- Buyers Guide
Introduction and Market Perspective
With declining voice revenue, but with a huge increase in demand for high speed data, cellular and wireless hotspot operators have a tremendous opportunity to develop their mobile broadband businesses. Successful execution means more than just providing high speed networks and figuring out how to best profit from them, it also means nurturing an application ecosystem, and delighting customers with attractive devices and a user experience that equals or exceeds their home broadband service.
Successive advances in cellular technology and system specifications have provided higher cell capacity and consequent improvements in single-user data rate – from 384kbit/sec in original WCDMA (3GPP release 99) to the 42Mbit/sec downlink, 11Mbit/sec uplink of High Speed Packet Access (HSPA) Dual Carrier and 150Mbit/sec downlink, 50Mbit/sec uplink of 3GPP Long Term Evolution (LTE) in 3GPP release 8, with the promise of more to come in further releases. Along with release 8, there’s a concurrent move to the Evolved Packet Core (EPC) – the simplified all-packet architecture designed specifically to improve data throughput and latency.
Yet, these improvements have produced a “chicken and egg” conundrum for mobile network operators: the more data capacity they make available, the more complex and data-hungry user equipment the device manufacturers offer, and the more sophisticated the demands of end-users become. Figure 1 shows the forecasted increase in data traffic, as presented to the LTE world summit in 2011. Finding the funding to keep improving network capacity, and ways of ensuring an acceptable revenue stream from high data users are real issues. For some operators, this means offering unlimited data plans, while others deliberately throttle back the speed available to users who exceed their monthly data allowance. Recently, one operator reported that 97% of the traffic carried on its network was data rather than voice, and both Amazon and ebay have reported huge increases in transactions originating from mobile devices.Analysts have also reported iPhone 4S owners consume more than 2.5 times as much data as iPhone 3G users, and that, while smartphone ownership enjoys double-digit year-on-year percentage growth, a mere one percent of wireless subscribers consume half of all downloaded data.
There’s a similar story with wireless LAN. By the turn of the Millenium, the first popular standards for wireless LAN (IEEE 802.11a and b, and 802.11g in 2003) were widely available, making connectivity “on the road” in airports, hotels, Internet cafes, and shopping malls a breeze. A study project gave us 802.11n in 2009, improving the maximum single-channel data rate from the 54 Mb/s of 802.11g to over 100 Mb/s, and introducing MIMO (multiple input, multiple output or spatial streaming), where up to 4 separate physical transmit and receive antennas carry independent data that is aggregated in the modulation/demodulation process. A new IEEE working group (TGac) aims to specify 802.11ac to deliver “Very High Throughput” (VHT) as an extension of 802.11n, providing a minimum of 500 Mb/s single link and 1 Gb/s overall throughput, running in the 5 GHz band. Bearing in mind the huge number of existing client devices – laptops, netbooks, tablets and smartphones – backward compatibility with existing standards using the same frequency range is a “must”. 802.11ac is scheduled to be finalized by the end of 2013, however devices complying with draft versions of the standards may appear before this.
We’re going green, so we want to spend at least part of our work week at home rather than commute. But if we’re going to work from anywhere, we want access to all our “stuff” – data, pictures, whatever – instantly. We’re already socially networked, 24 hours per day, 7 days a week. Now, we want to be able to share versions of our stuff with friends, colleagues, and customers – wherever they may be. And we don’t want to buy software applications we don’t need. Instead, we want to rent the applications we need to process our data for just as long as we need them. This is the vision of “cloud computing”, and its reality depends almost entirely on seamless high speed connectivity – via a cellular network, wireless hotspot or home broadband connection.
How does Agilent fit?
From the earliest days of wireless standards definition, Agilent Technologies has played a significant part. Working in the committees where test methodologies have been defined, Agilent has contributed test expertise and measurement science to ensure successive cellular and wireless LAN devices would meet performance and interoperability standards.
Let’s talk about 3 specific new test requirements and how Agilent is addressing them.
- Multi-standard radio
At the highest level, this is a development in cellular base stations where operators will use a single wideband power amplifier with multiple technologies – 2G, 3G and 4G – saving both initial capital and ongoing running costs. It’s estimated that over 50% of new base station installations will use this technology within the next 2 years.
Agilent’s new N7624B Signal Studio software allows users to create and transmit multi-carrier, multi -standard signals from a single signal generator. Engineers can analyze how their designs perform as they stress the amplifier with different signal combinations (for example, GSM & W-CDMA vs. GSM, LTE, W-CDMA) since they will have different peak-to-average ratio and intermodulation characteristics.
On the analysis side Agilent has two new test solutions, one for manufacturing and another for R&D. For manufacturing test, Agilent’s N9083A MSR Measurement Application uses an automatic sequencer for demodulation of these various signal types – capturing one format at a time – and eliminating the need for wide analysis bandwidth options (> 25 MHz), reducing the overall signal analyzer cost.Figure 2 shows a typical result.
For R&D, there’s a different challenge. MSR product developers face all the usual design problems of single-carrier systems, along with tremendous new potential for adverse interactions between dissimilar signals. This demands new tools providing new kinds of insights into signal characteristics. Multi-measurement capability allows users to pre-define a collection of VSA measurements, where measurements reside in memory, instantly ready to run. See Figure 3 for an example. Execution styles range from true simultaneous to fast-sequencing and powerful display tools combine and/or correlate multiple results. It’s key for developers whoneed to:
- Simultaneously verify all carriers in a multi-carrier power amplifier
- Simultaneously verify all signals in a multi-standard device
- Explore interactions between multiple transmitters in the same device
- Analyze uplink and downlink signals within a single frame, for either TDD or FDD
- Compare signals at different test points within the same signal path
- Perform several diverse measurements at once, such as measuring in-channel modulation quality while testing for spurious or harmonic emissions
- RF engineers need visibility into higher level operation
Interaction between the RF (or PHY – physical) layer andthe MAC (Medium Access Control) layer in 4G systems means that knowledge of the RF performance alone is no longer sufficient. See Figure 4.Introducing the new 89600 Wireless Link Analysis (WLA) Software! The 89600 WLA is a companion software to the 89600 VSA for RF Test, focused on helping solve problems of interactions between the MAC and PHY layers. It helps system integration engineers troubleshooting new BTS and UE designs for LTE – by decoding higher layer control messages and correlating them with the PHY layer signals they manage.
The 89600 WLA software supports both the release 8 and 9 versions of the LTE standard for both FDD and TDD formats, providing a graphical interface to give greater insight into unexpected behaviours. For example, the software provides a hybrid automatic repeat requests (HARQ) process monitor that quickly highlights if and when retransmission processes are occurring. If the system is working well, then these HARQ transmissions should cycle in a linear fashion. The 89600 WLA also provides multi-layer decoding with a built in ANSI compiler to show BTS signalling with the UE and displays CRC failures.
- Higher bandwidth test solutions for emerging wireless LAN standards
The new 802.11ac wireless LAN standard is in development today. It operates in the 5 GHz unlicensed ISM band (industrial, scientific and medical), andwill support data rates of greater than 1Gbps using advanced features over and above existing WLAN technology, but be fully interoperable with current 802.11a/b/g/n devices. Its very high throughput uses a wider RF bandwidth (up to 160 MHz), higher order modulation (up to 256 QAM) and more MIMO spatial streams than the current 802.11n.
While the standard isn’t complete, Agilent has a number of industry’s first design libraries and source and analyzer tools that can be used for system development today.
For design simulation, Agilent introduced industry’s first solution, the SystemVue WLAN library, in March 2011. To generate a contiguous 160 MHz wide signal, SystemVue works in combination with Agilent wideband arbitrary waveform generators (AWGs) and an RF signal generator. 160 MHz bandwidth signals are downloaded to wideband AWGs, and up- converted to RF using the external I/Q inputs of ESG, MXG, or PSG vector signal generators. Agilent has also introduced new 802.11ac signal creation software using its Signal Studio software. It enables the creation of 802.11ac waveforms with all modulation types and single- or multi-user MIMO of up to four streams. Generation of up to 80-MHz bandwidth signals is supported with one vector signal generator and 160-MHz bandwidth signal generation is possible using two vector signal generators.
For analysis, Agilent introduced industry’s first 802.11ac signal analysis software using 89600 VSA in March of 2011. It allows engineers to view and troubleshoot all 802.11ac bandwidths and modulation formats and up to 4x4 MIMO. It also provides flexible display for optimal viewing of MIMO results, allowing users to select from dozens of waveform, spectrum, error and summary trace types, and arrange up to 20 traces to show exactly what’s needed. See Figure 5.
89600 VSA software can work with a variety of hardware configurations to give the performance, bandwidth, and number of channels engineers need.InSeptember 2011, Agilent introduced the industry’s FIRST 160 MHz analysis bandwidth on the Agilent PXA signal analyzer.
Digital pre-distortion (or DPD) is an amplifier linearization technique to help address cost and performance issues in power amplifiers. It is widely used today in 3G and WLAN systems. The instantaneous bandwidth of the test equipment must be wide enough allow accurate characterization of the power amplifier: typically 3 to 5 times the nominal signal bandwidth – in this case up to 5 x 160 MHz or 800 MHz.Agilent’s SystemVue software provides an application that automates digital predistortion design. The software generates a stimulus waveform which is downloaded to an RF signal generator and applied to the power amplifier. The amplifier’s response is captured using 89600 VSA software,and compared with the desired signal to create the predistortion matrix. The VSA softwareworks with a variety of acquisition hardware, including Agilent oscilloscopes and digitizers that provide suitable measurement bandwidth.The predistorted signal is then sent to the power amplifier and the response checked. Figure 6 shows a typical DPD system.