09 Dec 2013

Multi Standard Radio: Analysing Interference Sources

Darren Tipton, Product Manager at Rohde and Schwarz looks at the multi-standard radios and how interference issues can be analysed.

With the advent of LTE, the communications industry is facing spectrum management challenges like never before. New generation mobile phones and other portable devices capable of transmitting and receiving GSM, WCDMA and LTE simultaneously, as well as base station infrastructure, are creating growing demand for advanced interference analysis.

The techniques that allow devices to transmit and receive in this manner are more generally referred to as Multi-Standard Radio, MSR.

A catch-all for the many challenges associated with placing several mobile communication systems side by side in a defined frequency area, multi-standard radio, MSR employs Radio Access Technology (RAT) to enable a range of signals to be transmitted from a single base station. This increases spectrum efficiency (in terms of SNR, data throughput and meeting capacity requirements) and ultimately reduces the network operator’s expansion costs.



Multi-standard radio test complexity

To illustrate the complexity of different RAT, at one point in time a base station transmitter might need to simultaneously transmit GSM signals with high power on/power off ratios, CDMA-like signals such as 3GPP WCDMA, and OFDM signals like LTE. Alone, these signals are challenging, but a common signal path, filters and power amplifiers in the chain complicates matters further.

Multi-standard radio MSR base station conformance testing is defined by the 3GPP global standardization body in 3GPP’s TS 37.141, whilst TS 37.104 describes the minimum requirements for the air interface. Notably, the TS 37.141 specification makes a distinction between multi- and single RAT operation. It defines multi-RAT operation as being when different technologies are used on different carriers in a multi standard base station; such as LTE paired with WCDMA. Multi-standard base stations can of-course also be operated using just one technology i.e. only one RAT is used, possibly in multi carrier mode, for example.

Multi-standard radio spectrum issues

Figure 1 Multi-standard radio spectrum issues

Different regulatory conditions mean that not every frequency band can be used for all standards, so the bands have been subdivided into three categories (BC1, BC2 and BC3) to define what can go where. Just four bands within BC2 allow for multi standard operation of LTE, WCDMA and GSM (LTE bands 2, 3, 5, and 8). BC1 and BC3 define simultaneous operation of LTE FDD and WCDMA, as well as LTE TDD and TD-SCDMA. Bands 17 and 18 are the exception, allowing LTE operation only, as well as band 6, which allows only WCDMA operation.

Multi-standard radio table 1

An additional requirement of TS 37.104 is that each band category has a minimum offset between the carriers and the upper and lower band edges. The offset depends on the band category, the RAT being used, and also on the channel bandwidth in the case of LTE.

Manufacturers configure a base station as single-RAT or multi-RAT according to ‘capability sets’, defined in TS 37.141. Each capability set specifies the test configurations to be used and the available band categories. To limit the number of RAT combinations in a multi standard base station, six different capability sets have been categorized. Each capability set differs as to which RAT it supports, whether it uses single-RAT or multi-RAT, and whether a single-carrier or multi carrier mode can be used.

Multi-standard radio table 2

Testing is required at the top edge, the middle and the bottom edge of the operating band for both multi-RAT and single-RAT situations.

Measurement of MSR interference

The combination of signal types and increased risk of interaction requires advanced measurement instruments, capable of analyzing problems at a higher level than ever before.

TS 37.141 provides test configurations with the goal of minimizing the complexity of the many possible test scenarios. However, these test configurations are based on the worst-case scenarios, with very strict criteria, an example being receiver tests in which two signals – a GSM carrier and an LTE carrier with a BWChannel = 5 MHz – are positioned at the lower and upper edge of BWRF while maintaining Foffset-RAT. This configuration fully utilizes the maximum bandwidth BWRF of a multi-standard base station.

To test such worst-case scenarios, traditional measurements such as frequency error, spurious emissions, out-of-band emissions, adjacent channel leakage, power ratio etc. need to be made. However, what makes MSRA measurement particularly challenging is the amount of testing required. Multiple standards means many more interactions between standards, so testing factors like intermodulation and interference from one standard to another is paramount. Speed of test becomes critical.

Identifying Signal Interactions

A key requirement for the system manufacturer is to ensure that there are no interactions between the various standards being transmitted in the band. This is a difficult task to achieve without the right tools. Due to the nature of multiple standards being transmitted simultaneously, the analyser must be able to demodulate all signals simultaneously from one single IQ capture to be able to identify from where the interference is coming from and at exactly which point in time.

The screenshot below shows how time correlation can be achieved when demodulating two different standards simultaneously. In this case, Rohde & Schwarz’ FSW signal and spectrum analyser is being used in MSRA mode and shows both GSM signal and WCDMA signal in one IQ capture. The bottom right hand graph shows a marker where the interference occurs in slot 1 of the WCDMA signal, which corresponds to the exact point at which the GSM signal is turned on. This demonstrates the power of being able to see all of the signals at the same time, something that applies to a whole host of signal types, including wireless LAN and Bluetooth and high bandwidth communications.

Multistandard radio analysis

Figure 2 Multistandard radio analysis

Conclusion

MSRA poses new challenges for signal and spectrum analyzers, both in terms of speed and the ability to measure different signals in parallel. High bandwidth, excellent dynamic range and genuine real-time capability are all ‘must-have’ features for this kind of measurement.

However, R&S®FSW’s multistandard radio analyser function meets the particular challenges of MSRA measurement head on by enabling simultaneous measurement of signals from different standards (GSM, WCDMA, LTE, etc.) at different frequencies within its 80, 160 and 320 MHz analysis bandwidth options reflecting continuously increasing demand for bandwidth.

Interaction between signals can be visualized by jointly displaying the correlated time alignment results for all demodulated signals in a capture – a unique capability among today’s signal analysers

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About the author

Darren Tipton is a Product Manager at Rohde & Schwarz UK. He received a BEng (Hons) Degree from The University of Liverpool in 2000 and started his career in RF, test development and later concentrating on field applications for mobile and base station test on GSM and WCDMA. Since 2006 he has been working for Rohde and Schwarz UK and is responsible for Spectrum Analysers, Signal Generators and Power Meters in the UK.

Rohde & Schwarz is an independent group of companies specializing in electronics. It is a leading supplier of solutions in the fields of test and measurement, broadcasting, radio monitoring and radio-location, as well as secure communications. Established more than 75 years ago, Rohde & Schwarz has a global presence and a dedicated service network in over 70 countries. It has approx. 7400 employees and achieved a net revenue of 1.2 billion (US$ 1.7 billion) in fiscal year 2008/2009 (July 2008 to June 2009). Company headquarters are in Munich, Germany.

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