Evolution of Cellular Systems 23
1.7.1 Key Technologies of 4G
1.7.1.1 Enhanced MIMO
Multiple‐Input Multiple‐Output (MIMO) is a key technique in the modern cellular
system, which refers to the use of multiple antennas at both the transmitter and receiver
sides. Therefore, base stations and terminals are equipped with multiple antenna elements intended to be used in transmission and reception to make MIMO capabilities
available at both the downlink and the uplink.
Enhanced MIMO is considered as one of the main characteristics of LTE‐Advanced
that will allow the system to meet the IMT‐Advanced rate requirements recognized by
the ITU‐R. The majority of the MIMO technologies already presented in LTE are
expected to remain playing a vital role in LTE‐Advanced, namely beamforming, spatial
multiplexing and spatial diversity. However, further improvements in peak, cell‐average
and cell‐edge throughput need to be obtained to significantly increase performance.
The above‐mentioned techniques need some level of channel state information (CSI)
at the base station, so that the system can adjust to the radio channel conditions and
substantial performance improvement can be attained. For TDD systems, this information is easily collected from the uplink, provided the channel fading is adequately slow,
due to the fact that the same carrier frequency is used for transmission and reception.
Again, due to the asymmetry of FDD systems, feedback information over the reverse
link is required. Full CSI could cause an additional overhead that might be too much, so
quantization or statistical CSI are preferable in practice. In addition, terminal mobility
can pose serious difficulties to the system performance, as the channel information
arriving at the eNB may be outdated.
Multi‐antenna techniques in a multi‐user situation has the role of delivering streams
of data in a spatially multiplexed fashion to the different users in such a way that all
the degrees of freedom of a MIMO system are to be used. The idea is to perform an
intelligent Space‐Division Multiple Access (SDMA), so that the radiation pattern of the
base station is adapted to each user to obtain the highest possible gain in the direction
of that user.
1.7.1.2 Cooperative Multipoint Transmission and Reception for LTE‐Advanced
4G cellular networks have to instantaneously provide a large number of diverse users
with very high data rates, and the capacity of the new radio access systems needs to be
enlarged. Conventionally, in cellular systems, each user is allocated to a base station on
the basis of principles such as signal strength. At the terminal side, all the signals arriving
from the rest of the base stations in the form of interference radically limit the
performance. The user also connects with a single serving base station while causing
interference to the rest of them. Due to the interference limitation of cellular systems,
the task of high data delivery cannot be accomplished by simply increasing the signal
power of the transmission. Each base station processes in‐cell users independently, and
the rest of the users are seen as inter‐cell interference whose transmission power would
also be increased.
CoMP in the framework of LTE‐Advanced involves several likely coordinating
schemes among the access points. Coordinated beamforming/scheduling is a simpler
method, where user data are transmitted only from a single cell. Joint processing
techniques require multiple nodes to transmit user data to the UE. Two approaches are
1.7.1 Key Technologies of 4G
1.7.1.1 Enhanced MIMO
Multiple‐Input Multiple‐Output (MIMO) is a key technique in the modern cellular
system, which refers to the use of multiple antennas at both the transmitter and receiver
sides. Therefore, base stations and terminals are equipped with multiple antenna elements intended to be used in transmission and reception to make MIMO capabilities
available at both the downlink and the uplink.
Enhanced MIMO is considered as one of the main characteristics of LTE‐Advanced
that will allow the system to meet the IMT‐Advanced rate requirements recognized by
the ITU‐R. The majority of the MIMO technologies already presented in LTE are
expected to remain playing a vital role in LTE‐Advanced, namely beamforming, spatial
multiplexing and spatial diversity. However, further improvements in peak, cell‐average
and cell‐edge throughput need to be obtained to significantly increase performance.
The above‐mentioned techniques need some level of channel state information (CSI)
at the base station, so that the system can adjust to the radio channel conditions and
substantial performance improvement can be attained. For TDD systems, this information is easily collected from the uplink, provided the channel fading is adequately slow,
due to the fact that the same carrier frequency is used for transmission and reception.
Again, due to the asymmetry of FDD systems, feedback information over the reverse
link is required. Full CSI could cause an additional overhead that might be too much, so
quantization or statistical CSI are preferable in practice. In addition, terminal mobility
can pose serious difficulties to the system performance, as the channel information
arriving at the eNB may be outdated.
Multi‐antenna techniques in a multi‐user situation has the role of delivering streams
of data in a spatially multiplexed fashion to the different users in such a way that all
the degrees of freedom of a MIMO system are to be used. The idea is to perform an
intelligent Space‐Division Multiple Access (SDMA), so that the radiation pattern of the
base station is adapted to each user to obtain the highest possible gain in the direction
of that user.
1.7.1.2 Cooperative Multipoint Transmission and Reception for LTE‐Advanced
4G cellular networks have to instantaneously provide a large number of diverse users
with very high data rates, and the capacity of the new radio access systems needs to be
enlarged. Conventionally, in cellular systems, each user is allocated to a base station on
the basis of principles such as signal strength. At the terminal side, all the signals arriving
from the rest of the base stations in the form of interference radically limit the
performance. The user also connects with a single serving base station while causing
interference to the rest of them. Due to the interference limitation of cellular systems,
the task of high data delivery cannot be accomplished by simply increasing the signal
power of the transmission. Each base station processes in‐cell users independently, and
the rest of the users are seen as inter‐cell interference whose transmission power would
also be increased.
CoMP in the framework of LTE‐Advanced involves several likely coordinating
schemes among the access points. Coordinated beamforming/scheduling is a simpler
method, where user data are transmitted only from a single cell. Joint processing
techniques require multiple nodes to transmit user data to the UE. Two approaches are