Analysis of WCDMA transmitter

Overview

In the early 80, the mobile communication system, which was mainly characterized by analog technology, was called the first generation mobile communication system. The digital mobile communication system, which was commonly used in the 90 by GSM and IS-95, is called the second-generation mobile communication system. The new generation mobile communication system, named IMT-2000, which is characterized by intelligent mobile communication technology, is called the third generation mobile communication system. Compared with the previous two generation systems, 3G systems have greater capacity, better communication quality and higher bandwidth utilization, which makes it possible to provide voice, data, conference TV and multimedia services for high-speed and Low-speed mobile users. And users can roam around the world seamlessly. (Computer science)

In 1992, the World Radio Conference (WRC) distributed a frequency band around 2GHz, and then the International Telecommunication Union Wireless Communications Department (ITU-R) began to define a list of requirements for a 3G system, proposing many techniques to meet these requirements: WCDMA, OFDM, TDSCDMA and ODMA. Technical entities of the third generation Partner Project (3GPP) are assigned to analyze these proposed technologies. The result of this work is that WCDMA has become the most preferred technology for 3G systems. 3GPP has written a technical specification, 25.101 of which includes the core performance requirements of the RF hardware part of the WCDMA mobile terminal. 3GPP also defines the two alternative modes of operation for WCDMA terminals:

Frequency Division multiplexing mode [FDD]:

• Physical channel is determined by two parameters: RF channel number and channel code

• Suitable for fast mobile applications

• Uplink and downlink links are separated in frequency domain

• Downlink links are larger than uplink capacity

• Both uplink and downlink are 100% duty-free

Time Division multiplexing mode [TDD]:

• Physical channel is determined by three parameters: RF channel number, channel code and time slot

• Suitable for indoor or slow moving applications

• Upstream and downlink links have similar capacity and occupy the same channel

• Both uplink and downlink have DTX

DTX (discontinuous transmission) is a method for optimizing the efficiency of a wireless voice communication system, which shuts down mobile or portable phones at any time without voice input. In a typical 2-way call, each party is slightly less than half the total time, so if the transmitter is only in the presence of voice input, the duty ratio of the telephone work can be less than 50%.

This situation can save the battery energy, reduce the workload of the transmitter components, make the channel more idle, allowing the system to use the free bandwidth and other signals to share the channel. DTX uses voice activity detection (VAD) circuits, which are sometimes called working voice transmissions (VOX) in wireless transmitters.

3GPP also regulates the FDD terminals using only 60MHz bandwidth, with a duplex interval of 190mhz:2110mhz-2170mhz for mobile Rx,1920mhz-1980mhz for mobile TX.

CDMA principle

Before discussing WCDMA transmitter, this part makes a brief introduction to the principle of CDMA. The signal extension mode used by CDMA system is "direct sequence" extension. In order to extend the signal, the CDMA system uses a unique code called an extension code multiplied by the not-modulated baseband data, which contains a certain number of code pieces.

The resulting extended data is modulated to the carrier for launch, and the modulated carrier bandwidth is directly affected by the spread-spectrum coding rate. WCDMA uses the 3.84MHZ chip rate to produce a wide bandwidth of the emission spectrum, so the term "broadband" is used.

In order to extract the original information, the CDMA receiver demodulation the information carrier and uses the Correlator (with the original transmitter spread spectrum code) to regenerate (extend) the desired signal. The extracted data is processed further as needed through a narrowband bandpass filter.

Specification requirements for WCDMA transmitters

3GPP specification of FDD3G Mobile Terminal RX/TX Electrical specification requirements. Before discussing the requirements of WCDMA transmitters, this section will describe several key transmitter parameters and their importance in the design of the transmitter.

Proximity channel power ratio [ACPR]:ACPR measures the size of the interference or adjacent frequency channel power. Typically defined as the ratio of average power in the adjacent channel (or offset) to the average power in the transmitting channel, ACPR describes the distortion caused by the nonlinearity of the transmitter's hardware.

ACPR is critical for WCDMA transmitters, because CDMA modulation produces closely adjacent spectral components in modulated carriers. The mutual modulation of these components results in the regeneration of the spectrum of the central carrier, and the nonlinearity of the transmitter will make these spectral regeneration elements enter the neighboring channel.

Error vector amplitude [EVM]: the error vector (including amplitude and phase loss) is the vector difference between the ideal error-free reference signal and the actual emission signal at a given moment.

This new parameter (EVM) is defined as the RMS value of the error vector for a period of time, because it is also changing as each symbol changes.

EVM is also very important for the performance of WCDMA transmitter, because it indicates the modulation quality of the transmitting signal. A large EVM value will result in poor detection accuracy, thus reducing the performance of the transceiver.

Frequency error: The difference between the specified carrier frequency and the actual carrier frequency. Due to the adjacent channel interference and low quality detection accuracy, large frequency error reduces the performance of transceiver.

Spurious and harmonics: spurious is the signal generated by different signal combinations in the transmitter, and harmonic is the distortion product produced by the nonlinearity of the transmitter. Harmonics are generated on the integer multiplier frequency of the transmitting signal frequency. After defining some key transmitter parameters, we now list some important requirements for specification and design of 3GWCDMA transmitter terminals.

Analysis of WCDMA transmitter

WCDMA radio frequency of the whole machine index relative to the 2G strict a lot, mainly in the signal purity (leakage), high dynamic range, modulation bandwidth (5MHz), complex code domain analysis. In order to realize the external performance of the whole machine to meet the requirements of protocol specification, it is very important to allocate the index of the system module. Therefore, in the development phase of the single Board and the main components of the indicators are more stringent requirements. Maxim provides a variety of WCDMA transmitter ICS, covering most of the common frequency range, where a scheme is selected as an example, see figure.

WCDMA Superheterodyne Transmitter

This transmitter is part of the complete WCDMA transceiver reference design and contains 4 major ICS:

MAX2388 Receive front end

MAX2309 IF quadrature demodulator

The MAX2363 orthogonal modulator/upper frequency converter transmitter IC

The MAX2291 RF power amplifier

The transmitter hardware uses the 380MHz if and 1920MHz to 1980MHz TX frequencies. The duplexer realizes full-duplex work by connecting the TX channel (with the RX channel) to the antenna.

At the back end of the TX circuit, the MAX2363 receives baseband transmission of I and Q differential signals as input, orthogonal modulation, if and Rflo frequency synthesis and RF frequency conversion. The Iflo is synthesized by internal VCO and PLL, and the frequency is 760MHz. The -7DBM signal provided by the external Rfvco module is input to the MAX2363 on the high end injection mode. On-chip RF drives enable the chip to directly drive external Pa.

At the front of the TX circuit, the Linear PA (MAX2291) in the chip-level package provides 28dB gain and output power up to +28dbm in this application. Since the insertion loss of PA is about 4dB, the maximum antenna output of the system is 24dBm.

After fully entering the working state, WCDMA systems work most of the time in medium power rather than full power. MAX2291 provides two optimized modes of output power to meet this requirement, extending the call time with the following expected performance:

VCC is 3.5VDC and is measured in high power mode:

pout=28dbm

Frequency =1.95ghz

ACP1=-39DBC (measured in 5MHz offset, 3.84MHz bandwidth)

Power Additional efficiency =37%

Standby Current Icc=97ma

VCC 3.5VDC, low power mode measured:

pout=16dbm

Frequency =1.95ghz

ACP1=-38DBC (measured in 5MHz offset, 3.84MHz bandwidth)

Power Additional efficiency =14%

Standby Current icc=30ma

The 3GPP specification provided in front stipulates that the output power of the WCDMA transmitter must be between +24DM and -50dbm to meet the required 74dB dynamic range. The v1.0 reference Design board is designed as a 80dB dynamic range, leaving some margin.