1,721,979 research outputs found
Multi-Stage Multi-User Detection Assisted Asynchronous Fast-FH/MFSK
Full text linkA multi-stage Multi-User Detection (MUD) scheme designed for asynchronous fast Frequency-Hopping/Multilevel Frequency-Shift-Keying (fast-FH/MFSK) systems is proposed, in which each signal detection interval is divided into sub-intervals and the MUD is applied to each sub-interval. In our scheme the MUD exploits the explicit knowledge of the hopping addresses assigned to users. The received signal level is attenuated by a constant scaling factor, when it is deemed to be overwhelmed by multi-user interference. For the sake of preventing erroneous detection events, when communicating over frequency-selective fading channels, the scheme advocated also invokes a space diversity technique. In the investigated scenario the achievable Bit Error Rate (BER) of the proposed scheme was reduced by as much as an order of magnitude in comparison to that of a conventional MUD scheme, when transmitting over an AWGN channel. For transmission over a channel exhibiting uncorrelated frequency-domain fading, selection diversity was used for the sake of achieving a reduced BER
Genetic Algorithm Aided Timeslot Scheduling for UTRA TDD CDMA Networks
No full textIt is demonstrated that genetic algorithms may be utilised 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 system’s time division duplex (TDD) mode. It is demonstrated that this novel GA-assisted UL=DL timeslot scheduling scheme is capable of avoiding the severe BS to BS intercell interference potentially inflicted by the UTRA TDD CDMA air interface owing to allowing all TSs to be used both in the UL and DL, hence potentially quadrupling the carried teletraffic and halving the uplink power
Improved Hybrid MMSE Detection for Turbo Trellis Coded Modulation Assisted Multi-User OFDM Systems
Full text linkA genetic algorithm assisted minimum mean-square error (MMSE) multi-user detector (MUD) is proposed for a turbo-trellis-coded modulation-based space division multiple access aided orthogonal frequency division multiplexing system. The simulation results show that the system employing the concatenated MMSE-GA MUD was capable of achieving a similar performance to that attained by its optimum counterpart employing the maximum likelihood MUD, while maintaining a significantly lower complexity, especially when supporting a high number of users
Multiuser MIMO-OFDM Systems using Subcarrier Hopping
No full textRecently space division multiple access (SDMA) assisted multiple-input–multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems invoking multiuser detection (MUD) techniques have attracted substantial research interest, which is capable of exploiting both transmitter multiplexing gain and receiver diversity gain. A new scheme referred to here as slowsubcarrierhopping (SSCH) assisted multiuser SDMA-OFDM, is proposed. It is shown that, with the aid of the so-called uniform SSCH (USSCH) pattern, the multiuser interference (MUI) experienced by the high-throughput SDMA-OFDM system can be effectively suppressed, resulting in a significant performance improvement. In the investigations conducted, the proposed USSCH-aided SDMA-OFDM system was capable of outperforming a range of SDMA-OFDM systems considered, including the conventional SDMA-OFDM system dispensing with the employment of frequency-hopping techniques. For example, at an Eb/N0 value of 12 dB, the proposed USSCH/SDMA-OFDM system reduced the bit error ratio (BER) by about three orders of magnitude, in comparison to the conventional SDMA-OFDM system, while maintaining a similar computational complexity
On the Computation of 16-QAM and 64-QAM Performance in Rayleigh-Fading Channels
No full textSUMMARY Quadrature Amplitude Modulation (QAM) schemes are attractive in terms of bandwidth efficiency and offer a number of subchannels with different integrities via both Gaussian and Rayleigh-fading channels, Specifically, the 16-QAM phasor constellation has two, while the 64-QAM possesses three such subchannels, which become dramatically different via Rayleigh-fading channels. The analytically derived bit error rate (BER) formulae yield virtually identical curves with simulation results, exhibiting adequate BERs for the highest integrity subchannels of both 16-QAM and 64-QAM to be further reduced by forward error correction coding (FEC). However, the BERs of the lower integrity subchannels require fading compensation to reduce their values for FEC techniques to become effective. This property creates ground for a variety of carefully matched, embedded mobile transmission schemes of different complexities. The practical implementation of such an embedded scheme is demonstrated by a low-cost, low-complexity and low consumption 50 kBd mobile video telephone scheme offering adequate speech and image quality for channel SNRs in excess of about 20 dB via Rayleigh-fading channels. key words: QAM theory; modulation for microcellular fading channels
Reverse-Link Performance of Synchronous DS-CDMA Systems in Dispersive Rician Multipath Fading Channels
No full textThe reverse-link performance of synchronous DS-CDMA is investigated in Rician multipath fading environments. The numerical results show that the performance benefits of synchronous transmission depend on the specular path’s power as well as on the multipath intensity decay factor of the channel. The achievable bit-error-rate reduction may become as high as an order of magnitude in comparison to the corresponding asynchronous system
Interference-Mitigating Waveform Design for Next-Generation Wireless Systems
No full textA brief historical perspective of the evolution of waveform designs employed in consecutive generations of wireless communications systems is provided, highlighting the range of often conflicting demands on the various waveform characteristics. As the culmination of recent advances in the field the underlying benefits of various Multiple Input Multiple Output (MIMO) schemes are highlighted and exemplified. As an integral part of the appropriate waveform design, cognizance is given to the particular choice of the duplexing scheme used for supporting full-duplex communications and it is demonstrated that Time Division Duplexing (TDD) is substantially outperformed by Frequency Division Duplexing (FDD), unless the TDD scheme is combined with further sophisticated scheduling, MIMOs and/or adaptive modulation/coding. It is also argued that the specific choice of the Direct-Sequence (DS) spreading codes invoked in DS-CDMA predetermines the properties of the system. It is demonstrated that a specifically designed family of spreading codes exhibits a so-called interference-free window (IFW) and hence the resultant system is capable of outperforming its standardised counterpart employing classic Orthogonal Variable Spreading Factor (OVSF) codes under realistic dispersive channel conditions, provided that the interfering multi-user and multipath components arrive within this IFW. This condition may be ensured with the aid of quasisynchronous adaptive timing advance control. However, a limitation of the system is that the number of spreading codes exhibiting a certain IFW is limited, although this problem may be mitigated with the aid of novel code design principles, employing a combination of several spreading sequences in the time-frequency and spatial-domain. The paper is concluded by quantifying the achievable user load of a UTRA-like TDD Code Division Multiple Access (CDMA) system employing Loosely Synchronized (LS) spreading codes exhibiting an IFW in comparison to that of its counterpart using OVSF codes. Both system's performance is enhanced using beamforming MIMOs
A Range of Low and High Delay CELP Speech Codecs Between 8 and 4 kbits/s
No full textIn this paper we study the performance and the error sensitivities of six CELP [I] based codecs operating between 8 and 4 kbits/s. Codecs using both forward and backward adaption of the linear prediction coefficients and the long term predictor (LTP) are described. Initially we describe four low delay codecs which all use backward adaption of the LPC coefficients but which differ in their use of LTP. These codecs all have frame-lengths of 3 ms or less, and their performance at various bit rates between 8 and 4 kbits/s is examined. Next the error sensitivity of these codecs, and means of improving it, are described. Then an algebraic CELP (ACELP) [2] codec operating at 6.2 kbits/s with a frame-length of 5 ms is described. Our final codec also uses ACELP and operates between 4.7 and 7.1 kbits/s, but it is forward adaptive and so it has a much longer frame-length of up to 30 ms. After describing this codec we compare the performance of our codecs in both error-free conditions and in the presence of channel errors. Surprisingly the error sensitivity of the low delay backward adaptive codec with no LTP is similar to that of the forward adaptive, high delay, ACELP codec
Reduced-Complexity Near-Optimum Genetic Algorithm Assisted Multiuser Detection for Synchronous Multicarrier CDMA
No full textIn this contribution, a Genetic Algorithm (GA) assisted Multiuser Detector (MUD) designed for MC-CDMA is investigated in the context of frequency selective Rayleigh fading channels. The achievable BER performance of the GA assisted MUD as well as its near-far resistance are investigated for a range of parameter values. It is shown that the proposed GA assisted MUD is capable of significantly reducing the complexity in comparison to that of Verdu's optimum MUD. For example, when supporting K = 20 users, the number of likelihood function evaluations is reduced by a factor of 1300
Iteratively detected multi-carrier interleave division multiple access
No full textIn this paper a novel transceiver is proposed, which amalgamates the benefits of multi-carrier transmissions with those of interleave division multiple access (IDMA), leading to the concept of multi-carrier IDMA (MC-IDMA), which invokes spreading in both the time as well as in the frequency domain. In the proposed MC-IDMA system the classic position of spreading and interleaving is reversed and hence the chips are interleaved, which are then mapped to multiple carriers. The different users are distinguished by their unique, user-specific interleavers. With the aid of turbo-style iterative joint detection and decoding, MCIDMA becomes capable of supporting a large user-population. This is particularly so, when sophisticated non-linear detectors are used. More quantitatively, it is demonstrated for example that the proposed MC-IDMA system is capable of supporting three times more users than the spreading gain of G = 16, while maintaining a modest complexity
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