User:Rmudproject/Sandbox

Multiuser diversity is a form of diversity scheme used in multiuser networks to achieve increased throughput/spectral efficiency in the system. It is based on the idea that in a multiuser network operating in a rich scattering & fast fading environment, at any given instant of time, atleast one user will have a good channel & scheduling the data transmission to this user will increase the achieved spectral efficiency. If in every frame, data transmission is scheduled to the user with the best channel, then the overall network throughput will increase compared to a network employing Round Robin scheduling, in which data transmission is scheduled to a randomly selected user. Since in every frame, user with the best channel is scheduled for transmission, this type of scheduling is called Opportunistic scheduling. Multiuser diversity schemes are implemented in the downlink of IS-856. Multiuser diversity makes sense only in channels with rich scattering & fast fading. Multiuser diversity is beneficial in SISO, SIMO & MISO downlink, but in MIMO downlink either using spatial diversity or spatial multiplexing, the multiuser diversity scheme doesn't yield benefits. This is because in MIMO schemes employing spatial diversity or spatial multiplexing, the channel variance decreases faster relative to the channel mean & this causes channel hardening meaning little scattering & fading in the channel.$$\gamma^*=\max_{1\leq k \leq K}\gamma_k$$ where $$\gamma_k$$ is user $$k$$ SNR.