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Difference between Antenna ports and Physical antennas | 5G NR,LTE

This page compares Antenna ports vs Physical antennas and mentions difference between Antenna ports and Physical antennas concepts introduced in LTE (i.e. 4G) and 5G NR wireless standards.

Introduction: The downlink frame from Base station to UEs in LTE and upcoming 5G NR ("New Radio") technologies are configured to use multiple antenna ports. They are used to transmit data from eNB/gNB to the UEs. These multiple antenna ports are used to achieve greater data reliability (using transmit diversity) or to increase data rate (using spatial multiplexing).

Antenna Ports

• The term "antenna port" is related to physical layer.
• It is the logical concept and not the physical one like RF antenna.
• Each antenna port represents specific and unique channel model.
• Dedicated reference signals are assigned for all the antenna ports individually, which helps in channel estimation and equalization. Reference signals are also known as pilot signals or pilot carriers.
• One resource grid is allocated per antenna port. There is one set of resource grids per transmission direction (uplink or downlink).
• There is one resource grid for a given antenna port (p) , subcarrier spacing configuration and transmission direction (downlink or uplink). Resource grid consists of number of RBs (Resource Blocks) for one subframe. Each RBs consists of 12 consecutive subcarriers in the frequency domain. Another element by name "RE" which refers to Resource Element spans 1 subcarrier on frequency domain and 1 OFDM symbol on time domain. Hence RBs will have one or many REs.

5G NR Resource Grid,Resource Block,Resource Element

• Each element in the resource grid for antenna port (p) and subcarrier spacing configuration (μ) is called a resource element and is uniquely identified by (k,l)p,μ where k is the index in the frequency domain and "l" refers to the symbol position in the time domain relative to some reference point. Resource element (k,l)p,μ corresponds to a physical resource and the complex value ak,lp,μ.
• Figure-1 depicts concept of resource grids, resource blocks and resource elements applied in LTE and 5G NR wireless systems.

Antenna ports vs Physical Antennas

Antenna ports mapping to physical antennas

• As shown in the physical layer below, antenna ports are logical ports which lies before and after precoding module.
• Number of layers in the physical layer is less than or equal to number of antenna ports.
• The figure-2 depicts mapping between antenna ports and physical antennas. One antenna port can be mapped to single physical antenna or multiple physical antennas. Hence modulated data symbols can be mapped to one or multiple physical antennas.

lte enodeb physical layer modules

• As shown in the figure-3 (derived from LTE Physical Layer >>), modulated data symbols from all the layers are combined and re-distributed to each of the antenna ports before pre-coding module.
• After precoding algorithms are applied each of the precoded data symbols goes through separate REs (Resource Elements). This is shown by multiple RE mapper modules.
• An independent resource grids are created for each of the antenna ports separately before IFFT operation during OFDM signal generations.
• Section "6.3.1.5 Precoding" in 3GPP TS 38.211 various Precoding matrix tables are mentioned. These tables are used for different combinations of (layers, antenna ports) as follows. One such table is mentioned below.
-Single layer/Two layer/Three layer/Four layer
-Two antenna ports/Four antenna ports
Note: Here for simplicity, "number of columns" refer to number of layers in a physical layer where as "number of rows" refer to number of antenna ports.

Precoding matrix for antenna ports

References:
3GPP TS 36.211 LTE
3GPP TS 38.211 5G NR (Physical channels and modulation)


5G NR Numerology | 5G NR Terminology


5G NR Control channels | 5G NR Traffic Channels | 5G NR Reference Signals and sequences

5G TECHNOLOGY RELATED LINKS

This 5G tutorial also covers following sub topics on the 5G technology:
5G basic tutorial    5G Frequency Bands    5G millimeter wave tutorial    5G mm wave frame    5G millimeter wave channel sounding    Difference between 4G and 5G    5G testing and test equipments    5G network architecture    5G NR Physical layer    5G NR MAC layer    5G NR RLC layer    5G NR PDCP layer

LTE RELATED LINKS

LTE tutorial   LTE features   LTE terminologies   LTE Frame   LTE TDD vs LTE FDD   LTE Channel types   LTE PSS SSS    LTE RS DMRS SRS    LTE PUSCH   LTE PUCCH   LTE PRACH   LTE PMCH    LTE PHICH    LTE PDSCH    LTE PDCCH   LTE PCFICH    LTE PBCH  

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