Spectral Efficiency and mmWave Bandpass Filter Temperature Stability.

Posted by Peter Matthews on Oct 12, 2018 12:06:33 PM
Peter Matthews

In an earlier Blog post we discussed the Shannon-Hartley Theorem in the context of 5G mmWave applications:

shannon-hartley theorem

Where:

C= Channel Capacity in bits/second

B = Bandwidth in hertz

S = transmit power, in Watts

N = noise on channel, in Watts

S/N = Signal to noise ratio

Now from this it is possible to ask: How many bits/second can we expect get for each chunk of Bandwidth?

The Shannon-Hartley bound says that the error free bit rate (Rb) must be less than the channel capacity:

Shannon Hartley bound

We can divide our bit rate Rb by the bandwidth B to get a definition of the bounds of Spectral Efficiency:

 spectral efficiency

Where:

η = Spectral Efficiency, Rb/B, in bits per second per Hertz

Eb/N0 = The ratio of energy per bit to noise power spectral density ratio, or normalized signal to noise ratio, where:

EbNo

Spectral efficiency, or bandwidth efficiency, tells us about the channel capacity over a 1Hz bandwidth. It is a measure of the efficiency of a physical layer protocol when it comes to utilizing the spectrum available. High spectral efficiencies come from innovative use of tools like modulation schemes and MIMO.

Wikipedia currently has a nice summary table of the Spectral efficiencies of common communication systems that is interesting to look at for historical context. 1G cellular, for example, exhibited a spectral efficiency of 0.45 bps/Hz, circa 1981 whereas LTE-Advanced (2013) can reach 30 bps/Hz with 8x8 MIMO.

5G is looking to make even more dramatic advances in the realm of spectral efficiency. In 2016 for example a team at the University of Bristol set a rate of 145.6 bps/Hz below 6GHz and a Nokia Bell Labs team reported an efficiency of 100bps/Hz at 28GHz.

At 500MHz bandwidth 100bps/Hz represents a data rate of 50Gbps, suggesting that the stated goal for 5G of 20Gbps is well within reach for mmWave systems. It should be noted though that not all of the 500MHz would be available for data – there are things like system overhead to take into account as well as system bandwidth tolerances that would eat away at this blazing 50Gbps.

This leads to an interesting question about how we should specify RF filters for such systems. Given that spectral efficiencies continue to increase, an incredible amount of data will packed into a humble 1Hz bandwidth and perhaps even up at mmWave frequencies, where bandwidth seems to be plentiful, the hard fought gains brought about by such advances in spectral efficiency should not be squandered by wasting bandwidth. We should begin to think about how much bandwidth we can tolerate losing to temperature variation, for example, if each MHz in bandwidth carries with it 100 Mbps.

At Knowles Precision Devices, DLI we have looked closely at Microstrip filter temperature stability and one key variable is the substrate material. Take for example this comparison between two 18GHz band-pass filter designs, one lot manufactured on our ‘CF’ substrate and the other on an Alumina board. We measured the filter response from -55°C to +125°C:

Figure 1.Response of Microstrip Bandpass Filters on Alumina.

alumina

Figure 2. Response of Microstrip Bandpass Filters on CF Dielectric

cf_dielectric

On the high side of the Alumina based filter we saw a 300MHz shift and on the low side a shift of 140MHz. A spectral efficiencies in the range of 100bps/Hz a shift in bandwidth of the order of just 10MHz could equate to 1Gbps of channel capacity, something that the data-rate focused world of 5G may find hard to tolerate.

Dealing with such high spectral efficiencies up in the mmWave range brings with it new concerns about the way in which an RF systems bandwidth responds to changes in temperature, for example. Talk to a DLI RF filter design expert at Knowles Precision Devices to ensure the substrate technology used to implement your filter structures is right for your application.

Additional resources are available to learn  more about Knowles Precision Devices DLI brand 5G solutions and our mmWave filters and microstrip technology.

Topics: mmWave, Filter, 5G

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