BREB Assistant Junior Engineer (IT) Written Test: 2019 (BREB)প্রশ্ন ও সমাধান

CDMA is a channel-access method that lets multiple transmitters share the same frequency band and the same time slot simultaneously, separating users purely by a unique spreading code instead of by frequency (FDMA) or time (TDMA).

How it works:

1. Each user is assigned a unique, (near-)orthogonal chip code (e.g., a Walsh code).
2. The transmitter multiplies (spreads) its narrowband data signal with its chip code, producing a much wider-bandwidth signal — this is spread-spectrum transmission.
3. All users' spread signals are transmitted together over the same band.
4. The receiver multiplies the received composite signal by the same code the sender used; because codes are orthogonal, every other user's signal averages to near-zero while the intended signal is recovered.

graph LR
  U1[User 1 data] -- code C1 --> AIR((Shared Frequency Band))
  U2[User 2 data] -- code C2 --> AIR
  U3[User 3 data] -- code C3 --> AIR
  AIR -- correlate with C1 --> R1[Receiver 1 recovers User 1]

Simple numeric intuition: if C1 = [1,-1,1,-1] and C2 = [1,1,-1,-1] (orthogonal, dot product = 0), the receiver correlating with C1 cancels out User 2's contribution and extracts only User 1's bit.

Where used: 2G/3G cellular standards (IS-95, WCDMA), GPS (each satellite has a unique code on the same frequency).

Exam takeaway: FDMA splits by frequency, TDMA splits by time, CDMA splits by code — allowing full-band, full-time simultaneous access with graceful capacity degradation as more users join.

Both are forms of acceptance testing performed before a public release, but they differ in who tests and where.

Aspect	Alpha Testing	Beta Testing
Performed by	In-house QA team / developers	Real end-users, outside the company
Location	Developer's site, controlled environment	Customer's real-world environment
Timing	Earlier — before beta	Later — final stage before public launch
Visibility of bugs	Finds functional bugs and crashes	Finds usability issues and edge cases missed internally
Reliability testing	Limited	Tests reliability, security and robustness under real use
Feedback source	Internal test cases	Direct customer feedback

Sequence in the release pipeline:

graph LR
  Dev[Development Build] --> Alpha[Alpha Testing - internal]
  Alpha --> Beta[Beta Testing - limited real users]
  Beta --> GA[General Availability Release]

Example: A software company finishes coding a mobile app. Its own QA team runs alpha tests to catch crashes and broken features. Once stable, the app is released to a small group of volunteer users (a "beta" release) who use it in daily life and report issues like confusing menus or battery drain before the app goes public.

Exam takeaway: Alpha = internal, controlled, earlier; Beta = external, real-world, later — both aim to reduce risk before general release, but beta specifically validates real-user experience.