Mobile and Personal Communications. Dr Mike Fitton, Telecommunications Research Lab Toshiba Research Europe Limited

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1 Mobile and Personal Communications Dr Mike Fitton, Telecommunications Research Lab Toshiba Research Europe Limited 1

2 Mobile and Personal Communications Outline of Lectures Personal communication system requirements Multiple Access Techniques Frequency Division Multiple Access Time Division Multiple Access Code Division Multiple Access Techniques to improve performance Equalisation Diversity and Diversity Combining 2

3 Evolution of personal cellular communications Cellular systems are expanding in capacity and services Increasing integration between wireless systems Wireless LAN, wireless PAN (Bluetooth), etc 3

4 Multiple Access 4

5 Multiple Access Requirements A cellular system employs a multiple access technique to control the allocation of the network resources. The purposes of a multiple access technique are: To provide each user with unique access to the shared resource: the spectrum. To minimise the impact of other users acting as interferers. To provide efficient use of the spectrum available. To support flexible allocation of resources (for a variety of services). 5

6 Frequency Division Multiple Access (FDMA) Each user is assigned a unique frequency for the duration of their call. Severe fading and interference can cause errors. Complex frequency planning required. Not flexible. Used in analogue systems, such as TACS (Europe), and AMPS (USA). 2 users shown 6

7 Time Division Multiple Access (TDMA) Each user can use all available frequencies, for a limited period. The user must not transmit until its next turn. High bit rates required, therefore possible problems with intersymbol-interference. Flexible allocation of resources (multiple time slots). Used in second generation digital networks, such as GSM (Europe), and D-AMPS (USA). 2 users shown 7

8 Frequency Hopping Code Division Multiple Access (FH-CDMA) Each user regularly hops frequency over the available spectrum. Users are distinguished from each other by a unique hopping pattern (or code). Interference is randomised. Used in Bluetooth TM only 1 user shown 8

9 Direct Sequence Code Division Multiple Access (DS-CDMA) All users occupy the same spectrum at the same time. The modulated signal is spread to a much larger bandwidth than that required by multiplying with a spreading code. Users are distinguished from each other by a unique spreading code. Very flexible, but complex. Currently used in 3G and 2 nd generation IS-95 only 1 user shown 9

10 Summary of Multiple Access Techniques: The Cocktail Party To illustrate the nature of the multiple access techniques, consider a number of guests at a cocktail party. The aim is for all the guests to hold an intelligible conversation. In this case the resource available is the house itself. FDMA: each guest has a seperate room to talk to their partner. TDMA: everyone is in the same room, and has a limited time to hold their conversation (so they must talk very quickly). FH-CDMA: the guests run from room to room to talk. DS-CDMA: everyone is in the same room, talking at the same time, but each pair talks in a different language. 10

11 Duplex Communication Two way communication is called duplex (eg. for cellular radio). One way is called simplex (eg. for paging). The link from the base-station to mobile is the down-link. from the mobile to base-station is the up-link. The up-link and down-link can exist simultaneously on different frequencies: Frequency Division Duplex (FDD). The link The up-link and down-link can exist on the same frequency at different times: Time Division Duplex (TDD). 11

12 Hierarchical cell structure Macrocells, up to 144kbps Microcells up to 384kbps Picocells up to 2Mbps Possible Application: MPEG4 video News download File transfer 12

13 Performance enhancements 13

14 The effects of Equalisation Power (db) Frequency (GHz) T rms = 2.67µs (i) Channel (Frequency Domain) Frequency-selective fading arises due to time-dispersion in the multipath channel. This type of wideband fading causes irreducible errors, unless its effects are mitigated. Equalisation is employed to remove the harmful frequency-selective fading. It acts as an adaptive filter, to produce an output signal with a flat frequency response. Consequently, error-free transmission at high data rates is possible. Power (db) Frequency (GHz) Noise Enhancing Amplification (ii) Forward Filter (Frequency Domain)

15 Linear Transversal Equaliser Forward Filter INPUT r(t) r(t) r(t-t) r(t-2t) r(t-nt) T T T C 0 C 1 C 2 C n ERROR e k TRAINING SEQUENCE DECISION DEVICE OUTPUT Z k The linear transversal equalisation (LTE) is one of the simplest forms of equaliser. The tap coefficients (C1 to Cn) are adapt to suit the current channel conditions. Normally this adaptation is done on a training sequence. In the presence of severe amplitude and phase distortion, the required inverse filter tends to result in an unacceptable degree of noise amplification. 15

16 INPUT C n Decision Feedback Equaliser r(t+nt) b m Forward Filter r(t+[n-1]t) r(t+[n-2]t) r(t) T T T C C C n-1 n b 2 DECISION DEVICE + b 1 ERROR e k - TRAINING SEQUENCE OUTPUT Z k The equaliser output signal is the sum of the outputs of the feedforward and feedback sections of the equaliser. The forward section similar to the LTE Decisions made from the output of the equaliser are now feed back through a second filter. If these decisions are correct, the ISI caused by these symbols can be cancelled without noise enhancement ^ X k-m T ^ X k-2 T Feedback Filter ^ X k-1 T ^ X k However, errors made in hard decisions are fedback through the equaliser and can cause error propagation 16

17 Diversity Diversity: the provision of two or more uncorrelated (or independent) fading paths between transmitter and receiver. The uncorrelated fading statistics are combined or selected in some form. Performance improvement results as it is unlikely that all the diversity paths will be poor at the same time. Consequently, the probability of outage is reduced. Methods for generating uncorrelated paths for diversity combining include time, frequency, polarisation, angle, and space diversity. 17

18 . 2 1 Space Diversity Tx m m... C 2 B 1 λ/2 A Power A λ/2 B C distance (i) Space Diversity (ii) Power Variation with Distance 18

19 Polarisation and Angle Diversity Branch 1 Branch 2 Power (Horizontal) time Branch 6 Branch 3 Power (Vertical) time Branch 5 Branch 4 (i) Polarisation Diversity (ii) Angle (Pattern) Diversity 19

20 Time and Frequency Diversity T c B c Power time Power freq Data TX 1 TX 2 Data f 1 f 2 A B C D E F G A B C D E F G A B C D E F G A B C D E F G T c time B c freq (i) Time Diversity (ii) Frequency Diversity Less desirable: extra signal bandwidth is required 20

21 Diversity Combining: Switched (or Scanning) Combining A Comparator Fixed Threshold RSSI A Comparator Average RSSI Sync Sync B RF Switch IF Demod To Detector B RF Switch IF Demod To Detector f c + f IF f c + f IF (i) Switch Diversity with Fixed Threshold (ii) Switch Diversity with Adaptive Threshold The current branch remains selected until a metric fails a certain threshold, usually the Received Signal Strength Indicator (RSSI). The next branch is then blindly selected. An adaptive threshold removes unnecessary switching. When the signal fades relative to the mean, switching occurs. This system is cheap and simple, but not ideal. 21

22 Diversity Combining: Selection Combining 1 Rx RSSI Detect the Best Signal The most appropriate branch is always selected. Slight performance advantage over switch diversity. 2 M Rx Rx RSSI RSSI Selection Logic from the M Receivers Output Detector The system is expensive, as all branchs have to be analysed. Using RSSI as a indication of quality is non-ideal, since it is unduly affected by interference. 22

23 Diversity Combining: Equal Gain Combining (EGC) Cophasing & Summing Detector Output M Post-detection combining. 1 All branchs are merely cophased and summed. 23

24 Diversity Combining: Maximal Ratio Combining (MRC) 1 2 a 1 a 2 Cophasing & Summing Detector Output Each branch is weighted before summation in proportion to its own signal-to-noise ratio. Slightly better performance than EGC, but requires the complexity of estimating signal-to-noise ratio. M a M a i NOTE: = 1 for Equal Gain Combining 24

25 The Effect of Diversity on Fading Statistics Cumulative Density Combiner None Switched Equal Gain The fading statistics are improved with the applications of diversity. It is much less likely for deep fades to occur Received Signal Level Relative to Mean (db) 25

26 The Effect of Diversity on Performance 10-1 BER Diversity The BER in a Rayleigh fading channel can be significantly reduced with the use of diversity. Diversity can offer an 8-12 db gain in Rayleigh channels. It can also increase the maximum bit rate in a dispersion limited environment by a factor of two E s / N I(dB) 26

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