1.Rx Sensitivity
Receiver sensitivity is one of the most fundamental concepts, representing the minimum signal strength that a receiver can detect without exceeding a certain bit error rate (BER). The term "error rate" here is a general term carried over from the circuit-switched (CS) era, although in most cases, the metrics used to evaluate sensitivity are BER (bit error rate) or PER (packet error rate). In the LTE era, throughput is directly used to define sensitivity—because LTE no longer uses circuit-switched voice channels. This marks a real evolution since, for the first time, sensitivity is not measured with standardized substitutes like the 12.2 kbps RMC (Reference Measurement Channel, which represents a 12.2 kbps speech codec), but with throughput that users can actually experience.
2.SNR (Signal-to-Noise Ratio)
When discussing sensitivity, we often relate it to SNR (Signal-to-Noise Ratio), typically referring to the demodulation SNR of the receiver. Demodulation SNR is defined as the threshold SNR at which a demodulator can properly demodulate a signal without exceeding a certain error rate. So, where do S and N come from?
S represents the Signal, or the useful signal; N represents Noise, referring to any signal that does not carry useful information. The useful signal typically comes from the transmitter in a communication system, while noise can come from various sources, with the most typical being the famous -174 dBm/Hz, the natural noise floor. It's important to note that this is independent of the type of communication system and is derived thermodynamically (so it's related to temperature). Additionally, it's actually a noise power density (with the unit dBm/Hz), meaning the wider the bandwidth we receive, the more noise we receive. The final noise power is obtained by integrating the noise power density over the bandwidth.
3.TxPower (Transmit Power)
Transmit power is crucial because the signal from the transmitter needs to travel through space and undergo attenuation before it reaches the receiver. Higher transmit power means a longer communication range.
Do we need to consider SNR for our transmitted signal? For instance, if our transmitted signal has a poor SNR, will the received signal also have a poor SNR?
This relates to the concept of natural noise floor. Assuming that space loss affects both signal and noise equally (which is not exactly true, as signals can resist fading through encoding, while noise cannot), if we assume space loss to be -200 dB, transmit signal bandwidth 1 Hz, power 50 dBm, and SNR 50 dB, what is the received signal's SNR?
The received signal's power would be 50 - 200 = -150 dBm (for 1 Hz bandwidth), while the transmit noise would be 50 - 50 = 0 dBm, and the noise received by the receiver would be 0 - 200 = -200 dBm. This noise is already "drowned" beneath the -174 dBm/Hz natural noise floor, so when calculating the receiver's noise, we only need to consider the "basic component" of -174 dBm/Hz. This principle holds for most communication systems.
4.ACLR/ACPR (Adjacent Channel Leakage Ratio/Adjacent Channel Power Ratio)
These metrics represent a part of the "transmitter noise" but refer to leakage into adjacent channels rather than the main transmission channel. We can collectively call this "Adjacent Channel Leakage."
ACLR and ACPR are essentially the same metric, with different terminology depending on whether it’s a terminal test or a base station test. Both terms refer to the interference a transmitter causes to other devices, specifically to nearby channels. They share a common feature in that the power calculation for interference signals is based on the bandwidth of the channel. This metric is designed to assess the impact of transmitter leakage on receivers of similar or identical systems.
In LTE, ACLR testing is set up in two modes: EUTRA and UTRA. The former measures interference from an LTE system to another LTE system, while the latter measures interference from LTE to a UMTS system. Therefore, we see different measurement bandwidths for EUTRA ACLR (based on LTE RB bandwidth) and UTRA ACLR (based on UMTS system bandwidth).
5.Modulation Spectrum/Switching Spectrum
In GSM systems, the Modulation Spectrum and Switching Spectrum play a similar role to adjacent channel leakage, but their measurement bandwidths are not the same as the occupied bandwidth of GSM signals.
Modulation Spectrum typically measures interference between synchronized systems, while Switching Spectrum measures interference between asynchronous systems (although, if not gated, switching spectrum will overshadow modulation spectrum).
GSM cells are asynchronous, meaning that the rise/fall of power in one cell may spill into the payload of another cell. This is why Switching Spectrum is used to assess transmitter interference in such scenarios. In contrast, in the GSM timeslot, these rise/fall transitions are brief, so most of the time, adjacent cell payloads will overlap temporally, and the Modulation Spectrum can be used to assess interference.
6.SEM (Spectrum Emission Mask)
SEM is a "within-channel" metric and should be distinguished from spurious emissions, which refer to signals outside the operational frequency band. SEM is more focused on emissions within the transmitter's operating frequency range.
SEM provides a "spectrum template," and when measuring emissions within the transmitter’s band, it checks if any points exceed the template's limit. SEM is related to ACLR but differs in that ACLR looks at average leakage power into adjacent channels, whereas SEM captures over-limit points in adjacent bands using a smaller measurement bandwidth (usually 100 kHz to 1 MHz).
7.EVM (Error Vector Magnitude)
EVM is a vector value, meaning it has both magnitude and phase. It measures the error between the actual transmitted signal and the ideal signal, which effectively indicates the "quality" of the transmitted signal. The larger the distance between the actual and ideal signal points, the greater the error, and the higher the EVM magnitude.
As discussed earlier, the SNR of the transmitted signal is often much higher than what the receiver needs. However, in some cases, such as short-range wireless communication (e.g., 802.11), the inherent SNR of the transmitter must be considered.
As systems evolve (like in 802.11ac, which introduced 256-QAM modulation), the need for a higher SNR becomes more critical. EVM is used by engineers in 802.11 systems to measure transmitter linearity, while engineers in 3GPP systems focus on metrics like ACLR/ACPR/Spectrum.
7.1 EVM and ACLR/ACLR Relationship
It’s hard to define a direct quantitative relationship between EVM and ACLR/ACPR. Amplifier nonlinearity can increase both EVM and ACLR/ACPR, but they do not always correlate. For example, clipping, a common method in digital intermediate frequencies, can reduce ACLR/ACPR by lowering peak-to-average ratio (PAR) but will degrade EVM.
7.2 PAR (Peak-to-Average Power Ratio)
PAR is typically expressed using the CCDF (Complementary Cumulative Distribution Function), which shows the probability of a signal exceeding a certain power level. PAR is crucial for modern communication systems, as high peak powers can push amplifiers into non-linear regions, creating distortion.
In the GSM era, due to the GMSK modulation’s constant envelope, PAR was zero. With EDGE and 8PSK, PAR increased, and amplifiers had to be designed differently to handle it. LTE systems manage PAR through SC-FDMA to reduce burstiness in the signal.
8.Interference Metrics Summary
The "interference metrics" refer to sensitivity tests under various interference conditions, beyond just the receiver's static sensitivity. Common interference metrics include Blocking, Desense, and Channel Selectivity.
8.1 Blocking
Blocking is an ancient RF metric, originally introduced during radar development. It refers to a large signal entering the receiver (typically the first LNA), causing the amplifier to enter a nonlinear or saturated state, which severely degrades the amplification of useful signals.
8.2 AM Suppression
AM Suppression is unique to GSM systems and refers to interference from a signal similar to a TDMA signal, synchronized with the GSM signal and with a fixed delay. It’s an indicator of the receiver's ability to tolerate interference from neighboring cells in a GSM network.
8.3 Adjacent (Alternative) Channel Suppression (Selectivity)
This metric refers to the receiver's ability to reject interference from adjacent or alternative channels. In cellular systems, adjacent channel interference arises from transmitter leakage into neighboring frequencies, and selectivity ensures that the receiver can distinguish between valid signals and interference.
8.4 Co-Channel Suppression (Selectivity)
This refers to interference from the same frequency channels, typically between co-located cells using the same frequency. A key factor in this is how well the receiver can distinguish signals from the same frequency channel and avoid cross-talk.
Summary
Blocking involves large signal interference with smaller signals, while AM Suppression, Adjacent Channel Selectivity, and Co-Channel Selectivity deal with small signal interference with larger signals. While RF itself can provide some mitigation, physical-layer algorithms are more crucial for addressing these issues.