Wireless Communication. Spread-Spectrum Communication System DSSS FHSS CDMA. Assoc. Prof. Hakan DOGAN Doç. Dr. Hakan DOGAN

Transcription

1 Spread-Spectrum Communication System Wireless Communication DSSS FHSS CDMA Assoc. Prof. Hakan DOGAN Doç. Dr. Hakan DOGAN

2 Spread Spectrum Spread Spectrum uses wide band, noise-like signals. Because Spread Spectrum signals are noise-like, they are hard to detect. Spread-spectrum signals are highly resistant to narrowband interference. Spread-spectrum signals are difficult to intercept Spread-spectrum signals are highly resistant to deliberate jamming, unless the adversary has knowledge of the spreading characteristics It is a favorite technology of the military because it resists jamming and is hard for an enemy to intercept. "Spreading" codes are called "Pseudo Random" or "Pseudo Noise" codes.

3 FHSS / DSSS There are two types of spread spectrum techniques are used Direct Sequence Frequency Hopping. Direct Sequence is adopted by CDMA

4 Frequency Division Multiple Access FDMA FDMA is a multiple access method in which users are assigned specific frequency bands. The user has sole right of using the frequency band for the entire call duration. The best example of this is the cable television system. The medium is a single coax cable that is used to broadcast hundreds of channels of video/audio programming to homes.

5 Time Division Multiple Access TDMA In TDMA an assigned frequency band shared among a few users. However, each user is allowed to transmit in predetermined time slots. Hence, channelization of user is achieved through separation in time. A good example is the widely used T1 transmission system, which has been used for years in the telecom industry. T1 lines carry up to 24 individual voice telephone calls on a single line

6 F/TDMA The method chosen by GSM is a combination of Time- and Frequency-Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 khz apart. One or more carrier frequencies are assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme

7 CDMA CODE DIVISION MULTIPLE ACCESS (CDMA) LARGE NUMBER OF TRANSMISSIONS ARE COMBINED ON THE SAME RF CHANNEL AT THE SAME TIME BUT ARE SEPERATED BY CODES.

8 DSSS DSSS(Direct Sequence Spread Spectrum) In telecommunications, direct-sequence spread spectrum (DSSS) is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that modulates the carrier or broadcast frequency. The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency

9 DSSS Direct Sequence Spread Spectrum (DSSS) is a spread spectrum technique whereby the original data signal is multiplied with a pseudo random noise spreading code. This spreading code has a higher chip rate (this the bitrate of the code), which results in a wideband time continiuous scrambled signal.

10 Spreading

11 Spreading DSSS significantly improves protection against interfering (or jamming) signals, especially narrowband and makes the signal less noticeable. It also provides security of transmission if the code is not known to the public. These reasons make DSSS very popular by the military

12 DSSS uses a signal structure in which the sequence of chips produced by the transmitter is already known by the receiver. If an undesired transmitter transmits on the same channel but with a different PN sequence (or no sequence at all), the de-spreading process has reduced effect of the undesired signal Therefore, it is the basis for the code division multiple access (CDMA), which allows multiple transmitters to share the same channel. (the cross-correlation properties of their PN sequences are important) DSSS can also be used as a multiple access technique, whereby several different pseudo random spreading codes are being used simultaneously. This multiple access technique is better known as Direct Sequence CDMA

13 CDMA Code division multiple access (CDMA) is a channel access method used by various radio communication technologies. One of the concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a single communication channel. This allows several users to share a band of frequencies (see bandwidth). This concept is called multiple access. CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA is a form of spread-spectrum signalling, since the modulated coded signal has a much higher data bandwidth than the data being communicated.

14 CDMA In Code Division Multiple Access (CDMA) systems all users transmit in the same bandwidth simultaneously. A s A s B s B s A d Data for User A B s Spreading Sequence for User B B d Data for User B A s Spreading Sequence for User A

15 CDMA As= (-1,+1,-1,-1,+1,+1) BPSK Durumu User A data is shown as Ad and equal to 1. (bit to symbol 1 1) Transmitted Signal is shown as At Ad=+1 At =(+1)*(-1,+1,-1,-1,+1,+1)=(-1,+1,-1,-1,+1,+1) As=(-1,+1,-1,-1,+1,+1) (Spreading Sequence)

16 CDMA Bs= (+1,+1,-1,+1,-1,+1) BPSK Durumu User B data shown as Bd and equals to 0 (bit to symbol 0-1) Vericisinden gönderilecek olan sinyal Bs olmak üzere Bd=-1 Bt=(-1)*(+1,+1,-1,+1,-1,+1)=(-1,-1,+1,-1,+1,-1) Bs=(+1,+1,-1,+1,-1,+1)

17 CDMA r = At + Bt (-1,+1,-1,-1,+1,+1) + (-1,-1,+1,-1,+1,-1) = (-2, 0, 0, -2, +2, 0) Alıcı hangi kullanıcının sinyalini ayırt etmek istiyorsa ilgili kullanıcının anahtar koduyla gelen sinyalin iç çarpımı yapılır ve çıkan sonucun <0 veya >0 olma durumuna göre gelen bilginin 1 mi 0 mi olduğuanlaşılabilir.

18 A e = A t + B t A s 6>0 Ae=1 (-2,0,0,-2,+2,0) * (-1,+1,-1,-1,+1,+1)= = 6 B e = A t + B t B s -6<0 Be=0 (-2,0,0,-2,+2,0) * (+1,+1,-1,+1,-1,+1)= = -6

19 CDMA Not: Max signal values are -6 and 6 if there is no power normalization. (-1,+1,-1,-1,+1,+1) + (-1,-1,+1,-1,+1,-1) = (-2, 0, 0, -2, +2, 0) = (-2, 0, 0, -2, +2, 0) = (+1, 0, 1, 1, 0, -1) Error =(-1,0,1,-1,+2,-1) Received Signal (-1,0,1,-1,+2,-1) * (-1,+1,-1,-1,+1,+1)= (-1,0,1,-1,+2,-1) * (+1,+1,-1,+1,-1,+1)=

20 FHSS Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.

21 It causes the carrier to hop from frequency to frequency over a wide band according to a sequence defined by the pseudorandom sequence Fast frequency hopping (FFHSS) FFHSS is characterized by several hops within each data bit. Low frequency hopping (LFHSS) LFHSS allows several consecutive data bits to modulate the same frequency IEEE a, Bluetooth

22 FHSS