1. S-72.227 Digital Communication Systems Overview into spread spectrum and CDMA communication

2. Timo O. Korhonen, HUT Communication Laboratory Overview into spread spectrum communication u Basic principles of spread spectrum communication u Types of DS modulation: principles and circuits –direct sequence (DS) –frequency hopping (FH) u Spreading code sequences –maximal length –Gold –Walsh u Asynchronous multiple access systems u CDMA diversity techniques

3. Timo O. Korhonen, HUT Communication Laboratory How the tele-operators* market CDMA: C overage For Coverage, CDMA saves wireless carriers from deploying the 400% more cell site that are required by GSM CDMA’s capacity supports at least 400% more revenue-producing subscribers in the same spectum when compared to GSM C apacity C ost $ $ A carrier who deploys CDMA instead of GSM will have a lower capital cost C larity CDMA with PureVoice provides wireline clarity C hoice CDMA offers the choice of simultaneous voice, async and packet data, FAX, and SMS. C ustomer satisfaction The Most solid foundation for attracting and retaining subscriber is based on CDMA *This example is from Samsumg’s narrowband CDMA (CDMAOne ® ) marketing

4. Timo O. Korhonen, HUT Communication Laboratory Spread spectrum (SS) characteristics u The bandwidth of the transmitted signal is much greater than the original message bandwidth u Transmitted signal bandwidth is determined by a spreading function (code), independent of the message, and known to the transmitter and receiver u Interference and noise immunity tolerance of a SS system is a function of the processing gain u Multiple SS system often co-exist in the same band. Users independent if they have (1) high code gain and/or (2) more orthogonal codes u Spreading sequences can be very long -> enable low PSD-> low probability of interception (especially in military communications) Narrow band signalWideband signal

5. Timo O. Korhonen, HUT Communication Laboratory Characterizing SS systems u Code gain, for BPSK –Larger G p improves noise immunity, but more transmission BW is required –For multiple access selection of appropriate code family is important u Jamming margin –Tells the magnitude of additional interference and noise that can be injected to the channel without hazarding system operation. Example:

6. Timo O. Korhonen, HUT Communication Laboratory Characterizing SS systems (cont.) u Spectral efficiency E eff : describes how compactly message is fitted into the transmission band: u Energy efficiency: The value of received SNR to obtain a specified error rate (often 10 -9 ). For BPSK the error rate is QPSK-modulation can fit twice the data rate of BPSK in the same bandwidth. Therefore it is more energy efficient than BPSK.

7. Timo O. Korhonen, HUT Communication Laboratory Spread spectrum flavors: Frequency hopping (FH) u In FH basic hopping pattern is determined by the code and it is deviated by the message: u This method is applied in BlueTooth ®

8. Timo O. Korhonen, HUT Communication Laboratory Slow (T C >> T b ) frequency hopping

9. Timo O. Korhonen, HUT Communication Laboratory Spread spectrum flavors: Direct Sequence (DS) u This figure shows a BPSK-DS transmitter and receiver (multiplication can be realized by RF-mixers) - DS-CDMA is used in WCDMA and IS-95 systems

10. Timo O. Korhonen, HUT Communication Laboratory DS-CDMA spectra in tone jamming u Assume DS - BPSK transmission, with a single tone jamming (jamming power J [W] ). The received signal is u The respective PSD is u At the receiver r(t) is multiplied with a local code c(t) yielding u The transmitted signal and the local code are aligned in-phase: data

11. Timo O. Korhonen, HUT Communication Laboratory DS spectra in tone jamming (cont.) u Despreading distributed the jammer power in frequency:

12. Timo O. Korhonen, HUT Communication Laboratory DS spectra in tone jamming (cont.) u Receiver filtering suppresses the jammer power:

13. Timo O. Korhonen, HUT Communication Laboratory DS and FH compared u FH is applicable in environments where there exist tone jammers that can be overcame by avoiding hopping on those frequencies u For large number of users DS is applicable because resource reallocation to other users can be easily arranged by power control u FH applies usually noncoherent modulation due to carrier synchronization difficulties -> modulation method degrades performance u Both methods can be used in military techniques –FH can be advantageous because the hopping span can be very large –DS can be advantageous because spectral density can be much smaller than background noise density u FH is an avoidance system: does not suffer on near-far effect! u By using hybrid systems some benefits can be combined: The system has a low probability of interception and low near-far effect at the same time. (Differentially coherent modulation is applicable)

14. Timo O. Korhonen, HUT Communication Laboratory Example of DS multiple access waveforms

15. Timo O. Korhonen, HUT Communication Laboratory FDMA, TDMA and CDMA compared Conceptually FDMA, TDMA and CDMA yield same capacity However, in wireless communications CDMA has improved capacity In addition CDMA has several advantages: ­ Enables easy usage of multirate ­ Performance can be improved also by adaptive antennas, multiuser detection forward error correction (FEC) ­graceful degradation

16. Timo O. Korhonen, HUT Communication Laboratory FDMA, TDMA and CDMA compared (cont.) u TDMA and FDMA principle: –TDMA allocates a time instant for a user –FDMA allocates a frequency band for a user u CDMA allocates a code for user u CDMA-system can be synchronous or asynchronous u Synchronous CDMA is very rare due to (multipath) channels that tend to destroy code orthogonality. u Wireless CDMA-systems as the IS-95 and WCDMA use asynchronous techniques where different user codes are not synchronous, especially after multipath u CDMA codes are by their nature –Orthogonal, as the Walsh-codes –Near-Orthogonal, as the Gold-codes –Non-orthogonal as the maximal length codes

17. Timo O. Korhonen, HUT Communication Laboratory Illustration of CDMA potential capacity u Consider reverse link u Each user transmits Gaussian noise whose deterministic characteristics are stored in RX and TX u Reception and transmission are simple multiplications u Perfect power control: each user’s power at the BS the same u Each user transmits using power P S that is other user’s interference power. Hence each user receives interference power where k u is the number of equal power users (1)

18. Timo O. Korhonen, HUT Communication Laboratory Illustration of CDMA potential capacity (cont.) u Each user applies a demodulator/decoder characterized by a certain E b /I o ratio (3 - 9 dB depending on channel coding, channel, modulation method etc.) u Each user is therefore exposed to an interference density where B T is the spreading bandwidth. u Received energy / bit at the signaling rate R is thus u Combining (1)-(3) yields the number of users u This can be increased by using voice activity coefficient G v = 2.67 (only about 37% of speech time effectively used), directional antenna system gain, for instance for 3-way antenna G A =2.5. (2) (3) (4)

19. Timo O. Korhonen, HUT Communication Laboratory Illustration of CDMA potential capacity (cont.) u In cellular system other cells introduce interference that decreases capacity. It can be shown that usually this coefficient is about u Hence asynchronous CDMA system capacity can be approximated by yielding with the given values G v =2.67, G A =2.4, 1+f = 1.6, u Assuming efficient error correction algorithms, dual diversity antennas, and RAKE receiver, it is possible to obtain E b /I o =6 dB=4, and then This is of order of magnitude larger value than the one obtained with the conventional TDMA systems!

20. Timo O. Korhonen, HUT Communication Laboratory DS is based on applying code correlation properties

21. Timo O. Korhonen, HUT Communication Laboratory Maximal length codes u Have very good autocorrelation but cross correlation can be very bad u Usually generated by feedback connected shift registers u Number of codes available depends on number of shift register stages: u Engineering code generator design by tables: 5 stages->6 codes, 10 stages ->60 codes, 25 stages ->1.3x10 6 codes

22. Timo O. Korhonen, HUT Communication Laboratory Design of a maximal length generator based on a design table entry u Feedback connections can be written directly from the table:

23. Timo O. Korhonen, HUT Communication Laboratory Maximal length code autocorrelation and PSD

24. Timo O. Korhonen, HUT Communication Laboratory Other spreading codes u Walsh codes: orthogonal, used in synchronous systems, also WCDMA downlink u Generation recursively: u All rows and columns of the matrix are orthogonal: u Gold codes: generated by summing preferred pairs of maximal length codes. Have a guarantee 3-level crosscorrelation: u For N-length code there exists N + 2 codes in a code family and u Both Walsh and Gold codes are used for multiple access systems. The Walsh codes are used usually in synchronous communications because their asynchronous crosscorrelations are not good

25. Timo O. Korhonen, HUT Communication Laboratory Characterizing asynchronous CDMA system u Normalize power for each user: u Integrate and dump yields at the decision time: u Intended signal for the j:th user: u User interference:

26. Timo O. Korhonen, HUT Communication Laboratory Characterizing asynchronous CDMA system (cont.) u Effect of user interference is a function of (1) applied codes crosscorrelation, (2) modulation method, (3) user power balance, (4) code synchronization u The background noise component is at the the decision time u Therefore the j:th user experiences the SNR:

27. Timo O. Korhonen, HUT Communication Laboratory Asynchronous CDMA system with AWGN (cont.) u Background AWGN noise is u Cross-noise average is zero due to uncorrelated AWGN and codes, eg u User interference induced noise

28. Timo O. Korhonen, HUT Communication Laboratory Asynchronous CDMA system (cont.) u User interference is comparable to code cross-correlation and can be expressed by u Assuming that the all users have the same code-correlation that yields for the maximal length codes u Defining now the effective bandwidth B eff in terms of signal power spectral density G(f) we get for BPSK

29. Timo O. Korhonen, HUT Communication Laboratory Asynchronous CDMA system: j:th user SNR u Thus the SNR for the j:th user is

30. Timo O. Korhonen, HUT Communication Laboratory Perfect power control u All users have the same power at the receiver input u Assuming U users we will have then for each user u Yielding the maximum number of users (SNR 1 : a single-user system SNR)

31. Timo O. Korhonen, HUT Communication Laboratory Near-far effect  Assume a single user of received power P i exists in a wireless communications at the distance d i having the propagation exponent  u For the i:th and j:th node at a power balanced network the received power is therefore and the single user SNR 0 is <- One-user system received SNR

32. Timo O. Korhonen, HUT Communication Laboratory Example of the near-far effect u Assume a set of parameters for a asynchronous CDMA system and that the RX-TX have equal distance except for the first transmitter u For the ideal power control, eg all received powers are equal the number of users is u Note that for d 1 <d j /2.78 only one user fits to the system!

33. Timo O. Korhonen, HUT Communication Laboratory Direct Sequence Techniques: QPSK-system u Serial-parallel transform moves the data stream to carriers u Orthogonal carriers convey the spreaded waveforms u Spreading codes c 1 and c 2 may or may not be orthogonal u What kind of circuit can make the demodulation? constellation diagram

34. Timo O. Korhonen, HUT Communication Laboratory RAKE receiver and wideband channel model

35. Timo O. Korhonen, HUT Communication Laboratory u RAKE receiver yields diversity gain, for large SNR RAKE-receiver utilizes diversity gain

36. Timo O. Korhonen, HUT Communication Laboratory Orthogonal Frequency Division Multiplexing (OFDM) communications u A dense FDM methods u Independent, orthogonal carriers arranged in frequency domain by 1/D, where D is the symbol period u Benefits:  OFDM is an efficient way to deal with multipath. For a given delay spread, the implementation complexity is significantly lower than that of a single carrier system with an equalizer.  In relatively slow time-varying channels, it is possible to significantly enhance the capacity by adapting the data rate per subcarrier according to the signal-to-noise ratio of that particular subcarrier.  OFDM is robust against narrowband interference, because such interference affects only a small percentage of the subcarriers.

37. Timo O. Korhonen, HUT Communication Laboratory Discrete Multi-Tone (DMT) modulation u Used in Digital Subscribe Line (DSL). u Usable frequency band is separated into 256 small frequency bands (or subchannels) of 4.3125 kHz each. u Within each subchannel, modulation uses quadrature amplitude modulation (QAM). u By varying the number of bits per symbol within a subchannel, the modem can be rate-adaptive. u DMT uses the fast Fourier transform (FFT) algorithm for modulation and demodulation.

38. Timo O. Korhonen, HUT Communication Laboratory OFDM system u Modules –serial to parallel converter (S/P) –IFFT (Inverse Fast Fourier Transformer) –parallel to serial converter (P/S) u Guard interval takes care of adaptation to multipath environment –added after each OFDM symbol –duration comparable to RMS delay spread –decreases transmission efficiency u Transmitted signal can have large dynamic range - linear amplifiers required -> power dissipation problems (battery problems)

39. Timo O. Korhonen, HUT Communication Laboratory OFDM modem Rx chainTx chain Contains channel

40. Timo O. Korhonen, HUT Communication Laboratory Addendum: Orthogonal and Non-Orthogonal Systems

41. Timo O. Korhonen, HUT Communication Laboratory Orthogonal and nonorthogonal systems u Consider the following waveforms and their presentations in frequency domain: u They are orthogonal dispute of their waveform overlap both in time and in frequency u Using orthogonal signals enables signaling of several users as they would be transmitted alone u How many orthogonal waveforms of duration T and bandwidth B T can be transmitted simultaneously? Based on certain bounds 1 it can be stated that about K = 2TB T polar waveforms. Therefore the respective bandwidth is for a channel having rate R=1/T 1. M. Petrich, "On the number oforthogonal signals which can be placed in a WT-product, " SIAM Journal, pp. 936-940, Dec. 1963

42. Timo O. Korhonen, HUT Communication Laboratory Orthogonal and nonorthogonal systems (cont.) u In nonorthogonal systems power of all users leaks as noise background to other users u However, using nonorthogonal systems is often appealing: –Non need to worry about maximum number of users in terms of TB product. Users will anyhow influence to each other that is manifested in some deterioration in their error rates. –No need to worry about inter-user synchronization. Thus relaxed hardware requirements. –Channel resource sharing becomes dynamic. Thus reception quality can be traded to system capacity u Which factors determine the capacity (number of users with a certain quality of service) in a nonorthogonal system? –Dimensionality of signal space (time- bandwidth product) or spreading signal cross correlations –Receiver sensitivity –Data redundancy (manifested via channel coding)