Part 3. Multiple Access Methods p. 1 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Review of Multiple Access Methods Aim of multiple access To simultaneously support communications between a base station and a number of users within a cell. TDMA (time division multiple access) All users are time-synchronized. A user is assigned a time slot (a finite time duration at a particular time) so that he or she can exclusively use the available frequency bandwidth to communicate. Other users are not allowed to transmit. FDMA (frequency division multiple access) Users are assigned with different segments of the available frequency bandwidth. A user has the exclusive right to use his/her allocated frequency bandwidth to communicate. Signals of other users are filtered by a bandpass filter. CDMA (code division multiple access) Users are assigned with different codes and these codes have low correlation. User signals are modulated by the assigned codes. The coded signals are detected at the receiver by correlations. SDMA (space division multiple access) p. 2 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Illustration of TDMA, FDMA, CDMA and SDMA Source: Rappaport s Wireless Communications TDMA CDMA FDMA SDMA p. 3 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
FDMA The number of channels that can be simultaneously supported is given by: N = B t 2B B guard B t is the total spectrum allocation B guard is the guard band allocated at the edge of the allocated spectrum B c is the channel bandwidth c p. 4 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
TDMA TDMA Frame structure Large overhead p. 5 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Higher capacity TDMA (cont.) The number of TDMA channel slots can be provided is given by: N = ( 2 ) t Bguard m B B c m is the number of TDMA slots per channel B t is the total spectrum allocation B guard is the guard band allocated at the edge of the allocated spectrum B c is the channel bandwidth p. 6 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
TDMA (cont.) Unperceivable to users Data transmission for users is not continuous, but occurs in bursts. p. 7 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Background of Spread-Spectrum Communications Frequency-hopping spread spectrum Is a digital communication technique in which the carrier frequency of a signal is varied in a (pseudo-)random fashion within a wideband channel. Multiple access is supported as carrier frequencies of multiple users most likely will not collide (can be done by design). Examples: Bluetooth, an option of IEEE 802.11 wireless LANs (not popular) Direct-sequence spread spectrum The signal is generated by multiplying the data with a (pseudo-)random sequence so that the resultant rate (chip rate) is high, resulting in a wideband signal. Multiple access is supported as random sequences used by multiple users have low correlations so that the interference due to other-user signals is reduced (but not eliminated). Examples: IS-95, WCDMA, cdma2000 p. 8 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Direct-Sequence Spread Spectrum (DSSS) A Data Symbol Incoming symbol sequence Spreading Sequence Periodic PN sequence x Chip Spread spectrum signal Spreading Sequence Block diagram: Data Symbol x Pulse Shaping Spread Spectrum Signal p. 9 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Spreading and Despreading Spread-Spectrum Signal Noise and Interference Data Symbols Spreading Modulation Channel + Demodulation Despreading Output Symbols High-rate spreading code Spreading code Incoming data stream (Narrowband signal) Spread-Spectrum Signal (Wideband signal) Original data stream x x Spreading Despreading p. 10 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multi-user Environment Spreading Seq. 1 Spreading Seq. 1 Info. Seq. 1 Despreader Info. Seq. 1 Spreading Seq. 2 Despreader Info. Seq. 2 Info. Seq. 2 Spreading Seq. 2 p. 11 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
How DSSS Technique Reduces Interference RF Signals Correlator Outuput desired signal after correlation (1) Spectral Density (db) narrowband interference signal Spectral Density (db) Processing Gain (db) Processing Gain (db) narrowband interference (becomes wideband) Frequency Frequency (2) Spectral Density (db) desired signal other-user signal Spectral Density (db) desired signal after correlation MAI (still wideband) Processing Gain (db) Frequency Frequency p. 12 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Direct-Sequence Spread Spectrum (Terminology) Spreading sequence (spreading code) Is the sequence used to spread the data symbol. A (pseudo) randomly generated sequence. Has sharp autocorrelation peak but low autocorrelation sidelobe. Examples: m-sequence, Gold sequences, Kasami sequences. Cross-correlation Is the correlation between two different spreading sequences. For SSMA, cross-correlation should be small. Examples of sequence sets with low cross-correlation: Gold sequences Processing gain Is the ratio of the spread-spectrum signal bandwidth to the data-signal bandwidth. Is, in most cases, the number of chips per symbol. p. 13 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
CDMA (1) CDMA (code division multiple access) Is not specifically referred to the use of direct-sequence spread-spectrum (DSSS) technique But is most often implicitly used in mobile communications to indicate the use of DSSS technique for multiple access. CDMA becomes attractive to mobile radio communications because the frequency reuse factor is 1. [c.f. reuse factor = 1/7 for many narrowband systems] The system capacity is increased. Different users use different spreading codes (i.e., spreading sequences) that are approximately orthogonal so that the receiver can decode each signal. p. 14 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
CDMA (2) The power of multiple users at a receiver determines the noise floor after correlation. If a CDMA signal has a higher power, it will generate more interference to other users. Near-far problem: If the power is not controlled, the CDMA signal of a user near to the base station will overshadow those signals originated from a distance away. Power control is necessary for CDMA systems. p. 15 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Features of CDMA Systems (extracted from Rappaport s Wireless Communications) Many users of a CDMA system share the same frequency. coexistence of multiple users on the same frequency band. CDMA has a soft capacity limit. Increasing the number of users in a CDMA system raises the noise floor. Thus, there is no absolute limit on the number of users in CDMA. The system performance gradually degrades (improves) for all users as the number of users increased (decreased). Inherent frequency diversity (multi-path diversity) can be exploited to mitigate the adverse effects of small-scale fading. p. 16 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Features of CDMA Systems (cont.) Adjacent cells can use the same frequency. A mobile station at the boundary of two adjacent cells can simultaneously receive signals from the two base stations. It is a diversity effect and can be used to improve the performance for mobile stations at the boundary. The handoff process is called soft handoff. Adjacent cells use different sets of spreading codes but can be operated at the same carrier frequency. The frequency reuse factor is therefore one. Multiple-access interference (MAI) is a problem in CDMA systems. MAI occurs because the spreading sequences of all users are not exactly orthogonal, leading to interference to other users signals. The near-far problem occurs at a CDMA receiver if an undesired users has a high detected power as compared to the desired user. p. 17 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multi-path Channel T p. 18 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
RAKE receiver to exploit multi-path diversity Spreading code generated with a time delay τ1. Path delays Path gains Extract the signal arrived from the path with delay τ1. Extract the signal arrived from the path with delay τ2. Extract the signal arrived from the path with delay τ3. Sourcr: T. Ojanpera and R. Prasad s WCDMA: towards IP mobility and mobile Internet p. 19 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
How RAKE receiver can exploit multipath diversity Spreading sequence has spike-like autocorrelation function: Autocorrelation peak Autocorrelation Autocorrelation sidelobe Shift One chip time A time shift more than or equal to one chip time yields very low autocorrelation value. Therefore, if the difference between path delays τ1 and τ2 is greater than the chip time, the signals arrived from the two paths can be resolved and extracted. p. 20 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Performance of SSMA Systems (1) Assumptions: BPSK is used. AWGN channel (not multi-path fading channel) is considered. Perfect power control is assumed so that the power levels of all users are the same. Rectangular chip waveform is used. Random sequences are used as spreading sequences. Gaussian approximation is used to approximate MAI so that the resultant BER is only an approximate one. Derivation See Rappaport s Wireless Communications p. 21 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
The bit error probability is where Performance of SSMA Systems (1) N is the processing gain of the system K is the number of users in the system E b is the energy per bit N 0 is the noise spectral density Observations: P Q K 1 N b = + 3N 2E A higher number of users degrades the performance. A higher processing gain reduces more on the MAI. F G H R S T Irreducible BER, which is the BER that cannot be reduced even if the signal power is increased, occurs and is given by F G H R S T P Q K 1 N b = Eb N0 + 3N 2E 0 b U V W 12 p. 22 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU I K E 0 b b U V W N 12 0 I J K J = Q F HG 3N K 1 I K J
Development on Spread Spectrum Techniques Source: T. Ojanpera and R. Prasad, WCDMA: Towards IP Mobility..., Table 1.1 p. 23 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
OFDM (Orthogonal Frequency Division Multiplexing) p. 24 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Introduction OFDM is a multi-carrier transmission scheme transform high-speed serial transmission to low-speed parallel transmission increase symbol duration, robust to multipath interference serial transmission parallel transmission 1 second 10 bits transmitted in 1 second, data rate: 10bits/s, bit duration: 1/10s bit duration: 1/10s 10 bits transmitted in parallel, bit duration: 1s, total data rate: 10bits/s, data rate per channel: 1bit/s different bit duration, same data rate p. 25 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Realization of parallel transmission Introduction (2) serial to parallel converter Multicarrier Transmission p. 26 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multipath Channels (1) Terrestrial Mobile Radio Communication Multipath channels Transmitted signals arrive at the receiver in various paths Illustration of multipath transmission p. 27 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multipath Channels (2) Measurement of multipath channel impulse signal time T 0 1/T Transmitter Multipath Channel T Receiver Channel impulse response τ max is the maximum delay spread T is the data symbol duration When T< τ max, frequency selective fading channel Multipath interference Desired signal interfered by τ max / T previous signals p. 28 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multipath Channels (3) Illustration of multipath interference Transmitter Multipath channel Receiver p. 29 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multipath Channels (4) Example: Broadband transmission, 100MHz Single carrier systems: DS-CDMA, chip duration (T) about 10ns Urban area, Microcell (<1km): τ max =1us path 0 path 1 T=10ns 0 0 path 100 (1us/10ns) τ max =1us 100 0 Each chip influenced by 100 previous chips Serious multipath interference, difficult to recover the desired signal at the receiver p. 30 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multipath Channels (5) Interpretation of multipath interference in frequency domain time freq. f Multipath channel frequency selective fading channel Signal Bandwidth ~100MHz f the signal experiences a frequency selective fading channel p. 31 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multipath Channels (6) Example: Broadband transmission, 100MHz Parallel processing Multicarrier system with 1000 subcarriers, T about 10us Urban area, Microcell (<1km): τ max 1us T=10us no delay 1000 delayed version 1000 τ max =1us Each data influenced by approximately 0.1 previous data symbol p. 32 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multipath Channels (7) Interpretation in frequency domain flat fading channel frequency selective fading channel f Multicarrier Transmission p. 33 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
OFDM Basics (1) Data symbol time freq. T filter at the receiver 2/T f Better spectrum efficiency 2/T Δf >=2/T Conventional Frequency Division Multiplexing f 2/T Δf=1/T Orthogonal Frequency Division Multiplexing (OFDM) f p. 34 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
OFDM Basics (2) Why could sub-carrier spacing be Δf=1/T? Received signal: T j2π f t j 2π f t 0 i= 0 i j ( ) () y t M 1 j2 ft i die π i= 0 = M is the total number of sub-carriers, d i is the data signal transmitted on the i th sub-carrier Sub-carrier down-conversion: T i = j T M 1 1 2 T M j π fjt j2π f 2 it j π fjt j2πδf ( i j) T rj = y() t e dt = d 1 0 i e e dt = d 0 i e i= 0 i= 0 i j j2πδf ( i j) M 1 j2π ( i j) ΔfT e 1 = d j T + di Interference from other sub-carriers j2πδf i j i j e e dt = 0 for i j can be obtained as long as Δf T is an integer The minimum sub-carrier spacing Δf=1/T! p. 35 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
OFDM Basics (3) Basic structure of OFDM systems Serial-to-Parallel Converter 0 0 0 T T T Oscillators are analog devices: expensive M up- and down- conversion: complicated p. 36 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
OFDM Basics (4) IFFT and FFT can be employed to realize the M sub-carrier upand down-conversion Digitalize the analog signal by sampling Sample rate: f s =MxΔf, duration: T s =1/f s () s t M 1 j 2 ft i die π i= 0 = t=nt s IFFT of d i M 1 M 1 M 1 j2 f nt j2π iδfn ( M Δf ) j2π in M s = i i = i s i= 0 i= 0 i= 0 π i s fi =Δ i f ( ) T = 1 ( M Δf ) s nt d e d e d e M sub-carrier up-conversion () s t M 1 j2 ft i die π i= 0 = M-point IFFT of d i M 1 j2 in M s( n) = die π i = 0 p. 37 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
OFDM Basics (5) Receiver M sub-carrier down-conversion T ( ) ( ) j2π fmt y m s t e dt 0 = M-point FFT of s(n) M 1 ( ) ( ) y m = m = 0 s n e j2π mn M Proof: ( ) y m ( ) M 1 j2πin M M 1 sn ( ) = de i j2π mn M i= 0 = n= 0 s e n M 1 M 1 M 1 j2π ( i m) j2π ( i m) n M 1 e Mdm i = m i i j2π ( i m) M FFT y m = d e = d = i= 0 n= 0 i= 0 1 e 0 i m p. 38 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
OFDM Basics (6) 0 T 0 T 0 T 0 T p. 39 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Cyclic Prefix of OFDM (1) OFDM symbol interference from the previous symbol received symbol OFDM symbol no interference from the previous symbol received symbol p. 40 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Cyclic Prefix of OFDM (2) Cyclic prefix is introduced to combat the inter-ofdm symbol interference (ISI) caused by the multipath channel. Multipath signals ISI due to multipath channel falls into the cyclic prefix; OFDM data symbols not affected by ISI. Adverse effects of ISI are eliminated. p. 41 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Transmitted signal: s( n) M 1 j2π in M die n M i= 0 Cyclic Prefix of OFDM (3) Why CP could help to avoid ISI? Mathematical interpretation =, = 0,, 1 M 1 j2π i( n Lg ) M s( n) = die, n = 0,, Lg 1,, M + Lg 1 i = 0 Multipath Channel L 1 ( ) = δ ( ) H n h n l l = 0 p. 42 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU l add cyclic prefix ( ) ( ) = ( ) + ( 1) + + ( 1) Received signal: y n h s n hs n h s n L 0 1 L 1 L 1 l = 0 l ( ) ( ) ( ) = hs n l = s n H n
Cyclic Prefix of OFDM (4) Multipath signals FFT after discarding cyclic prefix: M+ L 1 L 1 g j2π ( n Lg ) m M j2π ml M r( m) = y( n) e = Mdm he l n= Lg l= 0 H m Channel response on the m th sub-carrier Recovered data on the mth sub-carrier: ( ) ( ) d = r m MH = d m m m One-step equalization in frequency domain p. 43 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Cyclic Prefix of OFDM (5) Power efficiency total transmission power fixed the cyclic prefix: no new data information the system power efficiency is degraded Lg Definition of power efficiency: γ = M + L How to improve power efficiency reduce L g : if L g is shorter than the maximum channel delay, there will be ISI increase M: the bandwidth is fixed, larger M, narrower subbands, the system is vulnerable to inter-carrier interference caused by fast fading or frequency synchronization error g p. 44 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Cyclic Prefix of OFDM (6) Summary of CP As long as the length of CP is no less than the maximum channel delay, there is no ISI and data symbols can be recovered by using a simple one-step equalization in frequency domain Since the CP reduces the power efficiency of the system, the length of cyclic prefix is generally set to about 20% of the whole OFDM symbol length. Example: IEEE 802.11a & HIPERLAN/2 Data symbol length =3.2µs Cyclic prefix = 800ns [can absorb a channel dispersion of 800ns] Total length =4µs p. 45 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
As a multiple access scheme Flexibility of OFDM for user1 for user2 for user3 for user4 simple, flexible, make full use of the whole bandwidth p. 46 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Flexibility of OFDM (2) Easy to adapt to channel conditions Assume the channel condition is known at the transmitter (realized by feedback) sub-channel in good condition high-level modulations such as 64QAM 1.0 sub-channel in deep fading (bad condition) low-level modulations such as QPSK p. 47 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multiuser Diversity Flexibility of OFDM (3) A combination of the former two: channel conditions of all users are known to the transmitter for user3 for user2 for user1 user3 user2 user1 p. 48 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
PAPR: Peak-to-Average Power Ratio Definition: max s n PAPR= E s n ( ) 2 ( ) { } OFDM symbol is a sum of sinusoids When the number of sub-carriers M is large, PAPR is high 2 PAPR RF amplifiers: limited linear range, distort OFDM signals Signal Power in one OFDM symbol duration Source: http://www.ece.uvic.ca/~agullive/defence.pdf p. 49 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Summary of OFDM Advantages Easy to mitigate the adverse effects of channel dispersion by the use of cyclic prefix. Low-complexity implementation based on FFT/IFFT. Support high-rate transmission at a low implementation cost. Disadvantages High peak-to-average power ratio, so that highly linear power amplifiers are required at the transmitters in order to avoid intermodulation interference. The use of cyclic prefix reduces transmission efficiency. Some power is wasted by transmitting cyclic prefix, which are redundant. Good reference: R. van Nee and R. Prasad, OFDM for Wireless Multimedia Communications, Boston: Artech-House, 2000. p. 50 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU