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    The 4G test headache


    Innovation is unceasing, says Robert Green, but wider band-widths, faster transmission speeds and multiple protocols means developers face an increasingly heavy testing load


    Fourth generation (4G) technology is in the laboratory now and in field trials in certain areas of the world. Some people define the 4G goal as increasing data transfer rates to 100Mb/sec. Recently, NTT DoCoMo, the Japanese telecommunications giant and Japan's largest wireless carrier, has claimed to achieve a maximum packet transmission rate of approximately 5Gb/sec in a downlink transmission. The transmission used a 100MHz channel bandwidth and the target receiving device was a mobile device moving at 10km/hour. Since the maximum transmission rates closest to commercialization today are approaching 10Mb/sec, NTT's work is more like 5G technology.

    But more innovation is always on the horizon. Industry experts believe the next generation mobile phones will include navigational aids, enable purchasing, provide complete television programming, offer more innovative gaming for small screens, control environmental systems such as home heating/cooling systems, and assess health through monitoring of some physiological parameters.

    Yet while designers envision exciting opportunities, test engineers see tremendous challenges. The new devices will have more RF inputs and outputs to test. To obtain the higher transmission rates, signal bandwidths are getting wider. The 802.11 wireless LAN (WLAN) standards and the WiMax standards define channels with bandwidths as wide as 40MHz. As NTT DoCoMo's experiments indicate, bandwidths could get significantly higher. Furthermore, complex signal protocols for video and navigation services require test signals that are minutes in length. Adaptive coding and modulation techniques as well as smart antenna technology require the analysis of numerous types of signals.

    The test engineer is faced with much more sophisticated test instrumentation and increasing test times.  The new 4G technologies drive capital costs and test time upwards. Greater modulation bandwidths and signal capture bandwidths drive RF source and analyzer costs higher. Signals lasting minutes require large amounts of arbitrary waveform memory in the instrumentation and the test time to generate, capture, and analyze long data streams. Most significantly, the ability to test a number of transmission protocols in one device-under-test requires a large amount of signal processing power and firmware/software algorithm development. In addition, multiple, simultaneous RF transmitter and receiver modules may require multiple sets of instrumentation to generate and capture simultaneous data streams. While 4G and eventually 5G promise everything in the palm of the hand, the test engineer must face the challenge of keeping the total cost of test off an exponential curve. Here are some of the demands on test and instrumentation resulting from the technology in the newest wireless transmission standards.

    Multiple RF Streams

    One way to attack the insatiable desire for higher data rates is to increase the number of transmitters sending data. That is the objective of the 802.11n Wireless LAN standard. The standard has defined attainable data rates of 600Mb/sec with a minimum guaranteed rate of 100Mb/sec. Those data rates are achieved using a technology known as MIMO (Multiple Input Multiple Output). Several data streams are sent simultaneously which requires multiple transmitters and receivers. That is just one factor. The channel bandwidth can be either 20MHz or an extremely wide 40MHz bandwidth, adding substantial throughput potential compared with 3rd generation UMTS systems that use a 5MHz channel. The modulation scheme that the 802.11n standard employs is orthogonal frequency division multiplexing (OFDM). Small amounts of information are encoded on a large number of carriers as opposed to more conventional modulation methods which encode all the data on a single carrier. The number of carriers (or subcarriers) can be as high as 1000. The carriers are close together, but the encoding is orthogonal so that only the correct code for a carrier can extract the signal. The OFDM technique effectively increases the signal/noise ratio by spreading the information content over numerous carriers. To further enhance signal/noise ratio, 802.11n devices must be capable of determining the optimum channel conditions between the two devices and select the appropriate modulation coding scheme. That can involve varying the number of simultaneous transmission streams and the modulation method within the OFDM scheme. Modulation coding schemes can vary from BPSK and QPSK to16QAM and 64QAM.

    The MIMO transmissions are sent using two techniques, spatial multiplexing and beam forming. Spatial multiplexing divides the data into multiple data streams and simultaneously transmits the streams over different pathways. The receivers combine the received data streams into a single data stream for information recovery. Beam forming uses directional antenna elements to direct the beam over the optimum path after testing for the path of least signal loss. Currently chipset manufacturers have implemented schemes with 2 data streams. However the 802.11n standard specifies up to 4 data streams with 4 transmitters and 4 receivers (16 total paths).

    At the chipset level, manufacturers will need to test with multiple sets of RF signal generators and RF analyzers. These instruments must be RF vector signal generators and RF vector signal analyzers since the modulation and demodulation performance of the MIMO chipset must be tested and verified. These instruments need at least a 40MHz signal capture bandwidth (alternatively known as an IF bandwidth). That wide a bandwidth along with the digital modulation configurations requires high performance RF vector signal generators and RF vector signal analyzers.

    Even more significant for the chipset manufacturer is the need to have a high level of time synchronization between the representations of the transmissions. Timing is critical in generating and decoding the multiple data streams. In 802.11n devices, all the data streams will be synchronized to the same clock. Synching multiple clocks together in individual instruments or individual instrument modules requires high stability circuitry and tightly synchronized circuitry.  Carrier phase drift between multiple instruments must be minimized and there must be sub-nanosecond jitter between the transmission of the multiple I and Q digital data streams to the modulator. Figure 1 shows that three Vector Signal Generators can achieve peak-to-peak jitter between the output carrier waveforms of less than 1 degree. The modulating waveforms generated by an arbitrary waveform generator in each instrument can achieve time alignment and jitter of less than 1ns. Limited performance RF signal generators and moderate performance RF signal generators cannot achieve this type of performance. Even some high performance RF signal generators cannot achieve the necessary performance for simulating MIMO synched transmissions.

    The edges are aligned closer than 1 ns, and the jitter is much less than 1 ns.

    With OFDM modulation technology, the individual carriers can use different modulation techniques. Each narrow carrier transfers a small amount of information. Hence the modulation quality of each carrier in a 20MHz or 40MHz channel will need to be analyzed. Earlier generation technology analyzed a channel's modulation performance in a single parameter such as error vector magnitude (EVM). With all the carriers in OFDM signals, an independent analysis of each carrier's modulation performance is required.  Testing must grow from one EVM measurement on a transmission to 116 EVM measurements; and signals can have over 1000 carriers. Processing and analysis time must increase substantially to thoroughly analyze such a complex signal.

    The WiMax standard, 802.16d and 802.16e, uses OFDM modulation but also adds an additional level of complexity. WiMax includes OFDMA (Orthogonal Frequency Division Multiple Access) in which there is further division of the channels into sub-channels to increase user capacity.  OFDM accommodates multiple users by assigning an individual user a single time slot. In OFDMA, several users can transmit during the same time slot but on different sub-channel carriers. The RF generators and analyzers must be capable of sufficiently fast signal generation and signal capture to generate, acquire and analyze simultaneous time domain and frequency domain phenomena.

    Lengthy Signals

    For video transmissions, a number of transmission methods have been proposed and are under development and experimentation. The South Korean government has sponsored the development of WiBro, a 1Mb/sec transmission standard. The WiBro standard is being integrated into the latest versions of the WiMax standards. Qualcomm is commercializing a standard called MediaFlo. MedialFlo uses OFDM modulation technology with 5MHz to 8MHz bandwidth channels. Qualcomm claims 11Mb/sec downlink transmission rates. One aspect of the technology that helps to achieve this data transfer rate is the use of 4096 carriers in a channel. The signals are highly complex. Currently Qualcomm has offered test signals that are nearly 10 minutes in duration. In addition to requiring a large amount of memory in the instrument to store this type of signal, a test time of 10 minutes for one test is a long time on a high volume production test floor.

    Testing GPS locator requirements pose the same problems. Mobile phones which will be capable of processing signals from multiple satellites and compensating for Doppler shift when the device is in motion must receive and process another complex signal protocol. Testing GPS signals with the appropriate signal content will also require signals that may be 10's of minutes long.

    Power Saving

    Even DC measurements will get more complicated for the test engineer. The 802.11n standard has defined a power save protocol that enables a handset to transition into a sleep state when not transmitting or receiving voice data. VoIP-type voice data is transmitted in 100_s data packets with periods of silence (no packet transmission) that last as long as 20-30ms. Thus the mobile device may have periods of low duty cycle operation. Fast DC source instrumentation will be required to monitor the load cycle to ensure the device's power consumption meets the manufacturer's specifications.

    Engineers put to the test

    While users of 4G will not be able to live without the devices, mobile phone test engineers will have a hard time living with these devices. The phone designers may help by designing built-in verification schemes for some performance features, but the burden will fall on the test engineers. Test engineering will have to rise to the challenge with their own significant innovation. As it looks now, they will be buying top-of-the-line instrumentation with wide RF signal bandwidths, nsec level or better jitter, large memories, and extensive signal modulation-demodulation generation and analysis firmware/software. Furthermore the complex protocols will require much longer time sequences to ensure proper signal processing. Mobile phone test times are under two minutes now. With devices loaded with multiple transceivers and with complex signals to perform bit error rate tests on, test times could become substantial multiples of that.For test engineers, 4G phones will require 16G test technology.

    Robert Green is Senior Market Development Manager, Keithley Instruments, Inc.