Abstract
Abstract The combination of MIMO–OFDM is a very attractive solution for broadband wireless services. Thus, the two prominent fourth-generation (4G) cellular systems, WiMAX and LTE-advanced, have both adopted MIMO–OFDM transmission at the physical layer. OFDM signal however suffers from nonlinear distortions when passed through high-power amplifier (HPA) at the RF stage. This nonlinear distortion introduces out-of-band spectral broadening and in-band distortions on the transmitted signals. 4G cellular standards have placed strict limits on the allowable spectral broadening in their spectrum mask specifications, to insure that data transmission on a given channel is not interfering significantly with an adjacent channel user. In this article, we characterize the out-of-band spectral broadening introduced by HPA when MIMO–OFDM signals are transmitted over multiple relaying channels. Expressions for the power spectral density of MIMO–OFDM signals are derived over multiple relay channels, and the cumulative effects of HPA on the spectrum of the transmitted signals are estimated. It is shown that depending on the number of relays and the relaying configuration employed, it may happen that a transmitted MIMO–OFDM signal with the transmit spectrum mask initially within the allowable set limit at the source node arrives at the destination violating this limit due to the cumulative effects of the multiple HPA’s in a multihop relaying channel.
Highlights
Fourth-generation (4G) broadband communication systems need to provide ultra-high data rate services in order to meet the requirements of future highbandwidth multimedia applications over cellular systems such as the digital TV distributions and interactive videos planned in the WiMAX and LTE-advanced
We can draw a conclusion that even though the general mobile multihop relaying (MMR) system where data could traverse any number of relay hops from source to destination has widely been studied theoretically [20,31,32], only the 2-hop version such as those proposed by the IEEE 802.16j group [18] may be advised for any practical deployments in MIMO–OFDM transmissions because of the potentials for spectral mask violations when going beyond 2-hop transmissions as shown in these results
This article presents new insights on the out-of-band spectral re-growth due to high-power amplifier (HPA) nonlinearities when MIMO–OFDM signals are transmitted over multiple relay channels
Summary
Fourth-generation (4G) broadband communication systems need to provide ultra-high data rate services in order to meet the requirements of future highbandwidth multimedia applications over cellular systems such as the digital TV distributions and interactive videos planned in the WiMAX and LTE-advanced. Grad et al [12] studied spectral re-growth due to HPA nonlinearity in code-division multiple access (CDMA) systems They obtained analytical expressions for the power spectrum of the CDMA signal at the output of the HPA, using a complex power-series model for the HPA characteristics. We characterize for the first time in the literature, the cumulative spectral broadening effects of multiple HPAs when MIMO–OFDM signals are transmitted over multihop relaying channels. It is observed from this figure that Saleh model depicts the severest nonlinear distortions followed by the SSPA model. The closedform expressions for the in-band and out-of-band distortion parameters for the SEL HPA model are calculated by substituting f (r) into Equations (6) and (7) to obtain kj
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