Breakthrough Ideas (continued)
CDMA’s Unique Revolutionary Competitive Advantages.
The generative DNA in Qualcomm’s intellectual property¾both in its patent portfolio and its cdmaOne technology¾consisted of the commercialization of five unique features: (1) channelization¾spread spectrum coding that enables universal frequency reuse; (2) fast and accurate power control; (3) rake receivers; (4) soft handoff; and (5) RAN system design. Their architectural recipes for combining software-as-codified-knowledge into MSMs, CSMs, and RAN system designs synergistically amplified their added value, making the integrated CDMA RF system as a whole much more than a mere sum of parts.
1. Channelization. Universal frequency reuse is enabled by channelization. Qualcomm’s channelization uses coding to identify a unique user and a proprietary code generator that orthogonally (which means it maximizes the distinctiveness of each unique code from all others) spreads the modulated digital message across the spectrum of frequencies.
According to Viterbi, CDMA’s orthogonal coding has a significant side effect—“each user’s transmitted bandwidth is greatly enlarged by making the coded signal’s symbol rate, or clock, run much faster than the digital data rate of the source. For example, if the data rate is 10K bits/sec, the code clock symbol rate (which is often called the “chip” rate) may be 1M bit/sec or 100 times as fast.” Thus, coding at the higher clock symbol rate is how Qualcomm spreads the spectrum.
Beginning as anti-jamming and low probability of intercept solutions for the military, Viterbi noted that if the enemy does not know the code, the signal appears to be noise, but also, and more significantly, when the enemy tries to jam the signal, the friendly receiver’s demodulator transforms the enemy jamming signal into wideband noise, reducing its effectiveness by a factor known as the “processing gain” or “spreading factor,” which is the ratio of the clock code symbol rate to the original source’s bit data rate (100 times in the example just given). “Essentially, spread spectrum or CDMA is the ‘best’ signaling modulation for the ‘worst’ form of jamming signal.” This minimax law of physics yields a powerful competitive advantage¾it reduced interference and improved the signal-to-noise ratio by spreading coded signals across the spectrum.
Generating a CDMA signal proceeds through five discrete coding steps: (1) analog to digital conversion¾a codec chip, using Pulse Control Modulation, transforms analog voice into a digital stream; (2) variable rate vocoding (voice encoding)—located in the BTS and phone, the vocoder compresses speech at four different rates, dependent on the degree of voice activity; (3) encoding and interleaving¾uses coding algorithms in the MSM and CSM to reduce burst errors caused by interference and fading, both by building in redundancy (and using majority logic rules to recover) and scrambling (and subsequent de-scrambling) code by interleaving it; (4) channelizing the signals—the process of encoding signals before spreading them over the spectrum uses two important codes that each uniquely identify the user, first, by using orthogonal Walsh codes on the forward link that are recoverable only by a receiver applying the same Walsh code, and second, by using Pseudorandom Noise (PN) that yields 4.4 trillion combinations of code on the reverse link; and (5) conversion of the digital signal to a RF signal—the BTS multiplexes channelized data from all calls into one signal, then converts the digital signal to a UHF signal in two steps. First, the baseband signal is converted to an intermediate frequency (IF) that is, in turn, further converted and attached to a single carrier wave’s ultra high frequency signal. The digital to analog conversion in a mobile’s receiver reverses this signal generation process to recover voice.
Channelizing the signal is the process of modulating the RF signal to spread it across the spectrum. The core principle of spread spectrum is the use of a unique coded signal for each user attached to the same noise-like carrier across bandwidths much wider than required for point-to-point communication. The direct sequence method of spread spectrum (SS) artificially increases the bit rate by breaking each bit into a number of sub-bits called “chips.” Direct sequence spreading uses a higher chip rate PN-code to increase bandwidth and achieve the processing gain in performance. Breaking each bit into, say, 10 chips, called a spreading factor of 10, increases both the data symbol rate and the bandwidth by 10. Multiplying the original modulated signal by a higher spreading rate PN-code produces a wider bandwidth that is proportional to number of chips, producing an equivalent processing gain.
A SS receiver separates the desired coded information from all possible signals by multiplying it by a local replica of the PN code that, because it matches the assigned specific code, cancels it. When demodulating the signals, the SS correlator acts like a specially matched filter, responding only to SS signals with identical signal characteristics and PN-code to de-spread and recover the desired signal. The correlator collapses the SS signal back down to the original narrow bandwidth centered at the carrier frequency. This signal is passed through a bandpass filter that rejects all surrounding frequencies to create the processing gain. Processing gain refers to the level of improvement in the SNR from the spreading and de-spreading process. As you have learned, the increase in SNR is a function of the higher chip-spreading rate (number of chips per data bit) used.
In addition to the improved quality of the signal by using higher chipping rates, the unique spreading code for each receiver reduced interference from the many SS and traditional radios within a cell. Unlike traditional narrow band radio receivers, the SS correlator does not respond to many forms of natural, man made, or artificial noise or interference, including narrow band transmissions, which the correlator spreads as noise across its spectrum. Also, SS signals do not interfere with narrowband transmissions, for instance, in a hearing aid.
Because it spreads energy over a wide band (that is, it transmits at a lower spectral power density), spread spectrum is ideal for metropolitan areas with large blocking rates. A blocked call sends unwanted messages of “service lost or service not available.” With CDMA’s greater spectral efficiency, there is less blocking. Spectral efficiency is measured in Erlangs per unit service area, per MHz. Erlang blocking probability is equal to the calling rate times the average call length. The popularity of wireless telephony created demand that frequently exceeded the available channels in FDMA and TDMA modes, producing high blocking rates.
A basic principle of physic is that carrier-wave power decays with increasing distance. The drawback in narrow band data communication is its limited bandwidth, which requires signals to be transmitted at higher power to ensure correct reception in a channel corrupted by Gaussian noise. The gain in efficiency of spread spectrum creates competitive advantage through its ability to achieve quality data reception at lower power levels.
If you examine narrowband and spread waveforms in a graph with frequencies on the abscissa and Power Spectral Density on the ordinate, their shapes are respectively like an extremely tall tower (narrowband) and a low evenly sloping hill (wideband). The area under the spectral density curve represents total signal power. Thus, signals with equivalent total power may have either a large signal power concentrated in a small bandwidth or a small signal power spread over a large bandwidth. Thus, the spread spectrum solution increased spectral efficiency and reduced interference, error rates, and blocking probability.
Of course, this entire signal generation process is transparent to the user’s experience because overhead code channels are simultaneously transmitting the information required for command and control processes. A code channel is a stream of data designated for a specific use or person that utilizes code to separate functions or unique individuals.
Digital phones operate as a duplex system, which means forward and reverse links use different channels. In the forward link from BTS to mobile, there are four channels: (1) Pilot Channel—the BTS constantly transmits a higher dB pilot signal, using a specific Walsh code reserved for that purpose, that the mobile initially uses to acquire the system and subsequently uses to monitor pilot signal strength to adjust the optimal power required for transmission back to the BTS; (2) Synch Channel—the BTS constantly transmits over the sync channel, permitting the mobile to stay synchronized with GPS system time; (3) Paging Channel—the BTS transmits overhead information, commands and traffic channel assignments during call set-up, and the like on up to 7 paging channels; and (4) Forward Traffic Channel—during the call, CDMA uses between 55 and 61 forward traffic channels to send both voice and overhead control data during a call.
In the reverse link from mobile to BTS, the mobile uses two channels: (1) Access Channel—when not assigned to a traffic channel, the mobile uses the access channel to (a) register with the network, (b) originate calls, (c) respond to pages and commands, and (d) transmit overhead messages; (2) Reverse Traffic Channel—during a call, the mobile transmits voice data (or overhead information) to the BTS.
2. Power Control. Without fast and accurate power control to equalize reception at the BTS, the capacity of the system would be limited either by interference from unnecessarily strong signals to-and-from mobiles nearby or from weak and fading signals to-and-from far-away mobiles. To solve the interference problem created by overly strong signals and the fading problem of overly weak signals, feedback loops control power usage to get it just right. The optimal power for transmission is just strong enough to ensure reception.
Open loop power control is the mobile’s estimate of optimal power based on the strength of the signal the mobile receives from the pilot channel. Whenever the mobile transmits, open-loop-power-control adjusts power up or down to correspond to the received strength of the pilot signal. During a call, the BTS uses closed-loop-power-control of the reverse channel, up to 800 times per second, adjustable in 84-steps of 1 dB, based on its constant surveillance of signal-to-noise and error rates. In addition, the BTS uses open-loop- power-control on the forward link to adjust independently the power used to propagate its own signals.
In traditional wireless, the strongest signals from nearby mobiles capture the demodulator at the base station. The difference between near and far signals, called their propagation path loss, may be many tens of dB. By making the received powers from all users approximately equal, a nearby subscriber can no longer drown those far away. When the mobile is closer to the base station, it uses less power, which also prolongs battery life.
Moreover, this system of power-control dynamically changes the “size” of cells, increasing the ability of a congested cell to ensure reception by permitting an overflow of signals into less used cells. In a low use cell, power is so low the cell “shrinks,” permitting a congested cell to use its power to overcome interference. Universal frequency reuse and power control free the RAN to adjust its cell-size dynamically. However, because GSM/TDMA use completely different frequencies in adjacent cells, it is forced to handoff at rigid cell borders where, given congestion, blocking becomes a common hazard.
What’s more, once power control is available, the system designer and the operator have the freedom to trade quality of voice service for capacity because capacity and the SNR are reciprocal functions at a 3 to 2 ratio. Thus, by using a variable vocoder, the quality of sound can be traded off to increase cellular capacity. This can be done dynamically as needed from minute-to-minute. Such flexibility is always a significant asset to an operator.
Qualcomm led the innovation of superior vocoders, which convert the voice to digital signals or reconvert from digital to voice. Careful to include all human phonemes, Qualcomm’s inclusive and standardized solution can even convert the clicks of the Kung Bushmen of the Kalahari Desert into digits. After blazing the trail in variable rate vocoders, Qualcomm’s Selectable Mode Vocoder (SMV) is a breakthrough technology in itself, providing significant capacity and quality gains. The SMV algorithm chooses the optimal encoding rate from four options based upon a user’s speech input to ensure that sound quality remains high.
When compared to GSM’s new Adaptive Multi Rate (AMR) vocoder, which slowly switches between fixed encoding rates based on RF channel conditions, listening tests at an independent lab demonstrated that SMV delivered better quality than AMR, even when using the lowest coding rate that results in the most network capacity. Moreover, the SMV standardized solution gives the operator either static or dynamic flexibility to tradeoff small quality losses for large gains in system capacity, up to 75% more capacity when using the lowest coding rate.
3. Rake Receivers. Just as power control solved the near-far problem of interference, a rake receiver transforms the multipath problem into an improved signal. Radio waves follow multiple paths created as they bounce off obstacles. Obstructions cause micro-time-delays in the multipath signals. In FDMA, multipath problems created rapid fading for mobile users. In TDMA, multipath signals arriving at the wrong time interfered with someone else’s time-slot. Thus, the multipath problem typically degrades or cancels the signal.
However, by combining three receivers into one, a rake receiver detects the three strongest multipath signals and combines them into one clear signal, raking in time-diverse signals to integrate them. This summation of multipath signals both resists fading and maintains a favorable signal-to-noise ratio. Moreover, for every foot a mobile unit travels, these multipath complexities cause hundreds of gyrations in optimal power, meaning as low as possible but sufficient to ensure error-free reception. Both the mobiles and the BTS employ rake receivers to improve the SNR and control the power their transmitters use to send signals.
What’s more, using spread spectrum signals permits the accurate calculation of the relative arrival values of micro-delayed signals, permitting reasonably accurate measurement of the present location of a handset. Also, because it used information from diverse multipath signals, Qualcomm pioneered the rake receiver as one of the first uses of diversity technology. Thus, the rake receiver solution transformed the multipath problem that distorted or cancelled frequencies/time-slots in it rivals’ systems into an information-gain asset that facilitated CDMA system functioning.
4. Soft Handoff. Handoff is the procedure for transferring a call from one base station to the next. In traditional wireless, the new base station must allocate a new frequency channel to a new mobile as it enters its cell. When a channel is not available, the call is dropped. Whereas its rivals used a hard handoff (break-to-make), which dropped calls due to fading during the crucial time-window of searching for a new BTS to accept the handover, CDMA used a soft handoff (make-before-break). The spread spectrum architecture and its digit-signal-processing chips reduced worst-case interference and replaced the need for multiple radios used by TDMA/GSM, with each set to a single frequency.
Because all CDMA cells use the same SS RF carrier frequency, the MSM is designed to search for and detect pilot signals from new cells. The MSM establishes and maintains synchronous timing by simultaneous communications with the GPS-synchronized CSMs of multiple base stations. These same relations with pilot signals and timing also occur within a BTS’s sectors, where the handoff is classified as soft-soft. In a soft (or soft-soft) handoff, the mobile can be handed back and forth between BTSs (or sectors) as necessary to maximize reception. The soft handoff principle, which required system synchronization, eliminated the annoying dropping of circuit switched calls common in systems that required the acquisition and allocation of a new radio channel among a set of multiple radios, each designed to transmit a single frequency |
