An overview of CDMA Code - Division Multiple Access
Presented By Saimunur Rahman Matric No: C093003 Dept. of Computer Science & Engineering International Islamic University Chittagong
Presented to Abdullahil Kafi Assistant Professor Dept. of Computer Science & Engineering International Islamic University Chittagong And all of my classmates …
History of CDMA• In the USSR, the first work devoted to this subject was published in 1935 by professor D.V. Ageev.• CDMA was used in 1957, by the young military radio engineer Leonid Kupriyanovich in Moscow , he made a wearable automatic mobile phone called LK-1, with a base station.• In 1958, Kupriyanovich made the new experimental "pocket" model of mobile phone.
History of CDMA• 1958, the USSR started the development of the "Altay" national civil mobile phone service for cars• In 1963 this service started in Moscow and in 1970 Altay service was used in 30 USSR cities.
What is CDMA?• CDMA stands for Code division multiple access (CDMA)• Digital technology for delivering mobile telephone services• Uses spread-spectrum techniques• One channel carries all transmissions simultaneously• Communications with different codes
Idea of CDMALet us assume we have four stations 1, 2, 3 & 4.They are connected to the same channel. The data from station 1 are d1. The data from station 2 are d2. The data from station 3 are d3. The data from station 4 are d4.Code assigned to the first station is C1Code assigned to the first station is C2Code assigned to the first station is C3Code assigned to the first station is C4
Idea of CDMAWe assume that the assigned codes have two properties• If we multiply each code by another, we get 0.• If we multiply each code by itself, we get the number of stations. Always we must keep these two properties in mind
Idea of CDMA• Station 1 multiplies its data by its code to get d1.c1.• Station 2 multiplies its data by its code to get d2.c2.• Station 3 multiplies its data by its code to get d3.c3.• Station 4 multiplies its data by its code to get d4.c4. The data that go on the channel are the sum of all these terms, as shown in the box d1 . c1 + d2 . c2 + d3 . c3 + d4 . c4
Idea of CDMAIf station 1 and 2 are talking to each other. Station 2 wants to hear what station 1 is saying. It multiplies the data on the channel by c1, the code of station 1.Data = ( d1 . c1 + d2 . c2 + d3 . c3 + d4 . c4 ) . c1 = d1 . c1 . c1 + d2 . c2 . c1 + d3 . c3 . c1 + d4 . c4 . c1 = 4 ₓd1 [c1.c1 4, c2.c1 0, c3.c1 0 & c4.c1 0] = ( 4 ₓd1 ) / 4 = d1 d1 Was the datas of channel 1.
Chips• CDMA is based on coding theory.• Each station is assigned a code, which is a sequence of numbers called chips. Example of chip sequences
Chips• We can’t select chip sequences randomly, they were very carefully selected.• They are called ‘Orthogonal Sequences’ and they have some properties Lets discuss those properties that must be satisfied by chip sequences …
Chips sequence properties Each sequence is made of N elements, where N is the number of stations. If we multiply a sequence by a number, every element in the sequence is multiplied by that element. 2 ∙ [+1 +1 -1 -1] = [+2 +2 -2 -2] If we multiply two equal sequences, element by element and add the results, we get N. Where, N is the number of elements in each sequence. [+1 +1 -1 -1] ∙ [+1 +1 -1 -1] = 1 + 1 + 1 + 1 = 4
Chips sequence properties If we multiply two different sequences, element by element and add the results, we get 0. [+1 +1 -1 -1] ∙ [+1 +1 +1 +1] = 1+1-1-1 = 0 Adding two sequences means adding the corresponding elements. [+1 +1 -1 -1] + [+1 +1 +1 +1] = [+2 +2 0 0]
Chips Sequence GenerationFor generating chip sequences we will use Walsh Table.Walsh Table is a two- dimensional table with an equal number of rows and columns.Each row is a sequence of chips. Let’s see a figure for better understanding … …
Chips Sequence Generation Fig: General rule and examples of creating Walsh tables
Chips Sequence GenerationThe table for N sequences WNThe table for 2N sequences W2N Where N = 1,2,3 … …
Data representation in CDMAIf the station needs to send bits then For 0 bit it encodes it as -1. For 1 bit it encodes it as 1. For no signal it interpreted as 0. Fig : Data representation in CDMA
Encoding (By using example) • Lets assume, ▫ Channel 1 sending 0 bit. ▫ Channel 2 sending 0 bit. ▫ Channel 3 is silent. ▫ Channel 4 sending 1 bit • The data’s at the sender side are translated into -1, -1, 0, +1. • Each station multiplies the corresponding number by its chip, which is unique for each station.
Encoding (By using example) Channel No Data Chip Di * Ci Cn di Ci 1 -1 [+1 +1 +1 +1] [-1 -1 -1 -1] 2 -1 [+1 -1 +1 -1] [-1 +1 -1 +1] 3 0 [+1 +1 -1 -1] [0 0 0 0] 4 +1 [+1 -1 -1 +1] [+1 -1 -1 +1]• New results are sent to the channel• The sequence of the channel is the sum of all four sequences as defined before.[-1 -1 -1 -1] + [-1 +1 -1 +1] + [0 0 0 0] + [+1 -1 -1 +1]=[-1 -1 -3 +1]
Decoding (As per previous example)• Suppose, station 3 (which we said is silent) wants to listen station 2.• Now, station 3 multiplies the total data on the channel by the code for station 2, which is [+1 -1 +1 -1].• Then we get, [-1 -1 -3 +1] ∙ [+1 -1 +1 -1] = -1+1-3-1 = -4/4 = -1 -1 was encoded for station 2. -1 means 0. so, we got the value of station 2.
Let’s see the process in a figure Fig : Sharing channel in CDMA
Encoding (Analysis by using Signal lebel) It will be very clear if I use Signal level Lets see …
Encoding (Analysis by using Signal label) See the figure very carefully Fig: Encoding the digital signal created by four stations in CDMA
Decoding (Analysis by using Signal label) See the figure carefully Fig: Decoding of the composite signal for one in CDMA
Benefits of CDMAIncreased CapacityImproved QualityImproved QualityEnhanced PrivacyImproved CoverageIncreased Portable Talk TimeBandwidth on Demand
CDMA-ApplicationsCDMA for CellularCDMA-Short Message ServiceOver-the-Air ActivationCDMA Data and FaxSubscriber Access ControlCDMA for Personal Communications Services
ConclusionTechnology of choice for 3G generation because of it’s Greater total capacity Outstanding voice quality Fewer dropped callsIt’s use in satellite communication is of great importance