The mobile phone industry is one which has been characterized by a breathtaking speed of change and development, and anyone who has owned a number of handsets will be aware of the dramatic change evident between a phone of just a few years ago and the latest available models. In order to identify a set of core design issues which hold across generations of handset design, we need to set our sights higher than an analysis of the design of the latest high-end smartphone. We believe a very good place to start is with a review of the relatively short, yet thrilling, history of mobile handsets, providing an opportunity to understand the technological and market issues which have driven this phenomenal development.

Development of the first mobile handset

A famous telephone call

On April 3, 1973, Marty, a researcher at the US company Motorola, made a phone call from a Manhattan sidewalk to his colleague Joel Engel at the US telephone carrier AT&T.

The purpose of Marty’s call that particular Spring day was to inform Joel, that he, Marty, was calling him from the world’s first ever portable cellular telephone, beating AT&T in the technology race to develop a viable commercial portable cellular telephone. This first portable cellular phone was unlike anything we know today – consisting of about a kilogram of plastic and electronics, shaped something like a shoe, using analog radio technology, without any form of screen or menu buttons, and yet able to make and receive telephone calls “without wires” and when on the move. Marty, or, to give him his full name, Martin Cooper, is now revered by many as the father of the mobile phone.

The narrowband transmit beampattern design problem has been discussed in Chapters 13 and 14; see also [Forsythe & Bliss 2005][Stoica et al. 2007][Fuhrmann & San Antonio 2008][Stoica, Li & Zhu 2008][Guo & Li 2008]. Most of the proposed methods first relate the desired beampattern to the covariance matrix of the transmit signals (see, e.g., Chapter 13), and then aim to design the signals that approximate the covariance matrix determined in the first stage (see, e.g., Chapter 14). In the wideband case, similar approaches have been proposed to design the power spectral density matrix [San Antonio & Fuhrmann 2005], but no signals have been synthesized due to the difficulty of imposing the unit-modulus or PAR constraints.

In this chapter we propose an algorithm named WB-CA (wideband beampattern CA) to design unimodular or low-PAR sequences for transmit beampattern synthesis in wideband active sensing systems. We do not formulate the problem in terms of the transmit spectral density matrix (as was done in [San Antonio & Fuhrmann 2005]), but instead directly link the beampattern to the signals through their Fourier transform. The design criterion is formulated in Section 15.1, which is followed by the algorithm description in Section 15.2. Simulation examples are shown in Section 15.3 and concluding remarks are given in Section 15.4.

Problem formulation

We focus on far-field beampattern synthesis for uniform linear arrays (ULA) as illustrated in Figure 15.1.

In Chapter 18 we discussed covert UWA communications with a coherent RAKE receiver, in which case the CAN or WeCAN sequences were shown to be the probing waveforms of choice. As pointed out before, low chip SNR communication over a timevarying UWA channel precludes accurate channel estimation. This could render coherent detection schemes ineffective [Smadi & Prabhu 2004][Yang & Yang 2008] and thus make noncoherent schemes more favorable. In this chapter we examine covert UWA communications with noncoherent schemes and the waveform design issues thereof. More specifically, two types of noncoherent transceiver designs are addressed: orthogonal modulation and differential phase-shift keying (DPSK), both coupled with a DSSS technique and a noncoherent RAKE reception [Brennan 1959]. Although only binary information sequences are considered here, the derivations can be easily extended to a general M-ary case.

For orthogonal modulation, spreading waveform sets with low correlation properties (both auto- and cross-correlation) such as the Gold sequence set [Gold 1968] are desired for combating the multipath nature of UWA channels. For DPSK modulation, a common design is to adopt the cyclic prefix so that the receiver can eliminate the intersymbol interference by ignoring the prefix chips before proceeding with symbol detection [Pursley 1977][Tse & Viswanath 2005]. Such a design requires waveforms with low or even zero periodic correlations over certain lags, an example of which is the Frank sequence (see (1.23)). However, the aforementioned sequences (the Gold and Frank sequences) belong to the class of unimodular polyphase sequences that are constructed from fixed formulas with restricted lengths and/or fixed phase constellations.

The goal of an active sensing system, such as radar or sonar, is to determine useful properties of the targets or of the propagation medium by transmitting certain waveforms toward an area of interest and analyzing the received signals. For example, a land-based surveillance radar sends electromagnetic waves in the direction of the sky, where objects such as airplanes can reflect a (usually very tiny) fraction of the transmitted signal back to the radar. By measuring the round-trip time delay, the distance (called the range) between the radar and the target can be estimated since the speed of propagation for radio waves is known (3 × 108 m/s). Additional target properties can be obtained by performing further processing at the receiver side; e.g., the speed of a target can be estimated by measuring the Doppler frequency shift of the received signal.

In 1904, a German engineer named Christian Hülsmeyer carried out the first radar experiment using his “telemobiloscope” to detect ships in dense fog by means of radio waves. As to sonar, Reginald Fessenden, a Canadian engineer, demonstrated this in 1914 using a sound echo device, though not successfully, for iceberg detection off the east coast of Canada. It was amongst several other experiments and patents said to be prompted by the 1912 Titanic disaster.

Radar and sonar underwent considerable development during the two world wars and later on spread into diverse fields including weather monitoring, flight control and underwater sensing.