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