Wireless networks - Lecture 6: Multiple access techniques. The main topics covered in this chapter include: frequency division multiple access (FDMA); time division multiple access (TDMA); random access; random access methods in wireless networks; code division multiple access (CDMA);...
Wireless Networks Lecture Multiple Access Techniques Dr Ghalib A Shah Outlines Review of previous lecture #5 FDMA TDMA CDMA Random Access ► ALOHA ► Slotted ALOHA ► Reservation-based ALOHA Summary of today’s lecture Last Lecture Review Block Codes ► Hamming ► BCH ► Reed Solmon ARQ ► Sliding window ► Go-back-N Multiple Access Techniques Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Random Access Code Division Multiple Access (CDMA) User User User User Time y nc ue FDMA was the initial multiple-access technique for cellular systems Separates large band into smaller channels Each channel has the ability to support user Guard bands are used to separate channel preventing co-channel interference Narrow bandwidth (30 khz) Fr eq e C FDMA f1 Advantages ► Simple to implement in terms of hardware ► Fairly efficient with a small base population and with constant traffic Disadvantages ► Network and spectrum planning are intensive and time consuming ► Channels are dedicated for a single user, idle channels add spectrum inefficiency Us er U se r2 e C er s U Time nc y ue Entire bandwidth is available to the user for finite period of time Users are allotted time slots for a channel allowing sharing of a single channel Requires time synchronization Each of the user takes turn in transmitting and receiving data in a round robin fashion Fr eq U se r3 TDMA How it works? User presses Push-to-Talk (PTT) button A control channel registers the radio to the closest base station The BS assigns an available pair of channels Unlike FDMA, TDMA system also assigns an available time slot within the channel Data transmission is not continuous rather sent and received in bursts The bursts are reassembled and appear like continuous transmission Advantages ► ► ► Extended battery life and talk time More efficient use of spectrum, compared to FDMA Will accommodate more users in the same spectrum space than an FDMA system Disadvantages ► ► ► ► ► Network and spectrum planning are intensive Multipath interference affects call quality Dropped calls are possible when users switch in and out of different cells Too few users result in idle channels (rural versus urban environment) Higher costs due to greater equipment sophistication CDMA is a spread spectrum technique used to increase spectrum efficiency SS has been used in military applications due to anti-jamming and security e C CDMA User User User User Fr eq ue nc y User 10 Time CodeDivision Multiple Access (CDMA) Basic Principles of CDMA ► D = rate of data signal ► Break each bit into k chips • Chips are a userspecific fixed pattern ► Chip data rate of new channel = kD 11 CDMA Example If k=6 and code is a sequence of 1s and 1s ► For a ‘1’ bit, A sends code as chip pattern • ► For a ‘0’ bit, A sends complement of code • Receiver knows sender’s code and performs electronic decode function • • Su d = received chip pattern = sender’s code d1 c1 d c d c3 d c d c5 d c6 12 CDMA Example User A code = ► To send a 1 bit = ► To send a 0 bit = User B code = ► To send a 1 bit = Receiver receiving with A’s code ► (A’s code) x (received chip pattern) • • • User A ‘1’ bit: 6 > 1 User A ‘0’ bit: 6 > 0 User B ‘1’ bit: 0 > unwanted signal ignored 13 Advantag e s ► ► ► ► ► Greatest spectrum efficiency: CDMA improves call quality by filtering out background noise, crosstalk, and interference Simplified frequency planning - all users on a CDMA system use the same radio frequency spectrum Random Walsh codes enhance user privacy; a spread-spectrum advantage Precise power control increases talk time and battery size for mobile phones Dis advantag e s ► ► ► Backwards compatibility techniques are costly Currently, base station equipment is expensive Low traffic areas lead to inefficient use of spectrum and equipment resources 14 Rando m Ac c e s s Rando m Ac c e s s Me tho ds ► mo re e ffic ie nt way o f manag ing me dium ac c e s s fo r c o mmunic ating s ho rt burs ty me s s ag e s • in contrast to fixed-access schemes, each user gains access to medium only when needed -has some data to send • drawback: users must compete to access the medium (‘random access’) • collision of contending transmissions Rando m Ac c e s s Me tho ds in Wire le s s Ne two rks ► c an be divide d into two g ro ups : • ALOHA based-no coordination between users • carrier-sense based-indirect coordination -users sense availability of medium before transmitting 15 Random Access Collision Period User User User rescheduled User Time 16 ALOHAbas e d Rando m Ac c e s s us e r ac c e s s e s me dium as s o o n as it has a pac ke t re ady to trans mit ► after transmission, user waits a length of time >round-trip delay in the network, for an ACK from the receiver ► if no ACK arrives, user waits a random interval of time (to avoid repeated collision) and retransmits advantages: ► simple, no synchronization among users required disadvantages: ► low throughput under heavy load conditions ► probability of collision increases as number of users increases max thro ug hput = 18% o f c hanne l c apac ity 17 Pure-ALOHA 18 Slotted ALOHA time is divide d into e qual time s lo ts –whe n a us e r has a pac ke t to trans mit, the pac ke t is buffe re d and trans mitte d at the s tart o f the ne xt time s lo t ► BS transmits a beacon signal for timing, all users must synchronize their clocks advantages: ► partial packet collision avoided Disadvantages ► throughput still quite low! ► there is either no collision or a complete collision max thro ug hput = 36% o f c hanne l c apac ity 19 Slotted ALOHA 20 Example s lo tte d ALOHA in GS M ► Two type s o f c hanne ls in GS M: • Traffic c hanne ls (TCH): used for transmission of user data –based on FDMA/TDMA • Signalling channels, used for control and management of a cellular network • Rando m Ac c e s s Channe l (RACH): signalling channel for establishing access to the network (i.e BS) • employs Slotted ALOHA • only channel in GSM where contention can occur 21 Re s e rvatio n ALOHA Time s lo ts are divide d into re s e rvatio n and trans mis s io n s lo ts / pe rio ds ► during reservation period, stations can reserve future slots in transmission period ► reservation slot size