Wireless Communications

Spread Spectrum....& more about CDMA

Spread Spectrum: DS-SS, FH-SS, Coding Gain, Fading Margins

Key Digital Modulation Techniques: OFDM

Spread spectrum

Spread-spectrum In 1985, the FCC (Federal Communications Commission) allocated three

frequency bands for spread spectrum (SS)communications, originally developed by the military.

SS transmission technique has much greater immunity to interference and noise compared to conventional radio transmission techniques.

An increasing number of users can use the same frequency (similar to cellular).

Users of FCC certified spread spectrum products do not require a license from the FCC.

Spread Spectrum technology in three radio bands, 902-928 MHz, 2400-2483.5 MHz and 5752.5-5850 MHz for transmission under 1 Watt of power. This power limit prevents interference within the band over long distances.

SS data radios offer the opportunity to have multiple channels which can be dynamically changed through software. Allows for many applications such as repeaters, redundant base station and overlapping antenna cells.

Spread spectrum

3 ways to spread the bw of the signal: Frequency hopping: The signal is rapidly switched between different

frequencies within the hopping bandwidth pseudo-randomly, and the receiver knows before hand where to find the signal at any given time.

Time hopping: The signal is transmitted in short bursts pseudo-randomly, and the receiver knows beforehand when to expect the burst.

Direct sequence: The digital data is directly coded at a much higher frequency. The code is generated pseudo-randomly, the receiver knows how to generate the same code, and correlates the received signal with that code to extract the data.

Freq HopTime Hop

Bandwidth Usage in CDMA 2000 Depending on the factor N and on the type of spreading used, the

bandwidth that cdma2000 requires varies. The 3dB bandwidth for each band (transmit and receive) is defined here for the three types of spreading:

Single Carrier: BW3dB = 1.2288Mhz

Multi-Carrier: BW3dB = (N - 1) x 1.25Mhz + 1.2288Mhz, where N = 3, 6, 9, 12

Direct Spread: BW3dB = N x 1.2288Mhz, where N = 3, 6, 9, 12 cdma2000 allocates Guard regions of 625kHz, on each sides of the

allocated bandwidth, to prevent interference with neighboring bands. So the figure for the total bandwidth must include these two Guard regions.

Transmissions characteristics of cdma2000 (FDD) Modulation chip rate: N x 1.2288 Mcps Modulation:

Uplink: BPSK Data modulation QPSK Spreading modulation

Downlink: QPSK Data modulation QPSK spreading modulation

Detection: coherent for both uplink and downlink. Channel Spacing: (N+1) x 1.25MHz

Walsh codes

Walsh codes are defined as a set of N codes, denoted Wj, for j = 0, 1, ... , N - 1, which have the following properties:

Wj takes on the values +1 and -1.

Wj[0] = 1 for all j.

Wj has exactly j zero crossings, for j = 0, 1, ... , N - 1. Each code Wj is either even or odd with respect to its midpoint.

CDMA uses spread spectrumTechnology.cdmaOne™ identifies 2G and 2.5G cellular,PCS and wireless local loop (WLL) servicesbased on the IS-95A and IS-95B CDMA airinterface standards. IS-95A supports data delivery up to 14.4 kbps while IS-95B offers up to 115 kbps.CDMA2000 is an ITU-approved, IMT-2000 (3G) standardCDMA2000 1X can double voice capacity and delivers data rates up to 307 kbpsCDMA2000 1xEV is optimized for high-speed data:CDMA2000 1xEV-DO uses a designated channel for data at speeds of up to 2.4 Mbps in a single carrierCDMA2000 1xEV-DV integrates voice and data on a single channel with speeds of up to 4.8 Mbps

CDMA: the present and future

CDMA – Multiple Users

OFDM Frequency division multiplexing (FDM) is a technology that transmits

multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (text, voice, video, etc.).

Orthogonal FDM's (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other than their own.

Benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion.

Useful because in a typical terrestrial broadcasting scenario there are multipath-channels (i.e. the transmitted signal arrives at the receiver using various paths of different length).

Since multiple versions of the signal interfere with each other (inter symbol interference (ISI)) it becomes very hard to extract the original information.

OFDM The IEEE 802.11a/g/n standards are based on OFDM. The wideband wireless metro-area network (MAN) technology

WiMAX uses OFDM. The almost completed 4G cellular technology standard Long-Term

Evolution (LTE) uses OFDM. The high-speed short-range technology known as Ultra-Wideband

(UWB) uses an OFDM standard set by the WiMedia Alliance. OFDM is also used in wired communications like power-line

networking technology. One of the first successful and most widespread uses of OFDM was

in data modems connected to telephone lines. ADSL and VDSL used for Internet access use a form of OFDM

known as discrete multi-tone (DMT). And, there are other less well known examples in the military and satellite worlds.

Overall spectrum of the simple OFDM signal shown with four subcarriers within. Note that the zero crossings all

correspond to peaks of adjacent subcarriers.

conclusion IEEE 802.11 - FHSS/DSSS IEEE 802.11 a - OFDM IEEE 802. b (Wi-Fi) – DSSS IEEE 802.11 g - OFDM IEEE 802.11 n - MIMO (UWB) IEEE 802.16 (WiMax) - OFDM IEEE 802.15.1 (bluetooth) – GFSK ZigBee 802.15.4 systems (2.4GHz ) – DSSS-QPSK; 915MHz

DSSS-BPSK, in America and 868MHz DSSS-BPSK in Europe.

Effects of Fading

Irreducible Bit Error Rate due to multipath

Simulation of Fading and Multipath

Irreducible BER due to fading

Irreducible BER due to fading

BER due to fading & multipath