一、background

  • Current shortcoming-- the energy scarcity of wire-less networks

    With the skyrocketing increase of the number of intelligent devices such as wireless sensors,tales,wearables,and the widespread application of higgh-qualty multimedia services in the fifth-generation(5G) and beyond wireless networks,the limited and high-cost energy suppies from traditionally used batteries and wire cables have become the main bottleneck for the sustainablae and stable operation of massie wireless devices,which thus greatly restricts the performance enhancement of wireless netorks.

     

  • Methods A

    In the wireless powered communication networks(WPCN),operating in a "harvest- then-transmit"strategy.Namely,In the DL phase the users harvest the RF energy form HAP.And In the UL phase the users transmits information to HAP.

    But this strategy also leads to a performance bottleneck of WPCNs, namely the doubly near-far phenomena, which means that a user far from the HAP suffers from much reduced throughput due to the doubly distance-dependent signal power attenuation.such as [5].

     

  • Methods B

    With the decrease of wirless devices operating power and the recent deveopment of multiple-input multiple-output(MIMO) technology,WPCN become potentially suitable for low-power applications,such as radio frequency identi-fication (RFID) systems and wireless sensor networks.

    To improve the WPT efficiency ,most studies have considered MIMO technology( include MIMO relay using active RF chains and massive MIMO technologies ),which is unable to well address the double near-far phenomena of WPCNs.Even under the hybrid transceiver architecture,the deployed large-scale array with limited RF chains may still lead to high signal processing complexity and high power consumption.

     

  • Methods C

    By deploying the IRS in WPCNs , the energy harvesting zones to compensate for high signal power attenuation of users far from sthe HAP can be established to tackled the doubly near-far phenomenon.

    And the IRS-aided wireless communications have been extensively carried out foe various wireless systems,Such as MISO systems,point to point MIMO systems,multiuser MIMO systems,MIMO orthogoal frequency division multplexing systems(OFDM).

     

     

     

    • What is IRS?

      IRS is inessence a reconfigurable metal surface equipped with of a large number of passive reflecting elements without RF chains.

      智能反射面（IRS）是一种全新的革命性技术，它可以通过在平面上集成大量低成本的无源反射元件，智能地重新配置无线传播环境，从而显著提高无线通信网络的性能。具体地说，IRS的不同元件可以通过控制其幅度和/或相位来独立地反射入射信号，从而协同地实现用于定向信号增强或零陷的精细的三维（3D）无源波束形成。

       

    • Feature of IRS

      Since it cannot perform complex signal amplification/regeneration and precoding,the passive IRS is capable of yielding superior performance scaling with the number of reflecting elements(Though number of reflecting elements to change performance), and at the expense of substantially reduced hardware cost and power consumption.

      By adjusting the phase shift and ampliude attenuation of each reflected signal,called the IRS reflection coeffiient, the IRS is able to proactively construct the favorable source-destination propagation environment for both WPT and WIT.

      Compared to MIMO , MIMO usually operates in half-duplex mode and thus becomes less spectrum and energy efficient than the IRS operating in full-duplex mode.

       

    • problems of IRS

      1.Considering practical hardware limitations, both discrete phase shifts and discrete reflection amplitudes of the IRS were also studied.

      2.Channel state information(CSI) is required for the sake of fully exploiting the degrees of freedom(DoFs) introduced by the IRS.such as [13] [15] [17].

      3.The classical pilot-based channel estimation cannot be performed at the IRS due to its lack of baseband signal processing capability,then proposed variou scascaded transmitter-IRS-receiver channel estimation schemes for both frequency-flat and frequency-selective channels.

      4.Assuming the perfect IRS-related CSI,most works investigated the integration of IRS and some advanced techniques in 5G and beyond networks,such as simultaneous wireless information nd power transfer(SWIPT),non-orthogonal multiple access(NOMA),physical layer seurity,mobile edge computing(MEC) ,etc. [19] [20]

      5.Most existing WPT-related works focus on **conventional multiuser MIMO WPCNs without the IRS**, where the optimal MIMO transceivers and downlink/uplink (DL/UL) time allocation are jointly designed relying on **wireless channels with uncontrollable random fluctuations**.

二、motivation

To maximize the throughput of UL

To improve the WPT efficiency and double near-far phenomena of WPCNs.

 

三、contribution

  • Objective

    1.Construct the favorable DL/UL wireless channels by adjutig DL/UL IRS reflection coefficients.

    2.A higher throughput than that of conventional MIMO WPCNs is expected.

    3.Joint optimization of the DL/UL IRS reflection coefficients,the DL/UL time allocation and the transmit covriance matrices at both HAP and users for maximization in the IRS-aided MIMO WPCNs.

    4.The MIMO setup and common DL/UL IRS refection pattern(IRS refection coefficients need to be designed to make a trade-off between DL energy harvesting and UL information transfer).

     

     

  • The separate DL/UL IRS reflection pattern

    1.In the multi-user senario

    Because the Coupled optimization variables and non-convex unit-modulus constraints,we divide all optimization variables into three overlapped blocks using the harvest-then-transmit strategy, and then propose a 3-block alternating optimization algorithm.Namely the unified subgradient method, the semidefinite relaxation(SDR) based penalty function method and an element-by-element iterative algorithm.

    And for avoid the high complexity of solving a large-scale SDP problem,we propose classical successive convex approximation (SCA) strategy and the independent two-stage approach to optimize the IRS reflection matrix.

    2.In the single-user senario

    A 2-block alternating optimization algorithm using the Taylor-series expansion technique is proposed.

    And form the low computational complexity perspetive,we also consider another two alternative schemes for maximizing UL throughput in the asymptotically low-SNR and high-SNR regimes. It can be simplified to one associated with only the DL/UL IRS reflect beamforming and the DL/UL time splitting factor.

     

  • In the common DL/UL IRS reflection pattern

    1.low-complexity two-stage scheme to tackle the intensive coupling among the IRS- induced reflection,DL energy and UL information transfer.

    2.multi-user and single-user scenarios still applicable

    3.DL/UL bridged by the same IRS refect beamforming can be integrated into a single optimization objective.

     

四、conclusion

  1. Numerical results to illustrate the superior UL throughput performance of all proposed algorithms over the benchmarks.

  2. The large-scale propagation loss of the cascaded HAP-IRS-user channel can be remarkably reduced by properly deploying the IRS near the HAP or the user, thus leading to the significant improvement of the UL throughput.

  3. IRS is also helpful to overcome the well-known doubly near-far problem of WPCNs.

 

 

五、terminology（专业术语）

1.WPCNs 无线供电通信网络

2.WPT,WIT 无线功率传输、无线信息传输

3.SNR 信噪比--系统中信号与噪声的比例

4.HAP 混合接入点

5.phase shift,amplitude attenuation 相移、幅度衰减

6.half-duplex mode,full-duplex mode 半双工模式、全双工模式

7.MIMO，MISO 多入多出，多入单出

8.OFDM 正交频分复用

9.CSI 信道状态信息

10.TDD 时分双工--上下行用相同的频带，在一个频带内上下行占用的时间可根据需要进行调节，并且一般将上下行占用的时间按固定的间隔分为若干个时间段（时隙）

11.FDD 频分双工--上下行用不同的频带，一般上下行的带宽是一致的

12.tr() 求矩阵的秩

 

 

六、expression（特色表达）

 

七、idea（想法/点子）

 

 

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