SPACE-TIME BLOCK CODES APPLICATION IN LARGE CORE STEP-INDEX PLASTIC OPTICAL FIBERS N. Raptis, E. Pikasis, E. Grivas, and D. Syvridis Department of Informatics and Telecommunications, National & Kapodistrian University of Athens, Athens, Greece Corresponding author: raptis@di.uoa.gr Abstract: Discrete Multitone (DMT) modulation technique and Space-Time Block Codes (STBCs) diversity schemes are combined in an effort to achieve 1 Gbit/s bit rate over 100m of Large Core Step-Index Plastic Opti- cal Fibers (SI-POFs) with 980 ȝm core diameter. The combination of the two techniques is examined theoreti- cally, while simulation and experimental results are given for different diversity configurations based on Mul- tiple-Input Single-Output (MISO) schemes. Key words: Large Core Step-Index Plastic Optical Fibers, Discrete Multitone, Space-Time Block Codes. 1. Introduction Large Core Step-Index Plastic Optical Fibers (SI-POFs) with 980 ȝm core diameter have some tempting fea- tures, such as low cost, easy treatment etc. However, their large attenuation and modal dispersion limit their usage in “Short Area” applications. Due to their limited bandwidth, the Gbit/s target rate is not easily achieved at distances up to 100 m with conventional On-Off Keying (OOK) transmission. A solution is the use of Discrete Multitone (DMT) modulation, which can provide information squeeze in little bandwidth [1]. Another way to surpass the bandwidth limitation is by exploitation of the numerous modes in the fiber. This means that multiple streams of data can be inserted in the fiber and multiple receivers at the fiber end can re- trieve the data leading to Multiple-Input Multiple-Output (MIMO) transmissions technique and link capacity increase. A method to achieve MIMO transmissions is the Mode Group Diversity Multiplexing (MGDM) [2], but the distances covered are small due to the mode coupling phenomenon, which for SI-POF lengths larger than 30 meters, in general, makes the separation of the produced intensity ring patterns difficult. MGDM versions ap- plied in multimode fibers belong to a more general class of MIMO systems called spatial multiplexing tech- niques, where independent data streams are transmitted simultaneously by multiple sources [3]. Another class of MIMO systems includes diversity schemes where a number of replicas of the same signal are provided to the receiving side. With each replica suffering independent fading, the probability that the replicas fade severely and simultaneously gets reduced. In the SI-POFs case, a diversity scheme might be a good choice for performance improvement for over 50 m distances. Space-Time Block Codes (STBCs) are transmit diversity schemes that can strengthen the signal against Bit Error Rate (BER) degradation in frequency flat fading channels [4]. In this paper, DMT and STBCs are combined and several cases of Single-Input Single-Output (SISO) transmis- sion along with 2×1, 3×1 and 4×1 STBC schemes are investigated in terms of BER as a function of electrical SNR. Large core SI-POFs with a core diameter of 980 ȝm are considered in all cases throughout this paper. Each optical channel in the simulations is described by an analytical solution of Gloge’s time-dependent power flow equation with properly chosen parameter values assuming that Steady State Distribution (SSD) has been achieved. Some experimental results are also given. 2. Optical Channel – Impulse Response A SI-POF with 490 ȝm core radius, 10 ȝm cladding thickness and length of 100 m is considered. This length is longer than the length over which all individual disk patterns that correspond to different launch angles take the same intensity distribution across the fiber section and a SSD has been achieved [5]. The core refractive index (n), the fiber attenuation coefficient and the Numerical Aperture (NA) are equal to 1.4893, 0.16 dB/m (at Ȝ = 650 nm) and 0.5, respectively. The impulse response of the fiber which is a function of power and depends on the fiber length and time is given by the following formula [6]   , 4 exp 2 1 , 2 2 1 2                          T t t T z T z t t T t z h Ȗ Ȗ π Θ (1) where Ȗ ∞ is the minimum overall loss coefficient which was set equal to 0.0368 m -1 , Θ ∞ is the steady state diver- gence angle of the optical output beam at the end of an infinitely long fiber with Θ ∞ = (4D/Ȗ ∞ ) 1/2 , T is equal to n(Θ ∞ ) 2 /(2cȖ ∞ ), with D the constant coupling coefficient of the fiber set equal to 4.1×10 –4 rad 2 /m and c is the velocity of light in the vacuum (3×10 8 m/s). The values of the various parameters were properly set, in order the