LTE and 5G - Radio Frequency Planning and Optimization

[Signalian]

I will start with discussing the basic parameters of LTE and 5G cellular networks and later discuss RF troubleshooting cases for cellular networks.

eNodeB/gNodeB

What is the tower called ? eNodeB or eNB. It has a site ID. It was called BTS in 2G. Basically this is the site tower which has antennas, cables, feeders and BTS installed on it. It can be a GF (green field) site of RT (Roof top). The mobile phone connects to eNB to initiate the call process. The mobile phone is technically called UE (user equipment)

In 5G NR, cellular site/tower its called gNodeB (gNb). In 3G, the cellular tower was called NodeB.
Site ID can be PK1234, PK 2343 etc.

Physical layer Cell ID (PCI)

PCI in LTE is physical layer Cell Identifier. PCI is used for Downlink Synchronization including;

1. Frequency Synchronization
2. Timing Synchronization
3. Cell ID Detection

PCI includes two signals i.e., Primary Synchronization Signal and Secondary Synchronization Signal. (PSS and SSS).
PCI is calculated using the formula, PCI = (3 × SSS) + PSS

In ,LTE 4G:
PSS has 3 values i.e., 0,1,2.
SSS has 168 values i.e., 0 to 167

PCI = (3 * 167) + 2 = 503
So PCI values will vary from 0 to 503. LTE supports 504 unique PCIs

In, 5GNR:
PSS has 3 values i.e., 0,1,2.
SSS has 336 values i.e., 0 to 335.
In 5G-NR, the basic structure of PSS is the same but the number of SSS are increased
PCI = (3 * 335) + 2 = 1007
So PCI values will vary from 0 to 1007. 5GNR supports 1008 unique PCIs so 5G NR has double of the PCI’s compared to LTE 4G.

Each sector has different PCI value.
PCI Planning is itself a major topic and will discuss later.

RSRP

RSRP is Reference Signal Received Power is an RSSI type of measurement. It is the power of the LTE Reference Signals spread over the full bandwidth and narrowband. A minimum of -20 dB SINR (of the S-Synch channel) is needed to detect RSRP

RSRQ

RSRQ is Reference Signal Received Quality: Quality considering also RSSI and the number of used Resource Blocks (N) RSRQ = (N * RSRP) / RSSI measured over the same bandwidth. RSRQ is a C/I type of measurement and it indicates the quality of the received reference signal. The RSRQ measurement provides additional information when RSRP is not sufficient to make a reliable handover or cell reselection decision.

SINR/SNR

SINR- The signal-to-noise ratio of the given signal. SNR is defined as the ratio of signal power to the noise power, often expressed in decibels.
The Required SNR means that minimum level of SNR required to decode the type of LTE signal. LTE signal could be of different Code rate / Modulation etc.

RSSI

RSSI (Received Signal Strength Indicator) is a parameter which provides information about total received wide-band power (measure in all symbols) including all interference and thermal noise. RSSI is not reported to eNodeB by UE.

CQI

In the LTE system, the CQI (Channel Quality Indication) is used by the mobile to indicate the channel quality to the eNB. The CQI reported value is between 0 and 15. This indicates the level of modulation and coding the UE could operate

 

[jamahir]

Thanks. Bookmarked.

 

[Signalian]

Coverage Issue​

Verify UE’s (Mobile) RSRP from log.

If UE’s RSRP is bad, then Throughput is bad. This means UE is in bad coverage. UE which is either:
1. Far from site or
2. Deep indoor will have lower RSRP thus low throughput.

Troubleshooting:
1. Up tilt (electrical or mechanical) antenna for better coverage, basic step as of 2G.
2. Increase power of Cell eNb/gNb
3. Conduct analysis if its feasible to add a sector or site, like if this area is in a null.
4. Check if UE can be sent to a lower coverage layer. Say if UE is camped on 2600 or 3500 band, It can be shifted to 1800 or 800 band for better coverage. (This is the mobility strategy)

Interference or SINR Issue​

Basic observations from Graphs/Logs:

1. SINR decreases, Throughput decreases
2. Low RSRP (low coverage) means low SINR.
3. Higher Interference also means low SINR.
4. This leads to lower CQI which in turn leads to lower MCS (Modulation and coding schemes e.g. QAM to QPSK)
The CQI is a 4-bit integer and is based on the received signal-to-interference-plus-noise ratio (SINR) at the UE.

During cluster drive tests, if a Cluster has more interference so it will have lower SINR, lower CQI, lower MCS and lower throughput. The reason could be that Cluster may have more load, more traffic which will increase RSSI, increases interference.

Troubleshooting:

1. Down tilt certain sites to reduce noise flow and improve SINR however it will impact coverage.

2. Use features like “lean carrier Structure” to help with interference scenario. It is an LTE carrier with minimized control channel overhead and cell-specific reference signals. The Lean Carrier can enhance spectral efficiency, increase spectrum flexibility, and reduce energy consumption.

 

[Signalian]

LTE Physical Channels and Signals

Down link is from the tower to the cell, while the up link is from the cell/laptop to the tower.

Physical channels correspond to sets of time-frequency resources used for transmission of particular transport channel data, control information, or indicator information.

I think I personally miss the BCCH and TCH channels of 2G, but that is long gone in modern world. PDSCH in LTE was what TCH was in 2G.

 

[Signalian]

An overview of the channels (Uplink and Downlink)