Test & Measurement | February 06, 2007

New challenges for power amplifiers in UMTS user equipment

Mobile radio technology, based on UMTS, is developing at breakneck speed. Network operators worldwide are currently upgrading their networks with the new high speed downlink packet access (HSDPA) technology.
HSDPA optimizes the data transmission from the network to the user equipment (downlink). New transmission methods allow data rates of several Mbit/s. Theoretically, data rates of up to 14 Mbit/s are possible. In addition, HSDPA increases the capacity of mobile radio networks. Another advantage for mobile radio subscribers are faster service access times to a data service. HSDPA is part of release 5 of the 3rd Generation Partnership Project (3GPP) specifications, which are relevant for UMTS. But this by no means marks the end of the development of UMTS technology. Manufacturers of mobile radio infrastructure and user equipment are already working hard on implementing high speed uplink packet access (HSUPA), a part of the 3GPP release 6. HSUPA contains a large number of improvements for the data transmission from the terminal to the network (uplink): data rates of several Mbit/s, higher throughput and faster access times. HSUPA is expected to be launched commercially in 2007.

The fusion of HSDPA and HSUPA is also referred to as high speed packet access (HSPA). All data services in which large volumes of data are transmitted in both directions and which require a fast interaction between the downlink and the uplink will benefit from HSPA. Mobile office applications, voice over IP and videoconferencing are examples of such applications.

New techniques on the air interface
Advantages of HSPA will be achieved by using new techniques on the air interface. This includes, for example, the introduction of a fast data transmission protocol (hybrid automatic repeat request, HARQ) in the uplink and downlink which allows the recipient to automatically request that the transmission of errored packets be repeated. In contrast to UMTS, where new data packets can only be transmitted every 10 ms, HSPA allows transmission every 2 ms. Thus, the network is able to respond more quickly to changed channel conditions. The basic structure of the UMTS transmission frame with a length of 10 ms and 15 time slots is retained. Only subframes of 2 ms and 3 time slots each will be introduced for HSPA. One data packet per subframe can be transmitted .

Another major innovation in HSPA involves the base station. The base station is responsible for assigning the resources for the data transmission with HSDPA and HSUPA. It continuously performs measurements to determine the uplink transmission conditions, on the basis of which the. scheduling algorithm in the base station makes the decision regarding the assignment of the uplink resources available. The base station analyzes regular measurements of the mobile phones based on the channel quality to ensure an optimum assignment of the downlink resources.

HSPA not only requires numerous changes in physical transmission parameters and the introduction of new physical transmission channels but also changes in the protocol architecture. The implementation of HSPA for the manufacturers of net work elements, terminals, chipsets and modules is not an easy job at all.

The following pages describe the difficulties which have to be overcome particularly in the design of power amplifiers for HSPA-compatible terminals.

New uplink channels for HSPA
UMTS and also HSPA use the wideband code division multiple access (WCDMA) method for multiple access to the air interface. Instead of dividing the resources per frequency or time multiplex into channels, the signal is distributed across the entire frequency spectrum by code spreading. The spreading factor is indicated as a measure of the code spreading. Lower spreading factors allow the transmission of larger data volumes but less spreading gain.

Before introducing HSPA, basically two uplink channels were relevant for data transmission in UMTS: the dedicated physical data channel (DPDCH) for transmitting payload and the dedicated physical control channel (DPCCH) for transmitting control information. The DPDCH and DPCCH are transmitted using the IQ multiplex method in the uplink (see Fig. 1). The DPDCH is located on the I domain, the DPCCH on the Q domain. The DPDCH and DPCCH are individually spread; the DPDCH has a variable spreading factor cd depending on the data rate, while the DPCCH is always located on code 0 having the spreading factor 256.

Fig. 1 Transmission of the DPCCH and DPDCH uplink channels

A new uplink channel, the high speed dedicated physical control channel (HS-DPCCH), is needed for control purposes although HSDPA targets an enhancement of the downlinks. This channel is primarily used to transmit measurements for determining the channel quality from the terminal to the network. This information is also called channel quality indicator (CQI). In addition, the correct reception of the packets received in the downlink is confirmed on the HS-DPCCH. For this purpose, the terminal sends an ACK (acknowledgement). If an errored data packet is received, it can be requested again by using a NACK (negative acknowledgement) on the HS-DPCCH. The HS-DPCCH is transmitted as third code channel in the uplink and is located on the Q domain. As a result, a view of the code domain as shown in Fig. 2 is obtained. The HS-DPCCH is located on code 64 having the spreading factor 256, which corresponds to 15 ksymbols per second (ksps).

Fig. 2 Uplink code domain with HS-DPCCH

Due to its special structure, the HS-DPCCH causes some difficulties with the development of power amplifiers. The HS-DPCCH is not transmitted continuously but only when information has to be sent from the terminals. This results in a burst-like structure. Additionally, it can be predefined in the network that the transmit power is changed depending on the information to be transmitted on the HS-DPCCH . Thus, an ACK can be sent with a higher power than a NACK, for example. The CQI information can also be individually scaled. The HS-DPCCH is not necessarily based on the other uplink channels with respect to time. For this reason, the fluctuations in performance caused by the HS-DPCCH do not always occur on the timeslot borders of the dedicated channels DPDCH and DPCCH. Instead, the transmission of a HS-DPCCH subframe may suddenly start in the middle of a DPCCH timeslot. In this case, the terminal has to adapt the absolute transmit powers with respect to the single uplink channels to avoid exceeding the maximum permissible transmit power.

The introduction of HSUPA is accompanied by two other channel types in the uplink: the enhanced dedicated physical data channel (E-DPDCH) for transmitting user data with high data rates and the enhanced dedicated physical control channel (E-DPCCH) for transmitting control information. It should be noted that there can be up to four E-DPDCH channels per terminal at once. Depending on the data volume and transmission power available, the terminal selects how many E-DPDCH channels are required for transmitting the data packet and which spreading factor is necessary. The arrangement of the E-DPDCH channels and E-DPCCH channel on the IQ domain is defined for every possible configuration of the standard. Fig. 3, for example, shows the resulting code domain for four E-DPDCHs, measured on a spectrum analyzer. This configuration allows the maximum data rate of 5.76 Mbps for HSUPA. The E-DPCCH is always transmitted on the I domain and is located on code 1 with spreading factor 256. In this case, the HS-DPCCH is located on code 33 with spreading factor 256.

Fig. 3 HSUPA uplink signal with four E-DPDCHs

The new uplink channels for HSUPA also have to be taken into consideration with the development of the power amplifiers in the terminal.

Setting the channel power
A special method is used to set the power of the different channels in the uplink. Gain factors (also called beta factors) for the single channels are signaled from the network to the terminal during the connection setup. For the DPCCH they are referred to as c, for the DPDCH ßd, for the HS-DPCCH ßhs, for the E-DPCCH ßec and for the E-DPDCH ßed. The gain factor is used to describe a particular amplitude ratio that is assumed by the observed channel in relation to the DPCCH. From this amplitude ratio it is possible to determine the power value for the observed channel in relation to the DPCCH. Table 1 shows this using the E-DPCCH as an example.

Table 1 Amplitude ratio between E-DPCCH and DPCCH

The values that are signaled from the network to the terminal for the E-DPCCH are listed in the left-hand column of table 1. The standard stipulates how these values have to be interpreted by the terminal and which amplitude ratio, Aec = ßec / ßc, is to be set between E-DPCCH and DPCCH. The resulting power ratio, which can be easily determined by calculating 20 * log (ßec / ßc), can be read in the right-hand column of table 1.

The terminal can now scale all channels in the uplink to each other by using the gain factors. Please note that the gain factors describe the ratio of the channels relative to each other. This ratio remains unchanged irrespective to the total power of the uplink signal. The total power of the uplink signal is preset by mechanisms already known from UMTS. Fast power control in the uplink is crucial. To make this possible, the terminal has to follow the commands of the network, by which the transmit power can be incrementally increased or decreased. It is therefore essential that the maximum permissible output power for the terminal class observed is not exceeded.

Peak-to-average ratio
The ratio of peak power to average power of a signal (peak-to-average ratio, PAR) is a particularly important parameter for dimensioning power amplifiers. This topic was extensively discussed when the HSDPA standard was specified in 3GPP release 5 since the HS-DPCCH as third code channel at times significantly increases the PAR of the uplink signal. This results in larger dynamic variations in the signal and stricter requirements on the power amplifier's linearity. It becomes more difficult to remain distortion-free and to adhere to the limits for the permitted adjacent channel interference and for the modulation quality. Moreover, the power amplifiers used are supposed to operate efficiently also in terms of the costs. For this reason, the HSDPA standard allows a reduction of the maximum output power of the terminal for the periods during which the HS-DPCCH is sent. The required reduction of the power depends mainly on the amplitude ratio between DPCCH, DPDCH and HS-DPCCH. The permissible reduction of the maximum output power is therefore specified depending on the gain factor combinations ßc, ßd and ßhs. However, the standard only allows a reduction of the output power for very critical combinations, since a large reduction of the transmit power may affect the cell coverage.

In the 3GPP, 99.9 % PAR was used as a benchmark for evaluating the required reduction of the transmit power. Different HSDPA uplink signals were compared with a reference channel that consisted of only DPCCH and DPDCH. As a result, the 3GPP release 5 stipulated that the maximum transmit power of terminals of power classes 3 and 4 as specified in 3GPP is allowed to be reduced by up to 2 dB depending on the gain factor combination if a HS-DPCCH is transmitted. The result also took into account that even higher PAR values than those described by the 99.9 % PAR very often occur in a HSDPA signal.

Cubic metric
With the introduction of HSUPA, this procedure has been put to test again. The experience obtained through HSDPA showed that the increase of the PAR cannot be transferred 1:1 in dB to the required reduction of the transmit power. Therefore with the introduction of HSUPA, a new benchmark, the cubic metric (CM) was defined in 3GPP release 6. It is a more precise value for predicting the required reduction of the transmit power to ensure the desired uplink performance. The cubic metric applies equally to HSDPA and HSUPA uplink signals. Depending on the observed channel configuration, the cubic metric is determined in the uplink and describes the ratio of the cubic components in the observed signal to the cubic components of the reference signal. A simple voice signal was selected as reference signal. The detailed calculation formula can be found in the 3GPP specifications. The cubic metric is based on the fact that third-order nonlinearities in the characteristic of the power amplifier are the main reason for adjacent channel interference in the uplink. Therefore, in the 3GPP release 6 specifications the permissible reduction of the transmit power is determined as a function of the cubic metric of the observed signal in the event that HS-DPCCH and/or E-DPDCH / E-DPCCH channels are transmitted.

Compliance with the heightened requirements for HSPA depends on the implementation of the power amplifier, of course. Therefore, the 3GPP stipulates various test specifications for transmitter tests.

Measurements in accordance with the standard
A key test specification applies to checking the maximum output power of HSPA-compliant terminals. This test verifies whether the terminals adhere to the permissible transmit power depending on the observed signal configuration. Fixed values are defined for the 3GPP specifications of release 5, which have to be applied depending on the gain factor combination of the observed signal. As of release 6, the permissible reduction of the output power in the uplink is based on the cubic metric.

Another important test describes the permissible transmission mask of the HS-DPCCH, which as already mentioned, has a burst-like structure. For this reason, it is important that the changes in power remain within specific limits when the HS-DPCCH is switched on or off. Fig. 4 shows an appropriate measurement on a mobile radio tester.

The transmitter tests for the spectrum mask, adjacent channel interference (adjacent channel leakage ratio, ACLR), modulation quality and phase accuracy which were already known before the introduction of HSPA are accordingly expanded for HSPA. For example, an ACLR measurement on an uplink signal containing a HS-DPCCH is shown in Fig. 5. The measurement was carried out on a mobile radio tester. Fig. 6 shows a noteworthy measurement of the modulation quality, which is evaluated on the basis of the error vector magnitude (EVM). The effect shown is produced by starting the transmission of a HS-DPCCH subframe in the middle of a timeslot of the DPCCH.

Special uplink reference signals are defined for all test specifications for transmitter tests as specified in 3GPP. Some channel combinations have been representatively selected as subtests, since the combination of the gain factors considerably influences the signal characteristics. Transmitter tests for HSPA-compatible terminals have to pass all subtests, i.e. they have to fulfill all requirements for different gain factor combinations.

The test specifications described here are not only relevant for conformity tests. They can also be used to determine the quality of an implementation in the early stages of a development.

Future prospects
HSDPA and HSUPA present special challenges for the development of power amplifiers. HSDPA-compatible terminals have been on the market for some time and have proven their success in practice. The first commercial HSUPA-compatible terminals, however, are still being developed and optimized. With the help of powerful T&M equipment, these challenges in the early stages of the development can be tackled.

The development of HSPA is not yet completed. To make the new technologies much more efficient, the 3GPP release 7 will contain additional enhancements for HSPA.

Christina Geßner

Christina Geßner has been a technology manager for mobile radios at Rohde & Schwarz headquarters in Munich since 2004. Her tasks include the development and marketing of the T&M product portfolio for UMTS and HSPA. After completing her studies in electrical engineering with emphasis radio-frequency engineering at the University of Hannover in 1998, Ms Geßner first worked in the strategic product management of the mobile radio networks division at Siemens in Berlin and Munich.


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