58 research outputs found
PAPR Reduction in GFDM Systems Using an SLM Technique
Department of Electrical EngineeringIn the fifth generation (5G) cellular network system, user capacity should be improved com- pare with the current 4G network system. To this end, higher resource efficiency is an essential. Orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division mul- tiple access (OFDMA), which has high spectral efficiency resorting to orthogoanlity between subcarriers, is the most commonly used modulation technique in the current 4G network sys- tem. To maintain orthogonality, several types of frame structures are used for synchronized signal transmission and reception in Long Term Evolution (LTE). However, these fixed frame structures result in a fundamental limit for reducing latency. Thus an asynchronous commu- nication scheme has been emerged as one of the solutions to reduce latency. On the contrary, without synchronization, OFDM signals generate interference to each other. Recently, general- ized frequency division multiplexing (GFDM) has been proposed for the asynchronous multiple access. Many studies have evaluated that GFDM has higher sum-rate than OFDM for the asyn- chronous systems owing to the higher spectral efficiency and lower out-of-band emission (OOB). Despite the many advantages, GFDM also has disadvantages such as a high peak-to-average power ratio (PAPR). If the numbers of GFDM and OFDM subcarriers are equal, GFDM will get higher PAPR than OFDM due to multiple subsymbols. To reduce the PAPR, various PAPR reduction techniques have been studied on OFDM such as clipping, selective mapping (SLM), partial transmit sequence (PTS), Tone reservation (TR), and single-carrier frequency division multiple access (SC-FDMA) for LTE uplink. In GFDM, precoded GFDM and generalized fre- quency division multiple access (GFDMA) have been proposed as PAPR reduction techniques. Among PAPR reduction techniques, SLM is one of applicable techniques to the GFDM without signal distortions. In this paper, GFDM SLM is proposed as a PAPR reduction technique. In addtion, the performance analysis is compared in terms of the PAPR, OOB, and spectral efficiency among SC-FDMA, OFDMA, GFDMA, precoded GFDM, and GFDM SLM.ope
Novel Precoding Based PAPR Reduction Schemes for Localized OFDMA Uplink of LTE-A
High Peak to Average Power Ratio (PAPR) reduction is still a major challenge in Orthogonal Frequency Division Multiple Access (OFDMA) systems. The subcarrier mapping in OFDMA can be done in two modes, Localized mode and Distributed mode. The Localized mode is more suitable for the practical implementations than the Distributed mode due to its low sensitivity against the imperfect power control, phase noise and Inter Carrier Interference (ICI). In this paper, we propose two novel precoding based Multi-Carrier (MC) Multiple Access (MA) techniques for PAPR reduction, ZadoffChu precoding based Localized-OFDMA (L-OFDMA) technique and Generalized Chirp Like (GCL) precoding based L-OFDMA technique for L-OFDMA uplink systems of upcoming Long Term Evaluation-Advanced (LTE-A) system. These techniques are based on precoding the constellation symbols with Zadoff-Chu precoder and GCL precoder. The proposed techniques can reduce PAPR up to 6.5dB for L-OFDMA. It is noticeable from the computer simulation results, that the PAPR of our proposed precoding based MC systems have approximately equal PAPR as compared to the PAPR of competing technology called Localized Single Carrier Frequency Division Multiple Access SC-FDMA (LFDMA) system. LFDMA is implemented in release 8 of Long Term Evaluation (LTE). Additionally, these precoding based PAPR reduction techniques also takes the advantage of the frequency variations of the communication channel and can also offer substantial performance gain in fading multipath channel
Improved Hybrid Blind PAPR Reduction Algorithm for OFDM Systems
The ever growing demand for high data rate communication services resulted into the development of long-term evolution (LTE) technology. LTE uses orthogonal frequency division multiplexing (OFDM) as a transmission technology in its PHY layer for down-link (DL) communications. OFDM is spectrally efficient multicarrier modulation technique ideal for high data transmissions over highly time and frequency varying channels. However, the transmitted signal in OFDM can have high peak values in the time domain due to inverse fast Fourier transform (IFFT) operation. This creates high peak-to-average power ratio (PAPR) when compared to single carrier systems. PAPR drives the power amplifiers to saturation degrading its efficiency by consuming more power. In this paper a hybrid blind PAPR reduction algorithm for OFDM systems is proposed, which is a combination of distortion technique (Clipping) and distortionless technique (DFT spreading). The DFT spreading is done prior to clipping reducing significantly the probability of having higher peaks in the composite signal prior to transmission. Simulation results show that the proposed algorithm outperforms unprocessed conventional OFDM transmission by 9 dB. Comparison with existing blind algorithms shows 7 dB improvement at error rate 10–3 and 3 dB improvement at error rate 10–1 when operating in flat fading and doubly dispersive channels, respectively.Keywords: LTE Systems; OFDM; Peak to Average Power Ratio; DFT spreading; Signal to Noise Power Ratio
Busy burst technology applied to OFDMA–TDD systems
The most significant bottleneck in wireless communication systems is an ever-increasing disproportion
between the bandwidth demand and the available spectrum. A major challenge in
the field of wireless communications is to maximise the spatial reuse of resources whilst avoiding
detrimental co-channel interference (CCI). To this end, frequency planning and centralised
coordination approaches are widely used in wireless networks. However, the networks for the
next generation of wireless communications are often envisioned to be decentralised, randomly
distributed in space, hierarchical and support heterogeneous traffic and service types. Fixed
frequency allocation would not cater for the heterogeneous demands and centralised resource
allocation would be cumbersome and require a lot of signalling. Decentralised radio resource
allocation based on locally available information is considered the key.
In this context, the busy burst (BB) signalling concept is identified as a potential mechanism
for decentralised interference management in future generation networks. Interference aware
allocation of time-frequency slots (chunks) is accomplished by letting receivers transmit a BB
in a time-multiplexed mini-slot, upon successful reception of data. Exploiting channel reciprocity
of the time division duplex (TDD) mode, the transmitters avoid reusing the chunks
where the received BB power is above a pre-determined threshold so as to limit the CCI caused
towards the reserved chunks to a threshold value. In this thesis, the performance of BB signalling
mechanism in orthogonal frequency division multiple access - time division duplexing
(OFDMA-TDD) systems is evaluated by means of system level simulations in networks operating
in ad hoc and cellular scenarios. Comparisons are made against the state-of-the-art centralised
CCI avoidance and mitigation methods, viz. frequency planning, fractional frequency
reuse, and antenna array with switched grid of beams, as well as decentralised methods such as
the carrier sense multiple access method that attempt to avoid CCI by avoiding transmission on
chunks deemed busy. The results demonstrate that with an appropriate choice of threshold parameter,
BB-based techniques outperform all of the above state-of-the-art methods. Moreover,
it is demonstrated that by adjusting the BB-specific threshold parameter, the system throughput
can be traded off for improving throughput for links with worse channel condition, both
in the ad hoc and cellular scenario. Moreover, by utilising a variable BB power that allows a
receiver to signal the maximum CCI it can tolerate, it is shown that a more favourable trade-off
between total system throughput and link throughput can be made. Furthermore, by performing
link adaptation, it is demonstrated that the spatial reuse and the energy efficiency can be traded
off by adjusting the threshold parameter. Although the BB signalling mechanism is shown to
be effective in avoiding detrimental CCI, it cannot mitigate CCI by itself. On the other hand,
multiple antenna techniques such as adaptive beamforming or switched beam approaches allow
CCI to be mitigated but suffer from hidden node problems. The final contribution of this thesis
is that by combining the BB signalling mechanism with multiple antenna techniques, it is
demonstrated that the hybrid approach enhances spatial reusability of resources whilst avoiding
detrimental CCI.
In summary, this thesis has demonstrated that BB provides a flexible radio resource mechanism
that is suitable for future generation networks
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