The usual approach when designing an orthogonal frequency division multiplexing (OFDM) system, is to dimension the length of the cyclic prefix (CP) equal to the length of a typical "bad" (i.e., long) channel impulse response, so that both inter-symbol and inter-carrier interference are avoided for almost all channel realizations. However, such an approach does not maximize system capacity. It (a) wastes channel resources for relatively short channel impulse response realizations and (b) it is not necessarily optimal to completely eliminate interference. In this paper, we study the problem of designing the CP length for OFDM systems in a capacity-optimal way. To this end, we first present optimal and simplified metrics suitable to maximize capacity. Then, we apply those metrics and propose practical resource (bit-loading and OFDM sub-channel assignment) algorithms that include CP-length adaptation. We present numerical results for the example of a power line communication (PLC) OFDM system with typical indoor PLC channels that confirm the gains achievable with the proposed CP-length adaptation.

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Cyclic Prefix Design and Allocation in Bit-Loaded OFDM over Power Line Communication Channels

This paper proposes two novel strategies to extend the battery life of an orthogonal frequency-division multiplexing (OFDM) receiver by exploiting the concept of wireless power transfer (WPT). First, a new receiver architecture is devised that does not discard the cyclic prefix (CP) but instead exploits it to extract power from the received signal, realizing a WPT between the transmitter and the receiver. Subsequently, a flexible composite transmit strategy is designed, in which the OFDM transmitter transmits to the receiver two independent signals coexisting in the same band. It is shown that, by means of this approach, the transmitter can arbitrarily increase the power concentrated within the CP at the OFDM receiver, without increasing the redundancy of the transmission. Feasibility conditions for the self-sustainability of the transmission are derived, in terms of power consumption at the receiver, for both legacy and composite transmission. Numerical findings show that, under reasonable conditions, the amount of power carried in the CP can be made sufficient to decode the information symbols, making the transmission fully self-sustainable. The potential of the proposed approach is confirmed by the encouraging results obtained when the full self-sustainability constraint is relaxed, and partially self-sustainable OFDM transmissions are analyzed.

/pdf/a-composite-approach-to-self-sustainable-transmissions-3vwm9cfn7z.pdf

A Composite Approach to Self-Sustainable Transmissions: Rethinking OFDM

We propose to maximize achievable rate in OFDM systems via the adaptation of the cyclic prefix (CP) length jointly with tone and bit loading. We show that significant gains are obtained compared to the case of using a fixed CP with duration equal to the channel. Several metrics for the CP design are described. They tradeoff between performance and complexity. Both single user and multiuser OFDM are considered. Numerical results are reported for typical power line communication indoor channels.

Bit, tone and cyclic prefix allocation in OFDM with application to In-Home PLC

In this paper, we propose a multi-user aware frame structure for doubly dispersive channels in order to increase both spectral efficiency and frequency spread immunity of orthogonal frequency division multiple accessing (OFDMA) based systems. Unlike the conventional OFDMA based system where the fixed cyclic prefix duration and subcarrier spacing are utilized within the frame structure considering the worst case communication channel, in the proposed approach, multiple cyclic prefix durations and subcarrier spacings are employed. In order to build the proposed frame structure, the statistics of the mobility and the range of the users are mapped to inter-carrier-interference and maximum excess delay to obtain multiple subcarrier spacings and cyclic prefix durations. As a result, better frequency spread immunity and spectral efficiency are achieved by exploiting the doubly dispersive channel characteristics of the users.

Multi-User Aware Frame Structure for OFDMA Based System

We consider the problem of the joint allocation of power and cyclic prefix (CP) duration to maximize the system capacity in orthogonal frequency division multiplexing (OFDM). A closed form solution to the problem does not exist and the exhaustive search of the optimal values is practically unfeasible. Therefore, we propose a sub-optimal iterative power allocation algorithm jointly with the CP adaptation to see whether capacity improvements can be obtained w.r.t. the conventional choice of a uniform power distribution over the sub-channels and a predetermined long CP value. Numerical results show that via power and CP adaptation significant improvements can be obtained.

On power allocation in adaptive cyclic prefix OFDM

Conventional OFDM transmission system uses a fixed-length Cyclic Prefix to counteract Inter-Symbol Interferences (1SI) caused by channel delay spreading under wireless mobile environment This may cause considerable performance deterioration when the CP length is less than the channel RMS delay spread, or may decrease the system power and spectrum efficiency when it is much larger A novel Orthogonal Frequency Division Multiplexing (OFDM) transmission scheme is proposed in this paper to adapt the CP length to the variation of channel delay spread AOFDM-VCPL utilizes the preamble or pilot sub-carriers of each OFDM packet to estimate the channel RMS delay spread; and then uses a criterion to calculate the CP length, which finally affects the OFDM transmitter As illustrated in the simulation section, by deploying this scheme in a typical wireless environment, the system can transmit at data rate 115Mb/s higher than conventional non-adaptive system while gaining a 065dB power saving at the same BER performance

张朝阳

Papers

A novel OFDM transmission scheme with length-adaptive Cyclic Prefix