Wednesday, April 20, 2016

Link Budget, Joules Budget and User Capacity IV: Receiver Sensitivity and Internet of Small Things (IoST)

There are many parameters or specifications defined for quantifying the performance of a receiver design and implementation. Among them, the most notable includes the reference sensitivity power level (REFSENS) and the Total Isotropic Sensitivity (TIS), which are widely used for specify how sensitivity a receiver is. More sensitive a receiver is, less power it requires for maintaining a reliable communication and better performance it is believed to have.

In general, REFSENS measures the performance of the receiver module with considering its down-converting performance, demodulation/decoding capability and self-generated interference/noise. Per 3GPP definition, REFSENS specifically denotes the minimum mean power applied to each applicable receive antenna port at which the throughput shall meet or exceed the requirements, which is not less than 95% of the maximum throughput of the specified reference measurement channel. See §7.3, Annexes A.2.2 (for FDD), A.2.3 (for TDD) and A.3.2 of 3GPP TS 36.101 and 3GPP TS 36.521-1 .  For example, the FDD QPSK REFSENS for a two-antenna UE operating in a 10 MHz Band 13 channel is at least -94 dBm and in a 10 MHz Band 4 channel, it is -97 dBm.  In both cases, 50 resource blocks with payload 5160 bits and 1 code block per sub-frame are allocated.  This means the corresponding peak data rate is (5160+24)/0.001 = 5.184 Mbps, the maximum achievable spectral efficiency is 5,184,000 / (180,000 x 50) = 0.58 bit/second/Hz.

From the Shannon curves plotted above, the required minimum SNR is -3.0 dB.  Therefore, considering a RFIC with a noise figure of 7.0 dB, the best REFSENS number will be
Best REFSENS = Effective_Noise_Floor + Required_SNR
=Thermal_Noise_Floor + RFIC_Noise_Figure + Required_SNR
= -104.5 dBm + 7.0 dB - 3.0 dB
= -100.5 dBm
As such, there is a margin of 6.5 dB for the band 13 and 3.5 dB for the band 4.

Further, per 3GPP TS 36.101 and 3GPP TS 36.521-1, the QPSK REFSENS for category 0 HD-FDD UE is -92.3 dBm in a 10 MHz Band 13 channel and -95.3 in a 10 MHz Band 4 channel, both with 36 RBs (i.e., 6.48 MHz) and a target coding rate of 1/10.  As such, the corresponding peak spectral efficiency is 0.16 bit/second/Hz. On the other hand, per Shannon theory, the best category 0 REFSENS number is -108.3 dBm.  The QPSK REFSENS for category M1 HD-FDD UE is -100 dBm in a 10 MHz Band 13 channel and -103 dBm in a 10 MHz Band 4 channel, both with 6 RBs (i.e., 1.08 MHz) and a target coding rate of 1/3. The corresponding peak spectral efficiency is 0.58 bit/second/Hz and the best HD-FDD category M1 QPSK REFSENS number is -109.6 dBm, which is 1.3 dB better than the category 0 using 6 times of the bandwidth.

Different from REFSENS, which quantifies the receiver sensitivity at the output of the antenna and the input to the antenna port and RF connector, TIS quantifies the sensitivity at the input to the antenna. If everything else is the same, the difference between these two is the antenna and the connector to it. Fundamentally, TIS measures the average sensitivity of a mobile device in the downlink band. It is a function of the antenna, the receiver module, and operation environment defined by the test cases. It is equal to the conducted receiver sensitivity of a receiver when this is degraded by the radiation efficiency of the antenna as well as any other disturbances guided through the antenna.  As such, per §8.2 of 3GPP TR 37.902 and §6.8 of CTIA Test Plan for Wireless Device Over-The-Air Performance, TIS similarly denotes the spatially averaged minimum RF power level resulting in a data throughput not less than 95% throughput of the maximum throughput for each test case defined for REFSENS measurements. 

However, the

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