2. Wireless Transmission Frequencies for Communication

Frequencies for Communication (1)

twisted coax cable optical transmission pair

1 Mm 10 km 100 m 1 m 10 mm 100 µm 1 µm 300 Hz 30 kHz 3 MHz 300 MHz 30 GHz 3 THz 300 THz 2. Wireless Transmission VLF LF MF HF VHF UHF SHF EHF infraredvisible light UV

VLF = Very Low Frequency UHF = Ultra High Frequency Frequencies and Signals LF = Low Frequency SHF = Super High Frequency Multiplexing MF = Medium Frequency EHF = Extra High Frequency Modulation and Spread Spectrum HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency

Frequency and wave length: λ = c/f wave length λ, speed of light c ≅ 3x108m/s, frequency f

Frequencies for Mobile Communication (2) Frequencies and Regulations

VHF-/UHF-ranges for mobile radio ITU-R holds auctions for new frequencies and manages – Simple, small antenna for cars frequency bands worldwide (WRC, World Radio Conferences) – Deterministic propagation characteristics, reliable connections Europe USA Japan Cellular GSM 450-457, 479- AMPS, TDMA, CDMA PDC Phones 486/460-467,489- 824-849, 810-826, 496, 890-915/935- 869-894 940-956, SHF and higher for directed radio links, satellite communication 960, TDMA, CDMA, GSM 1429-1465, 1710-1785/1805- 1850-1910, 1477-1513 – Small antenna, focusing 1880 1930-1990 – Large bandwidth available UMTS (FDD) 1920- 1980, 2110-2190 UMTS (TDD) 1900- 1920, 2020-2025 Wireless LANs use frequencies in UHF to SHF spectrum Cordless CT1+ 885-887, 930- PACS 1850-1910, 1930- PHS Phones 932 1990 1895-1918 – Some systems planned up to EHF CT2 PACS-UB 1910-1930 JCT 864-868 254-380 – Limitations due to absorption by water and oxygen molecules (resonance DECT 1880-1900 frequencies) Wireless IEEE 802.11 902-928 IEEE 802.11 LANs 2400-2483 IEEE 802.11 2471-2497 • Weather dependent fading, signal loss caused by heavy rainfall etc. HIPERLAN 2 2400-2483 5150-5250 5150-5350, 5470- 5150-5350, 5725-5825 5725 Others R F-C ontrol RF-Control RF-Control 27, 128, 418, 433, 315, 915 426, 868 868

Signals (1) Signals (2)

Physical representation of data Different representations of signals: Function of time and location – Amplitude (amplitude domain) – Frequency spectrum (frequency domain) – Phase state diagram (amplitude M and phase ϕ in polar coordinates) Signal parameters: ϕ – Parameters representing the value of data A [V] A [V] Q = M sin Classification: – Continuous time/discrete time t[s] ϕ – Continuous values/discrete values I= M cos ϕ – Analog signal = continuous time and continuous values ϕ f [Hz] – Digital signal = discrete time and discrete values Composed signals transferred into frequency domain using Signal parameters of periodic signals: Fourier transformation – Period T, frequency f=1/T, amplitude A, and phase shift ϕ – Sine wave as special periodic signal for a carrier: Digital signals need: π ϕ – Infinite frequencies for perfect transmission s(t) = At sin(2 ft t + t) – Modulation with a carrier frequency for transmission (analog signal!)

1 Fourier Representation of Periodic Signals Antennas: Isotropic Radiator

Radiation and reception of electromagnetic waves: – Coupling of wires to space for radio transmission ∞ ∞ = 1 + π + π Isotropic radiator: g(t) c ∑an sin(2 nft) ∑bn cos(2 nft) 2 n=1 n=1 – Equal radiation in all directions (three dimensional) – Only a theoretical reference antenna Real antennas always have directive effects: 1 1 – Vertically and/or horizontally Radiation pattern: – Measurement of radiation around an antenna 0 0 tt z y z Ideal periodic signal Real composition yxIdeal isotropic radiator (based on harmonics) x

Signal Propagation Ranges Signal Propagation

Transmission range: Propagation in free space always like light (straight line) – Communication possible Receiving power proportional to 1/d² – Low error rate – d = distance between sender and receiver Receiving power additionally influenced by: Detection range: – Fading (frequency dependent) – Detection of the signal – Shadowing (Abschattung) possible Sender – Reflection (Reflexion) at large obstacles – No communication – Refraction (Brechung) depending on the density of a medium possible Transmission – Scattering (Streuung) at small obstacles Distance – Diffraction (Beugung) at edges Interference range: Detection – Signal may not be Interference detected – Signal adds to the background noise shadowing reflection refraction scattering diffraction

Real World Examples Multi-path Propagation

Signal can take many different paths between sender and receiver due to: – Reflection, scattering, and diffraction LOS pulses Multi-path pulses

Signal at sender Signal at receiver

Time dispersion: signal is dispersed over time Î Interference with “neighbor” symbols, Inter Symbol Interference (ISI) The signal reaches a receiver directly and phase shifted Î Distorted signal depending on the phases of the different parts

2 Effects of Mobility Multiplexing

Channel characteristics change over time and location: Channels ki – Signal paths change permanently Multiplexing in 4 dimensions: k k k k k k – Different delay variations of different signal parts 1 2 3 4 5 6 –Space (si) – Different phases of signal parts –Time (t) c Î Quick changes in the power received (short term fading) – Frequency (f) t c – Code (c) t s Power Long term 1 f Additional changes in: fading Goal: s 2 f – Distance to sender – Multiple use of a shared medium c – Obstacles further away t Î Slow changes in the average power received (long term fading) t Important: guard spaces needed! Short term fading s 3 f

Frequency Multiplexing Time Multiplexing

Separation of the entire spectrum into smaller frequency bands A channel gets the whole spectrum for a certain amount of time A channel gets a certain band of the spectrum for the full time Advantages: Advantages: – Only one carrier in the – No dynamic coordination k k k k k k medium at any time 1 2 3 4 5 6 k1 k2 k3 k4 k5 k6 necessary – Throughput high even c – Works also for analog signals for many users f c f Drawbacks: Drawbacks: – Waste of bandwidth – Precise if the traffic is synchronization distributed unevenly necessary – Common – Inflexible t –Guard spaces t clock

Time and Frequency Multiplexing Code Multiplexing

Combination of both methods Each channel has a unique code k k k k k k A channel gets a certain frequency band for a certain 1 2 3 4 5 6 amount of time All channels use the same spectrum –Example: GSM at the same time c

k1 k2 k3 k4 k5 k6 Advantages: Advantages: – Bandwidth efficient – Better protection against tapping c – No coordination and synchronization necessary – Protection against frequency f – Good protection against interference and selective interference tapping f – Higher data rates compared Drawbacks: to code multiplex – Lower user data rates – More complex signal regeneration But: t Implemented using spread spectrum t – Precise coordination required technology

3 Modulation Modulation and Demodulation

Digital modulation: Analog – Digital data is translated into an analog signal (baseband) Base-band – ASK, FSK, PSK - main focus in this chapter Digital Data Signal – Differences in spectral efficiency, power efficiency, robustness Digital Analog Analog modulation: 101101001 Modulation Modulation Radio Transmitter – Shifts center frequency of baseband signal up to the radio carrier Radio Motivation: Carrier – Smaller antennas (e.g., λ/4) – Frequency Division Multiplexing Analog – Medium characteristics Base-band Digital Signal Data Basic schemes: Analog Synchronization – Amplitude Modulation (AM) Demodulation Decision 101101001 Radio Receiver – Frequency Modulation (FM) radio – Phase Modulation (PM) carrier

Digital Modulation Advanced Frequency Shift Keying

Modulation of digital signals known Bandwidth needed for FSK depends on the distance between 101 as Shift Keying the carrier frequencies Special pre-computation avoids sudden phase shifts: Amplitude Shift Keying (ASK): t Î MSK (Minimum Shift Keying) – Very simple Bit-separated into even and odd bits, the duration of each bit is – Low bandwidth requirements 101 doubled – Very susceptible to interference Depending on the bit values (even, odd) the higher or lower t frequency, original or inverted is chosen Frequency Shift Keying (FSK): – Needs larger bandwidth The frequency of one carrier is twice the frequency of the other 101 Equivalent to offset QPSK Even higher bandwidth efficiency using a Gaussian low-pass

Phase Shift Keying (PSK): t filter: –More complex Î GMSK (Gaussian MSK), used in GSM – Robust against interference

Advanced Phase Shift Keying Quadrature Amplitude Modulation

BPSK (Binary Phase Shift Keying): Q Quadrature Amplitude Modulation (QAM): QPSK – Bit value 0: sine wave (0° shift) – Combines amplitude and phase modulation 10 11 – Bit value 1: inverted sine wave (180° shift) I Q It is possible to code n bits using one symbol: 1 0 – Very simple PSK –2n discrete levels, n=2 identical to QPSK – Low spectral efficiency I – Bit error rate increases with n, but less errors compared to comparable PSK –Robust,e.g., used in satellite systems BPSK schemes 00 01 QPSK (Quadrature Phase Shift Keying): A – 2 bits coded as one symbol Q 0010 Example: 16-QAM (4 bits = 1 symbol) 000 – Symbol determines shift of sine wave Symbols 0011 and 0001 have the same phase, 0011 1 – Needs less bandwidth compared to BPSK t 0000 but different amplitude. 0000 and 1000 have – More complex (regular synchronization, pilot signal) I different phase, but same amplitude. 11 10 00 01 1000 Î used in standard 9600 bit/s modems Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (American IS-136, Japanese PHS)

4 Hierarchical Modulation Spread Spectrum Technology

DVB-T (Digital Video Broadcast Terrestrial) modulates two Problem of radio transmission: separate data streams onto a single DVB-T stream – Frequency-dependent fading can wipe out narrow band signals for duration of the interference High Priority (HP) embedded within a Low Priority (LP) stream Solution: – Spread the narrow band signal into a broad band signal using a special code Multi-carrier system, about 2000 or 8000 carriers Interference QPSK (2 first bits), 16 QAM, 64 QAM Power Power Signal Q Spread Example: 64 QAM Signal Spread – Good reception: resolve the entire Detection at Interference Receiver 64 QAM constellation f f 10 – Poor reception, mobile reception: Protection against narrow-band interference I resolve only QPSK portion Side effects: – 6 bit per QAM symbol, 2 most – Coexistence of several signals without dynamic coordination significant determine QPSK 00 – Tap-proof – HP service coded in QPSK (2 bit), LP uses remaining 4 bit 000010 010101 Alternatives: Direct Sequence, Frequency Hopping

Spreading and Frequency Selective Fading DSSS (Direct Sequence Spread Spectrum) (1)

Channel Narrow-band Channels XOR of the signal with pseudo-random number (chipping Quality sequence): 1. 6 signals in FDM – Many chips per bit (e.g., 128) result in higher bandwidth of the signal 2 2. Quality changes over time 1 5 6 t 3 3. Spot view b 4 Advantages: f 4. 1, 2, 5, 6 ok, 3, 4 not ok User Data 5. Apply SS onto 6 signals! – Reduces frequency selective 01XOR Narrow-band Signal Guard Space fading t – In cellular networks: c Chipping Channel • Base stations can use the Quality Spread Spectrum Channels Sequence same frequency range 2 011010 1 011010 1 = 2 2 1. 6 signals utilize full f • Several base stations can 2 Resulting 2 2. No f planning required detect and recover the signal 1 Signal 3. Separation by codes per signals • Soft hand-over 011010 1 100101 0 f 4. CDM, unique per signal Drawbacks: t : Bit Period Spread 5. “Extension” of f b – Precise power control necessary t : Chip Period Spectrum c – Spreading factor s = tb/tc -> s*w bandwidth requirements © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 27 © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 28

DSSS (Direct Sequence Spread Spectrum) (2) FHSS (Frequency Hopping Spread Spectrum) (1)

Spread Discrete changes of carrier frequency: Transmit Spectrum Example: – Available bandwidth of small channels separated as in FDM User Data Signal Signal IEEE 802.11 X Modulator – Sequence of frequency changes determined via pseudo random number (TDM) Chip: 10110111000 Chipping Radio (Barker Code) Two versions: 1 MHz user data – Fast Hopping: Sequence Cs Carrier fr generate 11 MHz several frequencies per user bit Transmitter signal in 2.4 GHz – Slow Hopping: frequency several user bits per frequency Low-pass Correlator Advantages: Received Filtered Sampled – Frequency selective fading and interference limited to short period Signal Products Signal Sums Data – Simple implementation Demodulator X Integrator Decision – Uses only small portion of spectrum at any time (dwell time: Verweildauer) Drawbacks: Radio Chipping – Not as robust as DSSS Carrier fr Sequence C Receiver s – Simpler to detect (no knowledge of a chipping sequence as in DSSS required!)

5 FHSS (Frequency Hopping Spread Spectrum) (2) Cell Structure

Implements space division multiplex: t b – Base station covers a certain transmission area (cell) User Data Mobile stations communicate only via the base station 01 011 t Advantages of cell structures: f td – Higher capacity, higher number of users f3 Slow – Less transmission power needed f2 Hopping – More robust, decentralized f1 (3 bits/hop), – Base station deals with interference, transmission area etc. locally e.g., GSM f td t Problems: – Fixed network needed for the base stations f3 Fast – Hand-over (changing from one cell to another) necessary f2 Hopping – Interference with other cells f1 (3 hops/bit), e.g., Bluetooth, Cell sizes from some 100 m in cities to, e.g., 35 km on the t 1600 f changes/s country side (GSM): t : Bit Period t : Dwell Time into 79 carrier f b d – Even less for higher frequencies

Frequency Planning (1) Frequency Planning (2)

Frequency reuse only with a certain distance between the base f f f stations 3 3 3 f2 f2 f1 f1 f1 f3 f3 f 3 Cell Cluster f2 f3 f7 3 f2 f2 f2 f f f5 f2 5 2 f1 f1 f f f f4 f6 f5 Standard model using 7 frequencies: 4 6 5 f3 f3 f3 f1 f4 f1 f4 f3 f7 f1 f3 f7 f1 f2 f2 f3 f6 f5 f2

Fixed frequency assignment: 7 Cell Cluster – Certain frequencies are assigned to a certain cell – Problem: different traffic load in different cells f f f2 f 2 f 2 f 1 f h 1 f 1 f Dynamic frequency assignment: 3 2 3 h2 3 h1 h1 3 Cell Cluster g2 h3 g2 h3 g2 – Base station chooses frequencies depending on the frequencies already used in g1 g1 g1 g3 g3 g with 3 Sector Antennas neighbor cells 3 – More capacity in cells with more traffic – Assignment can also be based on interference measurements

Cell Breathing

CDM (Code Division Multiplexing) systems: – Cell size depends on current load • Low load: larger areas • High load: smaller areas

Reason: Additional traffic appears as noise to other users: – Increasing noise due to increasing number of participants – If the noise level is too high users, located at cell boundaries, drop out of cells

“Noisy area”