Song Ni

Network performance of asynchronous UTRA-like FDD/CDMA systems using loosely synchronised spreading codes

In this paper we characterize the capacity of an UTRA-like frequency division duplex (FDD) code division multiple access (CDMA,) system employing loosely synchronized (LS) spreading codes. Current CDMA systems are interference limited, suffering from inter-symbol-interference (ISI), since the orthogonality of the spreading sequences is destroyed by the channel. They also suffer from multiple-access-interference (MAI) owing to the non-zero cross-correlations of the spreading codes. LS codes exhibit a so-called interference free window (IFW), where both the auto-correlation and cross-correlation of the codes become zero. Therefore LS codes have the promise of mitigating the effects of both ISI and MAI in time dispersive channels. Hence, LS codes have the potential of increasing the capacity of CDMA networks. This contribution studies the achievable network performance by simulation and compares it to that of an UTRA-like FDD/CDMA system using orthogonal variable rate spreading factor (OVSF) codes.

On the network performance of UTRA-like TDD and FDD CDMA systems using adaptive modulation and adaptive beamforming

In this distribution we characterise the capacity of an adaptive modulation assisted, beam-steering aided TDD/CDMA system. In TDD/CDMA the mobiles suffer from interference inflicted by the other mobile stations (MSs) both in the reference cell the MS is roaming in (intracell interference) as well as due to those in the neighbouring cells (intercell interference). Furthermore, in contrast to FDD/CMDA, where the base stations (BSs) transmit in an orthogonal frequency band, in TDD/CDMA there is additional interference imposed by other BSs of the adjacent cells, since all time-slots can be used in both the uplink and downlink. In return for this disadvantage, TDD/CDMA guarantees the flexible utilization of all the available bandwidth, which meets the demand for the support of asymmetric uplink and downlink services, such as high data rate file download in mobile Internet services, etc. In wireless systems the link quality fluctuates due to either fading- and dispersion-induced channel impairments or as a consequence of the time-variant co-channel interference imposed by the teletraffic fluctuations due to the varying number of users supported. Due to these impairments conventional wireless systems often drop the call. By contrast, a particular advantage of employing adaptive modulation is that the transceiver is capable of automatically reconfiguring itself in a more error-resilient transmission mode, instead of dropping the call. This contribution studies the achievable network performance by simulation and compares it to that of the FDD/UTRA system.

Genetically enhanced performance of a UTRA-like time-division duplex CDMA network

In this contribution a dynamic channel allocation (DCA) algorithm is developed, which minimizes the amount of multi-user interference (MUI) experienced at the base stations (BSs) by employing genetic algorithms (GAs). A GA is utilized for finding a suboptimum, but highly beneficial uplink (UL) or downlink (DL) timeslot (TS) allocation for improving the achievable performance of the third generation UTRA systems time division duplex (TDD) mode. It is demonstrated that a GA-assisted UL/DL timeslot scheduling scheme may avoid the severe BS to BS inter-cell interference potentially inflicted by the UTRA TDD CDMA air interface owing to allowing all TSs to be used both in the UL and DL.

Adaptive Beamforming and Adaptive Modulation-Assisted Network Performance of Multiuser Detection-Aided FDD and TDD CDMA Systems

The network performance of a frequency division duplex and time division duplex (TDD) code division multiple access (CDMA)-based system is investigated using system parameters similar to those of the universal mobile telecommunication system. The new call blocking and call dropping probabilities, the probability of low-quality access, and the required average transmit power are quantified both with and without adaptive antenna arrays (AAAs), as well as when subjected to shadow fading. In some of the scenarios investigated, the systems user capacity is doubled with the advent of adaptive antennas. The employment of adaptive modulation techniques in conjunction with AAAs resulted in further significant network capacity gains. This is particularly so in the context of TDD CDMA, where the systems capacity becomes poor without adaptive antennas and adaptive modulation owing to the high base station (BS) to BS interference inflicted as a consequence of potentially using all time slots in both the uplink and downlink of the emerging wireless Internet.

Lossley Synchronized Spreading Code Aided Network Performance of Quasi-Synchonous UTRA-like TDD/CDMA Systems

In this paper we investigate the achievable capacity of a UTRA-like Time Division Duplex (TDD) Code Division Multiple Access (CDMA) system employing Loosely Synchronized (LS) spreading codes. The family of operational CDMA systems is interference limited, suffering from Inter-Symbol-Interference (ISI), since the orthogonality of the spreading sequences is destroyed by the frequency selective channel. They also suffer from Multiple-Access-Interference (MAI) owing to the non-zero cross-correlations of the spreading codes. By contrast, the family of LS codes exhibits a so-called Interference Free Window (IFW), where both the auto-correlation and cross-correlation of the codes become zero. Therefore LS codes have the promise of mitigating the effects of both ISI and MAI in time dispersive channels. Hence, LS codes have the potential of increasing the capacity of CDMA networks. This contribution studies the achievable network performance in comparison to that of a UTRA-like TDD/CDMA system using Orthogonal Vari- MSO able Rate Spreading Factor (OVSF) codes.