# Optimized DM-RS Configuration for Improved 5G New Radio Network Capacity and Performance.
## 3 Theoretical Background
### 5G/NR Radio Interface and DMRS Configuration  
Radio interface in 5G NR systems defines a number of reference signals that are used by the physical layer; however, unlike physical channels, they do not carry information originated by higher layers. The main purpose of these signals is related to channel status reporting and estimation and coherent demodulation. Unlike LTE, which relies on always-on cell-specific reference signals, 5G NR uses different reference signals for different purposes, which include demodulation reference signals (DM-RSs), phase-tracking reference signals (PT-RSs), CSI reference signals (CSI-RSs), tracking reference signals (TRSs) and sounding reference signal (SRSs). Demodulation reference signals are intended for channel estimation and equalization for better decoding and are included in the downlink channels _Physical Downlink Control Channel_ (PDCCH), _Physical Downlink Shared Channel_ (PDSCH) and _Physical Broadcast Channel_ (PBCH), where DM-RS is configured individually for each physical channel. When it comes to PDSCH DM-RS, which is the focus of this research as it has a major impact on system performance and capacity, the main parameters of its configuration defined by 3 GPP standard are dmrs-Type, dmrs-maxLength and dmrs-AdditionalPosition, which are provided by higher layers via DMRS-DownlinkConfig message. To achieve low latency, DM-RS is located early in the transmission, as the first OFDM symbol immediately after the PDCCH, which allows the receiver to obtain a channel estimate early. The parameter dmrs-Type defines whether Type 1 (DMRS Mapping Type A) or Type 2 (DMRS Mapping Type B) is used (Figure 2). The main difference is that in Type1, DMRS is configured on six out of twelve resource elements in the frequency domain during one symbol, or 50% of resources, and the used resource elements are non-contiguous. On the other hand, with Type 2, four out of twelve resource elements, or 33% of resources in that symbol, are used as two pairs of adjacent resource elements. The start position may vary for different antenna ports, to obtain some kind of orthogonality. Most of the 5G solutions available today have a configuration with predefined DMRS Mapping Type A.  
Figure 2: DM-RS configuration for different mapping types. Another important parameter is dmrs-maxLength, which defines the length of the DM-RS signal in the time domain and allows values of one or two consecutive symbols. The idea of deploying DM-RS length longer than one symbol is to allocate more DM-RS resources to support the multi-user MIMO (MU-MIMO) scenario, as a single symbol can support up to four antenna ports when DMRS Mapping Type A is deployed or six antenna ports in case of DMRS Mapping Type B. Adding an additional symbol for DM-RS doubles support for antenna ports to eight and twelve, respectively. Equipment manufacturers typically enable a configuration where the allocation of additional symbols happens dynamically, in accordance with needs, thus avoiding additional unnecessary overhead. The third parameter, dmrs-AdditionalPosition, defines additional symbols to be used for DM-RS in the slot, to cope better with challenging channel conditions caused by Doppler frequency shift primarily for high-speed using mobiles. Parameter dmrs-AdditionalPosition allows setting values of zero, one, two or three. Setting higher values sacrifices peak data throughput and capacity in favor of system robustness. A summary and overview of the impact of various parameter settings on DM-RS configuration are presented in Figure 3 through several examples. Non-optimal configuration of the parameter dmrs-AdditionalPosition may cause significant overhead increase and affect spectral efficiency; hence, it will be the main subject of optimization. On the other hand, the reduced number of symbols and insufficient density of DM-RS signals may compromise the quality of the connection for users who move rapidly.