Why Not Go Directly to Digital in Cellular

Why Not Go Directly to Digital in Cellular Radios, and Connect the A/D to the Antenna? Paul C. Davis (Retired from Bell Labs) pcdavis@ieee. org OR P 308 davis@aol. com SSCS Technical Meeting April 26, 2010

Cellular Radio is Like Finding a Pin in a Football Field PCD 4/2010

Outline 1. Intro. A/D to Antenna and RF “Secrets” 2. Heterodyne Receivers 3. Moving the A / D 4. Heterodyne Transmitters 5. Tracking Transmitter 6. 4/2010 PCD

What's so tough about designing a portable digital cellular radio? Receiver requirements 1. Sensitivity, which translates to IC Noise Figure 2. Blocker-immunity, which translates to Amplifier Linearity 3. Stand-by time, or current draw while waiting for a call Transmitter requirements 1. Noise in the receive band while transmitting 2. Spurious Signals at any frequency 3. Talk time, or battery life while actually talking PCD 4/2010

Legal vs. Market Requirements Legal requirements: Make a radio that works, and sounds good anywhere, but does not interfere with another radio. All cellular radios are "type approved" for sensitivity, interference resistance, and spurious signals. Market Requirements: Make it cheap. Make it small. Make it have a long battery life. Make it first. PCD 4/2010

Types of Radio Systems Cellular Radio Frequency: 900, 1800 MHz and higher bands Access: FDMA, TDMA (GSM), and CDMA or combination Cordless Phones Frequency: 45, 900, 1900, and 2400 MHz bands Access: FM and Spread Spectrum (frequency hopping) LAN (Bluetooth; IEEE 802. 11 x, could be a, b, g, etc. ) Frequency: 2400 MHz and 5. 2 - 5. 8 GHz Access: Spread Spectrum (frequency hopping SS, direct sequence SS and OFDM) PCD 4/2010

Types of Radio Systems (cont. ) Cellular Radio Sensitivity: < - 102 d. Bm Distortion: IP 3 > -10 d. Bm Handset RF Peak Power Out: 0. 6 to 2 W Cordless Telephone Sensitivity: < - 85 d. Bm Distortion: IP 3 > -25 d. Bm Handset RF Peak Power Out: 10 m. W to 250 m. W LAN (Bluetooth; IEEE 802. 11) Sensitivity: < -70 d. Bm; < -80 d. Bm Distortion: IP 3 > -16 d. Bm "Handset" RF Peak Power Out: 1 m. W, 100 m. W; 1 W PCD 4/2010

General Block Diagram of a Cellular Radio PCD 4/2010

Typical GSM (Radio) Receiver with Single IF • Sample GSM Receiver Requirements (Portable Radio) • Sensitivity: • -102 d. Bm at input of receiver chain must yield a 0. 01 BER • (Requires a signal-to-noise ratio (S/N) of ~9 d. B) • Interference Rejection • 0 d. Bm out-of-band single-tone blocking signals • -23 d. Bm in-band single-tone blocking signals • -43 d. Bm in-band two-tone blocking signals PCD 4/2010

Example GSM Receiver System Requirements for Type Approval Req. rej. of two-tone sig. >66 d. B. NF at LNA input < +8 d. B. PCD 4/2010

Two-tone Harmonic Generation C(E 1 cos at+E 2 cos bt)3 = fundamentals + 3 rd harmonics (Third-order IM products) + (3 CE 12 E 2)/4 [cos(2 at+ bt)+cos(2 at- bt)] + (3 CE 1 E 22)/4 [cos(2 bt+ at)+cos(2 bt- at)] From R. S. Carson, © 1990 Wiley PCD 4/2010

Two Tone IP 3 for Narrow Band Circuits Such as RF IC’s PCD 4/2010

PCD 4/2010

Phase Cancellation Scheme to Remove Sideband Noise Eliminates a filter at the cost of extra current and complexity. PCD 4/2010

“Direct Down” Receiver (No IF) Alcatel has sold millions since 1994. Watch for DC offset problems with high DC gain (80 d. B) and carrier leakage back through the antenna PCD 4/2010

A / D to Antenna For 900 MHz GSM System: Sample Rate : Jitter of S / H: A / D Linearity: Noise Figure at Antenna: Competitive A/D Power: > 1800 MHz < 3 ps ~19 bits < 10 d. B (< 4 n. Vpp / Hz) ~40 m. W PCD 4/2010

A / D After LNA and RF Filter For 900 MHz GSM System: Sample Rate : Jitter of S / H: A / D Linearity: Noise Figure at Antenna: Noise Figure at S/H Competitive A/D Power: > 200 MHz < 3 ps ~15 bits < 10 d. B (< 4 n. Vpp / Hz) < 27 d. B ( < 28 n. Vpp/ Hz) ~25 m. W PCD 4/2010

A / D After RF-IF Mixer For 900 MHz GSM System: Sample Rate : Jitter of S / H: A / D Linearity: Noise Figure at Antenna: Noise Figure at S/H Competitive A/D Power: > 142 MHz < 20 ps ~15 bits < 10 d. B (< 4 n. Vpp / Hz) < 20 d. B ( < 12. 5 n. Vpp/ Hz) ~10 m. W PCD 4/2010

Latest answer, wrong question? UCLA students have replaced the IF filter/mixer with a 10 m. W A/D, publ. in: “The Path to the Software-Defined Radio Receiver” Asad A. Abidi, Fellow, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 42, NO. 5, MAY 2007 © IEEE, 2007 PCD 4/2010

Indirect-up Transmitter Advantages: Phase shift is easier and uses less current at IF Disadvantages: n X IF spurs in the RF band. High powered output amplifier. PCD 4/2010

Direct-up Transmitter Single sideband modulator uses two mixers and phase shifter at 900 MHz. (Higher currents needed. ) Watch out for interference from power amplifier to VCO. PCD 4/2010

Direct-up Transmitter (with Offset Oscillator) High Frequency VCO different from modulated output signal. Filter for alternate side-band in LO path, not in signal path. PCD 4/2010

Block Diagram of Tracking Up-conversion Loop

Summary and Conclusions 1. Heterodyne receivers, used for decades, are still the most popular for cellular, cordless, and LAN. 2. Direct-down conversion has become a commercial reality and reduced the need for A/D at the IF stage. 3. Moving the A/D to the antenna would reduce the number of filters, and allow Software Defined Radio. However, the performance requirements are impractical in today's technology, to compete successfully. 4. The tracking transmitter technique reduces the receive band noise in the transmitted signal. However, it can only be used with constant envelope modulation. PCD 4/2010

Terms of Use The slides and notes in this Power Point presentation were created by Paul C. Davis, for a SSCS Section Presentation on April 26, 2010, and intended solely for the information and personal use of the audience and web-site users. PCD 4/2010