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`Analyzing WCDMA Performance During CompressedMode Handovers | EE Times
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`News & Analysis
`Analyzing WCDMA Performance
`During CompressedMode
`Handovers
`Analyzing WCDMA Performance During
`CompressedMode Handovers
`Koichi Sega, Tektronix, Inc.
`3/17/2003 03:10 AM EST
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`Analyzing the performance of a mobile radio network during
`handovers of handsets (UE) between base station (BTS)
`transceivers is a challenging task. Properly functioning handovers
`allow calls to continue without interruption and maximize
`resources through capacity sharing. Poorly executed handovers
`can disrupt calls, degrade qualityofservice (QoS), and reduce
`the operating ranges of cell sites—risking severe economic
`consequences.
`
`The ability for a UE to be handed over reliably between wideband
`CDMA (WCDMA) and GSM or TDD systems is particularly
`important as newer networks are phased into existing
`infrastructures. WCDMA systems manage handovers by briefly
`switching transmissions into compressed mode. Errors that occur
`during compressedmode handovers are unpredictable and must
`be captured in seamless blocks over long periods in order to
`analyze their characteristics and trace their sources.
`
`To effectively deal with errors occurring in the compressed mode,
`designers need to take a threedimensional approach to
`measuring the handover process, looking at measurements in the
`time, frequency, and code domains. Let's analyze why.
`
`Handling Handovers in WCDMA
`The handover is a process by which the radio access network
`changes the radio transmitters, radio access modes, or radio
`systems that are used to provide the bearer services to the UE,
`while maintaining a defined QoS.
`
`Handovers from one cell to another are required in several
`situations. The most common situation is when the UE moves
`from one base station coverage area to another. The UE may
`move between stations within the same radio system or into
`another system. The WCDMA standard supports handovers to
`any GSM or time division duplex (TDD) network frequency bands
`that meet the specifications.
`
`The multistandard UE may change its frequency or radio access
`mode, during a handover to a different cell. The UE may need a
`
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`handover if its requested service level exceeds the current cell
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`handover if its requested service level exceeds the current cell
`capacity. If a target cell cannot support the combination of bearer
`services (voice, data, multimedia, etc) that are provided by the
`current serving cell, some, or all, of the bearer services may be
`handed over to another cell.
`
`Within the WCDMA system, handovers are "soft" in order to
`minimize the interference on neighboring cells and to allow the
`use of identical carrier frequencies (intrafrequency handovers). In
`a soft handover, the UE transmits and receives the same signal
`from both cells simultaneously to make the transition as seamless
`as possible. Handovers are more complex when a multistandard
`UE moves between cells with different carrier frequencies or to a
`different network, such as GSM ("Interfrequency Handovers").
`Both types of handover are managed with an assist from the UE
`mobile unit.
`
`The multistandard UE continuously monitors for the presence of
`cells with other frequencies and radio access systems that it
`supports. When the network senses the need for a handover, the
`BTS measures some system parameters and commands the UE
`to measure other parameters and report the results. Key
`parameters include carrier frequency, system type, traffic volume
`and QoS levels.
`
`Working in the Compressed Mode
`When a handover is needed, the BTS directs the UE to operate in
`a compressed mode. The compressed mode is a method of
`turning off transmissions for a portion of the 10ms frame to
`create gaps that allow time for the UE and BTS to make a
`prescribed set of measurements. Compressedmode operation
`can be achieved by decreasing the spreading factor, removing
`bits from the data ("puncturing"), or using higher level scheduling
`to allocate fewer timeslots for user traffic.
`
`In compressed frames, the transmission gap slots are not used for
`data transmission and the instantaneous transmit power is
`increased in those slots to maintain quality (BER, FER, etc.)
`during the periods of reduced processing gain (Figure 1). The
`value of power increment depends on the transmission time
`reduction method.
`
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`Figure 1: Example of compressed mode transmission (3GPP
`TS25.215 V3.11.0).1
`
`Compressed frames can be set to occur periodically (as in Figure
`2 below) or on demand. The rate and type of compressed frames
`is variable and depends on the environment and the
`measurement requirements.
`
`Separate compressed mode signals must be defined for uplink
`and downlink paths as well as for each mode, radio access
`technology and frequency band supported by the UE. In typical
`applications, uplink data rates in compressed mode are set by
`higher protocol layers to twice the normal rate, downlink data
`rates to twice, or more.
`
`Figure 2 shows examples of a compressedmode frame structure
`for uplink operations. Figure 3 illustrates two types of downlink
`frame structures that differ in the location of the TPC bits. Type A
`would be used to provide the maximum transmission gap for
`measurements; Type B optimizes power.
`
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`Analyzing WCDMA Performance During CompressedMode Handovers | EE Times
`
`Figure 2: Example of a compressed mode frame structure for an
`uplink (3GPP TS25.212 V3.10.0) 1
`
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`Figure 3: Two examples of compressed mode frame structures for
`downlink (3GPP TS25.212 V3.10.0) 1
`
`In the compressed mode, a transmission gap pattern sequence is
`requested by higherlayer BTS protocols and the parameters are
`passed along to the UE by the BTS. The UE conducts only one set
`of measurements for each transmission gap pattern sequence.
`Figure 4 illustrates a compressedmode sequence of alternating
`transmission gap patterns while Table 1 lists the parameters that
`are used to define the sequence.
`
`Figure 4: Example of a compressed mode transmission gap
`pattern (3GPP TS25.215 V3.11.0) 2
`
`The First Two Measurement Dimensions
`The UE and BTS measure the Layer 1 protocol to determine and
`report the status of intrafrequency, interfrequency, intersystem
`handovers, traffic volume, and QoS levels. First, the BTS
`transmits a "measurement control message" to the UE including
`the measurement ID and type of measurement to initiate.
`
`When the reporting is complete, the UE sends a "measurement
`reporting message" to the BTS with the measurement ID and the
`results. The measurement control message is broadcast in idle
`mode within the system Information. When the UE monitors cells
`at other frequencies, modes, and radio access technologies, the
`BTS must direct the specific measurement needed to fulfill the
`requested handover. In WCDMA, the Layer 1 measurements are
`
`reported to higher layers of the protocol. In GSM, the
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`Analyzing WCDMA Performance During CompressedMode Handovers | EE Times
`reported to higher layers of the protocol. In GSM, the
`measurements are reported only to the GSM terminal.
`
`Table 2 lists some of the measurements made by the UE and the
`BTS during compressed mode.
`
`Error conditions that arise during compressed mode
`measurements and handovers can be brief and unpredictable. To
`be certain of catching these intermittent problems, power levels,
`frequency and modulation information must be monitored before,
`during and after they occur. Data must be captured seamlessly in
`order to preserve the signal characteristics and reveal the error
`sources. In depth analysis of error conditions often requires the
`correlation of signal states in the frequency, time, modulation, and
`code domains.
`
`The realtime spectrum analyzer employs advanced digital signal
`processing technology to acquire long seamless records of
`complex signals and display analysis results without the need for
`external data processing. For example, the analyzer can be set to
`acquire a full 10 seconds of signal in a 5 MHz span and analyze
`the results in multiple display formats. Time and frequency data
`are recorded simultaneously, revealing even brief, intermittent
`changes and when they occurred within the long records.
`
`The Third Dimension
`To compliment the time and frequency measurements, designers
`can start analyzing the performance of WCDMA handoffs in the
`code domain. To do this, designers should turn to spectogram and
`codogram measurements.
`
`A spectrogram provides a display of frequency vs. time vs. power
`density. This measurements shows how well the UE performed at
`different frequencies during handovers. In the spectogram shown
`in Figure 5, the vertical axis represents time and the horizontal
`axis represents frequency while the colors represent frequency
`and power density.
`
`Figure 5: Diagram of a typical spectogram measurements.
`The codogram is a 3D display of the orthogonal variable
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`Analyzing WCDMA Performance During CompressedMode Handovers | EE Times
`The codogram is a 3D display of the orthogonal variable
`spreading factor (OVSF) or channelization code vs. time slot vs.
`code power. In the codogram shown in Figure 6, the vertical axis
`is the time slot and the horizontal axis is OVSF while code power
`is represented by color.
`
`Figure 6: Diagram of a typical codogram measurement.
`
`With spectrogram and codogram measurements, more data is
`captured and no data is missed. Each measurement shows a
`seamless history of information.
`
`Wrap Up
`The compressed mode handover performance of WCDMA radio
`system UE and BTS transceivers and networks is complex and
`should be evaluated simultaneously in the frequency, time,
`modulation and code domains. The realtime spectrum analyzer
`can capture intermittent signals in long seamless records and
`provide measurement results that are correlated in multiple
`domains. Powerful 3D displays provide clear insight into the
`system quality and/or potential sources of errors.
`
`References
`
`1. 3GPP TS25.212 V3.11.0 Technical Specification Group
`Radio Access Network Multiplexing and Channel Coding
`(FDD) (Release 1999).
`2. 3GPP TS25.215 V3.10.0 Technical Specification Group
`Radio Access Network; Physical layer—Measurements
`(FDD) (Release 1999) .
`
`About the Author
`Koichi Sega is a product manager in Tektronix's Wireless Product
`Line. He received a degree in electrical engineering from Tokyo
`Denki University and can be reached at
`koichi.sega@tektronix.com.
`
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