Chapter 4: Concurrent Octa-Core RF Receiver Architecture
4.1 Introduction
4.1.1 Wireless Multi-Gbps Communication
Many applications require or benefit from high data rates far exceeding the capability of existing wireless technology. One of such example is the wireless transmission of uncompressed high definition video signals. The direct sending of uncompressed video signals greatly reduces power overhead for encoding and decoding video. Set-up boxes, Blu-Ray DVD players, and digital video cameras will the beneficiaries of this technology.
In general, the need for bandwidth is insatiable, much like the demand for CPU speed, static and dynamic RAM, flash memory, and external hard disk capacity.
To establish a high-speed wireless link, we need to first allocate communication channels. Each channel has a maximum achievable data rate, known as channel capacity C. Channel capacity is related to the bandwidth of the channel, BW, and the signal-to- noise ratio, SNR, in the following manner [49]:
log 1 , (4.1.1)
which shows that we can either use a large BW or a higher SNR to achieve a high communication data rate. However, based on the theoretic studies of digital communications [50], to achieve the same data rate using a smaller BW means to resort to
more efficient use of the spectrum. As an example, to extract 1 Gbps from 100 MHz of bandwidth channel obviously requires 10 bits per Hz, but only 1 bit per Hz from a 1 GHz bandwidth channel, and low-order constellations can be used to transmit and receive the data. The narrow-bandwidth system (100 MHz BW in this example), on the other hand, must use sophisticated signal modulation, often placing stringent demands on phase noise and power amplifier linearity (particularly for OFDM), and this translates into a system with less overall sensitivity. Much energy must be consumed in the baseband of the narrow-band systems to provide FFT and equalization functionality, which will end up consuming more energy per bit than the wide-bandwidth solution. Hence, a large bandwidth can reduce the complexity of communication system and reduce overall power consumption.
In the U.S., at the time this thesis is written, the Federal Communication Commission (FCC) has allocated three unlicensed bands with more than 1 GHz bandwidth. They are the 7.5 GHz bandwidth in the 3.1─10.6 GHz [51], the 7 GHz bandwidth in the 57─64 GHz (60 GHz band), and the 3 GHz bandwidth in the 92─95 GHz (90 GHz band) [52]. From RF circuits’ point of view, the implementation RF transceiver in 90 GHz band is similar to that in the 60 GHz band except at a higher center frequency. Between 60 GHz and 90 GHz, the 60 GHz attracts more research attention by far. In Section 4.1.2, we will compare the 3.1─10.6 GHz and the 60 GHz band.
4.1.2 Comparisons between the 3.1─10.6 GHz and the 60 GHz Band
Given an unlicensed 7.5 GHz bandwidth, the 3.1─10.6 GHz band has received less attention in the race of multi-gigabit wireless communication for two major reasons:
Firstly, this band’s wide baseband bandwidth over center frequency makes the conventional single-carrier-based analog frequency-translation scheme ineffective.
Secondly, the low equivalent isotropically radiated power (EIRP) limit enforced by the FCC makes the band unattractive to high performance applications. However, the 3.1─10.6 GHz RF signal has on the average 20 dB less channel path loss compared to the 60 GHz band to justify its low EIRP. In addition, at lower microwave frequency, RF signal would more easily penetrate through or diffract around obstacles along the wireless link, which makes non-line-of-sight communication possible. Furthermore, the 60 GHz band is much closer to the transistor fT compared to its 3.1─10.6 GHz band, and this implies a more inefficient power generation and amplification for the RF frontend circuitry.
Also, RF packaging in the 60 GHz will be more difficult due to its high frequency.
4.1.3 Previous Works using the 3.1─10.6 GHz Band
The unlicensed 3.1─10.6 GHz band has a 7.5 GHz total BW and can be used for short- distance multi-gigabit wireless communication. Previous works utilizing this band fall under two major categories: time-domain impulse-based [53][54][55], or WiMedia’s MB- OFDM compliant (frequency-hopping based) [56][57]. The impulse-based method has low spectral efficiency and is susceptible to inter symbol interference (ISI) for high data-rate
communication due to the relatively large multipath delay spreads over the pulse period, as shown in Figure 4.1(a). Equalization of the received impulse signals with ISI will be difficult to be implemented due to multi-GHz clocking speed of the digital signal processor. As a result, it is difficult to establish a reliable Gbps wireless link using the impulse-based approach.
Figure 4.1: Previous works on the 3.1─10.6 GHz band: (a) Impulse-band and (b) Frequency-hopping based
The MB-OFDM-compliant approach is essentially a diversity-improved narrow-band (frequency-hopping) method that utilizes a 528 MHz band out of the available 7.5 GHz RF spectrum at a given time window, as shown in Figure 4.1(b). The average spectrum over long measurement time window will follow the FCC’s radiation emission regulation on ultrawide-band signal. However, since only 528 MHz is effectively used for wireless
time time
(a)Impulse-based
freq. freq.
(b)Multi-band OFDM
3.1 average 10.6 3.1 10.6
spectrum average
spectrum ISI
communication, it requires a complex modulation scheme to achieve a multi-Gbps data rate. In addition, the requirement of frequency hopping will increase circuit overheads in the hopping LO implementation, and result in high power consumption.
We propose to divide the 3.1-10.6GHz into several RF channels and use them independently and concurrently. We will discuss this communication scheme and its RF receiver implementations.