What are the key pointers of low throughput and bad user experience in a 4G-LTE network?

There are many counters/indicators/parameters you can look at in a 4G/LTE RAN when troubleshooting low throughput and bad user experience. In no particular order, here are some of the pointers:

1. BLER (Block Error Rate) – The BLER should be less or equal to 10%. If the value is larger, then, there is an indication of bad RF environment. Typical causes of bad BLER are downlink interference, bad coverage, etc.
2. MIMO Parameters – MIMO(Multiple Input Multiple Output) it is a technique used to increase the data throughput by using multiple transmitter antenna and multiple reciever antenna. Rank Indicator (RI) is used in MIMO configurations as a measure of how well a MIMO communication works. For example, let’s assume that a UE is communicating with a eNB with 2×2 MIMO configuration and UE reports RI of ‘2’, it implies that the 2×2 MIMO is performing well. However, if UE reports RI of ‘1’, it implies that the communication is going on as if it is with single antenna. It means that the 2×2 MIMO is not performing with the best efficiency.
3. Downlink Interference (Bad CQI) – Cells with downlink interference are those whose CQI (cell quality indicators) values are low. Analyze the cell quality indicator values reported by the UE for; Transmit Diversity (typical values should be between 7 and 8), MIMO one and two layers (typical values should be between 10 and 12). If low CQI values are found after a CQI report is obtained, then downlink interference might be the cause of low throughput.
4. Uplink Interference – LTE uplink interference is caused by UEs near the cell border. When a UE is at the cell border it will need to transmit a strong signal to reach the tower. A stronger signal also includes more noise, and will thus increase the noise floor of all users inside the cell. When the noise floor (a.k.a “uplink noise rise”) increases, users closer to the cell tower will also have to transmit “louder” to reach the same cell tower. This screaming match can cause a sort of runaway feed back loop, which will directly impact the performance of the network. Uplink interference can also be caused by too much overlapping downlink coverage. The repercussions of Uplink interference include a decrease in total data throughput for the cell.
5. High VSWR – (Voltage Standing Wave Ratio) is a measure of how efficiently radio-frequency power is transmitted from a power source, through a transmission line, into a load (for example, from a power amplifier through a transmission line, to an antenna). A VSWR value under 2 is considered suitable for most antenna applications. The antenna can be described as having a “Good Match”. So when someone says that the antenna is poorly matched, very often it means that the VSWR value exceeds 2 for a frequency of interest. High values of VSWR result in low throughput due to losses.
6. LTE Cell Availability – Cell Availability identifies success rate of radio access network availability in a selected region. Ideally Cell Availability is >= 99%. In this case a Cell Availability report can help to identify; eNodeB having higher non-availability of radio network, failure reasons, etc.
7. SINR (Signal to Interference Noise Ratio) – is defined as the power of a certain signal of interest divided by the sum of the interference power (from all the other interfering signals) and the power of some background noise. “15 dB to 25 dB”: is typically considered the minimally acceptable level to establish poor connectivity. “25 dB to 40 dB”: is deemed to be good. “41 dB or higher”: is considered to be excellent.
8. RACH Failures – RACH (Random Access Channel ) is an Uplink Transport Channel which is used for initial Random Access. The major function of RACH is to allow the UEs to get Uplink Synchronization. RACH plays an important role in the transmission of Uplink Scheduling Requests. Data transmission between UE and eNodeB happens in two ways; 1) When there is a dedicated RRC (PUSCH resource available) and 2) When there is no dedicated connection (a Scheduling Request will be transmitted on RACH also known as Random Access Scheduling request). The process of accessing the network when no dedicated RRC is established or when the UE Transmits for the first time ( after power-on ) is called Random Access and the Channel that plays the major role in this aspect is called RACH channel.
9. Handover Failures – when a UE moves in LTE cellular wireless networks, it’s facing different environment and different levels of interference because the serving cell may nolonger be the optimal cell and the UE must be transferred to a new cell, this process is what is referred to as “Handover (HO)”. The serving cell is called “source cell” and the future cell is called the “target cell“. Handover failure can occur at any stage, but poor radio conditions and patchy coverage caused by fading and pathloss are the main reasons. Without handover, users will not be able to experience mobility and their connection is interrupted when exiting one cell and entering another cell. Handover Incoming Success Rate (HO IN SR) parameter is used to analyze the intra or inter-frequency Handover (HO) IN and includes both inter-eNodeB and intra-eNodeB scenarios. Look out for eNodeB in the network having low HO IN SR and cell level HO IN SR information.
10. UE Scheduling (how often is the UE scheduled?) – Scheduling in LTE is a process through which eNodeB decides which UEs should be given resources (RBs) and how much resource should be given to send or receive data. The module responsible for this is known as Scheduler. The LTE scheduler is responsible for; Link Adaptation (selects the optimal combination of parameters such as modulation, channel Coding & transmit schemes i.e. Transmission Mode TM1/TM2/TM3/TM4 as a function of the RF conditions.), Rate Control (resource allocation among radio bearers of the same UE which are available at the eNB for DL and at the UE for UL), Packet Scheduler (arbitrates access to air interface resources on 1ms-TTI basis amongst all active users), Resource Assigment (allocates air interface resources to selected active users on per “TTI – Transmission Time Interval” basis), Power Control (Provides the desired SINR level for achieving the desired data rate, but also controls the interference to the neighbouring cells) and HARQ (ARQ + FEC which allows recovering from residual errors by link adaptation)
11. RLC Retransmissions – The Radio Link Control (RLC) is a part of the LTE protocol stack in both the UE and the eNodeB which is defined on top of the Medium Access Control (MAC) layer and beneath other higher layers such as RRC (radio resource control) and PDCP (packet data convergence protocol). (RLC) layer has three functional modes: the TM (transparent mode), UM (unacknowledged mode) and
    AM (acknowledged mode). The RLC is responsible for; Segmentation, Reordering, Error correction, Protocol error detection and recovery and duplicate detection, Flow control and Retransmission (to recover from packet losses for radio beares which need error-free transmission). High RLC retransmissions is an indication of RF issues. A reasonable value of 0.5% or less should be your target.