参考《Capacity and Optimal Power Allocation for Fading Broadcast Channels With Minimum Rates》Nihar Jindal，Andrea Goldsmith
《无线通信》Andrea Goldsmith 对于多用户的衰落信道，香农容量的两个notions：遍历容量、中断容量 被提出。
前提：多用户、下行传输
1、一个基站给多个RX传输独立的信息；
2、每个RX遭受了time-varying flat-fading and additive Gaussian noise；
3、CSI假设：the transmitter and all receivers can track the channel fade perfectly, or in other words, that the transmitter and all receivers have perfect channel state information (CSI).
4、信道假设：the channel is slowly fading relative to codeword length, i.e., the channel is constant during transmission of a codeword。
底层信道的时变特性给无线通信系统的设计带来了显著地挑战；相比恒定的资源分配策略，动态的功率、带宽和速率分配可以导致明显的性能改善。
Ergodic capacity is concerned with achieving long-term rates averaged over all fading states，while outage capacity achieves a constant rate in all non-outage fading states subject to an outage probability.
遍历容量关心的是在所有衰落状态上，达到长期的速率平均（不考虑某些状态下，信道到底有多差）；中断容量是希望在所有未中断的衰落状态上，达到一个恒定速率（允许部分状态下，信道以一定概率中断）。
1、Ergodic capacity（为了获得在所有状态下的平均容量最大，让强的更强，弱的就不管了）
The ergodic capacity of a fading broadcast channel determines the maximum achievable long-term rates averaged over all fading states.
功率分配策略：注水法，也就是说，当用户信道强时，多分配功率；反之，少分配（未分配）。
users are allocated the most power when their channels are strong, and little, if any, power when their channels are weak.
缺点：Such an allocation scheme maximizes long-term average rates, but depending on the duration of channel fades, users with poor channels may not receive data for long periods of time while waiting for their channel to improve.信道差的用户在等待信道改善时，可能长时间无法接收数据。这显然不适合延迟敏感的应用，如 video or voice transmission
2、outage capacity（信道太差就直接舍弃，宣布中断；信道不至于太差，但低于恒常速率的，就多分配功率，拉其到恒定速率值；高于恒常速率的，就少分配功率）
each user maintains a constant rate some percentage of the time and no data is transmitted (i.e., an outage is declared) the rest of the time. 每个用户在一定的时间内保持一个恒定的速率，并且在其余时间不传输任何数据(即，中断被宣布)。
当信道特别差时，没有数据被发送给这个用户，因为这可能需要很大的功率去传输。
故一些状态中断，不传输，其他状态维持恒定的速率；
功率分配策略：信道反转；
This scheme eliminates all channel variation seen by the receivers by scaling the transmitted signal to invert fading so constant rates can be maintained during nonoutage. 信道弱的时候多分配功率。users are allocated the most power when their channels are weak.
the outage capacity region may be significantly smaller than the ergodic capacity region.区别：In ergodic capacity, the transmitter takes advantage of time variation in the channel by transmitting more data to users with strong channels, while in outage capacity the transmitter equalizes time variation by transmitting at constant rates in all non-outage states.
eg：《Adaptive Relaying Protocol for Wireless Power Transfer and Information Processing》2016
中断容量适用于延迟受限的传输模式（delay-limited transmission (DLT)），遍历容量适用于延迟容忍的传输模式（delay-tolerant transmission (DTT)）：
3、Zero-outage capacity refers to outage capacity with zero outage probability。 Zero-outage capacity is a special case of outage capacity in which no outage is allowed and constant rates must be maintained in all fading states. 也就是说，在所有状态下，恒定速率必须被维持，不允许中断发生。
4、simultaneously transmits delay-sensitive and delay-insensitive data
It is not desirable to shut off users for long periods of time as is possible in the ergodic capacity region, but forcing constant rates to be maintained subject only to an outage probability as is done in the outage capacity region severely reduces the set of achievable rates。 不希望像在遍历容量区域那样长时间地关闭用户，但是强制保持恒定的速率只受停机概率的约束，就像在中断容量区域所做的那样，这将严重减少可实现的速率集。
combine the notions of ergodic and zero-outage capacity
maximizing the ergodic capacity subject to minimum rate requirements for all users in all fading states. 特点：some power is used to maintain the minimum rates in all fading states while the remaining power is used to maximize the average rates in excess of the minimum rates.
一些功率用于提升至最小速率，一些功率用于进一步提高超过最小速率的
Users are never completely cut off due to the minimum rate requirements, but time variation of the channel is still taken advantage of by transmitting to users at rates higher than the minimum rates when their channels are strong and at exactly the minimum rates when their channels are poor.
《Proactive Eavesdropping Via Jamming for Ergodic Rate Maximization Over Wireless-Powered Multichannel Suspicious System》
1、the typical delay-sensitive transmission mode（即：延迟受限模式）, i.e., the suspicious transmitter adjusted its transmission rate to keep a target outage probability δ at the suspicious receiver
目标是：最小化中断概率
发射机可以自适应调节它的传输速率（通信速率）以维持接收机处的一个目标中断概率
2、the typical delay-tolerant transmission mode, which implies that to maximize the ergodic rate achieved at the SD, the ST should adaptively allocate its harvested power over the N channels based on the statistics channel state information (CSI) between itself and the SD.
目标是：最大化遍历速率
发射机可以根据TX-RX间统计的CSI来自适应地分配它的发射功率
统计的CSI？确定吗？
