Spread Spectrum Review/Recap

Spread Spectrum Review/Recap
Lecture 22

Overview Relationship between bandwidth of a signal (before and after and encoding SS) Benefits of SS FHSS Slow and Fast FHSS DSSS Relationship between Bit Rate of a Signal (Before and after DSSS encoding) CDMA

Bandwidth Before and After SS Encoding
Q:- What is the relationship between the bandwidth of a signal before and after it has been encoded using spread spectrum?

Definition of Spread Spectrum
Spread spectrum is a modulation method applied to digitally modulated signals that increases the transmit signal bandwidth to a value much larger than is needed to transmit the underlying information bits.

Spread Spectrum Signal Characteristics
They are difficult to intercept for unauthorized person. They are easily hidden, it is difficult to even detect their presence in many cases. They are resistant to jamming. They have an asynchronous multiple-access capability. They provide a measure of immunity to distortion due to multipath propagation.

Spread Spectrum Conditions
The signal occupies a bandwidth much larger than is needed for the information signal. The spread spectrum modulation is done using a spreading code, which is independent of the data in the signal. Despreading at the receiver is done by correlating the received signal with a synchronized copy of the spreading code.

Processing Gain : The spread spectrum increases the bandwidth of the message signal by a factor N, called the processing gain where β is the message signal bandwidth, βss is the corresponding SS signal bandwidth. , N > 1

Spread Spectrum Versus Narrowband Technology
Narrowband wireless communications can be defined as wireless communications using a single frequency center with no redundancy to communicate information at high power levels chosen to overpower interference in that frequency band. Spread spectrum wireless communications can be defined as wireless communications using a range of frequencies to communicate information at low power levels. Spread spectrum has also been defined as a wireless communications technology that uses more bandwidth than is required to deliver information. Spread spectrum also uses low power and can do so because all interference does not need to be overcome, due to the redundancy and/or error correction.

Spread Spectrum Versus Narrowband Technology

Spread Spectrum Spread spectrum

Characteristics of Spread Spectrum
Bandwidth of the transmitted signal W is much greater than the original message bandwidth (or the signaling rate R) Transmission bandwidth is independent of the message. Applied code is known both to the transmitter and receiver Interference and noise immunity of SS system is larger, the larger the processing gain Multiple SS systems can co-exist in the same band (=CDMA). Increased user independence (decreased interference) for (1) higher processing gain and higher (2) code orthogonality Spreading sequence can be very long -> enables low transmitted PSD-> low probability of interception (especially in military communications) Wideband signal (transmitted SS signal) Narrow band Signal (data)

Characteristics of Spread Spectrum (cont.)
Processing gain, in general Large Lc improves noise immunity, but requires a larger transmission bandwidth Note that DS-spread spectrum is a repetition FEC-coded systems Jamming margin Tells the magnitude of additional interference and noise that can be injected to the channel without hazarding system operation. Example:

Characteristics of Spread Spectrum
Spectral efficiency Eeff: Describes how compactly TX signal fits into the transmission band. For instance for BPSK with some pre-filtering: Energy efficiency (reception sensitivity): The value of 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.

Bandwidth Before and After SS Encoding
Q:- What is the relationship between the bandwidth of a signal before and after it has been encoded using spread spectrum? Ans:- The bandwidth is wider after the signal has been encoded using spread spectrum

SS Benefits Q:- List three benefits of spread spectrum.

Benefits of Spread Spectrum
Spread spectrum is being used in more and more applications in data communications. Security – need a wide BW receiver and precise knowledge and timing of the pseudorandom sequence Resistance to jamming and interference – jamming signals are usually restricted to one frequency Band sharing – many signals can use the same frequency band; but… many spread spectrum signals raise the overall background noise level Precise timing – can be used in radar where accurate knowledge of transmission time is needed

SS Benefits Q:- List three benefits of spread spectrum. Ans:-
(1) We can gain immunity from various kinds of noise and multipath distortion. (2) It can also be used for hiding and encrypting signals. Only a recipient who knows the spreading code can recover the encoded information. (3) Several users can independently use the same higher bandwidth with very little interference, using code division multiple access (CDMA).

Frequency Hopping SS

FHSS Q:- What is frequency hopping spread spectrum

Frequency hopping This dilemma was recognized prior to WWII.
In 1942, Hedy Lamarr and pianist George Antheil patented a “Secret Communication System”. Their scheme was for a frequency hopping remote control for torpedo guidance. In 1936 Hedy Lamarr became the first woman to grace the silver screen in a feature film wearing nothing but her birthday suit. Five years later, at a Hollywood dinner party, she engaged in a passionate discussion with an avant-garde composer about protecting U.S. radio-guided torpedoes from enemy interference. She scrawled her phone number in lipstick on the windshield of his car so they could develop their ideas further. In 1942, unbeknownst to her adoring public, the unlikely duo secured a patent and gave it to the United States government for a "Secret Communications System" expressly constructed to assist in the defeat of Hitler. The science presented in this patent serves as the basis for the technology we use today in cell phones, pagers, wireless Internet, defense satellites, and a plethora of other spread-spectrum devices. In 1933 Hedy achieved international renown when she played the first nude scenes in cinematic history in the Czech film "Ecstasy." The film was banned in many countries around the world, including the United States. On August 10, 1933, Hedy Kiesler married Austrian industrialist Fritz Mandl, CEO of one of the world's leading arms manufacturers. While her husband forbade her to pursue her acting career, (attempting to buy up all existing copies of "Ecstasy"), this proved to be one of the most valuable relationships to the scientific technology she would develop years later. From Strapless to Wireless Hedy Lamarr helped to set the groundwork for some of the most revolutionary technology of our time. Following the outbreak of World War II, Lamarr, a passionate opponent of the Nazis, wanted to contribute more to the allied effort. As Mrs. Fritz Mandl, she had closely observed the planning and discussions that went into attempting to design remote-controlled torpedoes. These never went into production, because the radio-controlled guidance system was too susceptible to disruption. She got the idea of distributing the torpedo guidance signal over several frequencies, thus protecting it from enemy jamming. The only weak point was how to employ the synchronization of the signal's transmitter and receiver. In 1940, Lamarr met the American avant-garde composer George Antheil of "ballet mécanique" fame. She described her idea to him, and asked him to help her construct a device that would enable this signal to be synchronized. Antheil laid out a system based on 88 frequencies, corresponding to the number of keys on a piano, using perforated paper rolls which would turn in sync with one another, transmitting and receiving ever-changing frequencies, preventing interceptance and jamming. In December of 1940, the "frequency hopping" device developed by Lamarr and Antheil was submitted to the national inventors council, a semi-military inventors' association. Lamarr and Antheil went on to file for a patent application for the "Secret Communication System," June 10, The patent was granted by the United States patent office on august 11, 1942. Lamarr and Antheil immediately placed their patent at the disposal of the US military. Though the us government did not deploy the "secret communication system" during World War II, the US Navy commissioned a project to acoustically detect submarines using sonar buoys remote-controlled from airplanes employing "frequency hopping" in the 1950s. Twenty years after its conceptualization, during the 1962 Cuban missile crisis, the first instance of large-scale military deployment of Lamarr and Antheil's frequency hopping technology was implemented-- not for the remote-controlled guidance of torpedoes, but to provide secure communications among the ships involved in the naval blockade. The early '60s saw the development of reconnaissance drones based on frequency hopping, which were later deployed in Vietnam. With the emergence of digital technology and the military's release of frequency hopping for public use in the 1980s, Lamarr and Antheil's invention took on new significance. Instead of "frequency hopping," today's term is "spread spectrum" but the basic idea is the same. The FCC recently allotted a special section of the radio spectrum for an experiment using the spread spectrum idea in a test designed to make cell phone calls more secure. A lot of corporate dollars have been invested in this process which has allowed more cell phone users to use the existing frequency spectrum. Hedy Lamarr Actress and co-inventor of frequency hopping spread spectrum

First spread-spectrum patent
By changing the transmitter frequencies in a “random” pattern, the torpedo control signal could not be jammed. Lamarr proposed using 88 frequencies sequenced for control. In 1936 Hedy Lamarr became the first woman to grace the silver screen in a feature film wearing nothing but her birthday suit. Five years later, at a Hollywood dinner party, she engaged in a passionate discussion with an avant-garde composer about protecting U.S. radio-guided torpedoes from enemy interference. She scrawled her phone number in lipstick on the windshield of his car so they could develop their ideas further. In 1942, unbeknownst to her adoring public, the unlikely duo secured a patent and gave it to the United States government for a "Secret Communications System" expressly constructed to assist in the defeat of Hitler. The science presented in this patent serves as the basis for the technology we use today in cell phones, pagers, wireless Internet, defense satellites, and a plethora of other spread-spectrum devices. In 1933 Hedy achieved international renown when she played the first nude scenes in cinematic history in the Czech film "Ecstasy." The film was banned in many countries around the world, including the United States. On August 10, 1933, Hedy Kiesler married Austrian industrialist Fritz Mandl, CEO of one of the world's leading arms manufacturers. While her husband forbade her to pursue her acting career, (attempting to buy up all existing copies of "Ecstasy"), this proved to be one of the most valuable relationships to the scientific technology she would develop years later. From Strapless to Wireless Hedy Lamarr helped to set the groundwork for some of the most revolutionary technology of our time. Following the outbreak of World War II, Lamarr, a passionate opponent of the Nazis, wanted to contribute more to the allied effort. As Mrs. Fritz Mandl, she had closely observed the planning and discussions that went into attempting to design remote-controlled torpedoes. These never went into production, because the radio-controlled guidance system was too susceptible to disruption. She got the idea of distributing the torpedo guidance signal over several frequencies, thus protecting it from enemy jamming. The only weak point was how to employ the synchronization of the signal's transmitter and receiver. In 1940, Lamarr met the American avant-garde composer George Antheil of "ballet mécanique" fame. She described her idea to him, and asked him to help her construct a device that would enable this signal to be synchronized. Antheil laid out a system based on 88 frequencies, corresponding to the number of keys on a piano, using perforated paper rolls which would turn in sync with one another, transmitting and receiving ever-changing frequencies, preventing interceptance and jamming. In December of 1940, the "frequency hopping" device developed by Lamarr and Antheil was submitted to the national inventors council, a semi-military inventors' association. Lamarr and Antheil went on to file for a patent application for the "Secret Communication System," June 10, The patent was granted by the United States patent office on august 11, 1942. Lamarr and Antheil immediately placed their patent at the disposal of the US military. Though the us government did not deploy the "secret communication system" during World War II, the US Navy commissioned a project to acoustically detect submarines using sonar buoys remote-controlled from airplanes employing "frequency hopping" in the 1950s. Twenty years after its conceptualization, during the 1962 Cuban missile crisis, the first instance of large-scale military deployment of Lamarr and Antheil's frequency hopping technology was implemented-- not for the remote-controlled guidance of torpedoes, but to provide secure communications among the ships involved in the naval blockade. The early '60s saw the development of reconnaissance drones based on frequency hopping, which were later deployed in Vietnam. With the emergence of digital technology and the military's release of frequency hopping for public use in the 1980s, Lamarr and Antheil's invention took on new significance. Instead of "frequency hopping," today's term is "spread spectrum" but the basic idea is the same. The FCC recently allotted a special section of the radio spectrum for an experiment using the spread spectrum idea in a test designed to make cell phone calls more secure. A lot of corporate dollars have been invested in this process which has allowed more cell phone users to use the existing frequency spectrum. Frequency switching pattern

FHSS Algorithm The initiating party sends a request via a predefined frequency or control channel. Once the receiving party have received the request, it sends a pseudo number called seed to the transmitting party via the same channel. The initiating party uses this seed as a variable in predefined algorithm and calculates the sequence of frequencies that must now be used to transmit the signals. Most often the period of frequency change is predetermined. The initiating party then sends the first piece of information via the lowest band of newly generated frequency spectrum. Thus acknowledging the receiving party that it has correctly calculated the sequence. The communication begins, and both the receiving party and the sending party change the frequencies along with the calculated order. Starting at the same point of the time.

FHSS Algorithm In FH Data is divided into chunks and transmitted at different frequencies at different times.

Frequency hopping spread spectrum
In a frequency hopping spread spectrum system, the carrier frequency is switched in a pseudorandom fashion. The transmitter and receiver know the pattern and are synchronized. 225 230 235 240 245 250 255 f (MHz) Time (ms) Dwell time In 1936 Hedy Lamarr became the first woman to grace the silver screen in a feature film wearing nothing but her birthday suit. Five years later, at a Hollywood dinner party, she engaged in a passionate discussion with an avant-garde composer about protecting U.S. radio-guided torpedoes from enemy interference. She scrawled her phone number in lipstick on the windshield of his car so they could develop their ideas further. In 1942, unbeknownst to her adoring public, the unlikely duo secured a patent and gave it to the United States government for a "Secret Communications System" expressly constructed to assist in the defeat of Hitler. The science presented in this patent serves as the basis for the technology we use today in cell phones, pagers, wireless Internet, defense satellites, and a plethora of other spread-spectrum devices. In 1933 Hedy achieved international renown when she played the first nude scenes in cinematic history in the Czech film "Ecstasy." The film was banned in many countries around the world, including the United States. On August 10, 1933, Hedy Kiesler married Austrian industrialist Fritz Mandl, CEO of one of the world's leading arms manufacturers. While her husband forbade her to pursue her acting career, (attempting to buy up all existing copies of "Ecstasy"), this proved to be one of the most valuable relationships to the scientific technology she would develop years later. From Strapless to Wireless Hedy Lamarr helped to set the groundwork for some of the most revolutionary technology of our time. Following the outbreak of World War II, Lamarr, a passionate opponent of the Nazis, wanted to contribute more to the allied effort. As Mrs. Fritz Mandl, she had closely observed the planning and discussions that went into attempting to design remote-controlled torpedoes. These never went into production, because the radio-controlled guidance system was too susceptible to disruption. She got the idea of distributing the torpedo guidance signal over several frequencies, thus protecting it from enemy jamming. The only weak point was how to employ the synchronization of the signal's transmitter and receiver. In 1940, Lamarr met the American avant-garde composer George Antheil of "ballet mécanique" fame. She described her idea to him, and asked him to help her construct a device that would enable this signal to be synchronized. Antheil laid out a system based on 88 frequencies, corresponding to the number of keys on a piano, using perforated paper rolls which would turn in sync with one another, transmitting and receiving ever-changing frequencies, preventing interceptance and jamming. In December of 1940, the "frequency hopping" device developed by Lamarr and Antheil was submitted to the national inventors council, a semi-military inventors' association. Lamarr and Antheil went on to file for a patent application for the "Secret Communication System," June 10, The patent was granted by the United States patent office on august 11, 1942. Lamarr and Antheil immediately placed their patent at the disposal of the US military. Though the us government did not deploy the "secret communication system" during World War II, the US Navy commissioned a project to acoustically detect submarines using sonar buoys remote-controlled from airplanes employing "frequency hopping" in the 1950s. Twenty years after its conceptualization, during the 1962 Cuban missile crisis, the first instance of large-scale military deployment of Lamarr and Antheil's frequency hopping technology was implemented-- not for the remote-controlled guidance of torpedoes, but to provide secure communications among the ships involved in the naval blockade. The early '60s saw the development of reconnaissance drones based on frequency hopping, which were later deployed in Vietnam. With the emergence of digital technology and the military's release of frequency hopping for public use in the 1980s, Lamarr and Antheil's invention took on new significance. Instead of "frequency hopping," today's term is "spread spectrum" but the basic idea is the same. The FCC recently allotted a special section of the radio spectrum for an experiment using the spread spectrum idea in a test designed to make cell phone calls more secure. A lot of corporate dollars have been invested in this process which has allowed more cell phone users to use the existing frequency spectrum.

Dwell Time FHSS systems include characteristics such as dwell time, hopping sequences, and hop time. These characteristics come together to make up how the FHSS system will function and the actual data throughput that will be available. The amount of time spent on a specific frequency in an FHSS hopping sequence is known as the dwell time. These channels, 1 MHz of bandwidth each, provide 79 optional frequencies on which to dwell for the specified length of the dwell time.

Hopping sequence The hopping sequence is the list of frequencies through which the FHSS system will hop according to the specified dwell time. The IEEE standard, section , states that 1 MHz channels should be used. These channels exist between and GHz in the United States and most of Europe. Every station in a Basic Service Set must use the same hopping sequence. Every station must also store a table of all the hopping sequences that are used within the system. These hopping sequences must have a minimum hop size of 6 MHz in frequency. If the device is currently communicating on the GHz frequency, it must hop to GHz at the next hop at a minimum.

Frequency hopping transmitter
The binary data to be transmitted is applied to a conventional two-tone FSK modulator. A frequency synthesizer produces a sine wave of a random frequency determined by a pseudorandom code generator. These two signals are mixed together, filtered and then transmitted. In 1936 Hedy Lamarr became the first woman to grace the silver screen in a feature film wearing nothing but her birthday suit. Five years later, at a Hollywood dinner party, she engaged in a passionate discussion with an avant-garde composer about protecting U.S. radio-guided torpedoes from enemy interference. She scrawled her phone number in lipstick on the windshield of his car so they could develop their ideas further. In 1942, unbeknownst to her adoring public, the unlikely duo secured a patent and gave it to the United States government for a "Secret Communications System" expressly constructed to assist in the defeat of Hitler. The science presented in this patent serves as the basis for the technology we use today in cell phones, pagers, wireless Internet, defense satellites, and a plethora of other spread-spectrum devices. In 1933 Hedy achieved international renown when she played the first nude scenes in cinematic history in the Czech film "Ecstasy." The film was banned in many countries around the world, including the United States. On August 10, 1933, Hedy Kiesler married Austrian industrialist Fritz Mandl, CEO of one of the world's leading arms manufacturers. While her husband forbade her to pursue her acting career, (attempting to buy up all existing copies of "Ecstasy"), this proved to be one of the most valuable relationships to the scientific technology she would develop years later. From Strapless to Wireless Hedy Lamarr helped to set the groundwork for some of the most revolutionary technology of our time. Following the outbreak of World War II, Lamarr, a passionate opponent of the Nazis, wanted to contribute more to the allied effort. As Mrs. Fritz Mandl, she had closely observed the planning and discussions that went into attempting to design remote-controlled torpedoes. These never went into production, because the radio-controlled guidance system was too susceptible to disruption. She got the idea of distributing the torpedo guidance signal over several frequencies, thus protecting it from enemy jamming. The only weak point was how to employ the synchronization of the signal's transmitter and receiver. In 1940, Lamarr met the American avant-garde composer George Antheil of "ballet mécanique" fame. She described her idea to him, and asked him to help her construct a device that would enable this signal to be synchronized. Antheil laid out a system based on 88 frequencies, corresponding to the number of keys on a piano, using perforated paper rolls which would turn in sync with one another, transmitting and receiving ever-changing frequencies, preventing interceptance and jamming. In December of 1940, the "frequency hopping" device developed by Lamarr and Antheil was submitted to the national inventors council, a semi-military inventors' association. Lamarr and Antheil went on to file for a patent application for the "Secret Communication System," June 10, The patent was granted by the United States patent office on august 11, 1942. Lamarr and Antheil immediately placed their patent at the disposal of the US military. Though the us government did not deploy the "secret communication system" during World War II, the US Navy commissioned a project to acoustically detect submarines using sonar buoys remote-controlled from airplanes employing "frequency hopping" in the 1950s. Twenty years after its conceptualization, during the 1962 Cuban missile crisis, the first instance of large-scale military deployment of Lamarr and Antheil's frequency hopping technology was implemented-- not for the remote-controlled guidance of torpedoes, but to provide secure communications among the ships involved in the naval blockade. The early '60s saw the development of reconnaissance drones based on frequency hopping, which were later deployed in Vietnam. With the emergence of digital technology and the military's release of frequency hopping for public use in the 1980s, Lamarr and Antheil's invention took on new significance. Instead of "frequency hopping," today's term is "spread spectrum" but the basic idea is the same. The FCC recently allotted a special section of the radio spectrum for an experiment using the spread spectrum idea in a test designed to make cell phone calls more secure. A lot of corporate dollars have been invested in this process which has allowed more cell phone users to use the existing frequency spectrum.

Frequency hopping transmitter
Typically the rate of frequency change is much higher than the data rate. The illustration below shows that the frequency synthesizer changes 4 times for each data bit. The time period spent on each frequency is called the dwell time (typically < 10 ms) In 1936 Hedy Lamarr became the first woman to grace the silver screen in a feature film wearing nothing but her birthday suit. Five years later, at a Hollywood dinner party, she engaged in a passionate discussion with an avant-garde composer about protecting U.S. radio-guided torpedoes from enemy interference. She scrawled her phone number in lipstick on the windshield of his car so they could develop their ideas further. In 1942, unbeknownst to her adoring public, the unlikely duo secured a patent and gave it to the United States government for a "Secret Communications System" expressly constructed to assist in the defeat of Hitler. The science presented in this patent serves as the basis for the technology we use today in cell phones, pagers, wireless Internet, defense satellites, and a plethora of other spread-spectrum devices. In 1933 Hedy achieved international renown when she played the first nude scenes in cinematic history in the Czech film "Ecstasy." The film was banned in many countries around the world, including the United States. On August 10, 1933, Hedy Kiesler married Austrian industrialist Fritz Mandl, CEO of one of the world's leading arms manufacturers. While her husband forbade her to pursue her acting career, (attempting to buy up all existing copies of "Ecstasy"), this proved to be one of the most valuable relationships to the scientific technology she would develop years later. From Strapless to Wireless Hedy Lamarr helped to set the groundwork for some of the most revolutionary technology of our time. Following the outbreak of World War II, Lamarr, a passionate opponent of the Nazis, wanted to contribute more to the allied effort. As Mrs. Fritz Mandl, she had closely observed the planning and discussions that went into attempting to design remote-controlled torpedoes. These never went into production, because the radio-controlled guidance system was too susceptible to disruption. She got the idea of distributing the torpedo guidance signal over several frequencies, thus protecting it from enemy jamming. The only weak point was how to employ the synchronization of the signal's transmitter and receiver. In 1940, Lamarr met the American avant-garde composer George Antheil of "ballet mécanique" fame. She described her idea to him, and asked him to help her construct a device that would enable this signal to be synchronized. Antheil laid out a system based on 88 frequencies, corresponding to the number of keys on a piano, using perforated paper rolls which would turn in sync with one another, transmitting and receiving ever-changing frequencies, preventing interceptance and jamming. In December of 1940, the "frequency hopping" device developed by Lamarr and Antheil was submitted to the national inventors council, a semi-military inventors' association. Lamarr and Antheil went on to file for a patent application for the "Secret Communication System," June 10, The patent was granted by the United States patent office on august 11, 1942. Lamarr and Antheil immediately placed their patent at the disposal of the US military. Though the us government did not deploy the "secret communication system" during World War II, the US Navy commissioned a project to acoustically detect submarines using sonar buoys remote-controlled from airplanes employing "frequency hopping" in the 1950s. Twenty years after its conceptualization, during the 1962 Cuban missile crisis, the first instance of large-scale military deployment of Lamarr and Antheil's frequency hopping technology was implemented-- not for the remote-controlled guidance of torpedoes, but to provide secure communications among the ships involved in the naval blockade. The early '60s saw the development of reconnaissance drones based on frequency hopping, which were later deployed in Vietnam. With the emergence of digital technology and the military's release of frequency hopping for public use in the 1980s, Lamarr and Antheil's invention took on new significance. Instead of "frequency hopping," today's term is "spread spectrum" but the basic idea is the same. The FCC recently allotted a special section of the radio spectrum for an experiment using the spread spectrum idea in a test designed to make cell phone calls more secure. A lot of corporate dollars have been invested in this process which has allowed more cell phone users to use the existing frequency spectrum.

Frequency hopping spread spectrum
The resulting signal, whose frequency rapidly jumps around, effectively scatters pieces of the signal all over the band. Someone else monitoring the spectrum would not recognize that a transmission is being made. In 1936 Hedy Lamarr became the first woman to grace the silver screen in a feature film wearing nothing but her birthday suit. Five years later, at a Hollywood dinner party, she engaged in a passionate discussion with an avant-garde composer about protecting U.S. radio-guided torpedoes from enemy interference. She scrawled her phone number in lipstick on the windshield of his car so they could develop their ideas further. In 1942, unbeknownst to her adoring public, the unlikely duo secured a patent and gave it to the United States government for a "Secret Communications System" expressly constructed to assist in the defeat of Hitler. The science presented in this patent serves as the basis for the technology we use today in cell phones, pagers, wireless Internet, defense satellites, and a plethora of other spread-spectrum devices. In 1933 Hedy achieved international renown when she played the first nude scenes in cinematic history in the Czech film "Ecstasy." The film was banned in many countries around the world, including the United States. On August 10, 1933, Hedy Kiesler married Austrian industrialist Fritz Mandl, CEO of one of the world's leading arms manufacturers. While her husband forbade her to pursue her acting career, (attempting to buy up all existing copies of "Ecstasy"), this proved to be one of the most valuable relationships to the scientific technology she would develop years later. From Strapless to Wireless Hedy Lamarr helped to set the groundwork for some of the most revolutionary technology of our time. Following the outbreak of World War II, Lamarr, a passionate opponent of the Nazis, wanted to contribute more to the allied effort. As Mrs. Fritz Mandl, she had closely observed the planning and discussions that went into attempting to design remote-controlled torpedoes. These never went into production, because the radio-controlled guidance system was too susceptible to disruption. She got the idea of distributing the torpedo guidance signal over several frequencies, thus protecting it from enemy jamming. The only weak point was how to employ the synchronization of the signal's transmitter and receiver. In 1940, Lamarr met the American avant-garde composer George Antheil of "ballet mécanique" fame. She described her idea to him, and asked him to help her construct a device that would enable this signal to be synchronized. Antheil laid out a system based on 88 frequencies, corresponding to the number of keys on a piano, using perforated paper rolls which would turn in sync with one another, transmitting and receiving ever-changing frequencies, preventing interceptance and jamming. In December of 1940, the "frequency hopping" device developed by Lamarr and Antheil was submitted to the national inventors council, a semi-military inventors' association. Lamarr and Antheil went on to file for a patent application for the "Secret Communication System," June 10, The patent was granted by the United States patent office on august 11, 1942. Lamarr and Antheil immediately placed their patent at the disposal of the US military. Though the us government did not deploy the "secret communication system" during World War II, the US Navy commissioned a project to acoustically detect submarines using sonar buoys remote-controlled from airplanes employing "frequency hopping" in the 1950s. Twenty years after its conceptualization, during the 1962 Cuban missile crisis, the first instance of large-scale military deployment of Lamarr and Antheil's frequency hopping technology was implemented-- not for the remote-controlled guidance of torpedoes, but to provide secure communications among the ships involved in the naval blockade. The early '60s saw the development of reconnaissance drones based on frequency hopping, which were later deployed in Vietnam. With the emergence of digital technology and the military's release of frequency hopping for public use in the 1980s, Lamarr and Antheil's invention took on new significance. Instead of "frequency hopping," today's term is "spread spectrum" but the basic idea is the same. The FCC recently allotted a special section of the radio spectrum for an experiment using the spread spectrum idea in a test designed to make cell phone calls more secure. A lot of corporate dollars have been invested in this process which has allowed more cell phone users to use the existing frequency spectrum. 225 230 235 240 245 250 255 f (MHz)

Frequency hopping receiver
The received signal is mixed using a local oscillator driven by the same pseudorandom sequence. The output produces the original two-tone FSK signal from which the binary data can be extracted. Timing is extremely critical in frequency hopping systems in order to maintain synchronization. In 1936 Hedy Lamarr became the first woman to grace the silver screen in a feature film wearing nothing but her birthday suit. Five years later, at a Hollywood dinner party, she engaged in a passionate discussion with an avant-garde composer about protecting U.S. radio-guided torpedoes from enemy interference. She scrawled her phone number in lipstick on the windshield of his car so they could develop their ideas further. In 1942, unbeknownst to her adoring public, the unlikely duo secured a patent and gave it to the United States government for a "Secret Communications System" expressly constructed to assist in the defeat of Hitler. The science presented in this patent serves as the basis for the technology we use today in cell phones, pagers, wireless Internet, defense satellites, and a plethora of other spread-spectrum devices. In 1933 Hedy achieved international renown when she played the first nude scenes in cinematic history in the Czech film "Ecstasy." The film was banned in many countries around the world, including the United States. On August 10, 1933, Hedy Kiesler married Austrian industrialist Fritz Mandl, CEO of one of the world's leading arms manufacturers. While her husband forbade her to pursue her acting career, (attempting to buy up all existing copies of "Ecstasy"), this proved to be one of the most valuable relationships to the scientific technology she would develop years later. From Strapless to Wireless Hedy Lamarr helped to set the groundwork for some of the most revolutionary technology of our time. Following the outbreak of World War II, Lamarr, a passionate opponent of the Nazis, wanted to contribute more to the allied effort. As Mrs. Fritz Mandl, she had closely observed the planning and discussions that went into attempting to design remote-controlled torpedoes. These never went into production, because the radio-controlled guidance system was too susceptible to disruption. She got the idea of distributing the torpedo guidance signal over several frequencies, thus protecting it from enemy jamming. The only weak point was how to employ the synchronization of the signal's transmitter and receiver. In 1940, Lamarr met the American avant-garde composer George Antheil of "ballet mécanique" fame. She described her idea to him, and asked him to help her construct a device that would enable this signal to be synchronized. Antheil laid out a system based on 88 frequencies, corresponding to the number of keys on a piano, using perforated paper rolls which would turn in sync with one another, transmitting and receiving ever-changing frequencies, preventing interceptance and jamming. In December of 1940, the "frequency hopping" device developed by Lamarr and Antheil was submitted to the national inventors council, a semi-military inventors' association. Lamarr and Antheil went on to file for a patent application for the "Secret Communication System," June 10, The patent was granted by the United States patent office on august 11, 1942. Lamarr and Antheil immediately placed their patent at the disposal of the US military. Though the us government did not deploy the "secret communication system" during World War II, the US Navy commissioned a project to acoustically detect submarines using sonar buoys remote-controlled from airplanes employing "frequency hopping" in the 1950s. Twenty years after its conceptualization, during the 1962 Cuban missile crisis, the first instance of large-scale military deployment of Lamarr and Antheil's frequency hopping technology was implemented-- not for the remote-controlled guidance of torpedoes, but to provide secure communications among the ships involved in the naval blockade. The early '60s saw the development of reconnaissance drones based on frequency hopping, which were later deployed in Vietnam. With the emergence of digital technology and the military's release of frequency hopping for public use in the 1980s, Lamarr and Antheil's invention took on new significance. Instead of "frequency hopping," today's term is "spread spectrum" but the basic idea is the same. The FCC recently allotted a special section of the radio spectrum for an experiment using the spread spectrum idea in a test designed to make cell phone calls more secure. A lot of corporate dollars have been invested in this process which has allowed more cell phone users to use the existing frequency spectrum.

Practical Example: Bluetooth
2.4 GHz – GHz Operating Range 79 Different Radio Channels Hops 1600 times per second for data/voice links Hops 3200 times per second for page and inquiry scanning 1 Mbps = Rb for Bluetooth Ver 1.1/1.2 3 Mbps = Rb for Bluetooth Ver 2.1 Gaussian Frequency Shift Keying (GFSK)

Frequency Hopping SS Signal is broadcast over seemingly random series of radio frequencies A number of channels allocated for the FH signal Width of each channel corresponds to bandwidth of input signal Signal hops from frequency to frequency at fixed intervals Transmitter operates in one channel at a time Bits are transmitted using some encoding scheme At each successive interval, a new carrier frequency is selected

Frequency Hopping SS

Frequency Hopping SS Hopping Sequence
Channel sequence dictated by spreading code Pseudorandom number serves as an index into a table of frequencies Chip Period Time spent on each channel FCC regulation  maximum dwell time of 400 ms IEEE standard  300 ms Chipping rate Hopping rate

Frequency Hopping SS Receiver, hopping between frequencies in synchronization with transmitter, picks up message Advantages Eavesdroppers hear only unintelligible blips Attempts to jam signal on one frequency succeed only at knocking out a few bits

FHSS Performance Considerations
Large number of frequencies used Results in a system that is quite resistant to jamming Jamming signal must jam all frequencies With fixed power, this reduces the jamming power in any one frequency band

FHSS Q:- What is frequency hopping spread spectrum Ans:- With frequency hopping spread spectrum (FHSS), the signal is broadcast over a seemingly random series of radio frequencies, hopping from frequency to frequency at fixed intervals. A receiver, hopping between frequencies in synchronization with the transmitter, picks up the message.

Slow and Fast FHSS Q:- Explain the difference between slow FHSS and fast FHSS.

(Frequency Hopping Spread Spectrum)
Discrete changes of carrier frequency sequence of frequency changes determined via pseudo random number sequence Two versions Fast Hopping: several frequencies per user bit Slow Hopping: several user bits per frequency Advantages frequency selective fading and interference limited to short period simple implementation uses only small portion of spectrum at any time

FHSS: Example tb: bit period td: dwell time user data slow hopping
(3 bits/hop) fast (3 hops/bit) 1 tb t f f1 f2 f3 td tb: bit period td: dwell time

Comparison between slow hopping and fast hopping
Pros: cheaper Cons: less immune to narrowband interference Fast hopping Pros: more immune to narrowband interference Cons: tight synchronization  increased complexity

Slow and Fast FHSS commonly use multiple FSK (MFSK)
have frequency shifted every Tc seconds duration of signal element is Ts seconds Slow FHSS has Tc  Ts Fast FHSS has Tc < Ts FHSS quite resistant to noise or jamming with fast FHSS giving better performance

Slow MFSK FHSS

Fast MFSK FHSS

Slow and Fast FHSS Q:- Explain the difference between slow FHSS and fast FHSS. Ans:- Slow FHSS = multiple signal elements per hop; fast FHSS = multiple hops per signal element.

DSSS Q:- What is direct sequence spread spectrum?

Direct Sequence Spread Spectrum
Spread spectrum increases the bandwidth of the signal compared to narrow band by spreading the signal. There are two major types of spread spectrum techniques: FHSS and DSSS. FHSS spreads the signal by hopping from one frequency to another across a bandwidth of 83 Mhz. DSSS spreads the signal by adding redundant bits to the signal prior to transmission which spreads the signal across 22 Mhz. The process of adding redundant information to the signal is called Processing Gain . The redundant information bits are called Pseudorandom Numbers (PN).

DSSS DSSS works by combining information bits (data signal) with higher data rate bit sequence (pseudorandom number (PN)). The PN is also called a Chipping Code (eg., the Barker chipping code) The bits resulting from combining the information bits with the chipping code are called chips - the result- which is then transmitted. The higher processing gain (more chips) increases the signal's resistance to interference by spreading it across a greater number of frequencies. IEEE has set their minimum processing gain to 11. The number of chips in the chipping code equates to the signal spreading ratio. Doubling the chipping speed doubles the signal spread and the required bandwidth.

Signal Spreading The Spreader employs an encoding scheme (Barker or Complementary Code Keying (CCK). The spread signal is then modulated by a carrier employing either Differential Binary Phase Shift Keying (DBPSK), or Differential Quadrature Phase Shift Keying (DQPSK). The Correlator reverses this process in order to recover the original data.

DSSS Channels Fourteen channels are identified, however, the FCC specifies only 11 channels for non-licensed (ISM band) use in the US. Each channels is a contiguous band of frequencies 22 Mhz wide with each channel separated by 5 MHz. Channel 1 = – (2.412 plus/minus 11 Mhz). Channel 2 = – (2.417 plus/minus 11 Mhz). Only Channels 1, 6 and 11 do not overlap

Spectrum Mask A spectrum Mask represents the maximum power output for the channel at various frequencies. From the center channel frequency, 11 MHz and MHZ the signal must be attenuated 30 dB. From the center channel frequency, outside MHZ, the signal is attenuated 50 dB.

DSSS Frequency Assignments
The Center DSSS frequencies of each channel are only 5 Mhz apart but each channel is 22 Mhz wide therefore adjacent channels will overlap. DSSS systems with overlapping channels in the same physical space would cause interference between systems. Co-located DSSS systems should have frequencies which are at least 5 channels apart, e.g., Channels 1 and 6, Channels 2 and 7, etc. Channels 1, 6 and 11 are the only theoretically non-overlapping channels. Channel 1 2.412 GHz Channel 6 2.437 GHz Channel 11 2.462 GHz 25 MHz

DSSS Non-overlapping Channels
Each channel is 22 MHz wide. In order for two bands not to overlap (interfere), there must be five channels between them. A maximum of three channels may be co-located (as shown) without overlap (interference). The transmitter spreads the signal sequence across the 22 Mhz wide channel so only a few chips will be impacted by interference.

DSSS Encoding and Modulation

DSSS Encoding and Modulation
DSSS (802.11b) employs two types of encoding schemes and two types of modulation schemes depending upon the speed of transmission. Encoding Schemes Barker Chipping Code: Spreads 1 data bit across 11 redundant bits at both 1 Mbps and 2 Mbps Complementary Code Keying (CCK): Maps 4 data bits into a unique redundant 8 bits for 5.5 Mbps Maps 8 data bits into a unique redundant 8 bits for 11 Mbps. Modulation Schemes Differential Binary Phase Shift Keying (DBPSK): Two phase shifts with each phase shift representing one transmitted bit. Differential Quadrature Phase Shift Keying (DQPSK): Four phase shifts with each phase shift representing two bits.

DSSS Encoding

Barker Chipping Code 802.11 adopted an 11 bit Barker chipping code.
Transmission. The Barker sequence, , was chosen to spread each 1 and 0 signal. The Barker sequence has six 1s and five 0s. Each data bit, 1 and 0, is modulo-2 (XOR) added to the eleven bit Barker sequence. If a one is encoded all the bits change. If a zero is encoded all bits stay the same. Reception. A zero bit corresponds to an eleven bit sequence of six 1s. A one bit corresponds to an eleven bit sequence of six 0s.

Barker Sequence

Direct Sequence Spread Spectrum ….

Complementary Code Keying(CCK)
Barker encoding along with DBPSK and DQPSK modulation schemes allow b to transmit data at 1 and 2 Mbps Complementary Code Keying (CCK) allows b to transmit data at 5.5 and 11 Mbps. CCK employs an 8 bit chipping code. The 8 chipping bit pattern is generated based upon the data to be transmitted. At 5.5 Mbps, 4 bits of incoming data is mapped into a unique 8 bit chipping pattern. At 11 Mbps, 8 bits of data is mapped into a unique 8 bit chipping pattern.

Complementary Code Keying (CCK)
To transmit 5.5 Mbps 4 data bits is mapped into 8 CCK chipping bits.. The unique 8 chipping bits is determined by the bit pattern of the 4 data bits to be transmitted. The data bit pattern is: b0, b1, b2, b3 b2 and b3 determine the unique pattern of the 8 bit CCK chipping code. Note: j represents the imaginary number, sqrt(-1), and appears on the imaginary or quadrature axis of the complex plane.

To transmit 5.5 Mbps 4 data bits is mapped into 8 CCK chipping bits.. The unique 8 chipping bits is determined by the bit pattern of the 4 data bits to be transmitted. The data bit pattern is: b0, b1, b2, b3 b0 and b1 determine the DQPSK phase rotation that is to be applied to the chip sequence. Each phase change is relative to the last chip transmitted.

To transmit 11 Mbps 8 data bits is mapped into 8 CCK chipping bits. The unique 8 chipping bits is determined by the bit pattern of the 8 data bits to be transmitted. The data bit pattern is: b0, b1, b2, b3, b4, b5, b6 ,b7 b2, b3, b4 ,b5, b6 and b7 selects one unique pattern of the 8 bit CCK chipping code out of 64 possible sequences. b0 and b1 are used to select the phase rotation sequence.

DSSS Modulation

Differential Binary Phase Shift Keying (DBPSK)
A Zero phase shift from the previous symbol is interpreted as a 0. A 180 degree phase shift from the previous symbol is interpreted as a 1.

Differential Quadrature Phase Shift Keying (DQPSK)
A Zero phase shift from the previous symbol is interpreted as a 00. A 90 degree phase shift from the previous symbol is interpreted as a 01. A 180 degree phase shift from the previous symbol is interpreted as a 11. A 270 degree phase shift from the previous symbol is interpreted as a 10.

DSSS Summary Data Rate Encoding Modulation
1 Barker Coding chips encoding 1 bit DBPSK 2 Barker Coding chips encoding 1 bit DQPSK 5.5 CCK Coding chips encode 8 bits DQPSK 11 CCK Coding chips encode 4 bits DQPSK

Another method of realizing spread spectrum is called direct sequence spread spectrum (DSSS). In a DSSS system the message bit stream is modified by a higher rate pseudonoise (PN) sequence (called a chip sequence). In 1936 Hedy Lamarr became the first woman to grace the silver screen in a feature film wearing nothing but her birthday suit. Five years later, at a Hollywood dinner party, she engaged in a passionate discussion with an avant-garde composer about protecting U.S. radio-guided torpedoes from enemy interference. She scrawled her phone number in lipstick on the windshield of his car so they could develop their ideas further. In 1942, unbeknownst to her adoring public, the unlikely duo secured a patent and gave it to the United States government for a "Secret Communications System" expressly constructed to assist in the defeat of Hitler. The science presented in this patent serves as the basis for the technology we use today in cell phones, pagers, wireless Internet, defense satellites, and a plethora of other spread-spectrum devices. In 1933 Hedy achieved international renown when she played the first nude scenes in cinematic history in the Czech film "Ecstasy." The film was banned in many countries around the world, including the United States. On August 10, 1933, Hedy Kiesler married Austrian industrialist Fritz Mandl, CEO of one of the world's leading arms manufacturers. While her husband forbade her to pursue her acting career, (attempting to buy up all existing copies of "Ecstasy"), this proved to be one of the most valuable relationships to the scientific technology she would develop years later. From Strapless to Wireless Hedy Lamarr helped to set the groundwork for some of the most revolutionary technology of our time. Following the outbreak of World War II, Lamarr, a passionate opponent of the Nazis, wanted to contribute more to the allied effort. As Mrs. Fritz Mandl, she had closely observed the planning and discussions that went into attempting to design remote-controlled torpedoes. These never went into production, because the radio-controlled guidance system was too susceptible to disruption. She got the idea of distributing the torpedo guidance signal over several frequencies, thus protecting it from enemy jamming. The only weak point was how to employ the synchronization of the signal's transmitter and receiver. In 1940, Lamarr met the American avant-garde composer George Antheil of "ballet mécanique" fame. She described her idea to him, and asked him to help her construct a device that would enable this signal to be synchronized. Antheil laid out a system based on 88 frequencies, corresponding to the number of keys on a piano, using perforated paper rolls which would turn in sync with one another, transmitting and receiving ever-changing frequencies, preventing interceptance and jamming. In December of 1940, the "frequency hopping" device developed by Lamarr and Antheil was submitted to the national inventors council, a semi-military inventors' association. Lamarr and Antheil went on to file for a patent application for the "Secret Communication System," June 10, The patent was granted by the United States patent office on august 11, 1942. Lamarr and Antheil immediately placed their patent at the disposal of the US military. Though the us government did not deploy the "secret communication system" during World War II, the US Navy commissioned a project to acoustically detect submarines using sonar buoys remote-controlled from airplanes employing "frequency hopping" in the 1950s. Twenty years after its conceptualization, during the 1962 Cuban missile crisis, the first instance of large-scale military deployment of Lamarr and Antheil's frequency hopping technology was implemented-- not for the remote-controlled guidance of torpedoes, but to provide secure communications among the ships involved in the naval blockade. The early '60s saw the development of reconnaissance drones based on frequency hopping, which were later deployed in Vietnam. With the emergence of digital technology and the military's release of frequency hopping for public use in the 1980s, Lamarr and Antheil's invention took on new significance. Instead of "frequency hopping," today's term is "spread spectrum" but the basic idea is the same. The FCC recently allotted a special section of the radio spectrum for an experiment using the spread spectrum idea in a test designed to make cell phone calls more secure. A lot of corporate dollars have been invested in this process which has allowed more cell phone users to use the existing frequency spectrum.

Direct Sequence Spread Spectrum –DSSS
In direct-sequence spread spectrum (DSSS), the serial binary data is XORed with a pseudo-random binary code which has a bit rate faster than the binary data rate, and the result is used to phase-modulate a carrier. chipping rate – bit rate of the pseudorandom code the faster you change the phase of a carrier, the more BW the signal takes up – looks like noise UNMODULATED CARRIER SLOW SPEED PSK HIGH SPEED PSK many clock (chipping rate) pulses in one data bit time

DSSS 1 data Pseudo Random Sequence data  PRS time of one data bit
UNMODULATED CARRIER SLOW SPEED PSK HIGH SPEED PSK “chip” time of one data bit frequency power carrier modulated by the data carrier modulated by the data  PRS XOR 

Direct Sequence Spread Spectrum (cont’d)
Observations A signal that would normally occupy a few kHz BW is spread out 10 to 10,000 times its BW. The fast phase modulation spreads the energy of the signal over a wide BW – appears as noise in a conventional receiver. Also called CDMA – Code Division Multiple Access used in satellites – many signals can use the same transponder used in cell phones – many users in same BW

Receiver Receiver must know the pseudorandom sequence of the transmitter and have a synchronizing circuit to get in step with this pseudorandom digital signal. The receiver using an identically programmed PN sequence compares incoming signals and picks out the one with the highest correlation. Other signals using different PN sequences appear as noise to the receiver and it doesn’t recognize them.

Processing gain The measure of the spreading is called the processing gain, G, which is the ratio of the DSSS bandwidth, BW, divided by the data rate, fb . The higher the processing gain, the greater the DSSS signal’s ability to fight interference. In 1936 Hedy Lamarr became the first woman to grace the silver screen in a feature film wearing nothing but her birthday suit. Five years later, at a Hollywood dinner party, she engaged in a passionate discussion with an avant-garde composer about protecting U.S. radio-guided torpedoes from enemy interference. She scrawled her phone number in lipstick on the windshield of his car so they could develop their ideas further. In 1942, unbeknownst to her adoring public, the unlikely duo secured a patent and gave it to the United States government for a "Secret Communications System" expressly constructed to assist in the defeat of Hitler. The science presented in this patent serves as the basis for the technology we use today in cell phones, pagers, wireless Internet, defense satellites, and a plethora of other spread-spectrum devices. In 1933 Hedy achieved international renown when she played the first nude scenes in cinematic history in the Czech film "Ecstasy." The film was banned in many countries around the world, including the United States. On August 10, 1933, Hedy Kiesler married Austrian industrialist Fritz Mandl, CEO of one of the world's leading arms manufacturers. While her husband forbade her to pursue her acting career, (attempting to buy up all existing copies of "Ecstasy"), this proved to be one of the most valuable relationships to the scientific technology she would develop years later. From Strapless to Wireless Hedy Lamarr helped to set the groundwork for some of the most revolutionary technology of our time. Following the outbreak of World War II, Lamarr, a passionate opponent of the Nazis, wanted to contribute more to the allied effort. As Mrs. Fritz Mandl, she had closely observed the planning and discussions that went into attempting to design remote-controlled torpedoes. These never went into production, because the radio-controlled guidance system was too susceptible to disruption. She got the idea of distributing the torpedo guidance signal over several frequencies, thus protecting it from enemy jamming. The only weak point was how to employ the synchronization of the signal's transmitter and receiver. In 1940, Lamarr met the American avant-garde composer George Antheil of "ballet mécanique" fame. She described her idea to him, and asked him to help her construct a device that would enable this signal to be synchronized. Antheil laid out a system based on 88 frequencies, corresponding to the number of keys on a piano, using perforated paper rolls which would turn in sync with one another, transmitting and receiving ever-changing frequencies, preventing interceptance and jamming. In December of 1940, the "frequency hopping" device developed by Lamarr and Antheil was submitted to the national inventors council, a semi-military inventors' association. Lamarr and Antheil went on to file for a patent application for the "Secret Communication System," June 10, The patent was granted by the United States patent office on august 11, 1942. Lamarr and Antheil immediately placed their patent at the disposal of the US military. Though the us government did not deploy the "secret communication system" during World War II, the US Navy commissioned a project to acoustically detect submarines using sonar buoys remote-controlled from airplanes employing "frequency hopping" in the 1950s. Twenty years after its conceptualization, during the 1962 Cuban missile crisis, the first instance of large-scale military deployment of Lamarr and Antheil's frequency hopping technology was implemented-- not for the remote-controlled guidance of torpedoes, but to provide secure communications among the ships involved in the naval blockade. The early '60s saw the development of reconnaissance drones based on frequency hopping, which were later deployed in Vietnam. With the emergence of digital technology and the military's release of frequency hopping for public use in the 1980s, Lamarr and Antheil's invention took on new significance. Instead of "frequency hopping," today's term is "spread spectrum" but the basic idea is the same. The FCC recently allotted a special section of the radio spectrum for an experiment using the spread spectrum idea in a test designed to make cell phone calls more secure. A lot of corporate dollars have been invested in this process which has allowed more cell phone users to use the existing frequency spectrum.

DSSS Signal The spread signal has the same power as the narrowband signal, but far more sidebands Amplitudes are very low and just above the random noise level Transmitter and receiver are using the same PN sequence, so signal will be recognized

Benefits of Spread Spectrum
Spread spectrum is being used in more and more applications in data communications. Security – need a wide BW receiver and precise knowledge and timing of the pseudorandom sequence Resistance to jamming and interference – jamming signals are usually restricted to one frequency Band sharing – many signals can use the same frequency band; but… many spread spectrum signals raise the overall background noise level Precise timing – can be used in radar where accurate knowledge of transmission time is needed

DSSS Q:- What is direct sequence spread spectrum?
Ans:- With direct sequence spread spectrum (DSSS), each bit in the original signal is represented by multiple bits in the transmitted signal, using a spreading code.

Bit Rate in DSSS Before and After
Q:- What is the relationship between the bit rate of a signal before and after it has been encoded using DSSS

DSSS Direct Sequence each bit in the original signal is represented by multiple bits in the transmitted signal, known as a chipping code the chipping code spreads the signal over a wider frequency band in direct proportion to the number of bits used.

Bit Rate in DSSS Before and After
Q:- What is the relationship between the bit rate of a signal before and after it has been encoded using DSSS Ans:- For an N-bit spreading code, the bit rate after spreading (usually called the chip rate) is N times the original bit rate.

CDMA Q:- What is CDMA?

CDMA Transceiver Block Diagram

Lets walk through an example?

Multiplication 1 x = 1 x -1 = -1 x 1 = -1 x = 1

CDMA example Low-Bandwidth Signal: High-Bandwidth Spreading Code:
...repeated...

CDMA example Low-Bandwidth Signal: High-Bandwidth Spreading Code:
Mix is a simple multiply … and transmit.

To Decode / Receive, take the signal:
CDMA example To Decode / Receive, take the signal:

CDMA example To Decode / Receive, take the signal:
Multiply by the same Spreading Code: … to get ... … which you should recognise as...

CDMA example To Decode / Receive, take the signal:
Multiply by the same Spreading Code: … to get ...

(Discuss noise) To Decode / Receive, take the signal:
Multiply by the same Spreading Code: … to get ...

What if we use the wrong code?
Take the same signal: Multiply by the wrong Spreading Code:

Take the same signal: Multiply by the wrong Spreading Code: … for example, let's just shift the same code left a bit:

Take the same signal: Multiply by the wrong Spreading Code: … you get ... … which clearly hasn't recovered the original signal. Using wrong code is like being off-frequency.

This obviously shows that
timing is critical. To receive a signal, you not only need to be generating the RIGHT code, but your TIMING needs to be locked very tightly to the received signal too.

CDMA Q:- What is CDMA? Ans:- CDMA allows multiple users to transmit over the same wireless channel using spread spectrum. Each user uses a different spreading code. The receiver picks out one signal by matching the spreading code.

Summary Relationship between bandwidth of a signal (before and after and encoding SS) Benefits of SS FHSS Slow and Fast FHSS DSSS Relationship between Bit Rate of a Signal (Before and after DSSS encoding) CDMA Chapter 9 summary.

Spread Spectrum Review/Recap

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