TAP Channel Measurement Fundamentals, Goals, and Plans

Outline Fundamentals Goals Plans

Outline Fundamentals –Channel Delay Spread Coherence Bandwidth Coherence Time Ricean K-factor Pathloss Coherence Distance –System Goals Plans

Fundamentals: Channel Multipath is common in the wireless channel (fading). Narrowband systems are unable to resolve separate multipath components. Wideband systems “see” multipath components arriving at different delays (delay spread).

Fundamentals: Channel Multipath = Frequency Selective In the frequency domain, some bands are better than others (frequency selectivity). The coherence bandwidth is inversely related to the delay spread of the channel. FFT/IFFT timefrequency

Fundamentals: Channel Mobility in the channel has two effects –Coherence time How quickly does the channel change? –Doppler shift How much is the carrier frequency shifted?

Fundamentals: Channel Each resolved multipath component has a certain ratio of direct to reflected signal power (Ricean K- Factor). Reflected and direct signals are attenuated to varying degrees depending on materials in the environment (pathloss).

Fundamentals: Channel Different antenna locations will see different channel impulse responses. The “coherence distance” is related to the number of scatterers in the environment. Small “coherence distance” indicates rich scattering.

Outline Fundamentals –Channel –System MIMO Directional antennas OFDM Cyclic prefix PAPR Goals Plans

Fundamentals: System Our system will have more than one antenna at both Tx and Rx (MIMO). Relationships between channels seen by different antennas determine the techniques we will use. CHANNELTXRX

Fundamentals: System Directional antennas can increase SNR. They can also reduce interference by directing nulls toward interferers. Highly directional antennas will see fewer multipath components.

Fundamentals: System Our system will operate in the frequency domain using OFDM. Each subcarrier can be seen as a separate narrowband signal that sees flat fading (single path) provided they are narrower than the coherence bandwidth.

Fundamentals: System Use of a cyclic prefix is standard in OFDM symbols. Multiplication in f-domain = cyclic convolution in t-domain. Convolution of a symbol with a cyclic prefix is equivalent to cyclic convolution of the original symbol. This gives us a scalar multiplicative channel for each subcarrier.

Fundamentals: System Amplifiers are nonlinear and don’t tolerate time- domain “spikes” well. If we send the same thing across all OFDM subcarriers, we get a spike in time-domain. We quantify these spikes with the peak to average power ratio (PAPR).

Outline Fundamentals Goals –Big picture –Parameters –Variables –Timeline Plans

Goals: Big Picture Channel measurements are a means to an end, not a final goal. System-specific parameters How to design beamforming algorithms How much multiplexing is possible Realistic performance bounds

Goals: Parameters to Measure Antenna correlation Pathloss exponents Coherence bandwidth Coherence time Delay spread Ricean K-factor

Goals: Experimental Variables Antenna separation Antenna polarity Antenna directionality Tx-Rx separation Measurement location

Goals: Timeline Begin assembling system May 04 Fully operational measurement system by August 04 Measurements completed by November 04 (earlier if at all possible)

Outline Fundamentals Goals Plans –Transmitted Signals –Signal Processing –Measurement Sequence –Hardware –Outstanding Issues –Actions

Plans: Transmitted Signals Design pilot signals according to the following criteria –Minimize PAPR (adjust phases accordingly) –Make the two transmit antennas orthogonal (alternate subcarriers) –Make cyclic prefix longer than delay spread –Make OFDM symbols much shorter than coherence time –Keep subcarriers much narrower than coherence bandwidth

Plans: Signal Processing Buffer several seconds of received samples from each radio Download to a PC for offline processing Find statistics using Matlab

Plans: Measurement Sequence Use narrowband measurements to find coherence time. Use extremely narrow subcarriers to find coherence bandwidth. Use extra long cyclic prefixes to find approximate delay spread. Find reasonable values for number of subcarriers and cyclic prefix length. Vary antenna separation, polarity, type, etc.

Plans: Outstanding Issues Carrier offset recovery (clock synchronization) Location, azimuth and distance measurements (GPS?) Antenna mounting hardware Antenna choices PAPR problems Other non-ideal hardware effects Limited buffer size for data storage Measurement locations

Plans: Actions Start writing System Generator code to control our measurements. Derive optimal pilot symbols. Write Matlab code to analyze data. Test with channel emulators. Buy higher capacity FPGA boards. Buy/make adjustable antenna mounting hardware. Find and buy several types of antennas. Begin field measurements.