Cdma

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ABSTRACT • CDMA -OFDM –MC-CDMA-PRINCIPLES-MERITS-DEMERITS.

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CHANNEL ESTIMATION-NEED-PRINCIPLETECHNIQUES(Pilot) TECHNIQUES(Pilot).

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PROBLEMS IN UPLINK TRANSMISSION TRANSMISSION.

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TDD MC-CDMA –PR –PRINCIPLES-POSSIBLE STRATEGIES.

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INTRODUCTION Over the past several years there has been increased interest in wireless communication systems that use code division multiple access(CDMA). The systems such as IS-95(Direct Sequence CDMA, DS-CDMA) and UMPTS (Wide band CDMA, W-CDMA) already use CDMA based techniques. On the other hand, thanks to excellent multi carrier(MC) modulation properties in the frequency selective fading radio channels, MC techniques gained a lot of popularity lately. The orthogonal frequency division multiplexing (OFDM) as the most common MC technique has been chosen as the European standard for digital audio broadcasting(DAB) and as the digital terrestrial video standard(DVB-T) (OFDM in DAB and DVB-T). Following this, a lot of research was devoted to the usage of hybrid schemes that use the benefits of both CDMA and MC. This led to the development MC-CDMA, MC-DS-CDMA and similar techniques that are suited for applications in future mobile communications. The MC-CDMA proved to be a suitable technique for the down link transmission . Uplink transmission, due to the more complex propagation conditions, introduces additional problems which results in harder applicability of MC-CDMA in uplink. The goal of this work is to explain the problems involved in uplink transmission and to offer some solutions which enable us to use modified MC-CDMA as an uplink transmission technique. More over, time division duplex (TDD) MC-CDMA as a promising transmission technique for next generation mobile radio systems is presented. The remaining part of the paper is organized as follows. In section II the basic principles of CDMA, MC and hybrid techniques will be reviewed. Then in section III the principle of the channel estimation in the downlink MC-CDMA is presented. In section IV main problems that occur in the uplink transmission are discussed. Section V introduces TDD MC-CDMA, that uses pre-compensation algorithm in order to overcome some of the 2 Email: [email protected]

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problems that occur in uplink transmission. Possible TDD MC-CDMA strategies are presented in section VI. Finally conclusions are given in section VII.

Basic principles of CDMA,MC and hybrid techniques • DS-CDMA

FIGURE 1: Direct sequence multiple access principle. The above figure shows the structure of DS-CDMA transmitter and receiver.In DS-CDMA each user is assigned a unique code sequence. information signal is multiplied by the code signal. For multiple access, the resulting signals for every user are added together and modulated by a RF carrier for transmission .upon reception,the received signal is first demodulated and then multiplied by the unique code signal again and finally passed through a low pass filter to recover the original data .

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. FIGURE 2. DS-CDMA transmitter/receiver The chip rate of the code sequence(1/Tc) is always much higher than that of information signal(1/Ts). Therefore by multiplying the two together, the spectrum of the information signal is spread across a large bandwidth , with the spreading ratio given by Ts/Tm. upon reception,since the code sequence are chosen such that the cross correlation between two different code sequences is small,by multiplying the received signal with the same code sequence used on the transmitter side, the original signal will be recovered. Most of the interference power from the other data signals will be rejected by passing the despread signal through a low pass filter. By using a number of different pseudo noise sequences,multiple users can transmit simultaneously using the same bandwidth. In all practical cases , at the receiver, all the users pseudo noise sequence will not be orthogonal and therefore the capacity of the system will be limited by this multiple access interference. A number of receiver techniques have been proposed to mitigate this effect , the two main categories being linear receivers and non linear interference cancellation receivers. With the linear , a linear transform is applied to the received signal try and restore the orthogonality between the users ,although the effect of additive noise also needs to be taken into account. With the non linear interference cancellation receivers,tentative estimates of users data are used to reconstruct interfering users signals and these are subtracted to leave an interference free signal. These can approach the performance of the maximum likelihood receiver. This optimum receiver compares the received signal with every possible combined transmitter signal, and requires an exponentially complex algorithm which prohibits its use if the number of CDMA users is larger than about ten. • ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING(OFDM) OFDM has been adopted as the European broadcast standard for digital video and digital audio broadcasting. It has also recently been included in high performance wireless local area network standards(HIPERLAN2 and IEEE 802.11). Multi carrier CDMA can use OFDM to provide multiple orthogonal carriers. 4 Email: [email protected]

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OFDM is a modulation technique which allows simple equalization of signals in frequency selective channels with severe inter symbol interference. This is achieved by increasing the symbol length at the cost of increasing the number of carriers used to transmit the information, thus keeping the same data rate. This can be easily and efficient implemented by taking the FFT of a block of data symbols. This results in a number of parallel(flat fading) narrow band channels which can be a equalized simple narrow band(single tap) equalizers. The inter symbol interference which could have been a number of symbol lengths is now only a fraction of a symbol length. This can be completely eliminated if the symbol is cyclicly

extended, by repeating the first part of the block at the end. The receiver can then operate on the part of the signal beyond the ISI with the only side effect a phase shift on each subcarrier. .

FIGURE 3:OFDM. There are however two disadvantages with using OFDM. Firstly the large number of carriers increases the peak to average power ratio of the 5 Email: [email protected]

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signal, causing problems when amplified by a non linear amplifier. Secondly by having the carriers much closer together, the system becomes more sensitive to frequency estimation errors.

Figure 4: Multi carrier modulation principle.

• MC-CDMA Since MC –CDMA is a combination of CDMA and OFDM techniques, its concept and structure are somewhat similar to DS-CDMA. The following figure shows the structure MC-CDMA transmitter and receiver. In this technique data signals are spreaded by code sequences first and then combined. The combined signal is then converted into a parallel data stream and transmitted over multiple sub-carriers , which can be effectively implemented using an inverse FFT. In general , the number of sub-carriers and the spreading ratio may or may not be the same ,depending on the situation. Since MC-CDMA mitigates ISI by having flat-fading over each sub-carrier,if the original symbol is too high, more sub-carriers are needed for modulation . Upon reception, an FFT operation is first applied to the received signal, and then the resulting signal is converted into serial data stream , which can be despread by the same code sequence to recover the original data signal.

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FIGURE 5: MC-CDMA Transmitter/Receiver. As mentioned , MC-CDMA also mitigates the effect of ISI. However unlike DS-CDMA whose ISI rejection is determined by auto correlation properties of code sequence,this is achieved by increasing the code symbol length (in time domain) an modulating each code symbol using different sub-carriers to keep the same data rate . This results in flat feeding over each sub-carrier.

The main advantage of doing this is that when the multiple access inteferefnce becomes a problem , the resulting linear detectors are much simpler to implement, as only a single tap equaliset is required for wach channel. Rake reception can also be employed to exploit the channel diversity by channel matching in the frequency domain allowing optimal reception for a single user. In the uplink another advantage of MC-CDMA can be exploited.If the signals can be synchronized within a small fraction of the symbol time(e.g. indoor , or very small cell environment) then this asynchronism can be overcome by cyclicly –extending the signal further allowing synchronous reception of all signals,with no ISI from other users.

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FIGURE 6: Multi carrier code division multiple access principle.

III CHANNEL ESTIMATION FOR THE MC-CDMA DOWNLINK In the case of coherent receivers it is necessary to obtain the information about the channel state at the receiver. The transmission over a mobile radio channel is, in the most common case, described by the influence of random noise(AWGN) and frequency selecting fading as given in (i) where S represents transmitted data matrix, R received data matrix, N the AWGN matrix and H the channel transfer function. The distrurbances caused by fading can partly be estimated and removed. This is usually done by pilot symbol aided channel estimation. R=HS+N.

The principle of pilot symbol aided channel estimation is to multiplexed pilot symbols in to the data stream. At the receiver the position and the original values of the pilot symbols are known and are used to make initial estimations of the channels. This information is used to estimate the complete channel impulses response by filtering. In MC-CDMA the multiplexing of the pilot symbols is done in both dimensions: Time and Frequency. The pilots and data symbols are multiplexed into a matrix structure called OFDM frame. 8 Email: [email protected]

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• TECHNIQUE I: PILOT AND DATA SYMBOLS TRANSMITTED IN SAME OFDM SYMBOL Pilot symbols are now arranged among data symbols at the transmitter similar to the pattern illustrated in fig. At the receiver channel estimates are formed at each pilot symbol location. The maximum frequency separation, Sf, of pilot symbols is set by the following equation. Sf < maximum delay spread of the multipath channel. For the UMTS vehicular channel B Model delay spread is 20 ms and hence Sf must be less than 50kHz. The subcarrier separation in this system is determined by the output bandwidth of the system divided by the number of carriers. The output sample rate of the system is 3.84 Msamples/s, hence the minimum output bandwidth is 1.92 MHz. With 128 subcarriers this gives a subcarrier separation of 15 kHz. Therefore the pilot symbols must be placed on every third subcarrier to give a subcarrier separation of 45kHz and hence obey equation. This system, therefore, has 44 subcarriers allocated to carry pilot symbols and 84 subcarriers of this sytem. Each user specific pilot symbol has the same power as aspread data symbol from a single user and pilot power is proportional to the number of users.

FIGURE 7:Arrangement of pilot symbols in channel estimation technique 1(a). 9 Email: [email protected]

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The loss in performance of this system compared to that with perfect channel knowledge in the receiver is apparent. A receiver with a perfect channel estimate, with a system load of 10 users, attains a BER of 3 * 10-2 at Eb/NO 7 dB, see Figure 7. However in a system with identical load and using this technique of channel estimation in the receiver a BER of 3 * 102 is now attained at Eb/NO 17 db, a performance degradation of 10Db. Technique 2: Pilot and data symbols transmitted in separate OFDM symbols An OFDM symbol which only consists of pilot symbols is transmitted at the start of each data burst. The channel equalizing coefficients derived from that channel estimate and are reused throughout that bust. A data burst consists of one OFDM symbol which carries exclusively pilot symbols followed by two data carrying OFDM symbols. The channel is assumed to be essentially in variant over the data burst as the duration of the burst is less than the coherence time of the channel. Each user specific pilot symbol has the same power as a spread data symbol from a single user and pilot power is proportional to the number of users. At a system load of 10 users a BER of 3 * 10-2 is attained at Eb/NO 12.5 db, a performance degradation of 5.5 db over the system with perfect channel knowledge in the receiver.

IV. UPLINK TRANSMISSION – IDENTIFYING THE PROBLEMS The MC-CDMA as presented in is very suitable for downlink transmission, but if we try to apply it directly to the uplink transmission we are facing some important problems.

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Visit: www.geocities.com/chinna_chetan05/forfriends.html THE MAIN PROBLEMS ARE:

 Channel estimation problem – Problem occurs because signals from

many mobile stations that arrive at the base station(BS) are affected by different channel transfer functions. Since each signal is, due to the frequency selective fading, influenced by different channel transfer function, pilot based channel estimation becomes difficult. Pilot symbols from different users must be placed on different positions in OFDM frame, otherwise severe multi user interference will prevent estimating the channel of desired user. But, if each user needs dedicated positions on OFDM grid for the pilot symbols most of the bandwidth will be occupied by the pilot symbols most of the bandwidth will be occupied by the pilot symbols and not enough bandwidth will be left for the data. As a result, the system capacity drastically decreases.  Synchronization problem – Base station tries to synchronize with

different users, but each signal has different Doppler shift. Also, at the base station we have signals from different users that try to synchronize to one time reference, but each user have different speed and different distance from the base station. Uplink synchronization for the MC-CDMA is difficult. There is a technique called TDD MC-CDMA which enables us to deal with some of the mentioned problems.

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FIGURE 9:The typical propagation scenario in the case of uplink transmission. V.TDD MC-CDMA WITH PRE-COMPENSATION TDD – Time Division Duplex  Division Duplex In every two-way communication system it is necessary to use separated channels to transmit information in each direction. This is called duplexing. In wireless systems either separating the channels in frequency or in time does this. Time Division Duplex (TDD) is a duplex technique where the same frequency band is used both by the base station and mobile stations (downlink and uplink), but at different time slots. The fact that the same frequency band is used in both directions will be exploited further on in order to develop TDD MC-CDMA that successfully deals with the channel estimation problem described in the previous section .

 Pre-compensation The idea is to estimate the channel only in the downlink transmission and to use this channel estimation also for the uplink transmission. Asumption is that the TDD time slot is short compared to the time-variance of the channel. Then, predictions of the channel are likely to be accurate for adjacent TDD time slots and we can adjust the transmission signal in order to achieve better performance. If this assumption is valid, pre-compensation will ensure the separation between users at the BS. For the downlink the channel is estimated by the pilot symbol aided channel estimation as described in Section III. However, for the uplink transmission no pilot symbol insertion is needed since we use the downlink estimation to pre-compensate the uplink frame.

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FIGURE 10: Precompensation , basic principle.  Pre-compensation TDD mode.

As already described, the TDD mode separates uplink and downlink transmission in the time domain. This enables us to deploy precompensation algorithms. Pre-compensation operates on the premise that there is a correlation between downlink channel conditions and uplink channel conditions. This is a reasonable assumption especially in the case of a slowly varying channel. As shown in figure 10.,Hi(I=1,2,3,4) represents the channel transfer function in base station to mobile station (BS->MS) direction while H I can be mainly eliminated with pilot symbol aided channel estimation at the MS as described in Section III. III. The calculated equalization matrix is also applied to the OFDMA frame that we wish to transmit. This pre-compensates the influence of Hi. The idea of pre-compensation can be further developed. It is also possible to pre-compensate Doppler shift or the time delays, by using the information acquired from the downlink frame.

FIGURE 10: The principles of pre-compensation

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VI.POSSIBLE TDD STRATEGIES TDD transmission can be realized in several ways. We propose three possibilities which can be used as a base for more complex strategies.For each of the strategies the advantages(marked with +), and disadvantages (marked with -) are given. The following abbreviations are used : Ttdd-TDD guard interval, Tprop – time of propagation ,Test – time of channel estimation,Twaitwaiting- wating time, Rx – receiver , Tx- transmitter,UL – uplink, DL – downlink.

1.Full TDD guard interval +complete time separation of downlink and uplink -long TDD – guard interval Ttdd = 2Tprop + Test -difficult frequency,time synchronization of Rx,UL 2.Reduced TDD-guard interval + shorter TDD – guard interval Ttdd= 2Tprop we are transmitting at least 2Tx, DL packets -Twait makes estimated channel value older

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3.No TDD guard interval,small cells Assumption:2Tprop + Test less than OFDM guard interval + no TDD guard interval , no twait + interference between Rx.UL and Tx,DL eliminated with OFDM guard interval -Test=0 , is that realistic ?

Assumption : processing delay Test larger than OFDM guard interval +advantages similar to that in above case -Twait>0,we use older channel estimation

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VII . CONCLUSION : This paper discusses TDD MC-CDMA as a promising transmission technique for next generation mobile radio systems. It is outlined where problems arise in uplink transmission , and how the channel pre-estimation can be successfully performed in the uplink case with the use of a pre-compensation algorithm at the transmitter.Next, possible TDD strategies have been discussed , with the special emphasis on the case of small radio cells with low time delay. In the future work presented TDD strategies will be further investigated, compared and developed.

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BIBILIOGRAPHY:

• WIRELESS BROADBAND NETWORKS HANDBOOK-JOHN. R.VACCA • WIRELESS NETWORKS-KAVEH PAHLAVAN-PRASHANT KRISHNAMURTHY. • A PROMISING NEW WIDEBAND MULTIPLE ACCESS SCHEME FOR FUTURE MOBILE COMMUNICATIONS-SCHNELL.M ,BROECK.I.,SORGER. • PILOT SYMBOL AIDED CHANNEL ESTIMATION IN TIME AND FREQUENCY- HOCHER .P. KAISER.

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