= C zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA which tail - PDF document

Anne Wiesler, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Software Radio structure for second generation mobile communication systems structure is described. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA student member IEEE and Friedrich Jondral, senior member IEEE Institut fur Nachrichtentechnik, Universitat Karlsruhe Abstract-Third generation mobile communication the air interface. In the next section a common transmitter A common receiver structure is systems like the European UMTS will enable a flexible communication, free from standard specific regulation proposed in Section I11 with main emphasis on a general of modulation, channel coding, baud rate and multiple equalizer. access schemes. This flexibility can only be reached by a radio structure which performs all baseband functions in 11. COMMON TRANSMITTER STRUCTURE software and is therefore totally software programmable [l]. To ensure that different software configurations can The generation of the transmission signal is exactly be understood and supported by all mobile terminal specified in the standards. Functions can be defined architectures a general programming language is required with a few parameters, for example the convolutional to describe the used air interface components. By the channel coding can be exactly described by the generator example of the second generation mobile communication polynomials (in binary representation), the code rate systems like the European GSM, the Japanese PDC, the (which is not always equal to the number of generator American IS-54 (respectively IS-136) and the European polynomials), the constraint length, a parameter that sets wireless communications system DECT, a common de- a termination on or off. In the case of termination the scription and implementation of the transceiver functions number of bits in one block and a parameter that specifies like channel coding, modulation and equalisation have = C zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA which tail bits are used must be set additionally. In most been developed. cases the tail bits are zeros, but in the IS-54 System for FACCH-bits there are the first information bits used. In the same way the block channel coding, the puncterer, the I. INTRODUCTION interleaver, the scrambler (for DECT) and a general burst Third generation systems like the European UMTS builder can be described and implemented. It is more will realize two goals with a more flexible communication difficult to find a common structure for the modulation technology: Firstly a global roaming will be possible. function. Secondly a dynamical adaption of the air interface to the time variant radio channel, the service and the cell type A. rl4-DQPSK s D Q p s K ( t ) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA can be executed during each communication session. This flexibility can only be achieved by a so called software The IS-54 and PDC use the .n/4-DQPSK, a special case radio, this means with a totally software programmable of QPSK [3]. The complex envelope of a n/4-DQPSK mod- radio structure. Changing to another mobile communi- ulated signal with the symbol duration T is cation system can be done by downloading the needed 03 software and reconfiguring the mobile terminal. To ensure .g(t - .T) 2 , (1) that the configurations can be understood and supported n=O by different mobile terminal architectures a general = C 00 . g ( t - n ~ ) . (2) used in a similar way and zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA exp [@(.)I programming language is required to describe the used air n = O interface components. The phases of the complex symbols z, = As a first approach a common description and implemen- exp [@(.)I are tation of the second generation mobile communication parameters, which enables a seamless and fast change of zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA are alternately out of {-1,1, - j , j } and zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA systems GSM, PDC, IS-54 and the wireless communi- cations system DECT have been developed. As they 2363 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA all are TDMA/FDMA systems a lot of functions are At a time two bits are assigned to one of four possible 0-7803-4320-4/98/$5.00 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA differential phases like it is shown in Tab. 1. so a parametrized software Thus the information is placed into the differ- implementation is proposed. This has the advantage that ential phase of two successive symbols. The sym- not the whole software of a system has to be downloaded. bols z, The reconfiguration can be done by exchanging a set of 1 + j ) , { z ( &(-l + j ) , $1. The - - j ) , *(-I V C 0 1998 IEEE ‘98

was chosen, that results in a IS1 over about 2 symbols but sian impulse zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA a small bandwidth. This IS1 is equalized together with the Table 1: Bit to differential phase assignment L 2 3. Thus at time t = n T the phase reply is zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA IS1 caused by the mobile channel at the receiver (Section pulse former zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA roll off factor zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 111). Indoor or micro cell channels do not cause long path delays and therefore at the DECT-system BT = 0.5 was (7) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA chosen which results in minor IS1 combined with a smaller t zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA bandwidth efficiency. Q zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA differential assignmen: zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA For the realisation of the theoretically infinite long Gaus- as FIR-filter it is cut to the length LT, with g ( t ) is a square root raised cosine filter with = 0.35 for IS-54 and Q = 0.5 for PDC. = J s ( r ) d r . q(t) This means that the two nyquist criteria are fulfilled -ca and no intersymbol interference (ISI) is produced by the q ( t ) is shown in Fig. 1 . filtering. The modulation is not linear because of the (3), but it can be realized with a I/Q-Modulator. A disadvantage of this theoretically very q(t) bandwidth efficient modulation is the high amplitude fluc- tuation. However the fluctuations are smaller compared to the one of the original DPSK [5], the requirements at the linearity of the end power amplifier are still very high. Economical C-power amplifiers can not be used in Japanese and American mobile terminals because of the high nonlinear distortions causing a strong increase of the side lobes. B. GMSII’ 4, BT = Fig. 1: Phase reply q ( t ) for GMSK with L = 0.3 The nonlinear GMSK modulation is used in the Euro- At t > LT q ( t ) is equal 0.5. With q(t) pean GSM and DECT systems. GMSK is a special kind the GMSK signal pulse zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA with the NRZ-stream dn zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of a 2-level FSK with modulation index h = 0.5 [ll]. The s ~ ~ s ~ ( t ) can be described as follows ] complex envelope of an GMSK modulated signal is [ 00 = exp j2nh d,q(t - S G M S K ( ~ ) nT) (8) n = O The nth NRZ-bit dn causes a change of phase about rq(t) or -nq(t), which is added to the changes of the previ- bitstream is build by transforming bi = 1 to di = -1 and zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA E {-1,l) and the frequency im- ous symbols. Here the information lies in the direction of g ( t ) . In the GSM-system the bits b; are first encoded the rotating complex signal vector. In [9] it is shown that can be builded by superposition of N, = 2L-1 differentially as follows S G M S K ( ~ ) impulses CK: b-1 = 1 bi = bi ebi-1, (5) - with @ means modulo-2 addition. After that the NRZ- - bi = 0 to di = 1. The DECT-system builds the NRZ- with bitstream directly from the incoming bits by transforming n L-1 b; = 1 to d; = 1 and b; = 0 to di = -1. Instead of a i=O 1=1 rectangular-impulse which is used at MSK, here the fre- quency impulse This representation can be used for all CPM-signals, for example the complex envelope of a MSK-signal (here is is used. h ~ ~ ~ ~ ~ ( t ) L = 1) can be written ist the known Gaussian impulse with the 2 time bandwidth product BT. This causes a reduction of 03 = bandwidth, but with the trade of a controlled intersymbol di] Co(t - s ~ s ~ ( t ) exp [ j ~ h nT) (12) interference (ISI). In the GSM-system the factor BT = 0.3 L J i=o n=O VTC ‘98 2364 0-7803-4320-4/981$5.00 0 1998 LEEE