpose of the paper is thus twofold: 1. to propose a new adjustable flexible hybrid spreading modulator - referred in the following as SHOFDM - suited for combination with SIC techniques (section II); 2. to derive and apply to SHOFDM a new MMSE version of the original ZF V-BLAST algorithm (section III). Section IV illustrates how the Hybrid SOFDM transmis- sion scheme can benefit from the improved new Succes- sive Detection (SD) MMSE-VBLAST algorithm for the ETSI BRAN HIPERLAN/2 (HL2) 5GHz local area broadband wireless system context in the uncoded scenario. II. NEW ADJUSTABLE HYBRID SOFDM TRANSCEIVER MODEL In the following, upper (lower boldface) symbols are used for matrices (column vectors) whereas lower symbols rep- resent scalar values, denote by transpose operator, conjugation and hermitian transpose. stands for the identity matrix. Overall system model: Since a carrier OFDM sys- tem [1] using a CP is equivalent in the FD to flat fad- ing parallel transmission channels, the baseband discrete- time block equivalent model of a SOFDM system can be de- picted in figure 1. Actually the received block vector can be expressed in the FD as a function of the emitted symbol and additive noise vectors using a MIMO flat fading channel