The most important challenge in wireless systems have always been faster and reliable wireless links. Recently, the need for higher data rates has grown exponentially over time leading to a problem called capacity crunch. Multi input multi output (MIMO) systems are one of the most important technologies to address this problem. Increasing the number of transmit and receive antennas to achieve higher wireless capacity has been the main trend in the wireless industry in recent years. High order MIMO and massive MIMO systems are typical examples of current MIMO technologies.
For MIMO systems to work successfully, accurate channel estimation is needed for each distinct link between receive and transmit antennas. To achieve this, a dedicated pilot signal is transmitted for each transmit antenna. As the number of transmit antennas increase, the number of pilot signals has to increase as well. This means more pilots occupy the transmit resources of the transmitter and significantly limit the transmit data rate. It is shown that the number of pilots required is a limiting factor of wireless capacity in high order or massive MIMO systems. Therefore, the problem is multiplexing as many pilots as possible into as few transmit resources as possible without creating interference between received pilot sequences. This requires design and selection of pilot sequences and their multiplexing scheme. The efficiency of such a scheme can be measured by the maximum number of pilots that can be transmitted per transmit symbol without any noticeable distortion in receiver channel estimation algorithms.
Ideally, the maximum number of pilots per transmit symbol is equal to the useful transmit symbol duration divided by the maximum channel delay spread. Note that the maximum number should be achieved without any compromise in channel estimation accuracy. In telecommunications, the delay spread is a measure of the multipath richness of a communications channel. In general, it can be interpreted as the difference between the time of arrival of the earliest significant multipath component (typically the line-of-sight component) and the time of arrival of the latest multipath components. In practice, achievable maximum number of pilots per symbol depends not only on the channel delay spread but also on the pilot sequences chosen and their multiplexing scheme.
Common sequences used as pilots for channel training include but not limited to pseudo noise like (PN), Walsh-Hadamard and perfect polyphase sequences (PPS). Another candidate is the cyclically shifted unit impulse sequence family. However, unit impulse sequence is difficult to use due to power amplifier ramp up problems and possible smoothing of the impulse function through transmitter front end processing among other problems. With the exception of the impulse sequence that is localized in time, all of the pilot sequences mentioned above are spread both in time and frequency without any localization.
In 3GPP UMTS and LTE systems, PN sequences are used for channel training in downlink such as Gold sequences. LTE uses PPS for the uplink. LTE uplink uses one transmit antenna; therefore, the maximum number of pilots per symbol is only one. In LTE downlink, there are only three pilots per OFDM symbol in average. Note that LTE pilots are staggered in time and frequency. Therefore, when calculating the maximum number of pilots per transmit symbol for LTE, the average number of distinct pilots per transmit symbol should be calculated from OFDM symbols that carry pilots within channel coherence time. For a normal cyclic prefix (CP), the channel with maximum delay spread is 5 μsec according to 3GPP standards. For a useful symbol length of 66.7 μsec this corresponds to 66.7/5˜13 pilots that can be transmitted within a symbol simultaneously. Taking into account the unused subcarriers and edge effects between transmit symbol boundaries, it realistic to assume 12 pilots per transmit symbol as a maximum goal. However, the pilots that are transmitted per OFDM symbol is only three for LTE downlink. The difference is very significant. This is due to the fact that long channel delay spreads cause highly frequency selective channels that requires more samples in the frequency domain, thereby limiting the maximum number of pilots that can be transmitted. More efficient channel training solutions are needed for future wireless systems that demand much higher number of pilots.