9/10/15 COMP 635: WIRELESS NETWORKS NOISE, INTERFERENCE, & DATA RATES Jasleen Kaur Fall 2015 1 Power Terminology q dB – Power expressed relative to reference level (P0) = 10 log10 (Psignal / P0) q Ø J: Can conveniently represent very large or small numbers Ø J: multiplication of ratios can be done by simply adding or subtracting Ø L: complicates addition and subtraction dBm = 10 log10 (Psignal / 1 mW) – Power in dB relative to 1 mW Ø 1 mW = 0 dBm Ø 30 dBm = 1 W Ø 40 dBm = 100 W Ø 80 dBm = 100 kW q SNR – Signal-to-Noise Ratio (dB) = Psignal / Pnoise q PAPR – Peak-to-Average Power Ratio (typically, dB) 2 © Jasleen Kaur, 2015 1 9/10/15 Typical Transmission Power Levels q 80 dBm (100 kW) – FM radio station with 50 km range q 60 dBm (1 kW) – Microwave oven (leak ~ -60 dBm, 1 nW) q 50 dBm (100 W) – total thermal radiation emitted by a human body q 40 dBm (10 W) – Power Line Carrier transmit power q 21 - 33 dBm (125 mW - 2 W) – Mobile Phones q 15 - 30 dBm (32 mW - 1 W) – Wireless LANs q 0 - 20 dBm (1 - 100 mW) – Bluetooth radio 3 Typical Receive Signal Power q q © Jasleen Kaur, 2015 Receive Signal Power: Ø 7 dBm (5 mW) – AM receiver Ø -100,-10 dBm (0.1 pW, 100 µW) – min,max of 802.11 variants Ø -60 dBm (1 nW) – magnitude +3.5 star, rcvd per m2 of Earth) Ø -127.5 dBm (0.178 fW) – from GPS satellite “Thermal Noise” Power (frequency-dependent): Ø -92 to -101 dBm – WLAN channels Ø -111 dBm – commercial GPS channel Ø -114 dBm – Bluetooth channel Ø -121 dBm – GSM channel Ø -132.24 dBm – one LTE subcarrier (15 KHz) 4 2 9/10/15 Shannon Limits: Noise & Data Rates q Channel capacity: Ø Ø Max rate of information transfer over a given channel When channel is only impaired by Gaussian white noise, C = BW * log2 (1 + S/N) § BW – bandwidth available for communication § S – received signal power § N – power of white noise q Factors fundamentally limiting data rate: Ø Ø S/N – signal-to-noise-ratio BW – available bandwidth 5 Shannon Limits: Noise, BW, Data Rates q Channel capacity: C = BW * log2 (1 + S/N) Ø q Factors fundamentally limiting data rate: S/N & BW Assume communication with information rate, R Ø Received signal power can be expressed as: S = Eb * R § Eb – received energy per information bit Ø Noise can be expressed as: N = N0 * BW § N0 – constant noise power spectral density (W/Hz) Ø R ≤ C = BW * log2[1 + (Eb*R)/(N0*BW)] § ρ ≤ log2[1 + ρ * Eb/N0] – ρ: radio-link bandwidth utilization (R/BW) Ø Lower bound on required received energy per info bit: Eb/N0 ≥ 2ρ – 1 / ρ 6 © Jasleen Kaur, 2015 3 9/10/15 Min Required Energy & BW Utilization Bandwidth u=liza=on (ρ = R/BW) q Eb/N0 ≥ 2ρ – 1 / ρ ; q When ρ << 1, Eb/N0 is relatively constant, regardless of ρ Ø q S = Eb * R; Given N0, min signal power increases linearly with data rate When ρ > 1, Eb/N0 increases rapidly with ρ Ø Increase in R (and not BW), implies much larger increase in S 7 Implications for Noise-limited Scenarios q When noise is the main source of radio-link impairment, Ø Data rate always limited by available S/N § Any increase in R will require at least same increase in S Ø q Power-limited Operation: when R << BW, Ø Ø q If sufficient S available, any R can be provided, given a BW Increase in R requires same increase in S Increase in BW doesn’t impact the S required for a given R Bandwidth-limited Operation: R > BW, Ø Any increase in R, requires a much larger increase in S § Unless BW is increased in proportion to increase in R Ø Increase in BW reduces the S required for a given R For efficient use of available S/N, BW should be at least same order as R to be provided 8 © Jasleen Kaur, 2015 4 9/10/15 How to Increase Received S/N? q By reducing distance between transmitter and receiver Ø Reduces attenuation as signal propagates Ø Achievable data rates can be increased by reducing range § Reduced cell sizes in cellular networks § Especially when R is same order (or larger) than BW Ø Alternatively, high R available only in center of cell § Not over entire cell area q By using additional antennas Ø Multiple receiver antennas (spatial diversity) § Reduces S/N Ø Multiple transmit antennas (beam-forming) § Boosts S 9 Increasing S/N: Multiple Antennas q By using additional receive antennas (spatial diversity) Ø Antenna diversity helps mitigate multipath loss situations Ø Offer several observations of the same signal § Can be combined to better estimate transmitted signal Ø q S/N can be increased proportional to # of receive antennas By using multiple transmit antennas (beamforming) Ø Focus a given total transmit power in receiver direction Ø Signals reach antenna elements at different times § Phases adjusted to achieve “constructive” superposition – In direction of receiver (only) 10 © Jasleen Kaur, 2015 5 9/10/15 Multiple-Input Multiple-Output (MIMO) q q Multiple transmit (or receive) antennas efficient only when R is power-limited (rather than bandwidth-limited) Spatial Multiplexing: Ø Use multiple antennas at both transmitter and receiver § Multiple-Input (M transmitters) Multiple-Output (N receivers) § M x N MIMO Ø At transmitter, data split into M sub-sequences § Transmitted simultaneously using the same frequency band – Data rate increased by factor M Ø At receiver, sub-sequences separates by interferencecancellation algorithms Ø Capacity of system grows linearly with min{M,N} § Typically, N ≥ M (for good error performance) 11 Interference-limited Scenarios q q q q Interference causes more radio-link impairment in cellular networks Ø Especially in small cells with high traffic load Ø Inter-cell as well as intra-cell Impact similar to that of noise Ø Max achievable R limited by available S/N Ø Inefficient use of power when R > BW Solutions also similar Ø Reducing cell size – reduces intra-cell interference Ø Multiple receive antennas – signal combining increases S/N Ø Beamforming – reduces interference from others Additionally, structure in interference allows suppression Ø e.g., interfering signal from a certain direction can be suppressed using spatial processing with multiple receiver antennas Ø Differences in spectrum properties also used to suppress interferer 12 © Jasleen Kaur, 2015 6 9/10/15 If BW Scarce; But High S/N Available… q Providing R > BW is inefficient – needs very high S/N Ø Ø But BW is often scarce and expensive! And high S/N can be made available sometimes! § Small cells with low traffic load § Phones close to the cell tower Ø q How to take advantage of such scenarios? Higher-order modulation Ø More bits of info per modulation symbol 13 Higher-Order Modulation q More bits of info per modulation symbol Ø Ø Ø q BW independent of size of modulation alphabet (4, 16, 64) Ø q Depends mainly on modulation rate (# of symbols / second) Max BW utilization (bits/s/Hz): Ø q QPSK – 2 bits of info in each modulation-symbol interval 16QAM – 4 bits per symbol interval 64QAM – 6 bits per symbol interval 16QAM (or 64 QAM) – twice (or thrice) that of QPSK L: Reduced robustness to noise and interference Ø Need higher Eb/N0 (energy per bit) for a given bit-error probability 14 © Jasleen Kaur, 2015 7 9/10/15 QAM: Over-Dimensioned Amplifier q High-order modulation – signal has larger variations in instantaneous transmit power Ø q Transmitter power amplifier must be over-dimensioned Ø q Also has larger peaks Avoids non-linearity at high power levels (signal corruption) Power amplifier efficiency will be reduced Ø Increased power consumption (and increased cost)! Alternatively, average transmit power needs to be reduced Ø L - phones need low power consumption and cost Ø § Reduced range! q Higher-order modulation more suitable for downlink (base station to phone) 15 Issues When Using Larger BW q BW is scarce and expensive q Wider BW needs more complex radio equipment Ø q Higher sampling rates, more power consumption, complex analogto-digital converters Increased corruption of transmitted signal due to time dispersion on the channel Ø Multiple propagation paths lead to delay spread § Yields a non-constant frequency response (frequency selectivity) § Corrupts frequency-domain structure of transmitted signal Ø Leads to higher error rates for a given S/N Ø Frequency selectivity has larger impact for wider-band transmission § Also impacted by environment – Less in small cells, and in obstruction free rural areas 16 © Jasleen Kaur, 2015 8 9/10/15 If Larger BW Available… q Receiver-side Equalization: Ø Counteract signal corruption Ø Works well for BW up to 5 MHz § For higher BW, complexity increases significantly q Multi-carrier Transmission: Ø Send signal as several narrowband signals Ø Frequency-multiplex the subcarriers Ø J: Less impact of frequency selectivity due to smaller BW Ø J: Easy evolution of radio equipment and spectrum Ø L: Subcarriers can not be tightly “packed” (interference) Ø L: Parallel transmission of subcarriers leads to large power variation § For legacy terminals, only individual subcarrier can be used § Lower bandwidth efficiency § Lower power amplifier efficiency; reduced range § More suitable for downlink 17 © Jasleen Kaur, 2015 9