NEW TRENDS IN WIRELESS COMMUNICATION TECHNOLOGY (WITH SUITABLE MULTIPLE ACCESS)
Manoj Kr. Shukla Assistant Professor Dept. of Electronics Engineering Harcourt Butler Technological Institute Kanpur 208002 Email:
[email protected] Email:
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Question • The EM spectrum is a limited resource • How can we ―share‖ it? – Time – Space – Frequency – Polarization – Spread Spectrum - use a wider bandwidth?
Question • The EM spectrum is a limited resource • How can we ―share‖ it? – Time – Space – Frequency – Polarization – Spread Spectrum - use a wider bandwidth?
Multiple Access techniques • Goal
• • • • • • •
allow many users to simultaneously share a communications resource
Time Division Multiple Access (TDMA) Space Division Multiple Access (SDMA) Frequency Division Multiple Access (FDMA) Polarization Division Multiple Access (PDMA) Code Division Multiple Access (CDMA) Interleave Division Multiple Access (IDMA) Orthogonal Frequency Division Multiple Access
Key Issue • separate the signals at the receiver to extract your information Two methods • Do not mix the signals in the first place – can use space or time (SDMA or TDMA)
• Use distinctive properties of each signal as a means to identify – Frequency spectrum (FDMA) – Polarization of waves (PDMA) – code sequence attached to each message (CDMA)
International Cocktail Party • FDMA – Large room divided up into small rooms with limited microphones. Each pair of people takes turns speaking.
• TDMA – Large room divided up into small rooms with limited microphones. Certain pairs of people per room, however, each pair gets limited seconds to speak.
• CDMA – No small rooms. Everyone is speaking in different languages with own microphones. If voice volume is minimized, the number of people is maximized.
Definitions • TDMA – Time Division Multiple Access • FDMA – Frequency Division Multiple Access • CDMA – Code Division Multiple Access • IDMA- Interleave Division Multiple Access
General Specification of TDMA • Rx: 869-894MHz Tx: 824-849MHz • 832 Channels spaced 30kHz apart • • • • •
(3 users/channel) DQPSK modulation scheme 48.6kbps bit rate Interim Standard (IS) – 54 Digital AMPS (Advanced Mobile Phone System) Uses Time Division Duplexing (TDD) usually
TDMA Details • The incoming data from each source are briefly buffered and scanned to to form a composite digital data stream mc ( t ) . m1 ( t )
U1 U2
UN
Buffer m2 ( t )
m N ( t )
Buffer
Frame mc ( t )
p r e a m b l e
1 2
Frame
N
information
Buffer Scan operation
...
p r e a m b l e
1 2
N
Time slot
Each slot may be empty or occupied. + has preamble & guard bits
TDMA System • Each user receives half of the frame and the full bandwidth. – Users can resolve both multipath
• Time allocation is independent of power allocation. • Nonlinear ISI cancellation. – Cancel edge effects as well.
Interval of Interest
s0 h1
s0 h2 s1 h1
s1 h2 s2 h1
s2 h2
TDMA Block Diagram User 1 Data
Estimate Channel
Channel
ISI Cancellation
Equalize
Detect and Decode
User 2 Data Output
Advantages of TDMA • • • •
Flexible bit rate No frequency guard band required No need for precise narrowband filters Easy for mobile or base stations to initiate and execute hands off • Extended battery life • TDMA installations offer savings in base station equipment, space and maintenance • The most cost-effective technology for upgrading a current analog system to digital
Disadvantages to using TDMA • Requires network-wide timing synchronization • Requires signal processing fro matched filtering and correlation detection • Demands high peak power on uplink in transient mode • Multipath distortion
General Specification of FDMA • Rx: 869-894MHz Tx: 824-849MHz • 832 Channels spaced 30kHz apart (3 users/channel) • DQPSK modulation scheme • 48.6kbps bit rate • Used in analog cellular phone systems (i.e. AMPS) • Uses Frequency Division Duplexing (FDD) • ISI (Intersymbol Interference) is low
Advantages of FDMA • Channel bandwidth is relatively narrow (30kHz) • Simple algorithmically, and from a hardware • • • •
standpoint Fairly efficient when the number of stations is small and the traffic is uniformly constant Capacity increase can be obtained by reducing the information bit rate and using efficient digital code No need for network timing No restriction regarding the type of baseband or type of modulation
Disadvantages to using FDMA • The presence of guard bands • Requires right RF filtering to minimize • • • •
adjacent channel interference Maximum bit rate per channel is fixed Small inhibiting flexibility in bit rate capability Does not differ significantly from analog system If channel is not in use, it sits idle
SDMA Space Division Multiple Access • Use highly directional – The receiver selects the beam that provides the greatest signal enhancement and interference reduction – Smart antenna systems can adjust their antenna pattern to enhance the desired signal, null or reduce interference.
Desired Signal Direction
SDMA Pros and Cons Advantages • BW increases with km2
• Simple system
Disadvantages • Restricted Geometry – terminals in same direction cannot share
• May have unused BW – if no terminals in given zone, bw not used
PDMA Polarization Division Multiple Access • Two methods – Two antennas with orthogonal polarizations – an antenna with dual-polarization (SATCOM)
• Each polarization provides one separate channel
PDMA Pros and Cons Advantages • doubles BW
Disadvantages • Large specialized Ae
Spread Spectrum CDMA - FHMA - DSMA - SSMA
Definition - Spread Spectrum • The transmission bandwidth must be much larger than the information bandwidth
• The resulting RF bandwidth is determined by a function other than the information being sent
Spread Spectrum - illustrated Power Density
Conventional Transmission
PDi
same total power Spread Spectrum Transmission
PDSS
f Bi B
How • Two main methods – Frequency Hopped Multiple Access (FHMA) – Direct Sequence Multiple Access (DSMA) – THMA does exist, but not common
• Both depend on pseudo random orthogonal codes • often called pseudo noise
FHSS
Frequency Hopping Multiple Access
• message is "cut" into small "chunks" • Each chunk is modulated by a different f c (determined by pseudo-random code)
• A band pass filter accepts the signals that follow the hopping sequence and rejects all other requires synchronization
• note - some early systems used short predictable patterns
FHSS - illustrated Frequency
Frequency Hop Tune Time Dwell Time
Time
DSMA Direct Sequence Multiple Access • Each bit is ―chipped‖ • Example - time domain Data
0.1 ms 1 bit
0.1 s
Chips 1000 chips Requires much wider bandwidth
Cross Correlation • Mathematical process used to determine the similarity between two signals
Received Signal
111101011001000 011110101100100
Modulo-2 sum
100011110101100
15-bit Code
Correlation = -1/15 (very poor)
Used for despreading
to determine start of code to lock onto correct code
Pseudo Random Orthogonal... • Different sequences are said to be orthogonal if they do not interfere with one another (ie have low cross correlation)
• A sequence is pseudo random if it is orthogonal with a time shifted version of itself
• note - this significantly reduces the number of codes available << 2 n -1
Spreading Process Noise Info Signal
Info
Baseband Signal Before spreading
Transmitted (Coded) Signal After spreading
How can you recover signal < noise
SNR gain of spread spectrum • The ratio of the ‗SNR out‘ to the ‗SNR into‘ the demodulator ( spreading factor).
GP =
SNRout SNRin
=
BWRF Rinfo
Example Given: 1 Mcps PN code 1 kbps information data signal BW
RF
= 2 MHz
6
G = 2 x 10 = 2000 = 33 dB p 3 10 This means that after de-spreading, signal is 33 dB (2000 times) bigger than the noise.
General Specification of CDMA • Rx: 869-894MHz Tx: 824-849MHz • 20 Channels spaced 1250kHz apart • • • •
(798 users/channel) QPSK/(Offset) OQPSK modulation scheme 1.2288Mbps bit rate IS-95 standard Operates at both 800 and 1900 MHz frequency bands
CDMA Operation • Spread Spectrum Multiple Access Technologies
CDMA in theory
• Sender A
– sends A d = 1, key A k = 010011 („0―= -1, „1―= +1) – sending signal A s = A d * A k = (-1, +1, -1, -1, +1, +1)
• Sender B – sends Bd = 0, key Bk = 110101 („0―= -1, „1―= +1) – sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, 1)
• Both signals superimpose in space – interference neglected (noise etc.) – A s + Bs = (-2, 0, 0, -2, +2, 0)
Decoding CDMA • Receiver wants to receive signal from sender A – apply key A k bitwise (inner product) • A e = (-2, 0, 0, -2, +2, 0) A k = 2 + 0 + 0 + 2 + 2 + 0 = 6 • result greater than 0, therefore, original bit was „1―
– receiving B • Be = (-2, 0, 0, -2, +2, 0) Bk = -2 + 0 + 0 - 2 - 2 + 0 = 6, i.e. „0―
CDMA Encode/Decode channel output Z sender
d0 = 1
data bits code
Zi,m= di.cm
1 1 1
-1 -1 -1
1 -1
1 1 1 -1 -1 -1
slot 1
-1
slot 1 channel output
1 -1
1 1 1 1 1 1
1
d1 = -1
-1 -1 -1
slot 0
i,m
1 -1
-1 -1 -1
slot 0 channel output
M
Di = S Zi,m.cm m=1
received input code receiver
1 1 1 1 1 1
1 -1 -1 -1
-1
1 1 1
1 -1
-1 -1 -1
-1
1 1 1 -1 -1 -1
slot 1
M
1
1 -1
-1 -1 -1
slot 0
d0 = 1 d1 = -1
slot 1 channel output
slot 0 channel output
CDMA: two-sender interference
MC-CDMA System • Complex orthogonal spreading codes. – Length 2 – Spread over two subcarriers. • Both users use full bandwidth and full frame. • Each subcarrier is flat fading • Code allocation and spreading length is independent of power allocation.
Full Bandwidth s1c11f1
s1c12f2
s2c21f1
s2c22f2
First Subcarrier
Second Subcarrier
Half Bandwidth
User 1 User 2
Multicarrier CDMA • • • •
The data is serial-to-parallel converted. Symbols on each branch spread in time. Spread signals transmitted via OFDM Get spreading in both time and frequency c(t)
S/P convert
s(t)
IFFT c(t)
P/S convert
MC-CDMA Block Diagram User 1 Data
Spread
S
Spread
User 2 Data
Estimate Channel
IFFT
IFFT
Channel
Interference Cancellation
FFT
Equalize
Despread
Detect and Decode
Output
DS-CDMA System • Complex, orthogonal spreading codes.
Symbol Interval
– Length 2
• Synchronous transmission • Users can resolve both multipath components. • Nonlinear interference cancellation – ISI – Other user
• Code assignment and
spreading length are independent of power allocation.
s1c11h
s1c12h
s2c21h
s2c22h
Chip Interval
User 1 User 2
DS-CDMA Block Diagram User 1 Data
S
User 2 Data
Estimate Channel
Spread
Spread
Channel
Interference Cancellation
Despread
Equalize
Detect and Decode
Output
Capacity • CDMA has the ability to deliver 10 to 20 times the capacity as FDMA for the same bandwidth. • CDMA also has a capacity advantage over TDMA by 5 to 7 times.
TD-SCDMA development • Datang Telecommunication technology (former China Academy of Telecommunication Technology) is the most active TD-SCDMA developer • The biggest manufacturers have formed number of Joint Ventures for TD-SCDMA R&D • The Chinese Government has already invested more than 1 billion (US$123.3 million) in the research and development (R&D) of TD-SCDMA • Domestic companies have got heavy public subsidies for TDSCDMA development
TD-SCDMA developer pool
TD-SCDMA
Cons:
Pros:
• ITU standard, belongs to • • • •
• • •
3GPP TDD technology, fully compatible with GSM and GPRS Easy to upgrade from existing infrastructure Efficient use of spectrum Effective data transmission. Asynchronous uplink – downlink, suitable for Internet traffic Use of Smart Antenna technology Good mobility: > 120 km/h Large cells, with diameter up to 40 km
• Standard development far • • • •
behind rivals. Standard is very immature, no commercial use so far No large scale support from industry. Only few TDSCDMA chips available Lack of equipments and handsets. No mass production. No uniform platform for applications -> No application developer “pool” Some unsolved technical problems: – – – –
Cell interference large cell area functions high speed mobility poor stability of existing IC’s – Power consumption of
5. Third Generation Mobile 5.3 TD-SCDMA
TD-SCDMA forum • Industry consortium devoted to develop and support TD-SCDMA technology • Established in Dec/2000 by China Mobile, China Telecom, China Unicom, Datang, Huawei, Motorola, Nortel and Siemens • More than 420 members – 16 Board Members – 18 Senior Members – 390 ordinary members
Advantages of CDMA • Many users of CDMA use the same frequency, • • • • • •
TDD or FDD may be used Multipath fading may be substantially reduced because of large signal bandwidth No absolute limit on the number of users Easy addition of more users Impossible for hackers to decipher the code sent Better signal quality No sense of handoff when changing cells
Disadvantages to using CDMA • As the number of users increases, the • • • •
overall quality of service decreases Self-jamming Near- Far- problem arise higher complexity of a receiver all signals should have the same strength at a receiver
near and far terminals • Terminals A sends and B receives – signal strength decreases proportional to the square of the distance – the signal of terminal B therefore drowns out A‘s signal
A
B
Comparison SDMA/TDMA/FDMA/CDMA Approach Idea
SDMA segment space into cells/sectors
TDMA
FDMA
CDMA
Terminals
only one terminal can be active in one cell/one sector
Signal separation
cell structure, directed antennas
segment sending time into disjoint time-slots, demand driven or fixed patterns all terminals are active for short periods of time on the same frequency synchronization in the time domain
segment the frequency band into disjoint sub-bands
spread the spectrum using orthogonal codes
every terminal has its own frequency, uninterrupted
all terminals can be active at the same place at the same moment, uninterrupted code plus special receivers
Advantages
very simple, increases capacity per km²
established, fully digital, flexible
simple, established, robust
Disadvantages
inflexible, antennas typically fixed
inflexible, frequencies are a scarce resource
Comment
only in combination with TDMA, FDMA or CDMA useful
guard space needed (multipath propagation), synchronization difficult standard in fixed networks, together with FDMA/SDMA used in many mobile networks
flexible, less frequency planning needed, soft handover complex receivers, needs more complicated power control for senders
typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse)
still faces some problems, higher complexity, lowered expectations; will be integrated with TDMA/FDMA
filtering in the frequency domain
CDMA Design Considerations •
Bandwidth
–
limit channel usage to 5 MHz
•
Chip rate depends on desired data rate, need for error control, and bandwidth limitations; 3 Mcps or more is reasonable
•
Multirate advantage is that the system can flexibly support multiple simultaneous applications from a given user and can efficiently use available capacity by only providing the capacity required for each service
–
–
CDMA2000 Pros and Cons • Evolution from original Qualcomm CDMA – Now known as cdmaOne or IS-95 • Better migration story from 2G to 3G – cdmaOne operators don‘t need additional spectrum – 1xEVD0 promises higher data rates than UMTS, i.e. W-CDMA • Better spectral efficiency than W-CDMA(?) – Arguable (and argued!) • CDMA2000 core network less mature – cmdaOne interfaces were vendor-specific – Hopefully CDMA2000 vendors will comply w/
W-CDMA (UMTS) Pros and Cons • Wideband CDMA – Standard for Universal Mobile Telephone Service (UMTS) • Committed standard for Europe and likely migration path for other GSM operators –Leverages GSM‘s dominant position • Requires substantial new spectrum – 5 MHz each way (symmetric) • Legally mandated in Europe and elsewhere • Sales of new spectrum completed in Europe – At prices that now seem exorbitant
TD-SCDMA • Time division duplex (TDD) • Chinese development – Will be deployed in China • Good match for asymmetrical traffic! • Single spectral band (1.6 MHz) possible • Costs relatively low – Handset smaller and may cost less – Power consumption lower – TDD has the highest spectrum efficiency • Power amplifiers must be very linear
IMT-2000 Radio Standards • IMT-SC* Single Carrier (UWC-136): EDGE – GSM evolution (TDMA); 200 KHz channels; sometimes called ―2.75G‖
• IMT-MC* Multi Carrier CDMA: CDMA2000 – Evolution of IS-95 CDMA, i.e. cdmaOne
• IMT-DS* Direct Spread CDMA: W-CDMA – New from 3GPP; UTRAN FDD
• IMT-TC** Time Code CDMA – New from 3GPP; UTRAN TDD – New from China; TD-SCDMA
• IMT-FT** FDMA/TDMA (DECT legacy)
Some Requirements for Future Wireless Systems • • • • • • • • •
low receiver cost de-centralized (i.e., asynchronous) control, simple treatment of ISI, cross-cell interference mitigation, diversity against fading, power efficiency (long battery life), multi-media services (e.g., mixed voice and IP), high user number, high throughput and high spectral efficiency,
Dept. of Electronics &
FDMA × TDMA × CDMA ×
Evolution of IDMA • A conventional CDMA system requires separate coding and spreading operations.
• Verdu and Viterbi [2]* has shown that the optimum multiple channel capacity (MAC) is achievable only when entire bandwidth is devoted to coding. This suggests combining the coding and spreading operations using low-rate codes to maximize coding gain. Dept. Electronics & *S. Verdú and S. Shamai, “Spectral efficiency of of CDMA with random
Evolution of IDMA….. Possible Solution for User Separation • Narrow band coded-modulation scheme using trellis code structures [4]
• To employ chip-level interleavers [3][4][5][6] Improvement in CDMA scheme by assigning different interleavers to different users [5]*[6]** *A. Tarable,et al, “Analysis and design of interleavers for CDMA systems,” IEEE Commun. Lett., vol. 5,, Oct. 2001. **S. Brück, U. Sorger, S. Gligorevic, and N. Stolte, “Interleaving for outer convolutional Dept. of Electronics & codes in DSCDMA Systems,” IEEE Trans. Commun.,July 2000.
