Muhammad Imadur Rahman

OFDMA vs. SC-FDMA: performance comparison in local area imt-a scenarios

The system requirements for IMT-A are currently being specified by the ITU. Target peak data rates of 1 Gb/s in local areas and 100 Mb/s in wide areas are expected to be provided by means of advanced MIMO antenna configurations and very high spectrum allocations (on the order of 100 MHz). For the downlink, OFDMA is unanimously considered the most appropriate technique for achieving high spectral efficiency. For the uplink, the LTE of the 3GPP, for example, employs SCFDMA due to its low PAPR properties compared to OFDMA. For future IMT-A systems, the decision on the most appropriate uplink access scheme is still an open issue, as many benefits can be obtained by exploiting the flexible frequency granularity of OFDMA. In this article we discuss the suitability of using OFDMA or SC-FDMA in the uplink for local area high-data-rate scenarios by considering as target performance metrics the PAPR and multiuser diversity gain. Also, new bandwidth configurations have been proposed to cope with the 100 MHz spectrum allocation. In particular, the PAPR analysis shows that a localized (not distributed) allocation of the resource blocks (RBs) in the frequency domain shall be employed for SC-FDMA in order to keep its advantages over OFDMA in terms of PAPR reduction. Furthermore, from the multiuser diversity gain evaluation emerges the fact that the impact of different RB sizes and bandwidth configurations is low, given the propagation characteristics of the assumed local area environment. For full bandwidth usage, OFDMA only outperforms SC-FDMA when the number of frequency multiplexed users is low. As the spectrum load decreases, instead, OFDMA outperforms SC-FDMA also for a high number of frequency multiplexed users, due to its more flexible resource allocation. In this contex different channel-aware scheduling algorithms have been proposed due to the resource allocation differences between the two blocks chemes.

License-exempt LTE systems for secondary spectrum usage: Scenarios and first assessment

This paper investigates the feasibility and broad implications of the use of 3GPP LTE systems in license-exempt situations. License-exempt use can occur in traditional unlicensed bands or in secondary usage of white space spectrum. Various scenarios for license-exempt use are listed with a particular focus on spectrum access as a secondary user. Regulatory requirements devised by Federal Communications Commission (FCC) for TV white space usage are described and tabulated. General principles and recommendations for license exempt spectrum usage are proposed with some important scenarios being identified. Additional requirements arise out of the need to share the spectrum with other potential users in a fair manner. System simulation results in an indoor setting are used to estimate spectral efficiencies under such conditions. We conclude that LTE can be used in license-exempt secondary usage scenario with only minor modifications, such as fair sharing procedures, and could benefit from support for higher order modulations.

Feasibility evaluations for secondary LTE usage in 2.7–2.9GHz radar bands

In this paper, we have studied the potential for secondary usage of radar bands by 3GPP LTE eNB devices in different scenarios, such as HeNB transmitters located at street level, HeNB transmitters located at high-rise buildings, macro LTE transmitter, etc. Different pathloss models are used to best suite the scenarios. By using different types of radar characteristics (e.g. radio navigations radars, meteorological radars, etc) and a protection requirement of −10dB Interference-to-Noise Ratio (INR), we have shown that in some scenarios, the required distances for adjacent channel radar usage in 2.7–2.9GHz band are quite reasonable. This means, in those scenarios, it could be possible to utilize the radar bands for secondary LTE systems. A protection margin of 18 to 20dB can be added for capturing the aggregate interference effects from multiple secondary interferers for downlink direction. More detailed system level investigations are required in this direction for further understanding the secondary usage in this band.

OFDM: Principles and Challenges

The nature of future wireless applications demands high data rates. Naturally dealing with ever-unpredictable wireless channel at high data rate communications is not an easy task. The idea of multi-carrier transmission has surfaced recently to be used for combating the hostility of wireless channel and providing high data rate communications. OFDM is a special form of multi-carrier transmission where all the subcarriers are orthogonal to each other. OFDM promises a high user data rate transmission capability at a reasonable complexity and precision. At high data rates, the channel distortion to the data is very significant, and it is somewhat impossible to recover the transmitted data with a simple receiver. A very complex receiver structure is needed which makes use of computationally expensive equalization and channel estimation algorithms to correctly estimate the channel, so that the estimations can be used with the received data to recover the originally transmitted data. OFDM can drastically simplify the equalization problem by turning the frequency-selective channel into a flat channel. A simple one-tap equalizer is needed to estimate the channel and recover the data. Future telecommunication systems must be spectrally efficient to support a number of high data rate users. OFDM uses the available spectrum very efficiently which is very useful for multimedia communications. For all of the above reasons, OFDM has already been accepted by many of the future generation systems [1].

Impact of MIMO Co-Channel Interference

In a real cellular system, existence of a number of multi-antenna schemes in neighboring cells means that different multi-antenna schemes will experience co-channel interference (CCI) from the same or other multi-antenna schemes. In this work, we have summarized our analysis and simulations related to cellular interference of some specific multiple input multiple output (MIMO) schemes on the same and other MIMO schemes. The goal is to study the impact of interference from MIMO schemes at a user located in the cell edge. Semi-Analytical evaluations of signal to interference and noise ratio (SINR) is done to find out the SINR statistics of different combinations of desired and interfering links. We have studied linear combining receivers for all the link combinations. Based on the current analysis, it is found that space-time block code (STBC) is a severe interferer compared to others, and specific attention is needed to counter the interference introduced by STBC interfering link.

Optimum pre-DFT combining with cyclic delay diversity for OFDM based WLAN systems

Cyclic delay diversity is applied in an OFDM receiver to increase the frequency-selectivity of the channel seen at the receiver for flat (or less frequency-selective) channels. Diversity combining is performed prior to the discrete Fourier transform (DFT) operation. A new method has been studied to optimize the pre-DFT diversity combining by selecting cyclic shifts and weight factors based on known channel state information. The optimum signal-to-noise ratio for maximum cyclic delay and optimum weight factors for multiple antenna receiver diversity is analyzed. Performance results in terms of bit error probability are presented. Finally, it is found that the proposed scheme drastically reduces computational complexity compared to traditional receiver diversity schemes.

Link Level Investigation of ACK/NACK Bundling for LTE TDD

In LTE TDD there is typically no one-to-one association between UL and DL subframes, and for DL heavy asymmetries with more DL than UL subframes, ACK/NACK reports for multiple DL subframes need to be transmitted in an UL subframe. To improve uplink control channel performance, ACK/NACK bundling, where multiple ACK- NACKs are combined to a single ACK/NACK response for several DL subframes, is supported. In this paper, we study the impact on downlink performance of ACK/NACK bundling on link-level for LTE TDD systems both for SIMO and MIMO systems. It is found that the expected loss due to ACK/NACK bundling is rather small and hence bundling can be a good solution in many scenarios.

Impact of Nonlinear Power Amplifier on Link Adaptation Algorithm of OFDM Systems

The impact of non linear distortion due to high power amplifier (HPA) on the performance of link adaptation (LA) - orthogonal frequency division multiplexing (OFDM) based wireless system is analyzed. The performance of both forward error control coding (FEC) encoded and uncoded system is evaluated. LA maximizes the throughput while maintaining a required block error rate (BLER). It is found that when OFDM signal, which has high PAPR, suffers non linear distortion due to non ideal HPA, the LA fails to meet the target BLER. Detailed analysis of the distortion and effects on LA are presented in this work.

Multi rate orthogonal frequency division multiplexing

A novel multi rate orthogonal frequency division multiplexing (OFDM) system is proposed in this paper. It is expected to provide a flexible physical layer support to next generation wireless networks. It is mainly suited to inter carrier interference limited situations, when a large number of sub carriers are used within the available bandwidth constraint. The proposal is to alter the otherwise uniform performance of the sub carriers in an OFDM system by using different bandwidths for different subcarriers. This is advantageous in the situation where different users and data channels in a wireless network will have different data rates and quality of service requirements. Different sub carriers with different performance figures can be mapped to the data channels with matching requirements. This scheme can enhance the conventional OFDM systems to become flexible. It is shown that, using different sub carrier bandwidths can improve the throughput of some sub carriers at the cost of the others. Results show that overall system performance also improves with the proposed scheme even without any optimal mapping of data stream to the sub carriers for slightly higher frequency offsets.

A Study of Precoding for LTE TDD Using Cell Specific Reference Signals

We investigate non-codebook-based multiple-input multiple-output (MIMO) precoding schemes for the time division duplex (TDD) mode of LTE where channel reciprocity could be exploited. Previously proposed non-codebook-based precoding schemes typically use UE specific reference signals for demodulation. Cell specific reference signals are however still transmitted for the transmission of common control signaling, mobility measurements and downlink channel quality measurements. In order to save the resources occupied by UE specific reference signals, and to simplify UE implementation, a non-codebook-based precoding scheme using cell-specific reference signals is considered. Link throughput simulations indicate that such scheme outperforms the scheme using UE specific reference signals in the scenario with high transmit antenna correlation and low UE velocity.