This snippit allows me to look at the signal strength of the cell towers used by my ZTE usb modem.

usb_gprs_signal_strength.rb

# quick hack to monitor signal strength
# based on info from: http://www.pcurtis.com/ubuntu-mobile.htm#signal_strength

require 'rubygems'
require 'serialport'
require 'time'

class GSM
  
  def initialize(options = {})
    @port = SerialPort.new(options[:port] || 3, options[:baud] || 38400, options[:bits] || 8, options[:stop] || 1, SerialPort::NONE)
    @debug = options[:debug]
    cmd("AT")
  end
  
  def close
    @port.close
  end
  
  def cmd(cmd)
    @port.write(cmd + "\r")
    wait
  end

  def wait
    buffer = ''
    while IO.select([@port], [], [], 0.25)
      chr = @port.getc.chr;
      print chr if @debug == true
      buffer += chr
    end
    buffer
  end
end

gsm = GSM.new(:debug => false, :port => '/dev/tty.ZTEUSBATPort_')
while true
  csq = gsm.cmd("AT+CSQ")
  # RSSI (Received Signal Strength Indicator)
  # 0 = 113 dBm or less
  # 1 = 111 dBm
  # 2 to 30 = 109 to 53 dBm
  # 31 = 51 dBm or greater
  # In practice, if the signal strength is below 10 then GPRS connections
  # are unreliable. Values around 15 are good, 25 is excellent.
  # mine hover between 9 and 12 in the Oropi
  puts csq.scan(/\+CSQ\:\s*?(\d+),/)
  gsm.wait()
end

Get's as low as 4, and often at 7 I'm still connected.... however the quality is poor.