Many research activities on massive MIMO-OFDM systems consider perfect or good enough knowledge of the channel matrix for each narrow-band sub-channel. However, for fast time-variant channel, the duration of the channel sounding increases. Due to the huge amount of information to be obtained, the duration of data transmission decreases. A low coherence time of massive MIMO channels could limit the number of antennas of these systems?
Channel estimation is indeed an important issue. One of the key benefits of Massive MIMO, as compared to conventional multi-user MIMO, is that the channel estimation is implemented in a clever way. Time-division duplex, uplink pilots and channel reciprocity are exploited to make the estimation overhead independent of the number of antennas. So to answer your question, the coherence time of the massive MIMO will not limited the number of antennas.
But the channel coherence is certainly important in order to determine how often we need to estimate the channel, over time and frequency. The number of channel uses per coherence block is the product of the coherence time and the coherence bandwidth. For example, 1 ms x 200 kHz gives you 200 channel uses.
Channel estimation is indeed an important issue. One of the key benefits of Massive MIMO, as compared to conventional multi-user MIMO, is that the channel estimation is implemented in a clever way. Time-division duplex, uplink pilots and channel reciprocity are exploited to make the estimation overhead independent of the number of antennas. So to answer your question, the coherence time of the massive MIMO will not limited the number of antennas.
But the channel coherence is certainly important in order to determine how often we need to estimate the channel, over time and frequency. The number of channel uses per coherence block is the product of the coherence time and the coherence bandwidth. For example, 1 ms x 200 kHz gives you 200 channel uses.
Dear, As Emil mentioned, the coherence time of the massive MIMO will not limited the number of antennas. As you know, In Massive MIMO system, M is large (Important: M ≫ K (number of users) ), and hence, TDD operation is preferable. For example, assume that the coherence interval is T= 200 symbols (corresponding to a coherence bandwidth of 200 kHz and a coherence time of 1 ms). Then , In TDD systems, the constraint on M and K is 2K
Thank you both for your helpful answers.
However, for the practical channel estimation, it seems to me that one-antenna user-equipments must be synchronized: the first user-equipment must send the pilots for channel sounding, after that the second user-equipment must send its pilots, etc. It is possible that several user-equipments send different pilots together. In a practical situation, if the number of users is not constant, this must create some synchronization difficulties.
Another limitation is the coherence time.The channel sounding takes time. The maximum duration of the channel sounding depends on the maximum number of users. For fast propagation channels, the coherence time decreases. If the coherence time is similar to the maximum duration of the channel sounding, the effective data rate is significantly reduced. Am I wrong ?
Dear,
In the massive MIMO context, the TDD assumption is mainly enforced to invoke channel reciprocity in order to simplify channel state information (CSI) estimation at the base station. Acquisition of the CSI fundamentally limits the capacity of any precoder. A system with M antennas that serves K user terminals (UTs) requires CSI between every base station antenna and UT (herein we consider one antenna per UT). Critically, all M*K channel links must be measured within a time span much shorter than the channel coherence time in order to be useful for the subsequent precoding stage. The coherence time of a wireless channel depends on how quickly the terminal speed and environment changes. According to 3GPP Release 9, this is typically on the order of a few milliseconds, but can drop below 500 μs.
For more information, check this link:
http://www.nutaq.com/blog/massive-mimo-–-part-3-capacity-coherence-time-and-hardware-capability
Gheorghe Zaharia: When the terminal connects to the network, the base station will help it synchronize itself to the base station and the use will be allocated a pilot sequence that it can use for channel estimation.
In a practical standard, the number of pilot sequences will be fixed, irrespective of how many users that are active for the moment. The size of the coherence interval will also be fixed, and thereby define the maximum velocity that the users may have.
If there are more users than pilots, they cannot all be active in every coherence interval. This will not be a big issue, because in a coherence interval of 200 symbols we can easily allocate 50 symbols for pilots. Hence, one can serve 50 users per cell, and still let everyone be active in every coherence interval. 100 users can also be handled, if you place 50 users in one half of the bandwidth and 50 users in the other half.
In today's cellular networks there is on average one user active at a time, so there is plenty of room for new users.
Thank you again for your helpful answers. Just one more question: which could be the future of differential MIMO-OFDM systems? Avoiding the knowledge of CSI would allow a continuous data transmission. Their performance could be similar to MIMO-OFDM systems using CSI, especially when the number of antennas used by the BS increases?
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