Emerging 3G CDMA Services Generate Performance Analysis Challenges
To ensure rapid, reliable deployment, CDMA2000 manufacturers and service providers must evaluate the new devices equipped with G features.By Rob Van Brunt, Spirent Communications, TAS Division
The deployment of the next-generation CDMA2000 1xMC air interface is underway and provides the ability for network operators to offer 3G services. While the initial benefit driving operators to rapidly deploy CDMA2000 1xMC (known commonly as CDMA2000) may be its critical increase in voice capacity, next-generation services such as high-speed packet data connectivity and location-based services promise to deliver a new user experience to subscribers.
The successful deployment of these complex services poses significant new challenges. No longer is analyzing the performance of the mobile device limited to measuring its voice quality under adverse conditions. To thoroughly evaluate 3G services, mobile device manufacturers and network operators must employ a new test plan designed to also characterize a mobile's data transmission metrics and its ability to take precise position location measurements. A next-generation of integrated, feature-rich performance analysis solutions are required to evaluate a mobile's complex combination of voice, data, and position location capabilities.
Always-on Data Connectivity
While circuit-switched data services have been offered on 2G (IS-95) CDMA networks for several years they have not been widely adopted by subscribers. This may be due to the fact that North American CDMA data services offer a maximum connectivity of 14.4 kbps. This access rate is many times lower than what consumers have grown accustomed to on even the slowest dial-up wired networks and makes Internet browsing a tedious experience.
CDMA2000 1xMC is designed to offer "always-on" data rates analogous to what can be achieved with typical wired network access. This includes high-speed packet data (HSPD) connectivity with rates up to 153.6 kbps in the first-generation of 1xMC and 307.2 kbps in later revisions. At these rates, network operators hope that subscribers will be more apt to utilize their mobile phone to access Web-based services. In addition, consumers will be presented with additional choices of wireless terminal form factor and functionality. Given the 3G network's ability to support more attractive wireless data rates, fledgling CDMA data card modem manufacturers believe there is a business model that supports wireless terminal platforms on Personal Digital Assistants (PDA's) or notebook computers.
The CDMA2000 air interface also adds support for a much wider variety of data services to be delivered to the phone. Included in the specification are operating modes that allow simultaneous transmission of voice and data. The specification also allows the data connection to move between active and dormant modes. This supports the "always-on" experience by permitting the higher-layer data protocols (e.g. PPP) to be left intact while data connections over the low-level air interface protocols are setup and torn down.
These services require the mobile to support flexible radio configurations (i.e. protocols) and additional traffic channels, such as a Supplemental Channel. Unlike a wireless voice call that can tolerate a certain level of lost voice packet transmissions, data connections must employ additional protocols to ensure a minimum of lost data packets. One example of an additional data-centric protocol is called Radio Link Protocol (RLP). RLP monitors the sequence of packet transmissions and coordinates re-transmissions when necessary. Figure 1 shows where RLP fits into the multitude of protocols involved in CDMA data transmissions.
Figure 1. CDMA Data Stacks
Figure 2. Spirent C2K-ATS CDMA2000 Automatic Test System
HSPD Test Requirements
Given the complexity and interaction of the numerous protocol stacks shown in Figure 1, accurately characterizing a mobile device's HSPD performance is a challenging task. The IS-898 CDMA Protocol Interoperability standard was developed by the 3GPP2 to provide a structured approach to the HSPD test process. IS-898 defines tests to characterize HSPD performance metrics such as data throughput under normal and impaired RF channel conditions. A variety of HSPD protocol interoperability tests are also specified by IS-898. These include validating the mobile's concurrent use of Fundamental and Supplemental Channels while connected to a single serving base station and while in a variety of hand-off scenarios.
To characterize a mobile's HSPD performance requires a test system that provides flexible access, control, and programmability over CDMA data network elements. Shown in Figure 2, the Spirent CDMA2000 Automatic Test System (C2K-ATS) enables lab-based performance analysis of CDMA mobile devices. At the heart of this system is the AirAccess C2K CDMA Network Emulator that supplies real-time emulation of all of the relevant CDMA network elements and as shown in Figure 3. AirAccess C2K supports all of necessary CDMA air interface and data protocol stacks required for HSPD testing. This includes the ability to perform HSPD tests in soft and hard-handoff conditions with up to six active base stations.
C2K-ATS is equipped with a TAS4500 RF Channel Emulator and TAS4600 Noise & Interference Emulator that can be configured to emulate a wide range of forward link radio channel environments. AirAccess C2K provides low-level visibility to key data transmission protocols including RLP performance statistics. These features enable developers and evaluators to make detailed cause and effect observations to correlate emulated radio channel conditions with device performance. The TASKIT/ C2K test executive software coordinates C2K-ATS equipment configuration, test execution, and results logging.
While AirAccess provides a network-centric view of CDMA and data protocol performance, the Spirent Universal Diagnostic Monitor (UDM) supplies a real-time analysis of mobile protocol performance to provide closed-loop testing. UDM also permits automated, low-level control over the mobile device increasing the level of overall test automation. To generate the packet data traffic required for HSPD performance analysis, Spirent's Smartbits data generation and analysis solution enables systematic data performance characterization.
While the first CDMA2000 HSPD services are currently being deployed in North America, Korean network operators such as SK Telecom have begun to trial a new even higher-speed packet data service called CDMA2000 1xEV-DO. This data-only service is optimized to provide peak data rates of 2.4 Mbps in the same 1.25 MHz bandwidth as the CDMA2000 1xMC air interface. Targeted at corporate users, 1xEV-DO Access Terminals (ATs) will be embedded into notebook computers and PDAs. Those touting 1xEV-DO also claim it provides a competitive advantage to service providers who are looking to reduce their cost per byte for wireless data services.
Although CDMA2000 1xMC and CDMA2000 1xEV-DO were designed to be very similar from a physical layer perspective, these standards have substantially different protocol layer implementations. To achieve transmission efficiency, 1xEV-DO uses both time and code division multiplexing on the forward link, features dynamic modulation types and data rates, and larger channel-coding interleavers. As such, 1xMC test solutions must be enhanced to address 1xEV-DO test applications and must offer support for inter-generation data handoffs between 1xEV-DO and 1xMC networks. Spirent's AirAccess EV Network Emulator extends the features of the AirAccess C2K product line to meet these additional requirements.
Location Based Services
Globally, location-based services are expected to be a $20 billion market midway through this decade. But what are location-based services and who wants them? Location based services can be split into two segments. The first segment, and most visible in North America, is the ability to provide enhanced emergency services to mobile device users. Defined by the FCC as E-911, the Phase II mandate of this health and safety service is motivated by the ability to locate a mobile user in an emergency situation, such as a car accident. The Phase II deployment of E-911 was originally targeted for last October in North America. Faced with complying with the challenging task of pinpointing mobile users to within 50 to 300 meters, network operators requested waivers from the FCC. Operators must now follow a compliance timeline starting October 1, 2001 and ending December 31, 2005, by which time 95% of all handsets sold must be location capable.
In addition to the need to comply with FCC regulations, network operators want to fill a growing demand for this value-added subscriber service. According to a recent survey of more than 1,000 Americans aged 18 or older, 60 percent said that the ability for rescue workers to locate them in an emergency was the most important feature for a mobile phone to have. Influenced by the events of September 11th, the demand for emergency location services in the survey was three times that of mobile e-mail access.
Figure 3. Spirent AirAccess C2K User Interface
Network Operators also believe that there will be a strong demand for commercial applications of position location technology, which fills the other segment of location-based services. Imagine walking down a city block while advertising and promotional content (e.g. coupons) is "pushed down" to your mobile phone. By generating newly found advertising revenue, operators believe they will be able to offer lower air time rates to individuals who agree to allow the display of their mobile device to be used as a location-sensitive billboard.
Both mobile and network based technologies can be used to implement location-based services. A list of key technologies is shown in Table 1. But field trials have indicated that mobile-based technologies offer the most promise towards meeting the stringent accuracy requirements imposed by the FCC. On 3G CDMA2000 networks, network operators are planning to use mobile-based techniques such as Advanced Forward Link Trilateration (AFLT) and Assisted-GPS (A-GPS) to determine a mobile's position in the network.
To supply the measurements required for AFLT, a mobile makes Pilot phase measurements on serving base stations and reports them back to the network using IS-801-1 signaling. In the case of A-GPS, the CDMA network assists the mobile by sending the position of the GPS satellites over the CDMA link. The mobile can than quickly make GPS pseudo-range measurements and report them back to the CDMA network. AFLT excels in urban (i.e. high base station density) and in-building applications while A-GPS proves useful in rural (i.e. low base station density) environments. Utilizing a combination of AFLT and A-GPS techniques benefits from their dissimilar strengths. A commercial example of a hybrid AFLT/A-GPS approach is called gpsOne.
Figure 4. Spirent Position Location Test System (PLTS)
In both AFLT and A-GPS applications, mobile-based measurements are reported back to the CDMA network. A new network element, called a Position Determination Entity (PDE), calculates the mobile's position based on the mobile measurements relayed back to the PDE by the serving CDMA base station. The PDE, implemented as a specialized information server, performs sophisticated positioning algorithms and reports the position result to support either an emergency or commercial location-based service.
Position Location Test Requirements
A combination of lab-based minimum performance tests and field-based tests have been developed to evaluate the mobile's ability to perform accurate position measurements and to report them back correctly to the serving network.
Minimum performance tests such as those being defined by 3GPP2 Working Group 4.3 (WG4.3) define baseline test procedures for lab-based mobile evaluation. These tests incorporate test scenarios for AFLT, A-GPS, and hybrid technologies. Network operators have defined field test scenarios that characterize a mobile's performance in representative environments such as indoors in a shopping mall or outdoors in an urban canyon. To perform adequately in both minimum performance and field test scenarios, mobile device manufacturers need test equipment that can be configured to meet the requirements of these broad test methodologies.
The required test system must implement all of the key terrestrial and satellite network elements including a multi-cell CDMA network, GPS constellation, and PDE. Since 3G networks will exist concurrently with their 2G predecessors, the test system must be capable of supporting both IS-95 (2G) and CDMA2000 protocols. An example of this type of system, developed by Spirent Communications, is shown in Figure 4. AirAccess C2K CDMA Network Emulator emulates up to six CDMA base stations in real-time and provides the IS-801-1 signaling required to exchange position location messages with the mobile under test during both static and hand-off conditions. The GSS5060 GPS Simulator can be configured to provide up to 12 GPS satellites complete with programmable impairments. PDE functionality is integrated into the system using the TAS3710 PDE.
To realize the demanding measurement accuracy required to characterize mobile performance, both the emulated CDMA and GPS signals must be synchronized to within nanoseconds. The Spirent Position Location Test System (PLTS) integrates and automatically synchronizes the system components to ensure accurate and repeatable test results. To maximize testing resources and to enable a large number of test scenarios to be performed in the minimum time, the PLTS utilizes TASKIT/ PLTS test executive software to automate all aspects of test campaign configuration, synchronization, execution, and results logging. Full automation is made possible by Spirent's Universal Diagnostic Monitor (UDM). UDM software provides simultaneous control and performance monitoring of the mobile device thereby minimizing manual user intervention.
Ensuring Rapid Deployment
In order to ensure the rapid and reliable deployment of value-added subscriber services, CDMA2000 mobile manufacturers and service providers must thoroughly evaluate the new wave of mobile devices equipped with 3G features. No longer is voice transmission quality the only performance metric measured by mobile evaluators. Emerging services such as high-speed packet data and location-based services require a new generation of more sophisticated test solutions to properly characterize their performance. Given the complexity and rapidly increasing number of 3G mobile test cases, mobile manufacturers and service providers require integrated systems equipped with full test automation are required to maximize valuable resources and minimize time to market.
Rob Van Brunt is the Director of Marketing for Spirent Communications, TAS Division. Rob has been involved in the development and marketing of wireless performance analysis solutions at Spirent for the past 10 years and can be reached at firstname.lastname@example.org.