Network Concepts and Standards
Section: Data and Multimedia Systems, Infopad,
Broadband CDMA Design,
Monolithic RF Circuitry
Over the past several years, wireless communications have seen dramatic advances in two distinct areas. On one hand, the demand for portable telephone services has resulted in intense research efforts to improve performance and increase capacity through digital transmission. Such systems focus on wide-area narrowband communications, providing low- bandwidth network services to individual users in a portable fashion. On the other hand, the need for more flexible computer networks has led to the advent of wireless LAN's such as the Motorola Altair. Such systems focus on local-area wideband communications, providing networking services to individual computers but usually not easily portable.
However, the distinction between these two systems is rapidly blurring. As laptop computers place mobile computing resources in the hands of individuals, wireless technologies capable of providing wide-area, wideband services will clearly be needed. With this merging of computation and communications, individual users will have instantaneous and portable access to fixed information networks via a lightweight mobile unit. Furthermore, users will be capable of transferring data to other users and accessing fixed computing resources without any constraints on where or when such access takes place. The mobile unit will support a myriad of services, including full-motion digital video and high-quality audio, and combines the functionality of today's analog mobile telephones, radio pagers, and laptop personal computers.
Since portability places severe constraints on the physical weight of the terminal, the available battery power is quite limited. Thus, power minimization is crucial; power reduction in both the digital and analog hardware must be achieved. To this end, the terminal should only carry the bare minimum of computing resources necessary to support its functionality; user computation should be mainly performed by large, non-portable computing facilities, with the high-speed wireless link serving as the terminal's means of accessing the fixed computation servers and data networks. Direct point-to-point wireless communication is not allowed; the link only provides the final interface into the wired data network, much like a conventional telephone handset serves as the link into the telephony system. Whereas the capability of moving massive amounts of digital data within networks already exists, the problem of easily getting data in and out of those networks is now addressed.
With the shift in emphasis from computation inside the mobile unit over to communications outside, it is evident that development of a wideband link capable of supporting the required user bandwidth becomes paramount. It is important to note that wideband video data is only supported in the downlink to the mobile; the uplink is used only for low-rate speech and control data. Although the system is still full-duplex, this asymmetry must be accounted for in the design, as the bandwidth requirements for maintaining the uplink are thus considerably less than those for the downlink.
Even with a reliable, guaranteed latency backbone network, the issues of user capacity and overall system bandwidth consumption still remain. With the best compression schemes developed to date, data rates in excess of 1 Mbps per user would be needed to support full- motion video. However, this data rate is not needed on a continuous basis; when regular computational tasks are being performed on compute servers back on the network, such as using a word processor or a spreadsheet, the screen changes only slightly on a frame-by-frame basis and over only a small region, usually on the order of a single character or a few pixels. Hence, the peak data rate required by a user may easily be much larger than the overall time-average data rate. Thus, minimizing the overall system bandwidth consumption while supporting a large number of users accessing data simultaneously is of paramount importance.
The advantages in improved spectral efficiency afforded by cellular systems have long been known; they have already been employed to a limited extent by existing analog mobile telephony systems, with cell sizes on the order of square kilometers. Due to the tremendous bandwidth requirements of such a portable terminal, it is inevitable that future wideband systems will exploit picocellular networks, with cell sizes on the order of meters to employ as much frequency reuse as possible.
The primary interference mechanism in such a cellular transmission environment is that of multipath fading, in which the transmitted signal interferes at the receiver due to reflections off of objects. From statistical measurements, the short-range indoor channel that we have been considering has typical delay spreads ranging from 20 nsec to 60 nsec with Rician-distributed fading characteristics. This is far different from the typical outdoor large- cell transmission environment that has much more severe Rayleigh fading characteristics. Likewise, the indoor transmission environment changes far more slowly than the outdoor, given that the mobile unit will likely be stationary during use.
With this in mind, direct sequence spread-spectrum, or code-division multiple access, becomes attractive for use in the downlink. It is naturally "immune" to multipath, since it can (with sufficient spreading) resolve the interfering multipath arrivals and combine them via a RAKE receiver as an intrinsic form of diversity. Also, CDMA can easily accommodate a wide range of user data rates by varying the transmit power for each user as a function of the required data rate. This concept of power modulation has already been exploited to improve the effective system capacity of next-generation digital cellular systems. It is important to realize that traditional impairments of CDMA systems such as near-far interference and unsynchronized codes do not exist for the downlink, since all downlink transmissions originate from a single point -- the base station. The broadcast nature of the signal, combined with the ability to resolve multipath arrivals and support variable data rates, makes CDMA extremely attractive for use in the high-performance wireless downlink in the system.
With these factors in mind, the system performance parameters are summarized in table 1. The system itself is a wideband extension of the U.S. IS-95 digital cellular CDMA standard, which utilizes a transmitted synchronization tone for timing recovery and Walsh orthogonal codes to multiplex users. A basic raw user data rate of 2 Mbps is assumed to allow a margin for channel error correction as well as the ability to explore various compression algorithms. The raw data is then modulated into a 1 Mbaud DQPSK symbol stream. In determining the cell size of 5 meters, a typical office environment consisting of soft-partition cubicles is assumed, with each cell typically containing 12-16 active users. Of those 16 users, it is assumed that approximately half are demanding the full 1 Mbaud data rate for video use, while the remainder are utilizing 128 kbaud (256 kbps) each for lower data rate applications such as voice or text/graphics. With a seven cell reuse pattern at maximum capacity, a chipping rate of 64 Mchip/sec is sufficient to support this. Furthermore, given an average delay spread of 40 nsec, the resolvable number of paths is given by
Tdelayspread N = ------------- + 1 Tchipwhich translates to 2-3 resolvable paths by the receiver and dictates the size and complexity of the RAKE receiver in the digital baseband hardware.
With a chipping rate of 64 Mchip/sec, a transmit bandwidth of 80 to 100 MHz will be required. Although this is a considerable amount of spectrum, this is amortized over large numbers of people using this spectrum simultaneously within multiple buildings. Considering that this bandwidth is designed to support full motion video and other multimedia network services for all users, this allocation of spectrum is not unreasonable given the level of service provided by the system, especially when compared to the spectrum allocated for existing systems such as NTSC television.
For development and system verification, a baseband equivalent model for the transceiver system has been developed in the U.C. Berkeley Ptolemy simulation environment. The system utilizes a delay-locked loop keyed to the timing pilot tone to achieve synchronization/lock, and furthermore uses the pilot tone to provide an estimate of the channel impulse response. To simulate multipath effects, a baseband equivalent channel model has been developed from measured statistics. This allows us to optimize the system for transmit power levels, number of parallel receivers, quantization levels in the receiver, etc.
As an example, the BER as a function of the number of users is shown, simulated under various conditions. By Monte-Carlo methods, a worst-case (extreme fade) channel and a "best-case" channel were determined from the model, and then both channels were simulated with and without maximum-ratio RAKE combining. The benefits of the three-tap RAKE receiver are apparent; under the worst-case fade condition, the RAKE combiner provided up to a factor of 1000 improvement in BER over the uncompensated case. As the number of users increases; however, this not only degrades the base signal-to- noise ratio, but also degrades the accuracy of the channel estimate, reducing the effectiveness of the RAKE combiner.
FIGURE: BER vs. Number of users
With high data rates and variable throughput requirements, developing the downlink has been of greatest importance, with CDMA providing an attractive means of both channel compensation and multiple access. The uplink, consisting of speech and pen input data, will require significantly lower data rates per user data rates (on the order of 32 kbps), and due to near-far effects and the unsynchronized nature of the uplink signals, CDMA is not nearly as attractive. Instead, more conventional TDMA, frequency-hopped, or orthogonal frequency- division multiple access techniques are being explored for use in the uplink.