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Downlink Channel Configuration for W-CDMA Bit Error Rate Testing

Thu, 09/26/2002 - 12:08pm

by David J. Schwartz, Agilent Technologies, dave_schwartz @agilent.com

The flexibility and complexity of the W-CDMA air interface present special challenges for the performance of Bit Error Rate (BER) testing. BER is the required method for determining the sensitivity of a W-CDMA receiver. The W-CDMA specifications provide a solution in the form of Reference Measurement Channels which, when combined with a properly configured test system, allow effective BER testing to be done.

Introduction
Receiver sensitivity is one of the most important characteristics of a radio communications device to test, both during development and in production. While the sensitivity of a receiver depends on many factors, it is the characteristics of the RF front end that produces the variation from unit-to-unit in production. Bit Error Rate (BER) is the test method used measure the sensitivity of a Wide band CDMA (W-CDMA) device.

Bit Error Rate can be performed in two ways. In the first method the data bits recovered by the device under test are returned to the system simulator at baseband, usually over a serial data link. The second method, Loopback BER, returns the recovered bits to the System Simulator using the uplink. This method has the advantage of requiring no additional connections to the device under test other than the radio link; but, adds the complexity of requiring the establishment of a compatible uplink.

PERFORMANCE BER TESTS ON A W-CDMA LINK

Challenges Presented by the W-CDMA Air Interface
While a bit error rate measurement itself is quite straightforward, using it on a system that performs significant manipulation of the data bits during transmission and reception, such as W-CDMA, presents more of a challenge. In this type of environment it is essential that the configurations of the System Simulator (SS) and the User Equipment (UE) are compatible. If these two are mismatched, the UE will be unable to recover the data bits correctly even if the signal level is strong, and the test results will be invalid.

The extreme flexibility and resulting complexity of W-CDMA makes this a much more interesting problem. To add to the challenge, the aspects of the W-CDMA system that affect radio link configuration for BER testing are spread throughout a large number of specifications documents. A list of the relevant documents is provided at the end of this presentation.

The W-CDMA test that utilizes BER is the Reference Sensitivity Measurement (see TS34.121, Section 6.2). When the UE is configured for this measurement the data bits to be tested are those at the top of Protocol Layer 2. The W-CDMA specifications provide for a method of looping these bits back to the SS via the uplink signal. That method use a special Layer 3 protocol entity known as the Radio Bearer Loopback Entity. (See TS 34.109, Section 5.3).

Along with variations in how a channel is configured, the W-CDMA system provides several different types of channels for carrying different types of information. In W-CDMA, "channels" can be physical, transport or logical, depending on where you are in the signal coding process.

Channel configurations vary considerably with the type of data being carried and whether the connection type is circuit switched or packet switched. In addition, many different channel configurations are possible for any given type of service.

A circuit switched connection on a Dedicated Physical Channel (DPCH) is used for the Reference Sensitivity Measurement.

W-CDMA Channel Coding and Structure
A great deal happens to the data bits as the circuit switched DPCH is built up. Radio bearer data arrives at Protocol Layer 2 and is passed to a Radio Link Control (RLC) entity. RLC provides block error detection and retransmission, if needed. Data from the RLC's is then handled by Medium Access Control (MAC) which can, again if needed, multiplex logical channels together before passing them to Layer 1. Layer 1 consists of one or more Dedicated Channels (DCH). Each DCH is individually configured for the type and amount of data it is expected to carry. The DCH provides the CRC that is used by RLC, error correction coding, rate matching, and interleaving of the transport blocks handed to it from Layer 2. Since the transport blocks can be longer than the 10 msec radio frame, the DCH's final step is to segment its data into appropriate sized segments to fit in the radio frames. The radio frame sized segments from the DCH's are then multiplexed together to a Coded Composite Transport Channel. At this point the data bits are interleaved once again and become the Dedicated Physical Data Channel (DPDCH), which itself is multiplexed with the Dedicated Physical Control Channel (DPCCH). Finally the bits are spread into chips by the channelization (OVSF) code and scrambled by the Node B (base station) primary scrambling code. Not shown is the final step of modulating the RF carrier with the scrambled chips.

Some of the link characteristics are semi-static; that is, they can only be changed by upper layer signaling. Others are dynamic and can be changed by the lower protocol layers at will. Changes to the semi-static characteristics occurs relatively slowly and infrequently. Changes to the dynamic ones can occur every radio frame. Semi-static characteristics include: number of channels, spreading rate (in the downlink), and error correction coding. Examples of characteristics with dynamic portions are rate matching, TrCH multiplexing, and the number of transport blocks to be sent at any given time.

W-CDMA Reference Measurement Channels
To deal with the high degree of flexibility of the W-CDMA air interface, a set of defined physical layer DPCH configurations are specified. These are the Reference Measurement Channels. When developing test solutions using these channels it is important to realize that these "channels" are not unique features of the air interface, they simply represent decisions about which of the already allowable configuration choices will be used for test purposes. All normal system physical layer behaviors and requirements still apply when the link is configured as per a reference measurement channel. Thus, the Reference Measurement Channel definitions include capacity for a Dedicated Control Channel (DCCH) as well as a Dedicated Traffic Channel (DTCH). Note the use of the term "capacity for"; this is an important concept that is part of the normal operation of the W-CDMA air interface. In normal system operation a channel is only present when it has information to carry and the reference Measurement Channels do not violate this principle.

The Reference Measurement Channels define the semi-static characteristics of the air interface. For example, the 12.2K Reference Measurement Channel defines:

* Number of DCH's: 2

* Logical Channels: DTCH and DCCH

* Spreading Factor: 128

* Error Correction Coding: Convolutional, rate 1/3

* TrCH Multiplexing Method: Fixed positioning

The data rate is a characteristic with both semi-static and dynamic parts. The semi-static part is the range of alternatives. This is the Transport Format Combination Set (TFCS). The dynamic part is the alternative currently in use, which is the Transport Format Combination (TFC). For the 12.2 RMC the TFCS is

     DTCH DCCH

     0 0 kbps

     0 2.5

     12.2 0

     12.2 2.5

The Reference Measurement Channel also defines the Layer 2 configuration for the DTCH. For this logical channel the RLC and MAC functions are set to "transparent" mode. In this mode they are effectively removed from the system. Layer 2 and above for the DCCH are not defined by the Reference Measurement Channel. Since all normal call processing operations must remain active while using a reference measurement channel, these portions of the air interface have the same structure as they would for any other circuit switched connection.

Real Time Channel Structure Variations
While the System Simulator is free to fill the DTCH with any test data it desires, the DCCH will only have data to carry when there is a valid need for system signaling; there is no defined "dummy" message with which to fill the downlink DCCH when no signaling messages are being carried. Thus, while the DTCH will always be present in a test environment, the DCCH will only be present intermittently. This adds to the challenge of achieving proper link configuration for BER testing.

The W-CDMA air interface has several ways of dealing with a channel being "not present". In order to understand the implications of these it is important to understand how the two logical channels, the DTCH and DCCH, end up combined into a single Dedicated Physical Channel (DPCH). The complete description of this is found in references 3 and 4. The best place to start to understand this is with the Transport Channel Multiplexing function, and then, to work out in both directions to rate matching and interleaving.

The number of possible arrangements of the W-CDMA air interface numbers in the millions. For any given connection only a small subset of these are needed. To make operation practical, that subset, known as the Transport Format Combination Set, is communicated from the network to the UE at the time of connection setup. The TFCS includes all of the allowable Transport Formats (TF) and the associated data capacity for each of the channels that can be present in the link, and all of the allowable Transport Format Combinations (TFC) for the link.

The Network's Radio Resource Control (RRC) entity provides this information to its lower layers. The UE's RRC entity does the same for its lower layers upon receiving the TFCS from the network.

Once this information is shared between the two, the transmitter can use it, along with the demands for transmission capacity from higher layers, to decide which channels shall be present and how each channel will be arranged in the radio frame. Likewise the receiver can use it to determine which channels are present and how to recover each channel that is present.

The W-CDMA system provides two methods for this determination to be made. The first of these is the inclusion of a Transport Format Combination Indicator (TFCI) in each radio frame. The second is Blind Transport Format Detection (BTFD).

When TFCI is used, the transmitting side determines which Transport Format Combination it will use. It then includes the TFCI, which is an index to the list of allowable combination in the TFCS, in the control portion of the DPCH. The receiver always knows how to recover the TFCI, which it then uses to determine which channels to try to recover how to decode each one.

When BTFD is used, the receiver must try every allowable TFC in the TFCS to determine which one results in the least errors.

The Reference Measurement Channels require the use of TFCI in both the up and down links.

Transport Channel Multiplexing and Rate Matching
For the downlink there are two modes of multiplexing the two channels: Fixed Transport Channel (TrCH) Positioning and Flexible TrCH positioning. For the uplink the TrCH multiplexing method is always flexible positioning. (The specifications do not bother to make the flexible vs. fixed distinction for the uplink). These figures illustrates the differences in the arrangement of the channels in the CCTrCH, at the point in the channel coding scheme where the multiplexing occurs, and then after the 2nd interleaving. (note that these figures are only illustrations, not a specific case)

Right away it is possible to see some significant differences between these two; and, some apparent problems. These differences and apparent problems have an effect on and are resolved by rate matching and Discontinuous Transmission (DTX).

The difference between these two forms of multiplexing is that with fixed positioning, a portion of each frame is dedicated to each of the channels and those portions do not change. With flexible positioning whichever channel is present occupies a position starting at the beginning of the frame, and it fills as much of the frame as it requires. Note that when both channels are present, the two forms of multiplexing produce the same result. (Note: this is not necessarily true for more complex channel structures than those used for the Reference Measurement Channels) One other interesting aspect of W-CDMA is that the channels are typically configured such that when both channels are present there are not enough bits available in the frame to carry all of the coded-up bits of both channels; thus, some amount of the bits for each are thrown away by the rate matching that precedes multiplexing. This is why TrCH positioning method, rate matching, and channel payload all interact to affect which locations in the radio frame are occupied by which channel.

Rate matching is one of the more difficult aspects of W-CDMA to understand. Is has both static and dynamic aspects, and the behavior is different for the uplink and downlink, with the downlink behavior also changing with TrCH multiplexing method.

The static aspects are set by RRC based on the type of service being carried by the radio link. The network RRC communicates these to its lower layers and the UE RRC, which then uses that information to set the static aspects of its rate matching. Both sides of the link then use this static information in combination with the capacity requirements of the various radio bearers to determine the capacity needed for each transport channel, and ultimately, the dynamic rate matching for each of those channels. The resulting rate matching parameters are one of the many things indicated by the TFCI, if it is used.

Both the up and down links are typically configured to have slightly less capacity than that required to carry all of the transport channels at their full capacity. Puncturing is used to reduce the bit count to an amount that fits in the physical channel.

In the downlink when the required total capacity is less than that of the physical channel, DTX occurs for the unused portions of the radio frame. If the TrCH multiplexing method is flexible, rate matching is first adjusted by reducing the amount of puncturing to allow the channels that are present to fill as much of the radio frame as possible.

Unlink the downlink; where the DPDCH and DPCCH are time multiplexed and spread by the same spreading function, the uplink DPCCH, which carries the TFCI, and DPDCH, which carries the transport channels, are IQ multiplexed and spread by different spreading functions that can (and usually do) operate at different rates. The DPCCH is always spread at a rate of 256. The DPDCH is given a minimum spreading factor during connection setup but the UE is allowed to increase its spreading factor up to 256 when the information rate drops low enough that the data capacity provided by the low spreading factor is no longer needed. For the 12.2 kbps Reference Measurement Channel the uplink DPDCH will be spread at 64 when both the DCCH and DTCH are present or when the DTCH alone is present; but, the spreading rate will go up to 256 when either the DCCH is present alone. When neither channel is present the DPDCH is not transmitted at all.

TEST SYSTEM CONFIGURATION The manner in which one must deal with these issues for BER testing depends on whether or not the tester in use supports the over-the-air signaling procedures used to establish a connection with the UE. When the test system utilizes a System Simulator without call processing, the test operator must control both the tester and the UE to achieve the correct configuration on both ends of the radio link. For the downlink the configuration parameters that must be correctly set are:

* Frequency (UARFCN) and Primary Scrambling Code

* DPCH Spreading Factor, TrCH Multiplexing, and Slot Format (as per the 12.2 kbps RMC)

* DPCH Channelization Code

* DPCH Transport Format Combination, at least so far as the DTCH bits are concerned.

* If the UE is set to respond to TFCI; then, the TFCI transmitted by the SS must at least indicate to the UE a transport format combination that allows it to correctly recover the DTCH.

If the tester is a System Simulator that does support signaling, then the correct configuration of the UE is achieved by instructing the SS to setup the required channel configuration. The SS then constructs and transmits connection setup messages that describe to the UE, among other things, the set of allowable transport format combinations and the mapping of those combinations to TFCI indications.

It is also important to remember when the DCCH is not present, the signal characteristics during the portions of the frame allocated to carrying the DCCH are irrelevant as long as they're not so strange as to confuse the receiver's ALC or its pilot detection.

REFERENCES References available upon request

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