Vankka J. - Digital Synthesizers and Transmitters for Software Radio (2000)(en)

Chapter 16

16. A GSM/EDGE/WCDMA MODULATOR WITH ONCHIP D/A CONVERTER FOR BASE STATIONS

16.1 Supported Communication Standards

The Global System for Mobile communication (GSM) is a second generation (2 G) system that has rapidly gained acceptance and a worldwide market share. As the mobile communications market develops, interest is building up in data applications and higher data rate operations. Short message services (SMS) were first added to the GSM system followed by high-speed circuit switched data (HSCSD) and the general packet radio service (GPRS). All of these services use the same modulation format as the original GSM network (0.3 Gaussian minimum shift keying (GMSK)), and change the allocation of the bits and/or packets to improve the basic GSM data rate. As a step towards 3G, enhanced data rates for GSM evolution (EDGE) provides a higher data-rate enhancement of GSM. It uses the GSM infrastructure with upgraded radio equipment to deliver significantly higher data rates. The primary objective of the EDGE signal is to triple the on-air data rate while taking up essentially the same bandwidth as the original 0.3 GMSK signal. The wideband code division multiple access (WCDMA) was selected by the European Telecommunications Standards Institute (ETSI) for wideband wireless access to support 3G services because of its resistance to multi-path fading, and other advantages such as increased capacity. This technology has a wider bandwidth and different modulation format from GSM or EDGE.

The first generation of the 3G base station modulator should include support for GSM, EDGE and WCDMA. The digital IF modulator is designed using specifications related to those standards [GSM99c], [TDD00], [FDD00]. The main requirements of the modulator are shown in Table 16-1. By programming the GSM/EDGE/WCDMA modulator, different carrier

A GSM/EDGE/WCDMA Modulator with On-Chip D/A Converter for 299 Base Stations

modulating data value [Dig99].

The differentially encoded data stream is filtered in a Gaussian pulse shaping filter. The impulse response of the pulse shaping filter is defined as

where denotes convolution and the rectangular function rect(x) is defined by

2ʌBT

where B is the 3dB bandwidth and T is the duration of one data bit. The GSM specification requires that BT = 0.3 [Dig99]. This definition is theoretical and the realization of (16.5) would be a filter of infinite length. This theoretical filter is associated with the tolerances defined in [GPP00c].

The phase of the modulated signal is

i

where the modulation index h = 1/2. This value implies that the maximum phase change during the data interval is /2 radians [GPP00c].

The modulated intermediate frequency (IF) signal at the useful part of the burst can be expressed as

where Ec is the energy per modulating bit, f is the carrier frequency and φ0 is a random phase, which is presumed to be constant during one burst [Dig99].

The modulation accuracy of the GSM is defined by the phase error, i.e. the difference between the phase error trajectory and its linear regression on the active part of the time slot. The spectral properties are defined by the spectrum mask and the timing of the burst is defined by the time mask. The required levels and the definitions of the performance metrics are specified in [GPP00c].

16.1.2 EDGE System

The Enhanced data rates for GSM evolution (EDGE) is a high-speed mobile data standard, intended to enable the second-generation GSM and TDMA networks to transmit data up to 384 kilobits per second. The EDGE provides the speed enhancements by changing the type of modulation. It triples the on-air data rate while meeting the same bandwidth occupancy (200 kHz) as the original GSM system. A linearized Gaussian 3 /8-8PSK modulation scheme [Lau86], [Jun94] is applied in the EDGE.

A block diagram of the EDGE system is shown in Figure 16-2. Although this section presents the whole signal generation chain in Figure 16-2, the blocks preceding the pulse shaping filters are left out from the implemented circuit presented in Figure 16-6.

In the EDGE system the data bits, arriving at a of 812.5 kbit/s, are Graycoded from groups of three bits into an octal-valued symbol l according to Table 16-2. The Gray coding ensures that if a symbol is interpreted erroneously as an adjacent symbol, the error occurs only in one bit. It thus reduces the bit error probability. This way the symbol rate 812.5/3 ksym/s = 270.833 ksym/s will equal the symbol rate in the GSM system. After the Gray coding, an 8PSK modulation is performed. The 8PSK symbols are achieved by the rule

where l is given by Table 16-2. The 8PSK symbols are continuously rotated with 3 /8 radians before pulse shaping. The rotated symbols are defined as

Figure 16-2 Generation of EDGE signal.

Table 16-2 Gray-coding of binary bit triplets into octal symbols

The in-phase and quadrature branches are obtained from the real and imaginary parts of y(t), respectively:

where φi is the angle of the rotated symbol dži. A constellation diagram of the baseband signal is presented in Figure 16-4. It can be clearly seen that the signal does not pass the region around zero.

The modulated IF signal is

where f ut is the carrier frequency.

The error vector magnitude (EVM), i.e. the magnitude of an error vector between the vector representing the actual transmitted signal and the vector representing the error-free modulated signal, is used to define the accuracy of the modulation in the EDGE system. The spectrum mask and the time mask define the spectral and timing properties of the signal, respectively. These performance metrics are defined and the required levels are specified in [GPP00c].

Equations (16.11) - (16.14) given by [Dig99] are a very complicated way

where P(t) is an M degree polynomial. Due to the even symmetry of the pulse c'0, it can be assumed that the odd coefficients are zero. The coefficients of the polynomial P(t) are achieved by calculating c'0 using (16.11) and fitting an M degree polynomial to the natural logarithm of the result. For

M = 6,

which is valid for all t and does not involve integrated error functions. The calculated peak and root mean square (rms) errors of the compared exact and approximate pulses over the interval -5T/2 ≤ t ≤ 5T/2 are 1.6% and 0.27%, respectively. This approximation is advantageous during the system level simulations when generating a reference signal for a device being designed. During this generation, the coefficients of the pulse shaping filter have to be recalculated for each symbol if the sampling rate is simultaneously converted with some rational number ratio, which is often the case with multimode systems. This can be efficiently computed using this approximation.

16.1.3 WCDMA System

The Wideband Code Division Multiple Access (WCDMA) system uses a Quadrature Amplitude Modulation (QAM). The modulating chip rate for WCDMA is 3.84 Mcp/s. From the modulator's point of view, the chip rate equals the symbol rate, and so the term symbol rate is used. Figure 16-5 presents the QAM modulation of the complex-valued chips sequence generated by the spreading process. The spreading process is described in [GPP00a], for example.

The I and Q branches, obtained from real and imaginary parts of the complex-valued chip sequence respectively, are pulse shaped using a Root- Raised-Cosine filter in order to reduce the signal bandwidth. The impulse

cos( t)

root raised I cosine filter ( = 0.22)

-sin( t)

Figure 16-5 QAM modulation.

used transmission bandwidth. The roll-off factor is specified as α = 0.22 in the WCDMA applications [GPP00b].

The signal is upconverted by multiplying it with a sinusoidal carrier according to the equation

where I(t) and Q(t) are filtered I and Q symbols and f ut is the carrier frequency [GPP00a].

The performance of the WCDMA signal is measured by the EVM, adjacent channel leakage power ratio (ACLR) and peak code domain error (PCDE). The required levels and definitions of the performance metrics are specified in [GPP00b].

16.2 GSM/EDGE/WCDMA Modulator

The block diagram of the modulator chip is shown in Figure 16-6. The use of different modulation formats requires programmable pulse shaping filter coefficients. The reconfiguration of new modulation formats can be achieved between bursts (e.g., GSM/EDGE). The two half-band filters increase preoversampling ratios, which reduces the complexity of the re-sampler (the order of the polynomial interpolator). The re-sampler circuit allows the sam-

Figure 16-6. GSM/EDGE/WCDMA modulator chip. The symbol rates (Fsym) are shown in

Table 16-1.