FDM and CDM pilot (DM-RS) multiplexing among antennas

The link between mobile and network is referred as “Air Interface”. This interface is defined by specification bodies like 3GPP (Third Generation Partnership Project). The air interface defines basic rules or protocols for transmission and reception between mobile and networks. These rules include but not limited to: What frequency spectrum will be used, what modulation method will be used, what all stages information bits will go through before transmitted, what waveform will be used for transmission and reception. 

The “air interface” waveform for 5G-NR is based on Orthogonal Frequency Division Multiplexing (OFDM). This means frequency band over which information needs to be transmitted is firstly divided into multiple narrow bands. These narrow bands are called sub-carriers and are mutually orthogonal. Therefore, multiple modulated symbols can be transmitted in parallel on different sub-carriers.  

In 5G-NR, the smallest time-frequency resource is known as Resource element and is consist of one subcarrier in one OFDM symbol. The transmissions are scheduled in group(s) of 12 subcarriers, known as physical resource block (PRB).

A wireless “channel” is medium over which information is conveyed. This channel is unpredictable because of many factors like multipath and shadow fading, Doppler shift, and time dispersion or delay spread. These factors are all related to variability introduced by mobility of the user and the wide range of environmental conditions that are encountered as a result.

It is important that receiver has information about properties of the communication channel for reliable exchange and detection of information conveyed. These properties of the channel are estimated at the receiver. To facilitate estimation of channel, OFDM systems use reference signals (or pilot symbols). These reference signals are embedded in resource blocks. These are also known as DM-RS (Demodulation reference signal).

When M transmit and N receive antennas are deployed, dedicated pilots for each transmit antenna are required to estimate a total of MN channels. Different pilot modes possible:

• FDM (Frequency Division Multiplexing): where pilots for different antennas occupy different tones and thus orthogonal in frequency domain. Pilot sequences of different antennas can use the same Chu sequence. The length of the Chu sequence, or the number of pilot tones per antenna, is Np/M, where Np is the total number of pilot tones.

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• CDM (Code Division Multiplexing): where pilots for different antennas occupy the same time/frequency resources but separated by different codes. Orthogonal pilot sequences can be constructed from shifted versions of the same Chu sequence (frequency domain CDM), possibly with additional Walsh code separation (time domain CDM). The number of pilot tones per block for any transmit antenna is always Np. The orthogonal pilot sequences can be constructed in two different ways, Chu sequence with different shifts, or Chu sequence with Walsh separation from two reference signal blocks.

  1. Frequency domain CDM: In the first construction, the orthogonality between pilot sequences of different antennas are achieved by exploiting the properties of shifted Chu sequences.
  2. Time domain CDM: In the second approach, each entry of the Chu sequence is further modulated by antenna-dependent Walsh codes.
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Both the schemes has its own advantages and disadvantages as far as channel estimation performance in various scenarios is considered. For example for 4×4 MIMO systems:

  1. MIMO system with CDM pilots consistently outperforms that with FDM pilots.
  2. At higher speed, CDM-f4-t1 scheme outperforms others because it does not rely on the time domain coherence between two long RS to separate the antenna streams.

~~Dheeraj