Today's mobile users are employing their handheld and wireless devices for far more than simple voice calls. Video streaming and other data applications that require greater bandwidth and better Quality of Service (QoS) are widely available, and bandwidth is growing continuously. To meet this demand for high-quality services, service providers are expecting their product manufacturers to develop and deliver products that give users the same access, user experience, and reliability from their portable wireless devices that they achieve with their wireline devices.
4G wireless technologies and standards including WiMAX and Long-Term Evolution (LTE) have become the future hope of the industry, promising throughput and range to support an expanded set of capabilities. These technologies will allow this new range of applications to operate on an evolving set of wireless devices.
Though beneficial, this evolution is not without challenges. To achieve the potential increase in functionality, 4G services and products must harness a complex Radio Frequency (RF) world, which requires smart antenna technologies based on Multiple Input/Multiple Output (MIMO) technologies.
A tricky testing environment
MIMO is the basis of emerging 4G wireless broadband technologies, including mobile WiMAX, LTE, and Ultra-Mobile Broadband (UMB). MIMO takes advantage of multiple transmit and receive antennas to employ techniques such as spatial multiplexing, adaptive antenna processing, and beamforming. As a result, products that employ MIMO technology provide higher throughput and range for voice, video, and data services. Figure 1 shows how a multiple antenna Base Station (BS) can steer the antenna in a specific direction to enhance range. The direction of the beam can change over time as the Mobile Station (MS) moves around. Channel emulation allows beamforming and other antenna techniques to be tested in the lab.
As MIMO-based 4G technologies evolve, product manufacturers and service providers are racing to find mechanisms that can test their services, infrastructures, and devices for carrier-grade reliability before they deploy in the field. The physical layer and open access requirements of new MIMO-based 4G technologies place greater demands on RF performance/interface quality, making 4G testing more difficult than previous technologies. In addition, the complex RF environment produces potential chaos for traditional over-the-air testing because of the breadth of complex scenarios that needs to be validated, the lack of repeatability, and the high costs involved.
Multipath and fading effects are essential to operate MIMO technology, as well as a source of interference that might degrade wireless performance. Testing in real-world multipath and channel conditions evaluates wireless devices‚Äô robustness and performance and challenges their dexterity to adapt to changing conditions by adjusting data rate, modulation, coding, and other parameters to the conditions. Consequently, manufacturers are looking for reliable, predictable, and repeatable testing methods that can re-create fading and multipath conditions to test their 4G products.
The importance of channel emulation
Because virtually hundreds of unpredictable scenarios with variable field conditions including noise and interference are inherent in 4G technologies, manufacturers need to re-create real-world channel conditions in a lab to establish the best possible testing environment for 4G base stations and mobile devices. Channel emulation simplifies and streamlines the testing process by creating these conditions in the lab, dramatically shortening the time, resources, and ultimately, expense of 4G testing. Through sophisticated, dynamic channel-modeling technology, channel emulators re-create the effects of radio transmissions in real-world setups to realistically reproduce the effects of disturbances, interference, reflections, noise, motion, and more.
Ideally, a channel emulator will support different test topologies, including direct connect (point-to-point) and handoff or multiclient setups (point-to-multipoint). The emulation should be bidirectional, re-creating the real-world reciprocal mapping of forward and reverse channel conditions as it will be seen in the field. Because MIMO capability is the key component, the channel emulator must support 2x2 for WiMAX and 4x4 for LTE, Wi-Fi, and 802.16m testing, as well as provide access to standard and custom channel models and antenna correlations on these configurations.
Furthermore, because the channel emulator sits between the mobile and base station device, it is critical that the device itself does not become a limiting factor in the test; the channel emulator must have RF characteristics that guarantee that the highest-quality signal is passed through end to end. 4G wireless broadband uses state-of-the-art Orthogonal Frequency Division Multiplexing (OFDM) modulation and multi-antenna MIMO. The channel emulator must accommodate RF factors such as dynamic range, Error Vector Magnitude (EVM), and noise floor much higher than previously required for 2G/3G data and voice technology.
As applications such as video streaming are developed, designers must consider whether their applications can operate in both optimal and marginal RF conditions. They should also strive to meet quality expectations for all RF conditions. A 4G-compatible channel emulator capable of simulating the RF environment and the resulting channel fading can assist developers in recognizing how the applications perform under different fading conditions. Channel-fading testing will give developers a clear understanding of how applications and devices will behave in real-world conditions.
Through MIMO, 4G offers a new, more advanced generation of applications to meet users ever-increasing demands. However, the challenges that service providers and equipment manufacturers must overcome to qualify these products and applications are far more complex than what they faced developing previous wireless access technologies. As a result, complete environmental testing with channel emulation has become a key factor in product and service development.