Transmit Gain Optimization for Space Time block Coding Wireless Systems with Co-channel Interference Nevio benvenuto and Stefano Tomasin Dipartimento di Elettronica e Informatica, Universita di Padova Via G Gradenigo 6A-35131, Padova(Italy) Tel:++39-0498277654,Fax:++39-0498277699 E-mail: nb, tomasin/@dei. unipd it ABSTRACT Flex project [8]. In these networks the devices are orga- The combination of Orthogonal Frequency divisio nized into synchronous piconets which can potentially in- terfere with each other and thus limit considerably the net- multiplexing and space-time block coding is a promising work throughput technique for wireless broadband transmission. In a sce- nario where other devices generate interference, we pro- In a STBC OFDM system, according to the particular pose a scheme where the transmit gains of each OFDM condition of both the channel and the interfering signals, subchannel are adaptively chosen. As a design criteria adaptation of the antenna gains could be done for each of we consider both the minimization of the interference and the OFDM subcarriers. However, a fully optimized sys- maximization of the signal to interference plus noise ra- tem turns out to be exceedingly complex, hence we fo- io at the detection point. As a particular case we con- cus our investigation only on the transmit gain adaptation sider also the situation of varying only the amplitude or Although in general, the transmit gains assume complex the phase of the gains. Indeed, it turns out that when in- alues, to limit complexity we also consider cases where terference Is present, an important role is played by the gains have the same phase or the same amplitude. More- phase of the transmit gains, and for the case of two trans- over, in order to limit complexity, the receiver adopts max- mit antennas we derive the optimum phase of the transmit imum ratio combining whose optimization depends only gains,under the assumption of equal amplitudes. As per- on the channel and not on the interference signal. Within formance measure we used the achievable bit rate of the this framework, we consider two cost functions for the various solutions for a broadband indoor system denoted choice of the transmit gains, namely minimization of the Windflex(European Project). Performance was compared power of the interference at the receiver(MI), and maxi- also with the system capacity obtained by a novel close mization of the signal to noise plus interference ratio form expression In order to have an upper bound on the system perfor- mance we derive a novel expression of the capacity of a L INTRODUCTION system with two receive antennas with adaptive transmit gains. In fact, previous results are limited to the system pacediversity has been recently considered with a where each transmitted signal is the linear combination of growing interest for its ability to significantly improve the all space-coded data [11] performances of wireless communications in non disper sive fading channels. In particular, space-time block cod- indeed there is a significant tradeoff between performance that benefits from spatial diversity. First introduced and computational complexity of the various solutions by Alamouti for a communication system with up to two receive and transmit antennas [6], STBC was further gen I SYSTEM DESCRIPTION eralized for a larger number of antennas [71 An OFDM wireless system is considered, where data broadband communications, where the transmission chan- and transmitted by Nt transmit antennas. The receiver is nel is dispersive. The benefits of both spatial and fre- equipped with Nr receive antennas and it receives both quency diversity can be easy achieved by the combination the useful signal and interference generated by N inter- of STBC and orthogonal frequency division multiplexing ferers. We assume that the interferers use OF DM and are (OFDM)[9], which divides the broadband channel into a synchronous with the useful transmitter. Hence, by number of orthogonal signals, which are modulated on suming that the cyclic prefix [9] is sufficiently long, the equally spaced subcarriers. The combined use of sTBc transmission and the interference channels are flat on each and OFDM has been recently considered for the deploy- OFDM subcarrier. Note that if the interference signals are ment of wireless indoor networks in the European Wind- not synchronous, the cyclic prefix may not absorb the de-Transmit Gain Optimization for Space Time Block Coding Wireless Systems with Co-channel Interference Nevio Benvenuto and Stefano Tomasin Dipartimento di Elettronica e Informatica, Universita di Padova ` Via G. Gradenigo 6A - 35131, Padova (Italy) Tel: ++39-0498277654, Fax: ++39-0498277699 E-mail: {nb, tomasin}@dei.unipd.it ABSTRACT The combination of Orthogonal Frequency Division Multiplexing and space-time block coding is a promising technique for wireless broadband transmission. In a sce￾nario where other devices generate interference, we pro￾pose a scheme where the transmit gains of each OFDM subchannel are adaptively chosen. As a design criteria we consider both the minimization of the interference and maximization of the signal to interference plus noise ra￾tio at the detection point. As a particular case we con￾sider also the situation of varying only the amplitude or the phase of the gains. Indeed, it turns out that when in￾terference is present, an important role is played by the phase of the transmit gains, and for the case of two trans￾mit antennas we derive the optimum phase of the transmit gains, under the assumption of equal amplitudes. As per￾formance measure we used the achievable bit rate of the various solutions for a broadband indoor system denoted Windflex (European Project). Performance was compared also with the system capacity obtained by a novel close form expression. I. INTRODUCTION Space diversity has been recently considered with a growing interest for its ability to significantly improve the performances of wireless communications in non disper￾sive fading channels. In particular, space-time block cod￾ing (STBC) is attractive as a simple and effective tech￾nique that benefits from spatial diversity. First introduced by Alamouti for a communication system with up to two receive and transmit antennas [6], STBC was further gen￾eralized for a larger number of antennas [7]. At the same time, the need of high bit rates favors broadband communications, where the transmission chan￾nel is dispersive. The benefits of both spatial and fre￾quency diversity can be easy achieved by the combination of STBC and orthogonal frequency division multiplexing (OFDM) [9], which divides the broadband channel into a number of orthogonal signals, which are modulated on equally spaced subcarriers. The combined use of STBC and OFDM has been recently considered for the deploy￾ment of wireless indoor networks in the European Wind￾Flex project [8]. In these networks the devices are orga￾nized into synchronous piconets which can potentially in￾terfere with each other and thus limit considerably the net￾work throughput. In a STBC OFDM system, according to the particular condition of both the channel and the interfering signals, adaptation of the antenna gains could be done for each of the OFDM subcarriers. However, a fully optimized sys￾tem turns out to be exceedingly complex, hence we fo￾cus our investigation only on the transmit gain adaptation. Although in general, the transmit gains assume complex values, to limit complexity we also consider cases where gains have the same phase or the same amplitude. More￾over, in order to limit complexity, the receiver adopts max￾imum ratio combining whose optimization depends only on the channel and not on the interference signal. Within this framework, we consider two cost functions for the choice of the transmit gains, namely minimization of the power of the interference at the receiver (MI), and maxi￾mization of the signal to noise plus interference ratio. In order to have an upper bound on the system perfor￾mance we derive a novel expression of the capacity of a system with two receive antennas with adaptive transmit gains. In fact, previous results are limited to the system where each transmitted signal is the linear combination of all space-coded data [11]. Simulation results for the Wind-Flex scenario show that indeed there is a significant tradeoff between performance and computational complexity of the various solutions. II. SYSTEM DESCRIPTION An OFDM wireless system is considered, where data of each subcarrier is coded by a space-time block code and transmitted by Nt transmit antennas. The receiver is equipped with Nr receive antennas and it receives both the useful signal and interference generated by N inter￾ferers. We assume that the interferers use OFDM and are synchronous with the useful transmitter. Hence, by as￾suming that the cyclic prefix [9] is sufficiently long, the transmission and the interference channels are flat on each OFDM subcarrier. Note that if the interference signals are not synchronous, the cyclic prefix may not absorb the de-