CDMA Technology : CDMA Technology Deepak Gupta 08416 Multiple access techniques: Multiple access techniques FDMA TDMA CDMA Slide 4: Channel Access Method CDMA frequency reuse planning: CDMA frequency reuse planning Slide 6: A/D Convertor Vocoder FEC D/A Convertor Spreader Code Generator Voice CDMA signal generation Analog to digital conversion: Analog to digital conversion The first step of CDMA signal generation is analog to digital conversion. CDMA uses a technique called Pulse Code Modulation (PCM) to accomplish A/D conversion . sampling quantization coding Voice Compression: Voice Compression CDMA uses a device called a vocoder to accomplish voice compression. People pause between syllables and words when they talk. Variable Rate Vocoder: Variable Rate Vocoder A CDMA vocoder varies compression of the voice signal into one of four data rates based on the rate of the user's speech activity . The four rates are: Full, 1/2, 1/4 and 1/8 . The vocoder uses its full rate when a person is talking very fast. It uses the 1/8 rate when the person is silent or nearly so Encoding and interleaving: Encoding and interleaving CDMA can effectively reject narrow band interference. Since narrow band interference affects only a small portion of the spread spectrum signal , encoding and interleaving can be used to assist in recovering this lost data How encoding works: How encoding works The type of encoding done at this stage is called "convolutional encoding." A simplified encoding scheme is shown here. A digital message consists of four bits (A, B, C, D) of vocoded data. Each bit is repeated three times. These encoded bits are called symbols. The decoder at the receiver uses a majority logic rule. Thus, if an error occurs, the redundancy can help recover the lost information. Interleaving: Interleaving Errors typically occur in bursts rather than independently. If the number of errors within a code word exceeds the error-correcting code's capability, it fails to recover the original code word. Error-free message: aaaabbbbccccddddeeeeffffgggg Transmission with a burst error: aaaabbbbccc ____ deeeeffffgggg Interleaving cont..: Interleaving cont.. Interleaving is a way to arrange data in a non-contiguous way to increase performance . Error-free code words:aaaabbbbccccddddeeeeffffgggg Interleaved : abcdefgabcdefgabcdefgabcdefg Transmission with a burst error: abcdefgabcd ____ bcdefgabcdefg Received code words after deinterleaving : aa_abbbbccccdddde_eef_ffg_gg Channelizing: Channelizing The encoded voice data is further encoded to separate it from other encoded voice data. The encoded symbols are then spread over the entire bandwidth of the CDMA channel. This process is called channelization. The receiver knows the code and uses it to recover the voice data. Two kinds of codes: Two kinds of codes CDMA uses two important types of codes to channelize users . Walsh codes channelize users on the forward link (BTS to mobile). Pseudorandom Noise (PN) codes channelize users on the reverse link (mobile to BTS). Digital to Radio Frequency (RF) conversion: Digital to Radio Frequency (RF) conversion The BTS combines channelized data from all calls into one signal. It then converts the digital signal to a Radio Frequency (RF) signal for transmission. Reception of signal: Reception of signal Conversion of RF signal to digital signal Despreading the signal Deinterleaving and decoding Voice decompression Digital to analog voice Code channels used in CDMA: Code channels used in CDMA Pilot channel: Pilot channel The BTS constantly transmits the pilot channel. The mobile uses the pilot signal to acquire the system. It then uses the pilot signal to monitor and adjust the power needed in order to transmit back to the BTS. Sync channel: Sync channel The BTS constantly transmits over the sync channel so the mobile can synchronize with the BTS . It provides the mobile with the system time and the identification number of the cell site. The mobile ignores the sync channel after it is synchronized. Paging channel: Paging channel CDMA uses up to seven paging channels. The paging channel transmits overhead information such as commands and pages to the mobile. The paging channel also sends commands and traffic channel assignment during call set-up. The mobile ignores the paging channel after a traffic channel is established. Reverse link channels: Reverse link channels The reverse link uses two types of channels to transmit voice and control data to the BTS Access Traffic Access channel: Access channel The mobile uses the access channel when not assigned to a traffic channel. The mobile uses the access channel to : Register with the network Originate calls Respond to pages and commands from the base station Transmit overhead messages to the base station Reverse link traffic channel: Reverse link traffic channel The reverse traffic channel is only used when there is a call. The reverse traffic channel transmits voice data to the BTS. It also transmits the overhead control information during the call. Power control: Power control Power control is a CDMA feature that enables mobiles to adjust the power at which they transmit. This ensures that the base station receives all signals at the appropriate power. Open loop power control: Open loop power control Open loop is the mobile's estimate of the power at which it should transmit. The open loop estimate is based on the strength of the pilot signal the mobile receives. As the pilot signal gets weaker or stronger, the mobile adjusts its transmission strength upwards or downwards. Closed loop power control: Closed loop power control In closed loop, the BTS sends a command to the mobile to increase or decrease the strength at which it is transmitting. The BTS determines this command based on the quality of the signal it receives from the mobile. Closed loop is only used during a call. Closed loop commands are sent on the forward traffic channel. Features of CDMA: Features of CDMA The following features are unique to CDMA technology: Universal frequency reuse Fast and accurate power control Rake receiver Different types of handoff Rake Receiver: Rake Receiver The rake receiver is a CDMA feature that turns what is a problem in other technologies into an advantage for CDMA How the rake receiver works: How the rake receiver works CDMA's rake receiver is multiple receivers in one. The rake receiver identifies the three strongest multi-path signals and combines them to produce one very strong signal. The rake receiver therefore uses multipath to reduce the power the transmitter must send. Both the mobile and the BTS use rake receivers. Handoff in CDMA: Handoff in CDMA CDMA is unique in how it handles handoff . soft hard idle Soft handoff: Soft handoff A soft handoff establishes a connection with the new BTS prior to breaking the connection with the old one. This is possible because CDMA cells use the same frequency and because the mobile uses a rake receiver. The CDMA mobile assists the network in the handoff. The mobile detects a new pilot as it travels to the next coverage area. Hard handoff: Hard handoff A hard handoff requires the mobile to break the connection with the old BTS prior to making the connection with the new one. CDMA phones use a hard handoff when moving from a CDMA system to an analog system because soft handoffs are not possible in analog systems CDMA idle handoff: CDMA idle handoff An idle handoff occurs when the phone is in idle mode. The mobile will detect a pilot signal that is stronger than the current pilot. It will simply begins attending to the new pilot. An idle handoff occurs without any assistance from the BTS. Advantages of CDMA: Advantages of CDMA security Capacity No frequency planning Clarity Power control Slide 39: Thank You!!