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Why we care about Residual EVM of a Signal Analyzer (802.11ac)

Error Vector Magnitude (EVM) is a measure used to quantify the quality or performance of a modulated signal from a transmitter or receiver. In simple terms, if we consider a constellation diagram the EVM is the magnitude of the difference between the measured vector and the ideal (reference) vector. This can be visualized as below.

It can be seen from this simple diagram that EVM is influenced by a number of parameters such as below:

• Phase Error
• Frequency Error
• Magnitude Error
• Noise that contributes to all of the above

Each of these areas are contributed to not only by the signal being measured, but also by the test instrument itself which has an effect on how well it can capture the signal, but also how it is able to generate an “ideal” reference signal to use for the calculation. If we take a look at this block diagram which shows a model of transmitter EVM contributions such errors also exist during the demodulation process. Therefor the limit of EVM demodulation performance can only be as good as the error contributions added during the demodulation process in the signal analyzer.

Many of the effects we are able to correct for in DSP as part of signal synchronization etc. Although phase noise is not so easy to correct for and has a direct impact on performance.

Below are some graphics showing a visual representation of the effects of the distortions described in the model on the constellation diagram.

EVM Performance when measuring the new 802.11ac 80MHz standard 256QAM

Below are two graphics showing screenshots taken from an R&S Vector Signal Analyzer on an 802.11ac signal with a specific EVM of the transmit signal. The good signal has 33dB of EVM and the bad signal only 30dB of EVM performance. It is clear to see on the bad signal that some symbols (constellation points) are much further away from the ideal point which would result in more error and a poorer EVM result.

Instruments with a worse EVM performance will contribute directly to this error and on signals over a wide bandwidth and such high order modulation schemes (256QAM / 1024QAM etc) EVM performance and phase noise performance of the test instrument becomes critical to measuring such parameters.

Graphics courtesy Rohde & Schwarz FSV-K70 user manual and some very helpful colleagues relating to the EVM plots above.

Posted in Test and Measurement, Tutorials, Wireless Technology

WiFi – Analysis of 802.11ac Wireless LAN signals with an OFDM VSA

The evolution of standards is fast paced. Wireless LAN A,B, G and N are in use and the market is now looking for more data rate, or shifts in where the channels can be situated.

There is very little spectrum left (if any) to slot in new channels, so the industry must think more about how space between channels or channels available from previously used technologies may be used. The 802.11af standard intends to do this by placing WiFi channels in TV white space. This is bandwidth left in between allocated channels to help avoid interference.

There is often the possibility of increasing the usage of currently allocated bands by increasing the bandwidth and altering the modulation schemes used by systems already in operation. This brings us to the next generation of Wireless LAN technology, 802.11ac.

Introducing 802.11ac

The 802.11ac standard aims to increase data rates to the region of 1Gbit/s by using wider channels compared to 802.11n such as 80MHz and 160MHz channels. Up to 8 MIMO spatial streams may be possible and includes higher order modulation such as 256QAM and will be situated in the 5GHz band.

Continue reading » » WiFi – Analysis of 802.11ac Wireless LAN signals with an OFDM VSA

Posted in Test and Measurement, Wireless Technology

Rohde and Schwarz – LTE Tutorial

In April 2009, Christina Gessner, Technology Manager from Rohde and Schwarz presented an online webinar discussing LTE technology and test equipment. This LTE Tutorial gives an overview of LTE technology including:

• LTE Technology Basics
• Key parameters
  • OFDMA and downlink frame structure
  • SC-FDMA and uplink frame structure
  • Network and protocol architecture
  • LTE UE categories
• Radio procedures
  • Cell search
  • System information broadcast
  • Random access
  • EPS bearer setup
  • Downlink and uplink data transmission
  • Mobility
  • MIMO

Rohde-Schwarz.com

Continue reading » » Rohde and Schwarz – LTE Tutorial

Posted in Test and Measurement, Tutorials, Wireless Technology

Rohde & Schwarz UK offers FREE full day LTE technical seminar

Rohde & Schwarz are holding a full-day LTE seminar in 3 different locations in November. The seminars will introduce LTE and LTE Advanced technology and explain the technical challenges when implementing chipset, UE and network equipment.

Whether new to LTE or experienced in the technology, you should get something useful from the day. The seminar will be interactive and address the following areas:

• Radio Link Aspects of LTE
• OFDMA Multiple Access Schemes
• MIMO Antenna Systems
• SC-FDMA Multiple Access Schemes
• Physical Layer Aspects of LTE
• Physical Channels in LTE
• Power Control in LTE
• Random Access Procedures in LTE
• TDD Mode in LTE
• LTE Protocol Aspects
• LTE Security Aspects
• Introduction to LTE Advanced

Continue reading » » Rohde & Schwarz UK offers FREE full day LTE technical seminar

Posted in Events & Conferences, RF and Microwave News, Wireless Technology

Welcome to TestRF.com

Welcome to my T&M Blog – TestRF.com. Just a little somewhere to post and share information I find useful relating to my day to day activities.

My name is Darren Tipton and I’m one of several Rohde and Schwarz Product Engineers / Applications Engineers based in the UK having over 10 years experience in the T&M industry working in RF and Microwave along with cellular (GSM, WCDMA, LTE) and non-cellular communications technologies (Wireless LAN, WiMAX etc). I mainly work with Spectrum / Signal Analysis (swept and real time), Signal Generators and Power Meters but have been known to get involved with other interesting stuff.

If I post something here that you find helpful in your work with test and measurement equipment that’s great but you should be aware that this is my personal T&M blog. I hope to not post anything that’s too stupid – but it could happen! The views expressed on these pages are mine alone and not those of my employer.

Posted in Wireless Technology