the foundations of the digital wireless world (selected works of a j viterbi) || the foundations of...
TRANSCRIPT
THE FOUNDATIONS OF THE DIGITAL WIRELESS WORLD
ANDREW J. VITERBI
University of Southern California and Viterbi Group, LLC
INDIAN INSTITUTE OF SCIENCE CENTENARY LECTURE
Bangalore, India
May 27,2008
Distinguished Faculty, Administration and Students of IISc Bangalore and Guests,
It is a distinct honor to have been invited to deliver the lecture on the hundredth anniversary of the founding of this renowned academic institution. I find it most fitting that as part of the celebration of this milestone, the Division of Electrical Sciences of IISc is hosting, this week, a conference entitled "Managing Complexity in a Distributed World". For India is an example to the world that a highly complex society integrating multiple cultures can be managed as a vibrant democracy. But beyond admiration, I have no credentials to comment further on this singular societal and political achievement.
Rather I will address one pillar of the Information Technology revolution which has profoundly affected the whole world, with India at its forefront. I refer to the wireless engineering contributions in which I have had the privilege of participating for half a century, in both academia and industry. Wireless technology is itself a complex creation which has evolved over little more than a century into the vehicle which can interconnect almost instantly the majority of the earth's population. Its origins lie in the late nineteenth century with the theoretical foundations of electromagnetic radiation by Maxwell and its experimental verification by Hertz. The early twentieth century witnessed its first uses as Marconis wireless telegraphy for ships at sea followed by broadcast radio, the first of an ever burgeoning source of information and entertainment. Military uses of radio, begun in the First World War and widely expanded in the Second, spawned another valuable technology, radar, to measure distances and track the motion of distant objects from aircraft to space vehicles and even planets. In the second half of the twentieth century television took over the role of mass media titan.
Information Theory, Satellite Communication and Moore's Law
All this is well known, but less well understood are the complex underpinnings of currently dominant digital technologies for mobile telephony and emergent ones for mobile data and
The
Fou
ndat
ions
of
the
Dig
ital W
irel
ess
Wor
ld D
ownl
oade
d fr
om w
ww
.wor
ldsc
ient
ific
.com
by U
NIV
ER
SIT
Y O
F B
IRM
ING
HA
M L
IBR
AR
Y -
IN
FOR
MA
TIO
N S
ER
VIC
ES
on 0
8/22
/14.
For
per
sona
l use
onl
y.
2
entertainment distribution and for accurate positioning and navigation. These developments resulted from the confluence of three technologies: information theory, satellite communication and digital circuit integration, roughly in that chronological order. Information theory, with roots in mathematical statistics, was founded by Claude Shannon in 1948 by means of a remarkable pair of papers in the Bell System Technical Journal, the highly respected publication of the Bell Telephone Laboratories where he performed his research. The application of mathematical statistics to communication and radar long preceded Shannon's papers, based on the work of C.R. Rao, Harald Cramer, Norbert Wiener and Stephen Rice, among the more notable. Yet Shannon's exposition of what are now known as the Source Coding Theorem and the Channel Coding Theorem were strikingly novel and largely unpredicted by previous work. More importantly, they established the potential as well as the limitations of all forms of telecommunications, wireless ultimately being the major beneficiary of the theory. But before wireless could be developed as a ubiquitous means of personal communication, two other physically based technologies needed to reach maturity.
The first was satellite and space communications. The Soviet launch of Sputnik in October, 1957 followed three months later by the U.S. Explorer I, ushered in the space age and the need for communicating over astronomical distances. Initially these requirements were for one-way telemetry of the spacecraft sensor data at very low bit rates, but within a few decades geostationary communication satellites transmitted at multi-megabit rates over distances on the order of 40,000 Kilometers. The attenuation at such distances results in an exceedingly low received signal power, so low that it is severely corrupted by background noise which is proportional to the receiver's temperature. There are four ways of improving the situation to achieve adequate performance and each has its limitations:
a) Increase the transmitted signal power, but this requires a proportional increase in the satellite transmitter's weight and consequently the very costly added launch thrust;
b) Increase the receiving antenna diameter, but this becomes extremely costly beyond 20 meters;
c) Reduce the front end receiver noise by cryogenic techniques; d) Reduce the signal-to-noise requirements by employing two information theory based
techniques known as source coding and channel coding.
The first two approaches increase the received signal power, the third reduces the receiver's noise power and the fourth affords the possibility of operating at a lower level signal-to-noise ratio without sacrificing performance. Once the evident limitations of the first three are reached, one can only resort to the fourth. It turns out that coding techniques can reduce the required signal-to-noise ratio for practically error-free digital transmission by an order of magnitude (10 dB) with far less cost and complexity than any of the other three methods. We should note that this became true for the first time in the context of space and satellite communication, for in terrestrial communication raising transmitter power usually does not present great difficulty. Another feature of space communication makes it amenable to coding methods; it closely resembles the so-called Gaussian channel which is perhaps the purest model for application of information theory. In fact, a number
The
Fou
ndat
ions
of
the
Dig
ital W
irel
ess
Wor
ld D
ownl
oade
d fr
om w
ww
.wor
ldsc
ient
ific
.com
by U
NIV
ER
SIT
Y O
F B
IRM
ING
HA
M L
IBR
AR
Y -
IN
FOR
MA
TIO
N S
ER
VIC
ES
on 0
8/22
/14.
For
per
sona
l use
onl
y.
of channel coding techniques, both block and convolutional, were developed and applied to space and satellite applications starting in the early sixties, with modest performance gains. But it was not until nearly two decades later that digital communication with all the benefits of channel coding came into common use in satellites. For this to happen, another more pervasive development had to reach maturity.
I refer, of course, to solid state circuit integration, the underlying technology which is the principal enabler of all digital electronic devices from the camera to television to cellular phones to the personal computer to the Internet. These are prime examples from a list too long to detail, all of which have changed the human work, entertainment and life experiences so extensively and rapidly over the past decade. Solid state electronics had its birth in the Bell Laboratories discovery of the transistor by Bardeen, Brattain and Shockley in 1947. However, successful circuit integration was only achieved almost two decades later by, among others, some of Shockley's early collaborators. One of these, Gordon Moore, predicted in 1965 that the number of electronic devices contained on a single silicon chip would double every eighteen months. Interestingly, a considerable fraction of the early U.S. government funded R&D for circuit integration was motivated by a need for miniaturized low power electronics on space vehicles and satellites. But the solid state evolution accomplished much more than that. Not only was size and power reduced, but circuit speed also grew exponentially and all this led to a proportional decrease in cost, all of which continue unabated. Such dramatic cost reductions are responsible for the Personal Computer, the almost universal access to the Internet and to mobile communication, for voice, data and video.
The Two Limits of Shannon
While the impact of this solid state technology evolution has spread over a wide collection of human pursuits, we continue to focus here only on wireless digital communication. Specifically we will consider the digitization and compression of voice, video and data, their transmission over a perturbed medium particularly in a mobile environment and related navigation technology. All these technologies owe their existence to the three stages of development which we have just outlined: information theory, satellite communication and solid state circuit integration, with origins respectively sixty, fifty and forty years ago. Claude Shannon, the founder of information theory, set forth two theorems, known as the source and channel coding theorems, which bracket the field. Stated colloquially, they establish the following two limits:
1) The minimum number of bits/second to accurately represent a source of data, voice or video;
2) The maximum number of bits/second which can be transmitted nearly error-free over a perturbed channel.
Let us address these sequentially. For written text, the digitization (to zeros and ones) is obvious and the compression to minimum number of bits is lossless, meaning that the
3
The
Fou
ndat
ions
of
the
Dig
ital W
irel
ess
Wor
ld D
ownl
oade
d fr
om w
ww
.wor
ldsc
ient
ific
.com
by U
NIV
ER
SIT
Y O
F B
IRM
ING
HA
M L
IBR
AR
Y -
IN
FOR
MA
TIO
N S
ER
VIC
ES
on 0
8/22
/14.
For
per
sona
l use
onl
y.
4
text can be reconstructed from its minimal representation with perfect accuracy. (Actually Shannon's theorem assumes known text statistics, but later work known as "universal source coding" bypassed this requirement.) For continuous sources, such as voice and video, the first step is to render them time-discrete by sampling the continuous waveform at a high enough rate. In 1924 Harry Nyquist at Bell Laboratories established that a sampling rate of twice the highest significant frequency of the waveform's spectrum would be sufficient for near-perfect reconstruction of the waveform from the samples. But the samples, though time-discrete, are still analogue. To digitize these, one must slice the samples into a number of levels, known as quantization. Each level is then assigned a binary code. This operation can not be reversed perfectly, meaning that the final version presented to the ear or eye will have some distortion. Of course, the finer the quantization, the less distortion, but the higher the bit rate. An extension of the original Shannon Source Coding Theorem, known as the Rate-Distortion Theorem, establishes the minimum rate required for a given level of distortion. This applies not just to the quantization step but to the subsequent step of compression to a minimum bit rate. The practical value of the theorem, however, is much diminished by the fact that the ear and eye can be fooled by psycho-acoustic and psycho-visual effects which through proper processing may permit much more compression than the theory would predict. In the case of voice, processing known as LPC (for linear predictive coding) begins by modeling the vocal tract as a linear filter. It then transmits the parameters of this filter model along with additional parameters describing the nature of the excitation (voice) driving the vocal chords. (An advanced version known as codebook excited LPC obtains even greater compression.) At the receiving end the voice replica is regenerated by driving a model of the excitation through the model of the vocal tract. Good reproduction of spoken voice, including speaker recognition, is achieved with no more than the equivalent of one bit per sample. Since most speech is reasonably contained within a 4 KHz frequency band, this requires a sampling rate of 8 K samples/sec. and consequently a voice coding rate of 8 Kbits/sec. Video digitization and compression is completely different. To begin with, it's actually 3-dimensional, two dimensions of picture and the third being time. There exist numerous and onerous standards for digital video, particularly the highdefinition (HD) variety. Most HDTV systems, however, sample each picture frame in about one million points (typically 768 rows of 1366 points each) known as picture elements or pixels. Temporally, to capture motion, 60 frames per second are formed for each of the three primary colors, resulting in a total of about 180 million samples per second. Compression is achieved by converting each sub-frame of 16 x 16 picture elements to a two-dimensional spectral domain and quantizing the lower frequency components more finely and the higher ones more crudely. Then taking into account the high degree of similarity among successive frames and among different primary colors, the compression succeeds in reducing the total number of bits required to about one-half of the number of picture elements of just one color, for a resulting bit stream of only 30 Megabits/sec. With advanced modulation methods, this bit stream can be transmitted in place of a conventional terrestrial analogue television channel which occupies just 6 MHz, but with a much superior resolution and immunity to interference.
The
Fou
ndat
ions
of
the
Dig
ital W
irel
ess
Wor
ld D
ownl
oade
d fr
om w
ww
.wor
ldsc
ient
ific
.com
by U
NIV
ER
SIT
Y O
F B
IRM
ING
HA
M L
IBR
AR
Y -
IN
FOR
MA
TIO
N S
ER
VIC
ES
on 0
8/22
/14.
For
per
sona
l use
onl
y.
Actually the immunity to interference of digital transmission depends on a different type of processing which follows the tenets of the second of Shannon's theorems, the Channel Coding Theorem, which unlike the first provides a firm limit and the hint of ways to approach it. The satellite and space channel, as we have said, is accurately modeled as a signal perturbed by additive Gaussian noise. In this case, the limit for accurate (nearly errorfree) transmission, which is known as the Channel Capacity, C, is related to the bandwidth occupied by the transmitted signal, W, and the ratio of signal power, S watts, to noise power, N watts, in the bandwidth of the signal, as follows:
C = WLog2(1 + S/N).
The requirement for near error-free performance is usually stated in terms of the minimum necessary energy per bit, Eb. This is just the ratio of the signal power to the rate R in bits/sec. Thus, S = EbR. Since the received noise in space communication receivers is essentially "white" meaning uniform over all frequencies , the noise density, No = N/ W is the usual interference measure. SIN is thus related to Eb/NO by
SIN = (Eb/NO)(R/W) .
Consequently, the maximum rate can be related to bandwidth and Eb/ NO by
R < C = WLog2[1 + (Eb/NO)(R/W)].
Inverting this expression yields the minimum value of Eb/NO for any given ratio of bit rate to occupied bandwidth for a digital communication system operating in Gaussian noise,
Eb/NO > (W / R)(2R/
W - 1) .
For various values of bandwidth-to-bit rate, we have the minimum values ,
Ln(2)(-1.6dB) as W/ R-too
Eb/No > 1 (0 dB) for W / R = 1
15/4(+5.7dB) for W/R=1/4
The first case applies to space communication where the available bandwidth greatly exceeds the rate; the second to both terrestrial and satellites with only moderate bandwidth constraints; and the third to highly constrained applications like digital terrestrial television. For over fifty years communication engineers have sought the means to approach this minimum requirement and hence either minimize the resources (transmitter power, antenna sizes or receiver noise temperature) or to maximize the transmission rate given constraints on these resources. All these so-called channel coding techniques employ redundancy to produce different signals for different messages so they can be distinguished from one another even after corruption by noise. One would expect that greater redundancy would strongly favor higher bandwidth-to-rate ratios . However, as W /R is increased above unity, one finds diminishing returns partly because the added symbols must contain less energy to keep the energy per bit low.
Two types of channel codes have been utilized, block and convolutional, and occasionally hybrids of the two. The more important issue than the type of code chosen is the process at
5
The
Fou
ndat
ions
of
the
Dig
ital W
irel
ess
Wor
ld D
ownl
oade
d fr
om w
ww
.wor
ldsc
ient
ific
.com
by U
NIV
ER
SIT
Y O
F B
IRM
ING
HA
M L
IBR
AR
Y -
IN
FOR
MA
TIO
N S
ER
VIC
ES
on 0
8/22
/14.
For
per
sona
l use
onl
y.
6
the receiver for deciding which message was sent. This processor is called the decoder and its degree of complexity often determines the choice of code. Early coding techniques employed prior to 1970 used block codes, wherein a block of bits is converted into a larger number of symbols which then drive a modulator which alters the carrier phase and amplitude in a manner dictated by the input symbols. The decoder at the receiver examines the noisecorrupted demodulated signal to determine the most likely block of bits input at the encoder. Given the rudimentary solid state circuitry prior to the seventies, the reduction in Eb/No relative to uncoded operation were no greater than a factor of 2 equivalent to 3 dB. For example, to achieve a probability of error on the order of 10- 6 (averaging one error in a million) requires an Eb/ No ratio of 10.5 dB; with the biorthogonal block codes of the 1960's early space probes reduced this required ratio to about 7.5 dB . Note that this is a small gain relative to the almost 12 dB potential gain predicted by t he Shannon limit for the high W / R values characteristic of space communication.
Convolutional Codes, Viterbi Decoding and Digital Satellite Television
A convolutional code differs in that each successive redundant symbol, or group of symbols, depends on a sequence of the previous bits input to the encoder. Its embodiment is a shift register , with information bits entering it one or a few at a time, and the outputs being formed as functions of the bits in the register . The number of outputs exceed the number of inputs, thus creating redundancy. Hence the encoder is essentially a finite-state machine, a fundamental concept of computer science. Just as for block codes, the decoder at the receiver determines the most likely sequence of information bits input to the decoder in spite of the detrimental effects of noise on the received signal. So-called sequential decoding algorithms for convolutional codes were proposed, analyzed and implemented experimentally from the late 1950's and a few were even implemented for reception of spacecraft transmissions. I published a different algorithm in 1967, which turned out to be optimal. Fundamentally it exploited the fact that a sequence formed by a finite-state machine and corrupted by an independent random noise sequence has the properties of a Markov sequence. This is a random sequence for which the probability distribution of each term depends only on the state of the underlying mechanism which generated the sequence. The first such decoders implemented with the rudimentary early solid state integrated circuits of the 1970's achieved gains of nearly 6 dB, half of the maximum coding gain in dB of the Shannon limit. This permits a reduction of a factor of 4 in transmitted power, and proportional spacecraft power amplifier weight. Alternatively, it permits a quadrupling of data rate or a reduction of receiving antenna diameter by half or a doubling of the range of the spacecraft , or a mixture of all the above which add up to 6 dB. Consequently this algorithm and its implementation technology gradually became favored , first in NASA missions and defense communication satellites , and eventually in commercial communication satellites as well. What made the algorithm truly practical was the progress of solid state integration (Moore 's Law) which reduced the decoder's size from a full drawer of electronics in the 1970's to a single silicon chip by the mid 1980's. By the 1990's it was implemented as a small fraction of a chip.
The
Fou
ndat
ions
of
the
Dig
ital W
irel
ess
Wor
ld D
ownl
oade
d fr
om w
ww
.wor
ldsc
ient
ific
.com
by U
NIV
ER
SIT
Y O
F B
IRM
ING
HA
M L
IBR
AR
Y -
IN
FOR
MA
TIO
N S
ER
VIC
ES
on 0
8/22
/14.
For
per
sona
l use
onl
y.
At the same time the processing speed and consequent bit rate which the decoder could handle was increased from a few Megabits/second to hundreds of Megabits/second. All this made it a valuable asset for Direct Broadcast Satellite television transmission and it is currently included in tens of millions of digital satellite television receivers. For advanced missions to the outer planets, a hybrid encoder involving concatenation of a convolutional code with a long Reed-Solomon block code further improved the performance by about 2 dB. This brought it within less than 3 dB of the Shannon limit. A number of standards, both governmental and commercial, have been issued based on these coding and decoding technologies. Needless to say, for digital television, while accurate transmission of the tens of megabits/second is important, the source coding discussed previously which compresses the information to be sent from many hundreds to a few tens of Megabits/sec, is even more so. These techniques have given rise to numerous standards as well.
The New Wireless Frontier: Personal Communication
As dramatic as the impact of space and satellite communication and direct broadcast television has been, it pales in comparison to the terrestrial wireless digital revolution. This began less than two decades ago and currently provides the means for mobile personal communication among nearly half of the world's population. Numerous hurdles needed to be surpassed for this to come about. To begin with, the terrestrial channel is more complex than the simple Gaussian channel which models the free-space relatively unencumbered electromagnetic propagation from satellites and spacecraft. In addition to the background thermal noise common to all receivers, it suffers more from fading due to multipath and from interference caused by other users which are near the communicator both in physical location and in spectrum occupancy. Multipath refers to the fact that the signal from the transmitter may arrive at the receiver via several paths, being reflected off urban impediments such as buildings or hills or even off rough terrain. If two paths arrive at the receiver with short relative delays, and similar power levels but nearly opposite phases, they may cancel each other out and hence cause deep fades; hence the term multipath fading. Possibly the greater hurdle for terrestrial communication, however, is multi-user interference. In a wireless wide-area network, personal communication among a large population of users requires more than separation in frequency because the number of calls requesting allocations in an urban environment greatly exceeds the spectral capacity for the service. This is accomplished by dividing a given region into numerous cells and providing each with a so-called Base Station consisting of a tower with transmitter and receiver capable of communicating with a large number of callers. Theoretically each base station communicates only with users in its own cell and all cells are interconnected into a network usually by wire, cable or fiber as well as to all fixed users of the public switched network. Thus any caller can access the network through the Base Station in its given cell and from there reach any phone user anywhere. The return voice or data signal arrives at the same Base Station from the destination which relays it to the original caller, all in real time with minimum delay. The several users within the cell can be isolated by frequency bands (frequency-division multiple access with
7
The
Fou
ndat
ions
of
the
Dig
ital W
irel
ess
Wor
ld D
ownl
oade
d fr
om w
ww
.wor
ldsc
ient
ific
.com
by U
NIV
ER
SIT
Y O
F B
IRM
ING
HA
M L
IBR
AR
Y -
IN
FOR
MA
TIO
N S
ER
VIC
ES
on 0
8/22
/14.
For
per
sona
l use
onl
y.
8
acronym FDMA) or by time slots (time-division multiple access with acronym TDMA), but the difficulty arises because the same frequencies must be allocated to other cells in order to obtain the user capacity multiple required. Interference from callers in these other cells can be overwhelming, particularly if both the intended caller is near the cell's edge and the interfering caller in the neighboring cell is also near the common edge. Note also that since each mobile user does not know the Base Station's location, its antenna must be omnidirectional and hence interferes uniformly in all directions. The ordinary solution to this interference problem is to assign different segments of spectrum to neighboring cells. For example, treating cells ideally as adjacent hexagons, assigning different spectral segments to each of the seven cells in a cluster (consisting of a central and six surrounding cells), guarantees a physical separation of at least one cell-diameter between any two users with the same allocation. This, of course, reduces the overall capacity by a factor of seven. With three-way cell sectorization, meaning that each Base Station has three antennas, each of which deals only with users in a 120 degree angle, the frequency bands can be reused every three sectors, thus reducing capacity by only a factor of three, but with more interference.
Spread Spectrum Technology
Besides spectral isolation through fractional frequency reuse, there are a number of mitigating techniques for both multipath fading and other-cell interference available in both FDMA and TDMA multiuser systems. However, none are as effective or will support as many users per cell for a given spectral allocation as a third technique, known generally as Spread Spectrum and more specifically as code-division multiple access with acronym CDMA. Long before its current use in commercial mobile communication, this concept originated in military communication where interference from a hostile jammer can overwhelm the intended receiver. One method to foil the jammer is to spread the communicator's signal over a wide frequency band, much greater than that required to contain the original digital stream. This can be achieved by multiplying the original binary sequence (of equal numbers of positive and negative values of equal magnitude) another such binary sequence which is switched much more rapidly than the original one. This spreads the signal by the ratio of the two sequence rates. The spreading sequence can be generated in a very precise manner so that if it is multiplied by a replica which lines up with it perfectly, the product results in constant positive value, but if it does not line up the product continues to switch equally between positive and negative values. Thus if the desired signal is spread at the transmitter and then multiplied by its replica at the receiver it returns to its original lower rate unspread form. Conversely, the jamming signal of any form and bandwidth will only be affected by the multiplying sequence at the receiver and hence spread out to appear as uniform noise. Finally, filtering the composite received signal to the bandwidth of the original desired signal suppresses the portion of the jamming signal which has been spread outside the desired signal's bandwidth. The communicator's advantage over the jammer is measured by the ratio of jammer's interference power to communicator's signal power at the receiver to prevent accurate transmission. Since the spreading tends to be uniform, given an interference power
The
Fou
ndat
ions
of
the
Dig
ital W
irel
ess
Wor
ld D
ownl
oade
d fr
om w
ww
.wor
ldsc
ient
ific
.com
by U
NIV
ER
SIT
Y O
F B
IRM
ING
HA
M L
IBR
AR
Y -
IN
FOR
MA
TIO
N S
ER
VIC
ES
on 0
8/22
/14.
For
per
sona
l use
onl
y.
of 1 watts, and a spread bandwidth of W s Hz, we may also define the interference density, 10 = l/WS watts/Hz, analogous to the noise density No for the pure background noise case. Then, as before, we may relate signal power to energy per bit and rate by S = EbR. Thus the communicator's advantage over the jammer is just
l/S = 10Ws/EbR = (Ws/R)/(Eb/lo).
We note that by employing codes which reduce the required level of Eb/lo, the advantage l/S is increased proportionally. The spreading ratio W s /R, known as the processing gain, is often on the order of 30 dB or greater for military uses in potentially hostile environments. Hence this technique has been used in military communication satellites for nearly fifty years. A particularly valuable corollary application of this technology is for position location. For if identical sequences are generated at both transmitter and receiver, lining up the receiver's locally generated sequence with the one extracted from the received sequence is facilitated by the fact that it will produce a constant value which upon integration yields a large number, while if the two are not exactly lined up (synchronized) the result will be a sequence of positive and negative values which when integrated will cancel each other out to produce a nearly zero value. From the lined up timing, assuming clock accuracy throughout, the propagation delay between transmitter and receiver can be measured with a precision which is inversely proportional to the spread bandwidth. This technique has also been employed for half a century to track spacecraft and satellites and is currently at the core of commercial satellite-based position location systems such as the Global Positioning Satellite System (GPS), the Russian GLONASS and in future the European Union's Galileo Satellite Network.
Returning to commercial wireless cellular communication, we observe that spread spectrum technology (code-division multiple access) can also be used for mitigating the effects of mutual user interference in this application as well as to remove the destructive effects of multipath. Considering the latter first, we note that the bandwidth expansion is achieved by narrowing the binary symbol duration. Thus for example, spreading to 10 MHz is achieved by multiplying by a binary sequence with a symbol duration of 0.1 microseconds. Then any multipath component which causes a delay greater than 0.1 microseconds from the main component will be greatly diminished by the receiver's correlator. Even better, by using multiple correlators, each lining up with a different multipath component, the results can be combined, thus augmenting the signal rather than allowing the components to combine destructively causing fading as in a narrowband situation. The other, and more important , advantage of spread spectrum techniques is in mitigating the effects of other user interference. Unlike frequency- and time-division multiple access which, as described above apportions all frequency or time slots within a given cell but must avoid those slots in adjacent cells, in CDMA all users in all cells share all the allocated spectrum and all other-user interference is mitigated by means analogous to those used to mitigate jamming. To achieve this it is first essential that each user 's transmitter power be controlled so that the power received by the base station is nearly the same for all users. To evaluate this approach we need to determine the maximum number of users per cell, M, assuming an equally fully loaded collection of cells. Then for any given user's signal received at a base station, there
9
The
Fou
ndat
ions
of
the
Dig
ital W
irel
ess
Wor
ld D
ownl
oade
d fr
om w
ww
.wor
ldsc
ient
ific
.com
by U
NIV
ER
SIT
Y O
F B
IRM
ING
HA
M L
IBR
AR
Y -
IN
FOR
MA
TIO
N S
ER
VIC
ES
on 0
8/22
/14.
For
per
sona
l use
onl
y.
10
will be M-1 equal power interfering users within the same cell (all using different timing of their spreading sequences so as to minimize the effect of mutual interference). Additionally, there will be interference from all the users in other cells . However, the effect of these is mitigated by the distances from t he given base station. Considerable analysis, simulation and experimental measurements have shown that the totality of received interference power from all other fully loaded cells is on the order of aM where a lies somewhere between 1/2 and 1, depending on the electromagnetic signal propagation path and antenna placement. But in any voice conversation, at least half the time on average there will be no transmission, only reception. Consequently, assuming an average user activity factor equal to p < 1/2 , the other-user interference power received at the base station is on the order of (1 + a)pM times the desired signal power.
Hence, proceeding as for the jamming case, we have
p(l + a)M = l/S = (Ws/R)/(Eb/lo).
Thus the maximum number of users/cell if all cells are fully loaded is
M = Ws/R p(l +a)Eb / 1o·
It can be shown that this loading is several times greater than achievable with either FDMA or TDMA, neither of which can reuse the frequency and time slots in adjacent cells or sectors . Note also the advantage provided by channel coding which reduces the required Eb/ Io ratio.
The brief history of digital cellular wireless , scarcely two decades in duration, began with carriers the world over adopting standards in the late 1980's based on TDMA technology. The North American carriers were slow to adopt digital because they already had millions of users of an analogue technology known as AMPS. The European Union, on the other hand, having no continent-wide analogue system with which to concern itself, readily adopted a new standard based on TDMA which came to be known as GSM , an acronym for Global System for Mobiles. Establishing roaming a.greements not only among all EU nations but with carriers in most nations worldwide, GSM has led the tide of cellular phone acceptance by consumers everywhere to the point that over two billion persons, over a third of the earth's population, are GSM subscribers. Meanwhile in North America and northern Asia, the CDMA technology also established a foothold, although initially with great difficulty. The proposal for CDMA to the North American cellular carriers was made at the end of the 1980's by Qualcomm Incorporated, a then small southern California company. It was rejected by all but a handful of companies led by Pacific Telesis, which agreed to consider it further. After several successful demonstrations and pilot programs, CDMA was finally established as an alternate standard in 1993. Today it has gained approximately an equal market share to GSM in North America, exclusivity in South Korea and a significant share in Japan and numerous other countries. Though it has only a bout a quarter as many subscribers as GSM, this still amounts to half a billion users. Moreover, where deployed its spectral efficiency, measured in users per cell per Megahertz of bandwidth, has significantly exceeded that of GSM. Both of these initial digital systems, GSM and CDMA,
The
Fou
ndat
ions
of
the
Dig
ital W
irel
ess
Wor
ld D
ownl
oade
d fr
om w
ww
.wor
ldsc
ient
ific
.com
by U
NIV
ER
SIT
Y O
F B
IRM
ING
HA
M L
IBR
AR
Y -
IN
FOR
MA
TIO
N S
ER
VIC
ES
on 0
8/22
/14.
For
per
sona
l use
onl
y.
are known as Second Generation (2G) Cellular technologies. In the past decade new cellular infrastructure (base stations and network equipment) has begun to be deployed for the so-called Third Generation (3G). For this new generation, which provides not only voice but data, messaging, video and multimedia services as well, practically all carriers worldwide are migrating to some form of CDMA. Although there are at least three varieties, all stem from the 2G CDMA system. Since technological progress, both in system concepts and solid state implementation, proceeds apace, already standards are being adopted for a Fourth Generation. 4G, which may become dominant in another decade, will also employ a spectrum spreading technique though quite different than CDMA. New features such as multiple antennas (MIMO), advances in channel coding (turbo and LDPC codes), as well as interference cancellation, will produce systems which are even more efficient than the current 3G systems. All this technological progress, however, will be overshadowed by the applications that will undoubtedly continue to be dreamed up.
Societal Benefits
In closing, it seems proper to ask what all this investment in brainpower and resources has provided for humanity. One is tempted to discount to near zero, if not below, all the phone chatter, inane messages, game playing, etc., that have been generated. On the other hand, it is evident that wireless has played an important role in the information technology transformation of the workplace and home, arguably not nearly as dominant as that of the Personal Computer and the Internet. But only wireless provides mobility and hence enables both PCs and the Web to be accessed anywhere and anytime. Wireless can also provide position information and thereby help to navigate and identify services in unfamiliar locations. Perhaps even more important than all else is the extent of its penetration. At most a billion persons have had access at one time to the Internet. Fewer own PCs. But half the earth's population has a cellphone. Contrast this percentage with that of wired telephone communication lines which never served more than one-tenth of the world's inhabitants. Now entire rural areas in some of the poorest countries are connected to the rest of the world through cellphone use. New generations of cellular devices will provide a vast variety of services at costs no greater than those presently for telephony alone, which will be made possible by further cost reductions from continued solid-state evolution. It is to be hoped that such progress will provide improved opportunities and quality of life for all the earth's people.
11
The
Fou
ndat
ions
of
the
Dig
ital W
irel
ess
Wor
ld D
ownl
oade
d fr
om w
ww
.wor
ldsc
ient
ific
.com
by U
NIV
ER
SIT
Y O
F B
IRM
ING
HA
M L
IBR
AR
Y -
IN
FOR
MA
TIO
N S
ER
VIC
ES
on 0
8/22
/14.
For
per
sona
l use
onl
y.